Bycatch species composition over time by tuna purse-seine fishery in the eastern tropical Atlantic Ocean

Abstract

Within the Ecosystem-based fisheries management framework, we evaluated the changes over time in bycatch species of the European tuna purse-seine fishery operating in the eastern Atlantic Ocean. Bycatch data was collected during two scientific observer programs conducted in the late 1990s and in the late 2000s. Over these two time periods, we compared the temporal trends in bycatch species composition, the probability of occurrence of functional groups per fishing set, the spatio-temporal species richness and the potential impact on several species listed in the red list of the International Union for Conservation of Nature (IUCN). The analyses were performed separately on the two main fishing modes of the fleet, i.e. sets on free-swimming school sets and on fish aggregating devices (FADs). Owing data quality constraints, we did not estimate bycatch rates. Ours results showed that the species composition of sharks caught on FADs decreased over time. The total species richness was higher for FAD sets than for free-swimming school sets (87 vs. 61 species respectively), such difference is common between fishing modes worldwide.

Biodiversity and environmental characteristics of the bycatch assemblages from the tropical tuna purse seine fisheries in the eastern Atlantic Ocean

“…Observer programmes provide the necessary information for that purpose, but with limitations and differences between countries and oceans. Our results are in concordance with the work performed by Torres‐Irineo, Amandè, et al, , Torres‐Irineo, Gaertner, et al, and Lezama‐Ochoa et al () focused on a Species richness estimation of the bycatch assemblages in the Atlantic and Western Indian Oceans, respectively. A greater sample size is necessary to obtain an accurate estimation of the total number of bycatch species in both fishing modes.…”

“…FAD sets have higher bycatch rates (241.5 t/1,000 t in FAD sets) (Amandè et al, ) and a larger number of species (Lezama‐Ochoa et al, ; Torres‐Irineo, Amandè, et al, ; Torres‐Irineo, Gaertner, Chassot, & Dreyfus‐León, ) than Free School sets (38.7 t/1,000 t in Free School sets). For example, Torres‐Irineo, Amandè, et al, , Torres‐Irineo, Gaertner, et al, reported 87 species in FAD sets and 61 species in Free School sets in the Atlantic Ocean. In the case of the Western Indian Ocean, Lezama‐Ochoa et al () reported 74 species in FAD sets and 56 in Free School sets.…”

“…The increase in FAD use from the 1990 s increased the purse seine catches of tropical tuna by nearly 60%, as well as the ecological problems associated with the bycatch managed by the International Commission for the Conservation of Atlantic Tunas (ICCAT) (Torres‐Irineo, Amandè, et al, ; Torres‐Irineo, Gaertner, et al, ).…”

Biodiversity and Habitat Characteristics of the Bycatch Assemblages in Fish Aggregating Devices (FADs) and School Sets in the Eastern Pacific Ocean

“…Our results showed that the sample size used in this study with nearly 100% coverage rate was sufficient to find almost all species in FAD and School sets, as shown by the shape of the accumulation curves where the asymptote is reached. Although it is known that the number of species of the bycatch in FAD sets is higher than in School sets (Amandè et al, 2010;Torres-Irineo et al, 2014), our work suggests that the total number of bycatch species (and not the number of species per set) caught in the tropical tuna purse seiners is the same for both set types provided that sufficient sample size and the coverage rate is reached irrespective of the fishing mode.…”

“…Our results suggest that Chao2 estimator, as used in the work of Torres-Irineo et al (2014) and Lezama-Ochoa et al (2015) is a good option for estimating the total species richness in both fishing modes. The Chao2 estimator showed that FAD and School sets could reach the asymptote with 71 and 68 species, respectively, which means that almost all species caught in the purse-seine fishery in the studied area were observed and the sample size was enough for obtaining bycatch diversity estimates.…”