Behavioural responses of sardines Sardina pilchardus to simulated purse‐seine capture and slipping

Abstract

The behavioural effects of confinement of sardine Sardina pilchardus in a purse seine were evaluated through three laboratory experiments simulating the final stages of purse seining; the process of slipping (deliberately allowing fishes to escape) and subsequent exposure to potential predators. Effects of holding time (the time S. pilchardus were held or entangled in the simulation apparatus) and S. pilchardus density were investigated. Experiment 1 compared the effect of a mild fishing stressor (20 min in the net and low S. pilchardus density) with a control (fishing not simulated) while the second and third experiments compared the mild stressor with a severe stressor (40 min in the net and high S. pilchardus density). In all cases, sea bass Dicentrarchus labrax were used as potential predators. Results indicated a significant effect of crowding time and density on the survival and behaviour of slipped S. pilchardus. After simulated fishing, S. pilchardus showed significant behavioural changes including lower swimming speed, closer approaches to predators and higher nearest-neighbour distances (wider school area) than controls, regardless of stressor severity. These results suggest that, in addition to the delayed and unobserved mortality caused by factors related to fishing operations, slipped pelagic fishes can suffer behavioural impairments that may increase vulnerability to predation. Possible sub-lethal effects of behavioural impairment on fitness are discussed, with suggestions on how stock assessment might be modified to account for both unobserved mortality and sub-lethal effects, and possible approaches to provide better estimates of unobserved mortality in the field are provided.

Behavioural responses to human‐induced change: Why fishing should not be ignored

“…Particularly interesting would be to apply experimental designs such as those of Huse and Vold (), Marçalo et al. () and Ingólfsson and Jørgensen () that can retain both captured individuals and escapees from active gears, where data are most limited. Then, the combined use of laboratory experiments and telemetry in natural conditions would allow assessing behavioural, physiological and life‐history differences between groups and the relative importance of different traits in gear selection in the wild.…”

School density affects the strength of collective avoidance responses in wild‐caught Atlantic herring Clupea harengus: a simulated predator encounter experiment

“…anoxia) and physical impairments (e.g. scale loss and skin injuries) increasing stress levels as well as delayed mortality as shown in Sardina pilchardus (Walbaum 1792) (Marçalo et al, 2008(Marçalo et al, , 2010(Marçalo et al, , 2013 and C. harengus (Tenningen et al, 2012). Among the reported behavioural effects caused by stressful manipulations, drastic changes in schooling tendency (e.g.…”

“…Among the reported behavioural effects caused by stressful manipulations, drastic changes in schooling tendency (e.g. reduced swimming speed, increased inter-fish distances, reduced school cohesion, incapability to maintain circular and co-ordinated swimming patterns) and lower responsiveness to predators resulting in a greater vulnerability to predation have been found in S. pilchardus (Marçalo et al, 2013). Despite there being no physiological measurements made on the fish (e.g.…”

Effects on schooling function in mackerel of sub-lethal capture related stressors: Crowding and hypoxia

“…These mechanisms include efficient gradient search [ 19 ], decision making [ 20 ], increased swimming efficiency [ 21 ], predator avoidance and information transfer [ 22 , 23 ], among others. Laboratory studies indicate that purse seine fishing and slipping may result in temporary behavioural impairment that could increase susceptibility to predation [ 24 ]. It has also been shown that hypoxia, one of the likely key stressors during purse seining, can have detrimental effects on anti-predatory behaviour of schooling species with demonstrated drastic changes in schooling tendency (e.g.…”