Comparisons of Trawl and Longline Catches of Deepwater Elasmobranchs West and North of Ireland

Abstract

A comparison was made between catches from deepwater trawl and longline surveys (1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000) in the Northeast Atlantic. Longline catches were dominated by elasmobranchs, particularly squalid sharks and species numbers were low. Trawl catches had higher species numbers, with more teleosts, though elasmobranchs were still an important component. Species composition of the catch was depth dependent. Comparative trawl and longline surveys of the eastern and southern slopes of the Rockall Trough (west and north of Ireland) were used to examine size-selectivity. Trawls and longlines selected for significantly different size frequency distributions of Centroscymnus coelolepis and Deania calceus, though not for Centrophorus squamosus. These data highlight some important aspects of behavior of the species studying relation to fishing gears. Smaller C. coelolepis were selected by longlines than trawls, suggesting that smaller sharks were present at a considerable height above the seabed, out of reach of trawls, but attracted to baited hooks. In the case of D. calceus, larger females were selected by hooks, but were not present in trawl catches, possibly indicating their ability to escape towed gears. Trawl selectivity ogives were constructed for D. calceus and ogives for C. coelolepis and C. squamosus were simulated, using available data. Results suggest that longlines are not as selective for C. coelolepis as trawls. Selectivity ogives for D. calceus were similar in form, but longlines selected bigger individuals. Life history studies suggest that these species cannot sustain high fishing pressures. The implications of these results for the management of fisheries taking elasmobranchs are discussed.

Feeding habits and niche separation among the deep-sea chimaeroid fishes Harriotta raleighana, Hydrolagus bemisi and Hydrolagus novaezealandiae

“…Hydrolagus affinis has been observed visiting baits at depths of 2020 m in the north Atlantic Ocean (Priede et al 1994), and several species have been caught on baited hooks, confirming scavenging behaviour, e.g. H. affinis (Forster 1964), Hydrolagus pallidus (Marques & Porteiro 2000) and C. monstrosa (Clarke et al 2005). Although the diet of chimaeras may be predominantly benthic fauna, scavenging behaviour suggests they retain a degree of opportunism.…”

Elasmobranch bycatch in a trammel net fishery in the Portuguese west coast

“…Most studies on bycatch and discards of elasmobranchs consider trawl and longline fisheries (Carbonell et al, 2003;Clarke et al, 2005;Coelho and Erzini, 2008;Megalofonou et al, 2005;Stobutzki et al, 2002). Erzini et al (2002) studied discards from the most important fisheries in southern coast of Portugal and found that, despite a great diversity of species caught, including seven chondrichthyan species, trammel nets were those with lower discards.…”

Population biology of the shark, Squalus megalops, harvested in the central-east Atlantic Ocean

“…Sexual dimorphism, or differences in mortality between sexes, could explain the biased sex ratio in the adults (Rago et al, 1998). Unequal sex ratios are quite common in longlines catches (Wirtz and Morato, 2001) because, although small fish taking bait from longlines may be scared away by larger fish (Clarke et al, 2005), the use of smaller hooks does not lead to catching small specimens (Veríssimo et al, 2003). Additionally, a higher natural mortality for males could have caused the biased sex ratio.…”