Three species of cetaceans are found in the Black Sea that are designated as endemic subspecies: the Black Sea harbour porpoise (Phocoena phocoena relicta Abel, 1905); the Black Sea bottlenose dolphin (Tursiops truncatus ponticus Barabash-Nikiforov, 1940); and the Black Sea common dolphin (Delphinus delphis ponticus Barabash, 1935) ( Figures 1 – 3 ). All of them are considered as vulnerable or endangered due to several historical and current adverse factors affecting their populations (Birkun et al., 2014). Among these factors, the primary one was the commercial hunting of cetaceans in the Black Sea which was highly intensive between 1929 and 1966 when a ban was adopted by the USSR, Bulgaria and Romania. It continued in Turkish waters until 1983 (Kleinenberg, 1956; Tonay and Öztürk, 2012). Additionally, a genetic study indicated a strong reduction in the population size of Black Sea harbour porpoise (approximately 90%) in the second half of the 20^th century, possibly due to massive dolphin fisheries and bycatch (Fontaine et al., 2012).

Nowadays, Black Sea cetaceans are protected in the riparian countries at both national and international levels. They are listed in the IUCN Red List, national Red Data Books, and (in EU waters) annexes II and IV of EU Habitats Directive 92/43/EEC and in Descriptor 1 (D1, Biodiversity) of EU Marine Strategy Framework Directive (MSFD) 2008/56/EC.

Incidental catch in fishing gear (henceforth referred to as bycatch) is a major threat for populations of small cetaceans in European seas, and the greatest source of non-natural mortality for many (Dolman et al., 2016; Amaha Öztürk, 2021). In Europe, cetacean bycatch is considered under the above-mentioned EU Directives, the EU Common Fisheries Policy (CFP) and the two regional CMS (Convention for Migratory Species) agreements on the conservation of cetaceans: ASCOBANS (Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas) and ACCOBAMS (Agreement on the Conservation of Cetaceans of the Black Sea, Mediterranean Sea and contiguous Atlantic area) - the latter one covering the Black Sea. Bycatch mortality is specifically identified as criterion D1C1 assessed in the current cycle of EU MSFD implementation for ‘good environmental status’ (GES) for cetaceans.

A quantitative understanding of the effect of bycatch on affected populations is key to being able to undertake (and later evaluate) effective bycatch mitigation measures (e.g. Moore et al., 2021; Wade et al., 2021). Determining an ‘acceptable’ removal rate (taking into account inevitable uncertainty) for a cetacean population is not easy and several suggestions have been put forward. For example, ASCOBANS agreed to a ‘limit’ of 1.7% of the harbour porpoise abundance as appropriate for the GES (Moffat et al., 2011; ASCOBANS, 2015). Other approaches are used elsewhere such as the estimated potential biological removal (PBR) used in the USA – this has been parameterised to be equal to 1% of the minimum abundance estimate for cetaceans (Wade, 1998). More recently, Manlik et al. (2022) proposed an approach they called sustainable anthropogenic mortality in stochastic environments (SAMSE): this method gives an estimate of sustainable bycatch level for bottlenose dolphins to be not more than 0.5% of abundance estimate.

Historically, several kinds of fisheries have been identified as primary sources of cetacean bycatch in the Black Sea including:

By far the largest bycatch (90 to 98% by number) reported by the above studies was that of the Black Sea harbour porpoise. Most cetacean bycatch was due to illegal, unreported and unregulated (IUU) fishing operations - this makes estimation of deaths difficult to obtain from the preferred method – direct observations. A previous estimation of cetacean bycatch numbers in the Black Sea (Birkun et al., 2014), based on historical data and extrapolation, suggested an annual catch of at least 20 000 animals (of which over 11 000 were in gillnets for turbot) – probably greatly exceeding any sustainable level. Tonay (2016), based on onboard observations of part of the fleet, estimated the annual bycatch of harbour porpoises to be around 2 000 animals (CV=0.37) in the Turkish western Black Sea which is the most precise sub-regional bycatch estimate available. Vishnyakova (2017) undertook a demographic study of the harbour porpoise population in the Azov Sea. It showed that bycatch was the main mortality factor for the Azov population, which declined by 60% over 13 years (2000-13). Clearly, this could be applicable to some other parts of the Black Sea.

The present study focused on developing and applying a standard approach to bycatch monitoring in the Black Sea with a view to filling the existing gaps on distribution, levels and effects of bycatch pressure on cetaceans in the region. The research was undertaken across several Black Sea riparian countries and the objective was to identify and evaluate patterns of cetacean interactions with fisheries. The methodological framework was developed for collecting field data and bycatch assessment, and the field surveys were conducted in Bulgaria, Romania, Türkiye and Ukraine between 2019 and 2021. The estimated bycatch level of the Black Sea harbour porpoise as the most affected species was compared to the most up-to-date abundance estimate derived from the summer 2019 aerial survey of the basin (Paiu et al., 2021a), which is the most comprehensive until now.

The study included information collected by questionnaire surveys and data from onboard observers, supplemented by data from cetacean stranding records and fishing effort (fleet size, annex 1). All these data were used to estimate total bycatch that was compared to total abundance estimated by the 2019 aerial survey (Paiu et al., 2021a) This approach using several sources of information is broadly following the recommendations of Wade et al. (2021). Each step is described below.

Overall basin wide bycatch rate of harbour porpoises in turbot gillnet fisheries found in this study in spite of the conservative assumptions used is apparently very high. Most previous studies of harbour porpoise bycatch in the Black Sea reported an average bycatch rate as individuals per km of nets (ind./km). The comparable statistic from the present study was 0.37 ind./km – similar to a previous 2014-2018 study in Bulgarian waters when 0.31 ind./km was reported (Zaharieva et al., 2022). Older studies conducted in different regions of the Black Sea reported a wide range of bycatch rates – these are summarised here for information without any comparison of methods and assumptions (e.g. how net height and soaking time were taken into account): 0.09 ind./km in Ukraine (Pavlov et al., 1996), 1.53 ind./km in Ukraine (Birkun et al., 2009), 0.22 ind./km in Bulgaria (Mihaylov, 2011), 0.33 and 0.19 ind./km in western Türkiye (Tonay, 2016), 4.14 ind./km in central Türkiye (Gönener and Bilgin, 2009) and 0.43 ind./km in eastern Türkiye (Bilgin et al., 2018).

Strandings can be a supplementary source of monitoring bycatch. In the current study, there were some carcasses indicating the evidence of bycatch in Bulgaria, Romania and Ukraine. Besides, between January 2019-May 2020, 50 stranded cetaceans (common dolphins 58%, harbour porpoises 36%, bottlenose dolphins 4%, and unidentified delphinids 2%) were reported in Turkish Black Sea coast, of which one harbour porpoise and six common dolphins indicated the evidence of bycatch (Paiu M. et al., 2021b). Strandings of common dolphins were in winter-early spring and not related to turbot fisheries but possibly to purse seines or midwater trawls. This implies that the interaction with fishing gears other than turbot gill nets needs further investigation. Besides, the result of Bulgaria indicated that it was not possible to robustly estimate the annual bycatch from the stranding record due to the low number of strandings compared to the data obtained through onboard observations. The advanced stage of decomposition also makes it impossible to determine the cause of death. As a result, it was understood that strandings themselves can indicate the occurrence of bycatch but hardly provide quantitative data without demographic modelling (Moore and Read, 2008).

During the current study pingers have been tested as mitigation measure in 27 of the hauls in Bulgaria. Three models of pingers have been tested: Future Oceans 10 kHz, Future Oceans 70 kHz and Porpoise Alerting Devices (PAL), frequency 10 kHz. Observed differences in bycatch levels in strings fitted with Future Oceans pingers and those without pingers were not significant in 2019 (Popov et al., 2020). The only model which showed significant decrease in bycatch (86%, p<0.05) was the PAL pinger deployed on six strings. These accounted for 10% of all observed hauls with resultant overall negative bias of 8% in recorded bycatch rate. Larger scale trials in terms of coverage and duration are needed to confirm these results and test potential habituation effect.

The bycatch estimates provided here concur with the overall bycatch in turbot gillnet and trammel net fisheries (including IUU fishing) calculated for the western Turkish Black Sea coast of 2 011 and 2 294 porpoises per year in 2007 and 2008, respectively, based on bycatch per haul and the total net amount in the area (Tonay, 2016). Using an earlier value of the number of active vessels in that area as 185 (Tonay and Öztürk, 2003), the average annual bycatch of harbour porpoises per boat varied between 10.9 and 12. These mean values are a little higher than those from this study (7.3-10 depending on data source). The differences probably reflect normal interannual and interregional variation, and the rough combined estimate of annual bycatch as 10 harbour porpoises per vessel can be assumed to be a robust estimate for further monitoring, given the consistency in values despite the different approaches. Widening sub-regional coverage, i.e. further research in the eastern Black Sea is important for enhancing the accuracy of scaling or further stratified modelling (Authier et al., 2021), since most of the data for both studies come from the western Black Sea. However, the estimates from the western subregion are particularly important due to the summer concentration of the major part of the Black Sea harbour porpoise population in this area (Paiu et al., 2021a).

The data obtained during this study from several independent lines of evidence corroborate earlier suggestions on bycatch seasonality and the potentially strong impact of IUU as well as legal fisheries. They also provide similar rough estimates of overall bycatch per vessel. However, the data presented here are somewhat limited due to the relatively small sample size of observed trips/hauls. Based on the calculator of Curtis and Carretta (2020), it can be suggested that the coverage by onboard observers needed to obtain abundance estimates with a CV value of about 0.3, is at least 220 trips for the whole Black Sea. The relatively small sample size (48 trips, 63 hauls) might have affected precision of the obtained result. However, unlike many situations elsewhere when bycatch sampling effort is considered low due to the rarity of bycatch events (Authier et al., 2021), bycatch frequency in the Black Sea is high (57.1% observed occurrence rate, 95% probability of observing bycatch is achieved at 0.2% observation effort: Curtis and Carretta, 2020), and here the high variance of cetacean bycatches is observed, which is due to outliers – extremely high bycatch events. Moreover, the highest bycatch incidents could be missed and thus increasing the observation effort would only increase the variance. Also, most part of the eastern Black Sea was not covered by this survey, data from that area are limited and probably more vessels involving bycatch are falling out of the GFCM statistics. Therefore, given the whole bulk of available evidence, despite the high figure obtained from this study, our estimates can be even underrated (rather than overrated).

Despite the small sample of observation effort, these results concurred with two previous independent studies (Birkun et al., 2014; Tonay, 2016). Birkun et al. (2014) provided a total estimated annual bycatch of porpoises in the Black Sea of 20 000 for all fishing gear types or 11 000 for gillnets, mostly based on surveys conducted in 2006-08 and 2012-13. Applying our method to the data of Birkun et al. (2014) increases the total for all gear to 25 000 porpoises. Alternatively, if we use the annual bycatch rate per vessel in 2007-08 calculated for western Turkish Black Sea fleet (after Tonay, 2016) for the whole basin’ turbot fleet, the annual bycatch estimate will be around 20 100 individuals. Therefore, assuming a stable bycatch rate during the last 15 years, the upper range of total bycatch estimate which can be taken into consideration is at least 20 000 individuals per year.

The earlier high overall porpoise bycatch estimates for the Black Sea (Birkun et al., 2009; Birkun et al., 2014) seemed incompatible with the data on overall abundance (65 000) of the Black Sea population of the harbour porpoise. That abundance though was based on an aerial survey that covered only the northwestern Black Sea (29% of total area). However, the CeNoBS aerial survey conducted in summer 2019 as part of ACCOBAMS Survey Initiative (ASI) has covered more than 60% of the basin. It is the most comprehensive basin survey so far providing an overall abundance estimate (uncorrected for g(0)) of 94 219 (CV=0.07) porpoises with the highest density in the southwestern part of the Black Sea (Paiu et al., 2021a). Using the correction factor for g(0) of 0.364 for good sighting conditions, calculated for the harbour porpoise in SCANS-III aerial survey of European Atlantic waters (Hammond et al., 2017), the abundance in the Black Sea would be some 258 900 porpoises. Thus, the bycatch rate is between 4.6% and 21.3% of the total abundance estimation, depending on the sources and methods of population and bycatch assessment ( Table 3 ). We consider the lower end of this range as quite a realistic estimate on basis of previous demographic study for the Black Sea harbour porpoise. Therefore, the new data obtained under the ASI have been crucial for reconciling bycatch and abundance estimates.

A previous demographic study (Vishnyakova, 2017) that suggested a long lifespan (23 years) and generation time (7.5 years) for Black Sea harbour porpoises was consistent with a relatively low bycatch rate, which indirectly supports the lower limit of estimates presented here. In terms of assessing impacts of bycatch on the Black Sea harbour porpoise populations, this suggests that if a PBR approach is to be considered, a review of the ‘traditionally’ used parameter values with respect to reproduction and recovery must be undertaken given the demographic information available for the Black Sea population which is known for its early maturation and high reproductive rates (Gol’din, 2004; Vishnyakova, 2017). However, it is clear that the information presented, even for the most conservative estimates of total bycatch, reveals population bycatch rates among the highest in the world (Read et al., 2006; Nelms et al., 2021), and greatly exceeds present agreed thresholds for sustainable levels (c.f. 1.7%, ASCOBANS, 2015) and thus poses a significant threat for this endemic subspecies.

Despite being a species of high conservation concern and under strict protection, conservation of harbour porpoise is failing in Europe (Carlen et al., 2021) including the Black Sea. The conservative estimates of bycatch levels for the Black Sea harbour porpoise in this paper raise serious concerns about the survival of this subspecies. Whilst there is a scientific need to continue to improve monitoring of bycatch and refining bycatch estimates in the light of abundance estimates and population assessment, it is quite clear that the available data are already sufficient to demand that the primary focus must be on establishing effective mitigation measures (see below) and ensuring that these are implemented and monitored for effectiveness (that will also entail population abundance monitoring). Cooperation with fishermen and fisheries authorities for enhancing bycatch reporting is crucial to increase the sample size and robustness of the estimate and to evaluate the effectiveness of mitigation approaches. More effective implementation of existing regulations and recommendations (ex. EU Habitats Directive, ACCOBAMS Recommendations 2.13, 4.9 and 7.11) is needed to minimize IUU and ghost fishing. Accurate and standardized spatio-temporal recording of fishing effort should be conducted, and spatio-temporal closure of fishing should be considered when necessary (this can be evaluated via population dynamics modelling approaches). Further aerial surveys on density, abundance and distribution of cetaceans in the Black Sea are needed to detect trends in population development and seasonal distribution patterns. Support of all Black Sea states for realization of recently adopted ACCOBAMS Resolution 8.10 on implementation of Long-Term Monitoring Strategy is crucial to achieve that. The retrieval of bycaught animals from fishing vessels should be encouraged by the authorities to obtain biological data for population structure assessment.

Thus, as a matter of urgency, bycatch mitigation measures should be further tested and introduced in the Black Sea. Elsewhere, acoustic approaches such as ADDs (pingers) have been used (Dawson et al., 2013). A few models have been tested in Türkiye, Romania and Bulgaria, some showing good results while others not (Gönener and Bilgin, 2009; Bilgin and Köse, 2018; Popov et al., 2020). In the current study, use of PAL pingers showed promising results. The sample size was small; thus, a large-scale trial is required to confirm these provisional results. Dolphin-safe fishing gears and technology are worth attention e.g., modified nets with acrylic glass spheres to improve acoustical detectability were tested in the Turkish Black Sea (Kratzer et al., 2021). In the implementation of mitigation measures and testing their continued effectiveness, in addition to common problems (habituation, habitat exclusion), local specific features should be considered carefully (e.g., assessment of effectiveness of pingers for the endemic Black Sea harbour porpoises). In conclusion, bycatch poses such a serious threat to the Black Sea harbour porpoises that all riparian countries engaged in turbot fisheries are required to implement urgent measures to reduce it immediately (ACCOBAMS, 2019; CMS, 2020).

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Ethical review and approval was not required for the animal study because No disturbance of animals occured.

DP, KV, MPai, AT and PG planned the study. DP, GM, KV, JI, MPai, CT, ED and PG collected data. PG led the analysis. DP, KV and PG analyzed data with input from GM, MPan, AAO, AT and ED. DP, GM, KV and PG wrote the initial manuscript draft with input from all the authors. All authors contributed to the article and approved the submitted version.