Sardine slipping during purse-seining off northern Portugal

Abstract

Observations aboard purse-seiners demonstrated that deliberate lowering of the net to allow pelagic fish to escape (''slipping'') was frequent off northern Portugal during the second semester of 2001. Some slipping occurred in 25 of 30 trips observed, and the quantities slipped were significantly higher when the net was set on dense echo-sounder marks. During the 12 weeks of the study, the sampled fleet (nine vessels) landed 2196 t and deliberately released an estimated 4979 t (CV 33.6%). More than 95% of the total catch was sardine. Data provided by the skippers in the absence of shipboard observers led to considerably lower estimates of slipped quantities. The main reason for slipping was daily quota limitations, but illegal size and mixture with unmarketable bycatch were also reported. These results demonstrate the existence and potential magnitude of slipping, but indications of large seasonal and regional variations make extrapolations for the entire fishery impractical.

Discards of the purse seine fishery targeting small pelagic fish in the eastern Mediterranean Sea

“…This is quite low and even if we consider omitting discards, it would have little effect on stock assessment estimates due to the high catches and abundance of small pelagics (Santojanni et al 2005). In contrast, in certain purse seine fisheries, in which slippage takes place, mortality of slipping fish may constitute an important fraction of the catch; Stratoudakis and Marcalo (2002) estimated that slipping fish make up 69% of the catch in Portuguese purse seines targeting sardine. Mortality of slipping fish (or non-catch mortality) is considered to be a source of error for stock assessment and fisheries management (Mitchell et al 2002, Stratoudakis and Marcalo 2002, Huse and Vold 2010 and it is a knowledge gap for all gears.…”

“…In contrast, in certain purse seine fisheries, in which slippage takes place, mortality of slipping fish may constitute an important fraction of the catch; Stratoudakis and Marcalo (2002) estimated that slipping fish make up 69% of the catch in Portuguese purse seines targeting sardine. Mortality of slipping fish (or non-catch mortality) is considered to be a source of error for stock assessment and fisheries management (Mitchell et al 2002, Stratoudakis and Marcalo 2002, Huse and Vold 2010 and it is a knowledge gap for all gears. Most of this mortality, which mainly occurs due to injuries of the escaping fish, may be species-specific, density-dependent ( Huse andVold 2010, Marcalo et al 2010) and may even vary across space and time.…”

Changes in the abundance and spatial distribution of the Atlantic chub mackerel (<i>Scomber colias</i>) in the pelagic ecosystem and fisheries off Portugal

“…Off Portugal, chub mackerel occurs down to 400 m depth (Martins and Cardador 1996) and its presence is noticed in the artisanal fishery that operates near those depths; fish size probably increases with depth as in several areas of the species range (Baird 1978, Perrota 1992. Since the acoustic surveys do not go beyond the shelf edge (~200 m) and purse seiners fish mainly within the 100 m contour (Stratoudakis and Marçalo 2002), large individuals are probably under-sampled in both. This may be aggravated by the fact that larger individuals are expected to swim faster than smaller ones, being therefore less catchable by the relatively small pelagic trawl with a slow trawling speed used in acoustic surveys (Valdemarsen and Misund 1995), as demonstrated for mackerel (Slotte et al 2007).…”

Mortality of mackerel (Scomber scombrus L.) after pursing and slipping from a purse seine

“…Catch regulation by slipping the whole or parts of a catch has traditionally been used in pelagic fisheries if catches are too large, or the size and/or quality of the fish are regarded as unsatisfactory (Stratoudakis and Març alo, 2002;Borges et al, 2008). This is particularly the case when there is a large price differential among fish sizes or qualities (high grading).…”