Estimating Bycatch From Non-representative Samples (II): A Case Study of Pair Trawlers and Common Dolphins in the Bay of Biscay

Abstract

Marine megafauna plays an important functional role in marine ecosystems as top predators but are threatened by a wide range of anthropogenic activities. Bycatch, the incidental capture of non-targeted species in commercial and recreational fisheries, is of particular concern for small cetacean species, such as dolphins and porpoises. In the North-East Atlantic, common dolphin (Delphinus delphis, Linné 1758) bycatch has been increasing and associated with large numbers of animals stranding during winter on the French Atlantic seashore since at least 2017. However, uncertainties around the true magnitude of common dolphin bycatch and the fisheries involved have led to delays in the implementation of mitigation measures. Current data collection on dolphin bycatch in France is with non-dedicated observers deployed on vessels for the purpose of national fisheries sampling programmes. These data cannot be assumed representative of the whole fisheries' bycatch events. This feature makes it difficult to use classic ratio estimators since they require a truly randomised sample of the fishery by dedicated observers. We applied a newly developed approach, regularised multilevel regression with post-stratification, to estimate total bycatch from unrepresentative samples and total fishing effort. The latter is needed for post-stratification and the former is analysed in a Bayesian framework with multilevel regression to regularise and better predict bycatch risk. We estimated the number of bycaught dolphins for each week and 10 International Council for the Exploration of the Sea (ICES) divisions from 2004 to 2020 by estimating jointly bycatch risk, haul duration, and the number of hauls per days at sea (DaS). Bycatch risk in pair trawlers flying the French flag was the highest in winter 2017 and 2019 and was associated with the longest haul durations. ICES divisions 8.a and 8.b (shelf part of the Bay of Biscay) were estimated to have the highest common dolphin bycatch. Our results were consistent with independent estimates of common dolphin bycatch from strandings. Our method show cases how non-representative observer data can nevertheless be analysed to estimate fishing duration, bycatch risk and, ultimately, the number of bycaught dolphins. These weekly-estimates improve upon current knowledge of the nature of common dolphin bycatch and can be used to inform management and policy decisions at a finer spatio-temporal scale than has been possible to date. Our results suggest that limiting haul duration, especially in winter, could serve as an effective mitigation strategy.

Behavioural Responses of Common Dolphins Delphinus delphis to a Bio-Inspired Acoustic Device for Limiting Fishery By-Catch

“…After the PIC project in 2018 (acronym of "analysis of PIngers used for Cetaceans in fishing activities of pelagic trawlers and gill netters") [32], showing a mean reduction of dolphin by-catch by 65% (CI: 15-98%) with the STM DDD03L repellent pinger (from STM-products) installed on French pair trawlers, the device was made mandatory on this fleet composed of about 12 active pair trawlers in winter. However, yearly estimates of common dolphin by-catches from this fleet in 2004-2020 reached a maximum of more than 600 individuals caught (median value, with large between-year variations in estimates, ranging from less than a hundred in 2018 to more than one thousand in 2017) [33]. This highlights the responsibility of other fisheries such as gill netters with regard to dolphin by-catch [21,25].…”

Spatio-temporal trends in cetacean strandings and response in the south-western Indian Ocean: 2000-2020

“…Our approach is important in view of the planned Ocean Economy developments in the region (Mannocci et al, 2013;Ren et al, 2016;Findlay, 2018;Mwango'mbe et al, 2021;Plön and Roussouw, 2022) which will likely result in an increase of stranding events as Ocean Economy activities increase (Plön et al, 2021a;OECD, 2023). Strandings can provide important information on the biology and health of marine mammals (Williams et al, 2011;Rouby et al, 2022) and provide baseline data on the species' range, life history parameters (e.g., age, reproductive status, diet) and even the occurrence and prevalence of disease. Given both the importance of stranding records as a valuable source of information regarding the threats to cetacean communities (Pyenson, 2010), and the need to better understand causes of mortality and factors affecting marine animal health (Tucker et al, 2018), this approach will help identify population threats and stressors to marine life, especially anthropogenic impacts that can be prevented or mitigated (Peltier et al, 2014).…”