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

Abstract

The selectivity of fishing gears with respect to fish species and size is important, both for fisheries management and fishing operations. Purse seining is an efficient, environmentally friendly fish capture methodology generally targeting single species aggregations, but once a fish school has been selected and surrounded by the seine, there is no selections for individual size, species or catch quantity. A common practice for evaluating the catch is to haul the seine to a point where physical samples or inspections of catch composition can be made. The release process is called slipping and may lead to mortality in the released fish. The objective of this study was to simulate a crowding situation and investigate how the behaviour was affected in response to increased fish density, decreased oxygen levels, or a combination of the two, and to see if there is a behavioural measure that can be used to set safe crowding limits. The experiment was conducted on Mackerel (Scomber scombrus) held in net pens. The volume of the net pen was reduced to increase fish density, and a tarpaulin bag was wrapped around the pen to reduce the oxygen levels. Oxygen, fish density and space occupancy was monitored during the experiment, and the behavioural reactions was assessed using an imaging sonar. The main result was that the schooling function, i.e. the response to a predator model, was significantly reduced during crowding but not in response to hypoxia. There were some indications of a slow recovery of the function post-treatment. We conclude that crowding causes behavioural responses that occur before densities that induce fish mortality. Consequently, there is a behavioural response that could be used as a proxy for setting safe crowding limits.

Physiological and flesh quality consequences of pre-mortem crowding stress in Atlantic mackerel (Scomber scombrus)

“…Our results are therefore best considered as a demonstration of the potential for crowding stress to influence quality parameters and further work would be required within the fishery to establish if the results are generalisable. Arguably, the responses we observed in the net pen experiment would be less than a wild caught population due to the effects of habituation (as noted by [14,15]). Accordingly, future work should set out to determine any physiological and quality effects from commercial catches.…”

“…Duration of crowding is dependent on catch size, but pumping can take up to an hour or more to be completed [13]. Experimental observations suggests that purse seine crowding is associated with increases in physical activity levels [12,14,15], and anecdotal evidence from the fishery indicates that this activity can become intense during the final stages of capture. Behavioural activity at this stage is characterised by sustained and intense escape behaviour from the fish.…”

Behavioural and welfare implications of a new slipping methodology for purse seine fisheries in Norwegian waters

“…Efforts to maximise post slipping survival through better catch control earlier in the capture process (such as those described by [ 55 ] and [ 56 ]), may be negated if the slipping process itself further compromises the welfare of released catches. As such, it is important to understand how slipping methodology affects welfare.…”

An Adapted Slipping Process to Exclude Jellyfish in the Sea of Marmara Purse Seine Fishery

“…Purse seining is energy efficient, environmentally friendly fish capture methodology generally targeting large and small pelagic fish shoals (Handegard et al, 2017) with sonar detection, light sources or Fish Aggregation Devices (FAD). Purse seining solely accounts for about 30% of the world's total fisheries catch (Watson et al, 2006) and approximately three out of four domestic catch of Turkey (TurkStat, 2018).…”