Priorities for ecological research on cetaceans in the Galápagos Islands

Abstract

Located in the eastern tropical Pacific, the Galápagos Islands are an oceanic insular ecosystem subject to strong environmental variability driven by local and regional processes. Past research has shown that such conditions can attract and sustain at least 23 cetacean species, out of which 14 are common, including nine Delphinids, one Ziphiid, one Physeterid, and three Balaenopterids. These species occupy both coastal and oceanic habitats, most are present year-round, and a few are migratory. However, research on cetaceans in Galápagos has been sporadic and chronically underfunded and is not currently considered a priority in the research agenda for Galápagos. Based on a review of existing information and an assessment of knowledge gaps, here we identify priorities for ecological research on cetaceans in Galápagos along five topical areas: 1) spatiotemporal occurrence, 2) population assessment, 3) health assessment, 4) social ecology, and 5) trophic ecology. Addressing these knowledge gaps will also help inform actions to preserve cetacean biodiversity and to manage human activities involving or affecting cetaceans in Galápagos. Given the logistical and funding challenges of conducting cetacean research in Galápagos, we recommend optimizing data sampling and accessibility via integrated research protocols and open data repositories. We also recommend capitalizing on local citizen science activities, such as those conducted from cruise ships and whale-watching tours, which can serve as platforms of opportunity for obtaining basic data, thereby contributing to long-term data acquisition. Our proposed priorities should be assessed by Ecuadorian and Galápagos governmental institutions in broad and inclusive consultation with stakeholders and the scientific community prior to development and implementation of a research agenda. Collectively, these efforts will advance our understanding of the ecological role that marine megafauna, such as cetaceans, play in Galápagos and other oceanic islands, including maintaining large-scale connectivity and mitigating climate change.

1 Introduction

A casual glance at oceanic islands might suggest a collection of isolated terrestrial habitats in an otherwise blue desert. A view from underwater, however, reveals that these insular ecosystems are much more than remote oases for terrestrial biota (e.g., Hasegawa et al., 2009; Gove et al., 2016). They provide habitats that attract a wealth of marine life from larval to adult stages and serve as steppingstones for migratory species (e.g., Pinheiro et al., 2017; Fontoura et al., 2022). Due to their long life spans and wide-ranging movements, marine megafauna in particular can play a key role in linking coastal and oceanic insular ecosystems (e.g., Hindell et al., 2020; Klimley et al., 2022). Indeed, ecological connectivity via local, regional, and long-distance movements is increasingly recognized as an essential process in the life history of large fishes, sea turtles, and marine mammals inhabiting oceanic insular ecosystems (e.g., Ketchum et al., 2020; Rooker et al., 2019; Ferreira et al., 2021) that is worth understanding and preserving in and of themselves (Game et al., 2009; Dunn et al., 2019; Klimley et al., 2022), as well as to help preserving the ecological communities these “umbrella” species are part of (Caro and O’Doherty, 1999).

Belonging to Ecuador, the Galápagos Islands are an oceanic insular ecosystem located in the eastern tropical Pacific (ETP) 1,000 km off mainland South America (Figure 1). Despite their equatorial location, the Galápagos are subject to seasonal variability resulting from the annual intensification of the southeast trade winds and equatorial upwelling (Palacios, 2004; Sweet et al., 2007; Forryan et al., 2021), and to strong interannual variability driven by the El Niño-Southern Oscillation (Palacios et al., 2006; Fiedler and Lennert-Cody, 2019), which is intensifying with climate change (Mendelssohn et al., 2005; Dueñas et al., 2021). Variable exposure of the islands to this environmental forcing results in a marked environmental zonation (Harris, 1969; Wellington et al., 2001; Schaeffer et al., 2008), which gives rise to a distinctive biogeographic patterning across the archipelago (Edgar et al., 2004; McKinley et al., 2022). Regarding migratory connectivity, the Galápagos are a well-known stop-over for marine megafauna, including teleosts, elasmobranchs, and chelonians that regularly move among oceanic archipelagos in the ETP (e.g., Todd and Grove, 2010; Cambra et al., 2021; Silver‐Gorges et al., 2020; Klimley et al., 2022).

Figure 1

For the equally highly mobile cetaceans, however, this regional connectivity has not been directly established, except in a few cases (Torres-Florez et al., 2015; Cantor et al., 2016; Hucke-Gaete et al., 2018; Pacheco et al., 2019). Nevertheless, the slow life history, high trophic position, and large-scale movements of cetaceans are such that their ecological roles (Bowen, 1997; Kiszka et al., 2015; Kiszka et al., 2022) could have broader implications within and beyond Galápagos waters. In their role as predators, cetaceans participate in top-down control of the marine communities they inhabit and link oceanic and coastal ecosystems (e.g., Estes et al., 1998). Further, by acting as nutrient recyclers and carbon reservoirs, cetaceans provide ecological services that are key for oceanic productivity and climate change mitigation at global scales (Martin et al., 2021; Pearson et al., 2022).

A collaboratively derived environmental research agenda was recently developed for Galápagos based on a participatory process involving governmental entities, universities, and non-governmental organizations (Izurieta et al., 2018). This process prioritized 50 research questions, including several with a marine or conservation theme, although these questions were selected for their importance for policy makers and practitioners, and the authors acknowledged the value of conducting a parallel exercise for identifying fundamental research questions (Izurieta et al., 2018). This paper is an answer to this call, by identifying research questions relevant to cetaceans in Galápagos. Toward this goal, here we summarize the existing legislative and protective framework of relevance to cetaceans in Galápagos, review the available scientific information on cetacean occurrence in Galápagos, identify knowledge gaps, and provide recommendations for advancing ecological research.

2 Legislative and protective measures

Despite having been settled by humans only relatively recently, the Galápagos Archipelago has a long history of indiscriminate human exploitation, primarily driven by extractive pressures on the rich marine resources, which has resulted in serious conservation issues that continue through today (e.g., Ruttenberg, 2001; Boersma et al., 2005; Awkerman et al., 2006; Jacquet et al., 2008; Sonnenholzner et al., 2009; Schiller et al., 2014; Ruiz et al., 2016; Usseglio et al., 2016; Alava and Paladines, 2017; Cerutti-Pereyra et al., 2020; Bonaccorso et al., 2021). Starting with 18^th century whaling, the sperm whale (Physeter macrocephalus) was among the first species to be targeted, and by the mid 19^th century the local population had become depleted (Shuster, 1983; Hope and Whitehead, 1991; Whitehead and Hope, 1991; Whitehead et al., 1997). To prevent further whaling, in 1990 the government of Ecuador declared a “whale refuge” in all its jurisdictional waters, that is, the 200-nautical-mile Exclusive Economic Zone (EEZ) (Acuerdo Ministerial No. 196, 1990; Evans, 1991; Merlen, 1992), and subsequent legislation prohibited whale hunting indefinitely in its EEZ (Registro Oficial, 2000; Registro Oficial, 2002).

In 1998, Ecuador created the Galápagos Marine Reserve (GMR; Figure 1A), extending 40 nautical miles seaward from the coastal baseline and covering an area of 138,000-km² (Ley Orgánica de Régimen Especial Para la Provincia de Galápagos, 1998), making it one of the largest marine protected areas in the world. In 2001, UNESCO designated the GMR as a Natural World Heritage Site, in recognition of the astonishingly rich and diverse marine communities inhabiting it (Heylings et al., 2002). The boundaries of the GMR were revised in 2022, resulting in a slightly larger area of 142,759 km² (Acuerdo Ministerial No. MAATE-2022-039, 2022).

Despite the protections offered by the GMR, industrial fishing pressures targeting squid, high-trophic level fish, and sharks have continued to increase just beyond the reserve—and sometimes within it due to weak enforcement—and are driving a regional conservation crisis (Edgar et al., 2011; White et al., 2020; Bonaccorso et al., 2021; Global Fishing Watch, 2021). This led the Ecuadorian government to announce in January 2022 that it would create a new marine protected area within its EEZ, named “Hermandad Marine Reserve,” a 60,000-km² corridor adjacent to the GMR (Decreto Ejecutivo No. 319, 2022; Acuerdo Ministerial No. MAATE-2022-019, 2022; Acuerdo Ministerial No. MAATE-2022-041, 2022) (Figure 1A). The new marine reserve is part of a regional initiative that seeks to connect the insular marine protected areas of Ecuador, Costa Rica, Panama, and Colombia to achieve greater protection of their shared marine resources (Decreto Ejecutivo No. 319, 2022; Acuerdo Ministerial No. MAATE-2022-019, 2022).

Finally, in November 2022, the IUCN Marine Mammal Protected Areas Task Force announced that the Galápagos Archipelago would become one of its Important Marine Mammal Areas in the southeast temperate and tropical Pacific Ocean, recognizing that its waters harbor small and resident populations of endemic pinnipeds, provide habitat for reproductive and feeding activities of vulnerable whale species, and support aggregations of a high diversity of medium and small cetaceans (IUCN-MMPATF, 2022). These criteria provide a basis for further prioritization of conservation measures of relevance to marine mammals in Galápagos (Tetley et al., 2022).

3 A review of cetacean occurrence in Galápagos

Contemporary research on cetaceans in Galápagos waters has been largely sporadic and chronically underfunded. Collection of information on cetacean presence began in the 1970s, primarily as a casual but growing interest by local naturalist guides working aboard cruise ships touring the islands (Day, 1994; Merlen, 1995), an activity that continues through today (e.g., Palacios and Salazar, 2002; Denkinger et al., 2013; Denkinger et al., 2020). Also in the 1970s, a program to monitor dolphin mortality incidental to purse-seine tuna fishing operations in the ETP was established by the U.S.’s National Oceanic and Atmospheric Administration (NOAA) and subsequently implemented by other nations participating in the fishery (Ballance et al., 2021). These programs placed fisheries observers on tuna vessels, who collected cetacean sightings throughout the ETP, including Galápagos waters (Fiedler and Lennert-Cody, 2019). Additionally, between the mid 1980s and mid 2000s, NOAA also conducted dedicated research vessel surveys to estimate dolphin abundance and trends in the ETP, including Galápagos waters (Wade and Gerrodette, 1993; Hamilton et al., 2009).

A long-term study of sperm whales in Galápagos waters began in the mid 1980s, which has provided the basis for much of the contemporary knowledge on the species (e.g., Whitehead, 2003; Eguiguren et al., 2021). Several other research expeditions have been undertaken in Galápagos with the explicit purpose of studying cetaceans since then (Lyrholm et al., 1992; Smith and Whitehead, 1999; Palacios, 1999a; Palacios, 2000; Wise et al., 2009; Félix et al., 2011; Biggs et al., 2017; O’Hern et al., 2017). With the establishment of the GMR, since the early 2000s collaborative efforts between Galápagos-based and international scientists have continued to generate crucial knowledge about marine mammals in the area (Palacios and Salazar, 2002; Palacios et al., 2004; Denkinger et al., 2013; Biggs et al., 2017; O’Hern et al., 2017; Eguiguren et al., 2019; Denkinger et al., 2020; Eguiguren et al., 2021; Páez-Rosas et al., 2021). Finally, cetacean presence in Galápagos has also been gleaned from documentation of live-stranding events, beach-cast specimens, and osteological specimens in museums (Palacios, 1996; Palacios et al., 2004).

Several studies have attempted to characterize cetacean species diversity in Galápagos waters (Day, 1994; Merlen, 1995; Smith and Whitehead, 1999; Palacios and Salazar, 2002; Palacios, 2003; Denkinger et al., 2013). At least 23 species have been documented, although the relative sighting frequency of each species varies between studies because of differences in geographic coverage, sampling effort, and methodological approaches. To objectively assess the spatial and temporal patterns of cetacean occurrence in Galápagos waters, for purposes of this review we used the regional compilation of marine mammal sightings by Palacios (2003) and augmented it with more recent compilations of sightings for humpback whales (Megaptera novaeangliae) by Félix et al. (2011) and sperm whales by Cantor et al. (2017). We defined the geographic extent of our study area as a 4×4-degree box bounded by 88.5-92.5°W and 2°S-2°N (Figure 1B) and limited the marine mammal sightings data to this extent, resulting in a total of 3,227 sightings spanning the period 1973-2014 (Table 1, Figure 2A). As these sightings were collected using different methods, no standardized measure of effort was possible and instead we used the general spatiotemporal pattern in the data as a proxy for coverage (Figures 2A and 3). Out of the 20 cetacean species in our sightings compilation (Table 1), 14 were most common (i.e., species with 10 or more sightings). These included nine Delphinids (Stenella attenuata, Stenella longirostris, Stenella coeruleoalba, Delphinus delphis delphis, Tursiops truncatus, Grampus griseus, Globicephala macrorhynchus, Pseudorca crassidens, and Orcinus orca), one Ziphiid (Ziphius cavirostris), one Physeterid (P. macrocephalus), and three Balaenopterids (Balaenoptera edeni brydei, Balaenoptera musculus, and M. novaeangliae). Among the less commonly seen species, there were three Delphinids (Peponocephala electra, Lagenodelphis hosei, Feresa attenuata), one Balaenopterid (Balaenoptera acutorostrata), one Ziphiid (Mesoplodon peruvianus), and one Kogiid (Kogia sima). The relatively low sighting frequency of the Ziphiids and Kogiid is likely due to their long dive times coupled with their cryptic behavior at the surface, as the stranding record suggests that they may be more common (Palacios et al., 2004; MacLeod and Mitchell, 2006). In fact, one additional Ziphiid species, ginkgo-toothed beaked whale (Mesoplodon ginkgodens), is only known from strandings (Palacios, 1996; Palacios et al., 2004).

Figure 2

Figure 3

The overall spatial distribution of marine mammal sightings in our compilation indicated that the study area was covered reasonably well, although it is apparent that much of the effort has been on the deep waters (> 1500 m) of the western part of the archipelago, especially around Isabela and Fernandina islands (Figures 2A and 3). In contrast, the coastal shallow waters (< 500 m) of the central and eastern part of the archipelago have received comparatively much less effort (Figures 2A and 3). Among the commonly seen species, two had a primarily coastal (< 500 m) distribution (T. truncatus and M. novaeangliae), two occurred in both coastal and oceanic habitats (O. orca and B. edeni brydei), and the remainder were primarily found in deep (> 1000 m) waters (Figures 2 and 3).

In terms of seasonal coverage, more sightings in our compilation were collected in the first part of the year (January-May), when sea state conditions are calm, than in the second part (June-December), when sea conditions are rougher (Figure 4). Nevertheless, most species appear to use waters of the archipelago year-round, with sightings in most or all months of the year (Figure 4). Two of the Balaenopterids (B. musculus and M. novaeangliae) are seasonal migrants, being reported primarily during the second part of the year (Figure 4), coinciding with the low-latitude phase of the migratory cycle for Eastern South Pacific populations (Palacios, 1999b; Félix et al., 2011; Torres-Florez et al., 2015; Hucke-Gaete et al., 2018). Additionally, although the Bryde’s whale (B. edeni brydei) is present year-round, a larger number of sightings during the second part of the year (Figure 4), indicates a preference for the cooler and more productive conditions characteristic of this season, as also reported by Denkinger et al. (2013). Finally, several species had more sightings in the first part of the year (S. attenuata, S. longirostris, S. coeruleoalba, D. delphis delphis, G. griseus, G. macrorhynchus, P. macrocephalus; Figure 4), but this pattern is likely influenced by increased sampling effort when sea conditions are calmer.

Figure 4

Marked fluctuations in species occurrence and distribution at interannual and long-term scales have also been described. For example, Denkinger et al. (2013) documented the apparent disappearance of Bryde’s whales and short-beaked common dolphins (D. delphis delphis) from the archipelago during the 1997-98 and 2010 El Niño events, when upwelling, productive conditions were suppressed. In the case of the sperm whale, a long-term study spanning the period 1985-2014 revealed large inter-decadal fluctuations in the number of animals visiting Galápagos each year (Cantor et al., 2017; Eguiguren et al., 2021), a result of the movements in and out of the archipelago undertaken by this highly social and widely roaming species (Whitehead et al., 1997; Cantor et al., 2017).

Finally, Palacios (2003) investigated the community ecology of Galápagos cetaceans in relation to oceanographic conditions through non-metric multidimensional scaling, based on nine species with sufficient occurrence data (Figure 5). The ordination provided a two-axis solution (86% and 4.2% of the information in the data, respectively), with axis 1 representing the dominant environmental gradient in the study area, contrasting upwelling conditions close to the islands at the negative end with warm and phytoplankton-poor conditions away from the islands at the positive end, and with sample units sorting out along this gradient (Figure 5A). Species showed distinct functional responses along this gradient, with pantropical spotted (S. attenuata) and spinner dolphin (S. longirostris) occurrence increasing towards the positive end; common bottlenose dolphins (T. truncatus), Risso’s dolphins (G. griseus), and Bryde’s whales increasing towards the negative end; and short-beaked common dolphins, short-finned pilot whales (G. macrorhynchus), sperm whales, and striped dolphins (S. coeruleoalba) peaking somewhere along these two extremes (Figure 5B). Axis 2 only explained a small amount of the information in the data and was unrelated to the environmental variables considered (Palacios, 2003). These results shed insight on how the complex oceanographic conditions around Galápagos support a diverse cetacean community with distinct habitat preferences and distribution patterns (Palacios, 2003).

Figure 5

4 Research priorities

The above review demonstrates that the biologically productive Galápagos Archipelago is an area that attracts and sustains an outstanding cetacean diversity. However, beyond information on spatial and temporal occurrence, little is known about the biology, ecology, and stock structure of most cetacean species in Galápagos waters or about the connectivity of their movements between the archipelago and other habitats in the ETP. This information is necessary for assessing the status of cetacean populations in Galápagos and for developing appropriate management and conservation measures. In this section, we identify 10 critical knowledge gaps along five topical priority areas for future ecological research on cetaceans in Galápagos based on feasibility and scientific merit: I) spatiotemporal occurrence, II) population assessment, III) health assessment, IV) social ecology, and V) trophic ecology. These knowledge gaps and priorities are summarized in Table 2, with consideration of relevant methodological approaches, species most amenable to these approaches, and the outcomes that may be expected from short- and long-term efforts. Although most of the knowledge gaps require long-term efforts (years to decades), short-term efforts (months to years) will be crucial for generating the necessary baseline information. We emphasize that these priorities are based on our combined experiences and perceptions as international academic scientists conducting cetacean research in Galápagos, as detailed in our Positionality Statement in Section 6. By advancing these priorities, our goal is to catalyze discussion and consultation leading to setting a participative research agenda among relevant institutions, local stakeholders, and the broader scientific community interested in advancing cetacean research, management, and conservation in Galápagos.

4.1 Priority I: Spatiotemporal occurrence

4.1.1 Knowledge gap 1: Distribution

The shallow (< 500 m) and coastal waters of the central and eastern part of the archipelago have received comparatively less effort than the deep waters (> 1500 m) of the western part (Figure 2A), and therefore our understanding of cetacean occurrence and habitat use in neritic waters is incomplete. For example, common bottlenose dolphins appear to occur primarily in neritic waters throughout the archipelago, but sightings tend to be strongly clustered (Palacios and Salazar, 2002; Palacios, 2003; Figure 2B). Bryde’s whales are common in the highly productive western part of the archipelago (Palacios and Salazar, 2002; Palacios, 2003; Figure 2G), but a temporally variable “hotspot” of occurrence has been also reported off San Cristóbal Island, on the eastern part, that appears to be driven by a localized upwelling (Denkinger et al., 2013; Biggs et al., 2017). These illustrative examples indicate that further work is needed to characterize cetacean distribution and their relationship to fine-scale oceanographic and topographic processes (Houvenaghel, 1978; Schaeffer et al., 2008; Figueroa, 2021; Neave et al., 2021).

4.1.2 Knowledge gap 2: Local and regional movements

Many studies of cetaceans around oceanic islands throughout the world have identified distinct island-associated populations (e.g., Baird et al., 2009a; Baird et al., 2013a; Oremus et al., 2012; Quérouil et al., 2013; Kiszka et al., 2014; Estrade and Dulau, 2020; Panicker et al., 2022; Sambolino et al., 2022). Further work is needed to determine whether there are island-associated cetacean populations in Galápagos, to what islands, their inter-island movements, and their differentiation with pelagic populations. Studies are also needed to characterize the movements of Bryde’s whales within the archipelago and their connection to other areas where the species is seen along the mainland (Castro et al., 2017; Rasmussen and Palacios, in press¹) and the offshore ETP (Wade and Gerrodette, 1993; Hamilton et al., 2009).

4.1.3 Methodological approaches

Future studies could combine multiple sampling approaches to advance our understanding of cetacean spatiotemporal occurrence in Galápagos (Table 2). First, for most species, this can be achieved with continued collection of sighting data (time, geographic position, species, group size, and photography for confirmation and archiving). These observational data are straightforward to record by researchers as well as by naturalists and citizen scientists aboard cruise ships (e.g., Palacios and Salazar, 2002; Denkinger et al., 2013; Alves et al., 2018; Denkinger et al., 2020). Second, photo-identification efforts will allow for the study of local and regional movements of individuals within and beyond Galápagos (e.g., Baird et al., 2013a; Baird, 2016; Alves et al., 2018; Alves et al., 2019; Pacheco et al., 2019; Ferreira et al., 2021; Dinis et al., 2021), particularly for species that are easy to identify and often have distinctive markings (T. truncatus, G. macrorhynchus, P. crassidens, O. orca, P. macrocephalus, and M. novaeangliae) (e.g., Cantor et al., 2016; Denkinger et al., 2020). Third, electronic tagging will allow detailed tracking of the movements and migrations of the large whales (B. edeni brydei, B. musculus, M. novaeangliae, P. macrocephalus) as well as the larger-bodied Delphinids (G. griseus, G. macrorhynchus, P. crassidens) and Ziphiids (Z. cavirostris and Mesoplodon spp.) that use Galápagos waters. While more costly, logistically demanding, and requiring welfare considerations, tagging is the most feasible way to reveal the routes of migratory whales (e.g., Silva et al., 2013; Prieto et al., 2014; Hucke-Gaete et al., 2018; Fonseca et al., 2022), which will enable researchers to create connectivity maps across coastal and oceanic habitats in the ETP and beyond (e.g., Davies et al., 2021; Boteler et al., 2022). For the Delphinids and Ziphiids, tagging can provide crucial information on residency, site fidelity, and habitat use (e.g., Baird et al., 2009b; Baird et al., 2013b; Abecassis et al., 2015; Baird, 2016; ). Finally, passive acoustic monitoring with towed or moored hydrophones (Mellinger et al., 2007) would be especially useful for detecting deep-diving and cryptic species (K. sima, Ziphiids), as well as Odontocetes or Balaenopterids with distinctive acoustic signatures, such as clicking sperm whales and singing humpback and blue whales. Relative to visual methods, acoustic data may not always provide accurate species identification or group size estimates, but recent advances on automated detection of broad-band echolocation clicks and tonal sounds (e.g., Gillispie et al., 2009; Frasier et al., 2017), as well as machine learning tools (e.g., Beslin et al., 2018; Bermant et al., 2019; Woodward et al., 2020) offer promising solutions.

Such multiplatform data collection is increasingly being used to generate predictive habitat suitability maps (e.g., Thorne et al., 2012; Bouchet et al., 2015; Tobeña et al., 2016; Tardin et al., 2017; Fiedler et al., 2018; Tardin et al., 2019; Correia et al., 2021; Fernandez et al., 2021), even with short-term efforts (months to years). In the long term (years to decades), the accumulation of such data will increase the coverage, reliability, and scope of these maps and help refine the characterization of species-specific hotspots of occurrence and their fluctuations in response to environmental variation such as El Niño Southern Oscillation and climate change (e.g., Llapapasca et al., 2018; Becker et al., 2019; Becker et al., 2022). This information could be used to identify cetacean areas of significant importance within Galápagos, as has been done for other locally threatened marine taxa (Edgar et al., 2008; Ventura et al., 2019) as well as for the highly migratory elasmobranchs (Hearn et al., 2010; McKinley et al., 2022). Beyond helping researchers improve their ecological understanding of cetacean occurrence patterns, predictive habitat suitability maps are increasingly used as management tools (e.g., Sahri et al., 2021), and they could help to delineate areas of importance within the GMR that have particularly high conservation significance and yet may be subject to ongoing anthropogenic disturbances.

4.2 Priority II: Population assessment

4.2.1 Knowledge gaps 3 and 4: Density and abundance

Unlike pinnipeds, for which population assessments have been conducted on a regular basis throughout Galápagos (e.g., Páez-Rosas et al., 2021), very few studies have reported population assessments for cetaceans in Galápagos (Palacios and Forney, 2008; O’Hern et al., 2017; Whitehead and Shin, 2022), and there is no formal ongoing program for monitoring cetacean populations. Yet, baseline information on demographic parameters, such as density, abundance, survival, and recruitment, is essential for understanding population status and dynamics, and to unravel the relative importance of competition and predation in structuring the local cetacean community relative to the influences of environmental variability. Ultimately, monitoring temporal trends in population parameters provides information that is immediately useful for guiding conservation decisions (e.g., Taylor et al., 2007; Boyd and Punt, 2021).

4.2.2 Knowledge gap 5: Population structure

Studies are needed to assess the relationship of common bottlenose dolphins in Galápagos to the recently recognized subspecies of offshore bottlenose dolphin (T. truncatus nuuanu) or other ecotypes in the ETP (Palacios et al., 2004; Costa et al., 2022) for appropriate management. Other examples include elucidating the relationship of Bryde’s whales and sperm whales seen in Galápagos to those that occur in offshore waters and in the jurisdictions of countries bordering the ETP.

4.2.3 Methodological approaches

To estimate demographic parameters of species whose individuals are likely to be re-identified multiple times, such as local populations (T. truncatus, B. edeni brydei) or those observed in long-lasting groups (O. orca), we suggest designing mark-recapture studies based on photo-identification data (Urian et al., 2015; Wickman et al., 2020; Hammond et al., 2021) (Table 2). In the short term, analyzing photo-identification data with simple mark-recapture models (White and Burnham, 1999) is sufficient to generate one-off estimates of abundance or minimum population size (e.g., Baird et al., 2008; Baird et al., 2009a; Baird et al., 2009b). With an appropriate sampling design for photo-identification effort, long-term data will allow more elaborate mark-recapture modeling to estimate a range of other demographic parameters (e.g., survival, emigration, recruitment; see Pollock et al., 1982; White and Burnham, 1999). Our understanding of population structure and dynamics of cetacean species in Galápagos can be much improved by considering individual covariates, such as sex, age, and/or maturity classes, when estimating parameters with mark-recapture models. This information can be obtained using a variety of field and laboratory approaches: some species are sexually dimorphic (O. orca, P. macrocephalus) and others can be sexed genetically with remote biopsy sampling; hormonal analyses can indicate reproductive condition; epigenetic methods can be used for aging individuals (e.g., Polanowski et al., 2014); and boat-based photogrammetry and drone-based morphometry can generate body length estimates, especially of large Odontocetes (e.g., Kotik et al., 2022).

Although some species are common and/or easy to identify, many are unsuited for photo-identification-based mark-recapture modeling, either because the populations and/or their ranging areas are very large (M. novaeangliae, B. musculus), or because they are typically seen in large, fast-moving groups (D. delphis delphis, S. coeruleoalba, S. attenuata, S. longirostris). For these cases, archipelago-wide efforts would be better suited to distance sampling using line-transect surveys (Buckland et al., 2001; Buckland et al., 2015). This approach can generate one-off density or abundance estimates in the short-term (Palacios and Forney, 2008; O’Hern et al., 2017), as well as trends over time (Table 2). Distance sampling requires dedicated surveys with properly pre-defined transect lines, trained personnel, and careful consideration of assumptions and caveats (Thomas et al., 2007; Faustino et al., 2010), but it remains a cornerstone of cetacean population monitoring. This methodology is typically conducted from large research vessels capable of open-ocean operation (e.g., O’Hern et al., 2017), but has also been adapted to sailboats (e.g., Palacios and Forney, 2008), small boats (Dawson et al., 2008), and aircraft (e.g., Panigada et al., 2011). Line-transect surveys, either boat-based or aerial, can be cost-effective solutions for estimating density and abundance of a wide range of cetacean species in coastal areas (Aragones et al., 1997; Williams and Thomas, 2009; Lambert et al., 2019).

4.3 Priority III: Health assessment

4.3.1 Knowledge gaps 6 and 7: Body condition and general health status

Other than basic documentation of stranding events (Palacios et al., 2004), health assessments for cetaceans in Galápagos are absent. Studies have shown that environmental perturbations such as El Niño as well as exposure to anthropogenic activities near human population centers have impacts on the body condition and general health of teleost fish (Lamb et al., 2018) and pinnipeds (Brock et al., 2013a; Brock et al., 2013b; Paéz-Rosas et al., 2016; Páez-Rosas et al., 2021), suggesting that cetaceans may also be affected by these stressors.

4.3.2 Methodological approaches

Health status in cetaceans can be assessed through monitoring of the nutritional status (Joblon et al., 2014; Christiansen et al., 2020) and the prevalence of cutaneous conditions of individuals and populations (Hart et al., 2012; Soares et al., 2022), which can reveal key insights about population status, the quality of the surrounding environment, and interactions with human activities (e.g., boat traffic, fisheries). A straightforward way to investigate body condition and external lesions and injuries in cetaceans is through visual analyses of photographic and video data. For instance, footage recorded from drones offers a privileged bird’s-eye view from which researchers can extract accurate quantitative morphometric data to evaluate body condition and health status (e.g., Christiansen et al., 2020; Horton et al., 2019; Bierlich et al., 2021) (Table 2). Drones can also be adapted to sample the microbial and viral communities in the breath of small and large cetaceans as a complementary approach to assess their health (e.g., Centelleghe et al., 2020). Drone-based photogrammetry and respiratory microbiota sampling may be more suitable for large whales in Galápagos (B. edeni brydei, B. musculus, M. novaeangliae, P. macrocephalus) (e.g., Dawson et al., 2017), but coupling drone images with laser altimeters may generate suitable high-resolution aerial photogrammetric data for smaller cetaceans as well (see Bierlich et al., 2021).

While drone sampling requires dedicated research efforts and specific safety and sampling protocols (e.g., Raoult et al., 2020), boat-based photography can provide a quick assessment of general body condition (e.g., “emaciated”, “thin”, “good”) based on fat deposition in specific body parts (e.g., Joblon et al., 2014; Soares et al., 2022; Wachtendonk et al., 2022). Such photographic records can be collected even from opportunistic platforms and can generate insights on the cutaneous lesions and diseases of a range of cetacean species. Visual analyses of the gross characteristics of skin conditions from photographs can indicate cases that are potentially related to viral, fungal, or bacterial aetiology (e.g., Van Bressem et al., 2007; Van Bressem et al., 2009). Photographic records can also be used to assess injuries indicative of traumatic events; wounds and scars, for instance, can suggest predation pressure (Best and Photopoulou, 2016; Baird et al., 2022; Capella et al., 2018; Corsi et al., 2022) as well as negative interactions with anthropogenic activities (e.g., Harnish et al., 2019), entanglement in or interactions with fishing gear (e.g., Baird et al., 2014), or collision with boats (e.g., Toms et al., 2020).

Investigating the underlying causes of body and cutaneous conditions in cetaceans, however, requires further laboratory analyses to determine nutritional status and diagnose disease or injury (Table 2). Therefore, we recommend including biological tissue sampling during dedicated surveys whenever feasible. Proper evaluation of cutaneous conditions includes electron microscopy or molecular analyses of skin tissue to confirm a potential pathology (e.g., Groch et al., 2020). Additional analyses of tissue samples should include pollutant loads (e.g., Alava et al., 2009; Remili et al., 2020). On the other hand, although fresh stranded animals can yield tissue samples opportunistically, any dedicated biopsy sampling requires specialized research efforts (i.e., personnel, equipment, storage, and permitting) and are most feasible for the larger-bodied species (i.e., T. truncatus, G. griseus, G. macrorhynchus, O. orca, P. macrocephalus, B. edeni brydei, B. musculus, M. novaeangliae).

4.4 Priority IV: Social ecology

4.4.1 Knowledge gaps 8 and 9: Social organization and social structure

Long-term studies of social ecology in Galápagos marine mammals are among the most world-renowned, including pinnipeds (Wolf et al., 2007), sperm whales (Whitehead, 1985; Whitehead, 2003; Cantor and Whitehead, 2015), and killer whales (Denkinger et al., 2020). While we advocate for the continuation of these efforts, studies on other species or aspects of sociality would advance our understanding of social complexity in cetaceans. Investigating the various social systems present in cetaceans (Mann et al., 2000; Rendell et al., 2019) requires understanding four axes: social organization, social structure, care system, and mating system (Kappeler, 2019). This work requires long-term efforts and can be particularly challenging in the marine realm; however, two of these axes, social structure and social organization, are feasible to measure, even with short-term studies.

The diverse Odontocete fauna of Galápagos represents a variety of unique social systems. Matrilineal species like short-finned pilot whales and killer whales (Denkinger et al., 2020) or species with high levels of fission-fusion social dynamics like common bottlenose dolphins and Risso’s dolphins are found in Galápagos. Studying their social organization, in terms of group size and composition, as well as their social structure, in terms of number and strength of social associations, will make for interesting comparisons with other oceanic islands where these species have been studied (Pinela et al., 2009; Hartman et al., 2015; Alves et al., 2019; Servidio et al., 2019; Dinis et al., 2021). Even the social structure of the well-studied sperm whale would benefit from continued investigation. Despite being widespread in deep waters (> 1000 m) around Galápagos (e.g., Palacios and Salazar, 2002; Palacios, 2003; Figures 2F and 3), sperm whale habitat use has shifted over the past four decades between the western, northern, and southern parts of the archipelago (Cantor et al., 2016; Eguiguren et al., 2019). These fluctuations possibly reflect different space use by socially distinct clans over time, but further studies are needed to elucidate their cultural and environmental drivers.

4.4.2 Methodological approaches

Basic information on social organization, such as group size (number of individuals estimated in the field and double-checked with photography) and composition (age and/or sex classes, at least for sexually dimorphic species: P. macrocephalus, O. orca), can always be recorded during both dedicated surveys and from opportunistic platforms (Table 2). Describing social structure, on the other hand, requires tracking social interactions or associations among individuals identified in groups over time (Whitehead, 2008). The need for recurrent re-sighting makes this effort more suitable for species that are reliably identifiable through standard photo-identification techniques and that are common in Galápagos, such as T. truncatus, P. macrocephalus, O. orca, and G. macrorhynchus (Table 2). As such data accumulate, a major goal will be to describe patterns of social structure, for instance subdivisions of the population into stable or more socially connected social communities (Weiss et al., 2021). For species living in medium to high levels of fission-fusion dynamics (e.g., T. truncatus), subdivision into communities can reflect important aspects of their ecological environment and the level of competition among individuals (e.g., quality and distribution of resources) and reveal key aspects of their social environment (e.g., social preferences and avoidances, spatial overlap, social clustering around behavioral or biological traits; e.g. Machado et al., 2019). For sperm whales, quantifying temporal stability, geographic range, behavioral variation, and acoustic communication will further reveal how social learning and cultural transmission influences the lives of individual whales and erects social barriers across sympatric clans (Rendell and Whitehead, 2003; Cantor and Whitehead, 2015; Cantor et al., 2015; Eguiguren et al., 2019; Eguiguren et al., 2021; Hersh et al., 2022). The delineation of sperm whale populations into culturally driven vocal clans is a key attribute of their population structure to be considered in regional and international conservation efforts to conserve this broadly roaming species (Brakes et al., 2019).

4.5 Priority V: Trophic ecology

4.5.1 Knowledge gap 10: Foraging and diet

Trophic interactions are central for mapping marine food web structures and understanding ecosystem functioning and dynamics, including nutrient cycling and energy flow. Empirical data on predator-resource interactions are the backbone of food web and ecosystem models, which are typically inferred from foraging observations and analysis of stomach contents and feces. For instance, undigested beaks in fecal samples of sperm whales provide a window into the inaccessible guild of demersal and mesopelagic cephalopods (Smith and Whitehead, 2000; Smith and Whitehead, 2001; Whitehead et al., 2001), and are a reliable proxy of foraging success (Whitehead and Rendell, 2004). In contrast to pinnipeds (Páez-Rosas et al., 2012; Urquía and Páez-Rosas, 2019), elasmobranchs (Páez-Rosas et al., 2018; Salinas-de-León et al., 2019), and seabirds (Jiménez-Uzcátegui et al., 2019), studies of diet in cetaceans in Galápagos have been rarely performed (Smith and Whitehead, 2000; Smith and Whitehead, 2001; Palacios et al., 2004). While fresh specimens (e.g., stomach contents) from stranding events are rarely available in Galápagos (Palacios et al., 2004), the establishment of basic protocols for sample collection from stranded cetaceans by designated and properly trained personnel could help fill this gap, while also informing monitoring and conservation strategies (e.g., Peltier et al., 2014).

Among the oceanic Delphinids occurring in Galápagos, ecologically similar species pairs have somewhat distinct distribution patterns, while mixed-species aggregations are relatively rare. For example, short-beaked common dolphins are predominant in the western and southern parts of the archipelago, while striped dolphins occur more often in the northwestern and northern part (Palacios and Salazar, 2002; Palacios, 2003; Figures 2C and 3). Similarly, Risso’s dolphins and short-finned pilot whales are both found in the deep waters surrounding the margins of the archipelago, but Risso’s dolphins appear to be more closely associated with the steep slopes of the western part of the archipelago (Palacios and Salazar, 2002; Palacios, 2003; Figures 2E and 3). These intriguing patterns suggest some level of habitat selection or niche partitioning, possibly mediated by foraging specializations (e.g., Whitehead et al., 2001; Quérouil et al., 2008) that deserve further study through methodologies such as those described below.

4.5.2 Methodological approaches

The investigation of the role of prey and predators in ecosystem dynamics can be largely expanded by biochemical tracer analyses (e.g., Boecklen et al., 2011; Pethybridge et al., 2018). Stable isotopes, fatty acid signatures, and other trace elements can complement our understanding of the role of cetaceans as both prey and predators. For instance, combining carbon and nitrogen stable isotopic analysis of bulk tissue and their constituent amino acids can reveal key insights on cetacean diet at the population level, and trophic position and interactions at the community level, over time (Teixeira et al., 2022). Such biomarkers can be accessed from stranded and preserved animals in scientific collections, but also from tissue samples of living animals (e.g., Teixeira et al., 2022). We suggest increased effort for collecting skin and blubber tissues of medium species (T. truncatus, G. griseus, G. macrorhynchus) and large species (B. musculus), both with remote biopsy systems and non-invasively, such as collecting sloughed skin of Odontocetes that frequently shed epithelial tissue (P. macrocephalus; e.g., Marcoux et al., 2007) (Table 2). Long-term data on biochemical tracers will provide a unique opportunity for assessing trends in resource use over time and space, and for relating these trends to environmental variation (e.g., El Niño Southern Oscillation; Arnés-Urgellés et al., 2021). Beyond contributing to mapping the local Galápagos food webs (e.g., Okey et al., 2004; Alava, 2009), this approach will also allow us to quantify trophic niche overlap between and within species (Enríquez-García et al., 2022), and investigate individual and ontogenetic variation in foraging, and the extent to which such variation is driven by resource partitioning and intra- and interspecific competition (e.g., Teixeira et al., 2022).

5 Discussion

The biologically productive waters surrounding the Galápagos Archipelago form a distinct oceanic insular ecosystem within the greater ETP. Our review confirmed that this ecosystem supports a large and diverse cetacean community, but beyond presence and distribution information, for most species not much more is known about habitat use, population structure, health, and social ecology. In contrast, significantly more efforts have been devoted to the study and monitoring of Galápagos invertebrates (e.g., Edgar et al., 2004; Edgar et al., 2008; Edgar et al., 2011), fishes (e.g., Schiller et al., 2014; Lamb et al., 2018; Salinas-de-León et al., 2019), pinnipeds (e.g., Páez-Rosas et al., 2012; Brock et al., 2013a; Brock et al., 2013b; Paéz-Rosas et al., 2016; Páez-Rosas et al., 2018; Urquía and Páez-Rosas, 2019; Páez-Rosas et al., 2021), and seabirds (e.g., Anchundia et al., 2014; Tompkins et al., 2017; Jiménez-Uzcátegui et al., 2019; Arauco-Shapiro et al., 2020). Admittedly, cetacean research is comparatively more expensive and more logistically demanding. However, at present, the study of cetaceans in Galápagos lags other tropical and subtropical oceanic insular ecosystems around the world, especially the Hawaiian (e.g., Baird et al., 2013b; Baird, 2016; Kratofil et al., 2023) and Macaronesian (e.g., Silva et al., 2014; Hartman et al., 2015; Fais et al., 2016; Tobeña et al., 2016; Alves et al., 2018; Alves et al., 2019; Romagosa et al., 2020; Dinis et al., 2021; Ferreira et al., 2021; Herrera et al., 2021) archipelagoes, where long-term studies have been generating significant new information in recent years. We acknowledge that, being located in the global north, the latter two ecosystems have benefited from established expertise, access to financial resources, and robust legislation mandating marine mammal research and protection that are generally not available in the global south. Further, as cetaceans are not considered extractive resources in Galápagos and additionally receive some level of legal protection throughout Ecuador (whales, at least), they are generally not “on the radar” of government entities in charge of marine resource management.

Given the logistical and funding challenges of conducting cetacean research in Galápagos, we suggest that the development of a collaborative research agenda should include specific actions aimed at establishing and strengthening meaningful partnerships between foreign and local researchers, and include the involvement of governmental and non-governmental entities, as well as stakeholders. Some existing examples of such partnerships include the long-term monitoring of cetacean presence using opportunistic sightings collected by naturalist guides working aboard cruise ships through a program implemented by the Galápagos National Park and the Charles Darwin Foundation (Palacios and Salazar, 2002; Denkinger et al., 2013; Denkinger et al., 2020), or the population assessment surveys for cetaceans conducted through a collaboration between U.S. academics and the Ecuadorian Navy through INOCAR, its institute for oceanographic and Antarctic research (Biggs et al., 2017; O’Hern et al., 2017). Continued partnership with the local tourism industry, in particular, would provide additional opportunities for participation by trained naturalists and citizen scientists in opportunistic but valuable data collection that would enhance and enrich research goals in a relatively inexpensive manner (e.g., Van Cise et al., 2021). An increased presence of researchers and observers in the field would have a side benefit related to the management of human activities in Galápagos by promoting better-guided ecotourism, better regulated vessel traffic, and enhanced surveillance and reporting of illegal fishing activities within the GMR (e.g., Cerutti-Pereyra et al., 2020; Bonaccorso et al., 2021; Global Fishing Watch, 2021).

Other aspects to be considered during development of a collaborative research agenda include capacity building and training of local researchers; optimization the research permit application required by the Galápagos National Park based on the specific needs and requirements cetacean research, including vessel operation and sample collection and export (if appropriate); and fostering the adoption of integrative data sampling and data sharing protocols. On the topic of data sharing, efforts can be optimized if research groups contribute to centralized repositories that provide the technological infrastructure, data access permissions, and terms of use. Such data repositories include those that are taxa-specific but otherwise accept a wide variety of data types (e.g., Halpin et al., 2006), as well as those specializing on photo-identification (e.g., Olson et al., 2020; Berger-Wolf et al., 2017) or tracking data (e.g., Block et al., 2016; Kays et al., 2022). Centralized repositories have the additional advantage of facilitating the curation of citizen science data; for instance, photographs from the public could be perused by specialists to verify species identification (e.g., Jarić et al., 2020), individual photo-identification (e.g., Cheeseman et al., 2021), and assessment of cutaneous and body conditions (e.g., Hart et al., 2012). Ultimately, these collective efforts will foster collaborative research, accelerate discovery, maximize the use of funds, promote the training of local scientists, and create opportunities for the professional development of early-career researchers as well as established scientists and conservation practitioners.

In conclusion, an improved understanding of Galápagos cetaceans will help quantify the ecological role that megafauna plays in connecting oceanic insular ecosystems (Boteler et al., 2022) and further establish the importance of marine reserves for transboundary conservation (e.g., Halpern, 2003; Game et al., 2009; Tetley et al., 2022). Indeed, in the marine realm, cetaceans are firm favorites for raising public awareness about the need for conservation actions (i.e., “flagship species”; Veríssimo et al., 2011) and for expanding these conservation actions to their habitats and ecological communities (“umbrella species”; Caro and O’Doherty, 1999). The information generated from our proposed approaches will also inform the management of the effects of climate change (Dueñas et al., 2021) and of human activities in Galápagos, including the impact of illegal fishing within the GMR, which not only can lead to direct negative interactions (such as megafauna bycatch; Alava et al., 2019; Cerutti-Pereyra et al., 2020) but also contribute to the overexploitation of marine resources (e.g., Usseglio et al., 2016; Bonaccorso et al., 2021). Avoiding such downstream cascading effects on Galápagos’ unique biota, environment, and socio-ecological systems (Denkinger and Vinueza, 2014; Walsh and Mena, 2016) will require tailored conservation plans that consider cetaceans as integral components of the ecosystem.

6 Positionality statement

We are biologists and academics in the fields of marine ecology, population ecology, and behavioral ecology, with a focus on marine mammals and other marine megafauna. We share a keen interest in the study of Galápagos cetaceans. As international scientists, we have a variety of experiences conducting cetacean research in Galápagos, but we are neither Ecuadorian citizens nor Galápagos residents. As such, we are not active participants in these communities, nor do we depend, economically or otherwise, from research conducted in Galápagos. We do share cultural aspects with the Ecuadorian people and have professional ties with its research community.

Born and raised in Colombia, DP is a marine ecologist and oceanographer with Ph.D. studies and post-doctoral appointments from institutions in the U.S., where he has resided for 27 years. He has participated in research expeditions in Galápagos in 1993-94, 2000, and 2005, and has additionally visited Galápagos on multiple occasions to participate in scientific workshops. DP is currently an Associate Professor at Oregon State University and remains actively involved in scientific research and conservation throughout Latin America.

MC Is a behavioral ecologist, born and raised in Brazil, who completed undergraduate and graduate degrees at Brazilian institutions before undertaking Ph.D. studies and post-doctoral appointments in universities in the global north. MC has been participating in research expeditions in Galápagos since 2013, starting with his Ph.D. research on the social ecology of sperm whales, as part of a multi-decadal collaboration between a Canadian university and several Ecuadorian and Galápagos institutions. MC is currently an Assistant Professor at Oregon State University in the U.S. and remains an active collaborator in Latin American research projects, including the long-term sperm whale research program in Galápagos.

Despite our collective experience in Galápagos, we acknowledge that we are still external actors. As such, our understanding of the priorities for research in the area is unintentionally influenced by our worldviews, which are themselves biased by our own social and cultural origins, privileges, academic trajectories, and personal experiences. For these reasons, we emphasize that our proposed priorities should be assessed by Ecuadorian and Galápagos governmental institutions in broad and inclusive consultation with stakeholders and the scientific community prior to development and implementation of a research agenda.

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