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Effects of fishing pressure on biomass and size composition of main commercial species in the “Penisola del Sinis-Isola di Mal di Ventre” MPA (Sardinia, Italy)_ A first assessment

Stefano Marra1, 2*, Andrea Camedda2, Stefania Coppa2, Giorgio Massaro3 and Giuseppe A. De Lucia2
  • 1 Università della Tuscia, Italy
  • 2 CNR-IAMC (Consiglio Nazionale delle Ricerche - Istituto per l’Ambiente Marino Costiero ), Italy
  • 3 Area Marina Protetta "Penisola del Sinis - Isola di Mal di Ventre", Italy

Marine protected areas (MPAs) are considered useful tools for an ecosystem based approach to conservation and their effectiveness has been widely demonstrated in the literature for many marine species, especially fish (García-Charton et al., 2004; Guidetti et al., 2014). In spite of this, some studies highlighted that many MPAs have produced weak effects for the conservation of exploited stokes (e.g. Guidetti et al., 2008). The “Penisola del Sinis-Isola di Mal di Ventre” is an Italian multiple-use MPA established in 1997. The analysis of temporal trends of abundance and biomass of commercial fish species in the area showed how this MPA has been ineffective for the recovery of exploited stocks and how commercial and recreational fishing pressure seems to be an important source of impact (Marra et al., 2016). Spatial and temporal variability resulted more important than the protection level (i.e. the level of fishing restriction acting in the different protection zones) in determining the response of the biological variables. The present work aims to add details on the effects of the fishing pressure acting in the area. Data of abundance and size of fish - belonging from four video visual census surveys conducted in 2004, 2007, 2010 and 2012 - were used. The two main fishing techniques acting in the area (gillnetting and longlining) were considered, and the temporal trends in biomass of their target species were analysed to produce a preliminary impact assessment. Furthermore, an analysis of trends in size distribution of a subset of the selected target species – the most abundant - was performed to test if a decline of the mean size can be detected across years. Three sites were selected representing respectively the offshore part (Mal di Ventre_ Mv) at 5 nautical mile from the mainland, the southern (Torre Seu_Ts) and the northern part (Su Pallosu_SP) of the Sinis Peninsula. The species used for the assessment of gillnetting impact were Labrus merula, Labrus viridis, Mullus surmuletus and Serranus Cabrilla, these were indeed the most common species observed in the area that are fished almost exclusively with such technique. As all the species affected by longlining can also be fished with gillnets, only the combined impact of these two techniques could be assessed (species used were Diplodus sargus sargus, D. vulgaris and D. puntazzo). Biomass of species target of gillnetting was on average higher at MV (338.88±114.85 g/125m2 ), with the lowest values observed in 2004 (153.36±77.946 g/125m2) and the highest in 2007 (871.05±254.04 g/125m2). The site with the lowest mean biomass was SP (56.73±21.154 g/125m2) that ranged from a minimum value of 11.61±11.06 g/125m2 (2004) to a maximum of 90.80±56.165 g/125m2 (2007). Finally TS has a mean value of 290.65±89.592 g/125m2 ranging from 136.59±71.43 (2007) to 567.10±276.31 g/125m2 (2004). Temporal trends shows no clear direction: in each site values of biomass were similar across the years (with the exception of the extreme values recorded in 2007 for MV and 2004 for TS). Species affected both by gillnetting and longlining show a similar pattern, with the highest mean value observed at MV (3961.18±802.18 g/125m2), intermediate in TS (2105.07±586.86 g/125m2) and the lowest value observed in SP (1150.62±372.02 g/125m2). Biomass in MV ranged from 2078.67 ±415.58 (year 2010) to 4707.81±2182.6 g/125m2 (year 2004), in TS it range from 843.61±181.09 (year 2012) to 3463.16±1348.5 g/125m2 (year 2007) and in SP from 185.78±185.78 (year 2004) to 2760.13±1012.7 g/125m2 (year 2010). Temporal trends were different in the three sites: in MV biomass decreased in 2010-2012 in comparison with 2004-2007; in TS biomass reached a maximum in 2007 and then constantly decreased in the following years; in SP values of 2010 were particularly higher in comparison of the other years. The mean length recorded in 2004, 2007, 2010 and 2012 for D. vulgaris were respectively: 15.0 cm, 18.1 cm, 18.8 cm and 15.4 cm (MV). In TS mean length were 16.3 cm, 15.2 cm, 16.9 cm, 13.5 cm. In SP 15.0 cm 12.4 cm 17.4 cm 13.7 cm. As for D. sargus sargus, the mean length in the different years was respectively 17.0 cm, 19.1 cm, 22.1 cm, 15.6 cm (MV). In TS: 26.5 cm, 15.1 cm, 17.4 cm and 13.2 cm. In SP: 17.0 cm, 13.8 cm, 18.4 cm and 13.8 cm. Specimen of L. merula were not detected in 2004 in MV, their mean length in the following surveys was respectively 27.5 cm, 25.0 cm and 16.0 cm. In TS the mean lengths were: 22.0 cm, 20.0 cm and 15.0 cm (no specimen were observed in 2012). In SP only one specimen of L. merula was observed (20 cm_ 2010). Finally, the mean lengths of S. scriba were 21.3 cm, 19.7 cm, 20.0 cm and 13.7 cm in MV. In TS the mean lengths were:18.3 cm, 13.6 cm, 17.7 cm and 14.3 cm. In SP: 14.0 cm, 16.0 cm, 12.0 cm, 14.8 cm. No clear temporal trends have been detected but differences were observed regarding the spatial scale: biomass values were higher in MV, intermediate in TS and lower in SP, both when the target species of gillnetting and of gillnetting and longlining are considered. This result suggests that in the three sites different fishing pressures are acting. The different fishing pressure among the three sites is also suggested by the fact that D. vulgaris, D. sargus sargus, L. merula and S. scriba showed an higher mean length in MV while in SP tend to have the lowest mean length. Anyway all the areas seem to be affected by a strong fishing pressure, as these species tend to have small mean length in all sites declining across years. This work constitutes a first assessment of the fishing impact acting in the area on a site scale. Although the study is preliminary, it represents an useful baseline to be improved with ad hoc studies on fishery effort, in order to produce more appropriate spatial planning in the area.

References

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Marra, S., de Lucia, G.A., Camedda, A., Esinosa, F., Coppa, S. (2015). New records of the distribution and conservation status of the endangered limpet Patella ferruginea in Sardinia (Italy, W Mediterranean). Aquat. Conserv. Mar. Freshw. Ecosys. doi: 10.1002/aqc.2615

Keywords: Mediterranean, Small-scale fishery, Temporal Trends, commercial species, long-term monitoring, fish

Conference: XIX Iberian Symposium on Marine Biology Studies, Porto, Portugal, 5 Sep - 9 Sep, 2016.

Presentation Type: Poster Presentation

Topic: 1. ECOLOGY, BIODIVERSITY AND VULNERABLE ECOSYSTEMS

Citation: Marra S, Camedda A, Coppa S, Massaro G and De Lucia GA (2016). Effects of fishing pressure on biomass and size composition of main commercial species in the “Penisola del Sinis-Isola di Mal di Ventre” MPA (Sardinia, Italy)_ A first assessment. Front. Mar. Sci. Conference Abstract: XIX Iberian Symposium on Marine Biology Studies. doi: 10.3389/conf.FMARS.2016.05.00149

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Received: 29 Apr 2016; Published Online: 03 Sep 2016.

* Correspondence: Mr. Stefano Marra, Università della Tuscia, Viterbo, Italy, stefano.marra@iamc.cnr.it