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MARINFORSK-Marine ressurser og miljø

RedSlip: Reducing slipping mortality in purse seines by understanding interactions and behaviour

Alternative title: RedSlip: Reducing slipping mortality in purse seines by understanding interactions and behaviour

Awarded: NOK 5.5 mill.

Project Number:

243885

Application Type:

Project Period:

2015 - 2018

Partner countries:

Current practices for releasing unwanted catches from purse seines (slipping) can kill a large proportion of the released fish. NFR-Project RedSlip aimed to reduce slipping mortality by improving our understanding of how the purse seine performs and how Mackerel (Scomber scombrus) behave during the capture process. Mackerel supports valuable fisheries in Norway, but is highly sensitive to crowding density and duration in purse seines. RedSlip investigated whether capture-related stresses (i.e. crowding, oxygen depletion and physical injury) can alter fish behaviour, which may be used as warning signals, before the fish are fatally harmed. RedSlip also investigated whether understanding gear performance and net-volume during hauling could be used to develop safe release thresholds in purse seine fisheries. In controlled experiments, we investigated how mackerel responded to increased crowding and oxygen depletion. We monitored the schooling response in mackerel in the net pens, while increasing crowding densities up to 80-100 fish per cubic metre and lowering oxygen to 40% saturation. The strongest sub-lethal collective behavioural signal was a reduced response to a predator model at high crowding densities. In addition, tail beat frequency was shown to increase in response to the stressors, which could have application as an indicator of sublethal stress in commercial fishing. In commercial fisheries, the behaviour of mackerel, was monitored using cameras in relation to oxygen concentrations and crowding levels. Early in the capture process, the captive fish were generally unconstrained by the net and formed ordered, polarised schools. However, as the volume of the net became more constraining, the outer edges of the school were unable to avoid contact with the netting, and increased crowding densities were observed, associated with frequent fish-to-fish and fish-to-net contact and disordered behaviour. Oxygen concentrations were variable but generally declined throughout the hauling period, as the catch became more crowded; although, no oxygen concentrations less than 40% saturation were observed. We also used acoustic techniques to monitor school structure, crowding densities and gear performance in commercial fisheries. Behavioural reactions to early stage capture show that the mackerel schools are generally polarised and have dynamic and variable reactions to capture. During seine hauling, an increase of about 50% in density was measured, compared to pre-catch levels. With the aim of predicting the range of possible crowding densities experienced in different fishing situations at the current limits for slipping, a method for reconstructing the three-dimensional shape and volume of a purse seine was developed. The results indicated that in many situations fish densities are within safe levels at the current release limits, in both the EU and in Norway. However, in large catches fish densities may approach or exceed detrimental levels, especially when caught with a smaller seine. To provide advice on the effective management and regulation of slipping in purse seine fisheries, in collaboration with colleagues from the fishing industry and international research community, these results were compared against current slipping regulations, in Norway and the EU. It was concluded that evidence from this project supports the principle that releasing, or 'slipping', unwanted catches from the purse seine can be a responsible catch control practice; if done before the captive fish become too crowded and in a way that maintains their ordered behaviour. That is, mackerel tolerated moderate crowding densities and relatively low oxygen concentrations without significant mortality. Furthermore, such conditions were generally not observed until late in the haul-back phase, particularly in slipped catches. However, a predetermined fixed limit for catch release, based on purse seine length, is not an optimal method for regulating slipping. This is because the variability of seine dimensions and catch sizes, as well as the complex net geometry under varying operational conditions, makes it difficult to predict the volume of free space available to the catch, and hence the potential crowding density. Changes in behaviour have been shown to occur at sub-lethal (e.g. increased activity and disrupted schooling function) and potentially lethal crowding densities (e.g. disordered breakdown of schooling structure). These could be used as potential indicators of the welfare status of the catch, but practical challenges for effectively monitoring such indicators remain. To this end, further development of innovations developed in this project could prove fruitful, for example: monitoring individual behaviour directly in the catch, using a dedicated probe; and inferring school behaviour from acoustic backscattering, using sonar technology on the drop keel.

Current practices for releasing unwanted catches from purse seines (slipping) can cause considerable unaccounted mortality in these fisheries. In this project we aim to mitigate for this by developing the necessary tools to improve our understanding of purse seine performance, fish density structures and fish behavior during the capture process. Building on the knowledge that slipping mortality is related to crowding density and duration, we hypothesize that the risk of mortality is related to the stresses associated with hypoxia and skin injury, and that these will manifest in disruption to the school structure before the fish are fatally crowded. We further hypothesize that, in addition to understanding how the fish interact with the seine, by understanding gear performance and being able to predict seine volume during hauling it will be possible to develop safe release thresholds in purse seine fisheries. Acoustic and photogrammetric techniques will be used to monitor school structure, crowding densities and gear performance. The experiments will be carried out in net pens and in commercial fisheries, allowing us to translate the information gathered from the mesocosm experiments into real commercial fisheries. We will work in collaboration with colleagues from the fishing industry and international research community to integrate of our increased knowledge from this project into practical advice on the effective management and regulation of slipping in purse seine fisheries. Mackerel (Scomber scombrus) will be used as the study species. This species has been shown to be highly sensitive to crowding in the purse seine and supports valuable fisheries in Norway. The outcomes of this proposal are expected to enable the safe release of unwanted catches in mackerel fisheries, but also gain knowledge and develop gear and catch monitoring techniques that will be useful for further development of efficient and sustainable purse seine fisheries on a general basis.

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Funding scheme:

MARINFORSK-Marine ressurser og miljø