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

Abstract

The release of unwanted fish from purse seines whilst still in the water is termed slipping and may lead to significant mortality following release. The objective of this study was to determine the fish welfare implications of a new slipping methodology in which fish are released via a discharge opening formed in the bunt end of the purse seine net. Video analyses of collective and individual level fish behaviour were undertaken in the Norwegian mackerel and herring purse seine fisheries, to quantitively describe slipping behaviour and to determine its driving factors. The majority of fish escaped the purse seine with the schooling structure intact as part of large groups towards the end of slipping process, increasing their speed following escape. However, there was also a tendency (24% of all escapes) to escape in a manner likely to impact negatively upon their welfare, with a breakdown in schooling structure and physical contact with the fishing gear and conspecifics. The tendency to express such welfare compromising behaviour was higher for mackerel than for herring, but was also influenced by the vessel releasing the fish, the amount of fish being slipped, how long the discharge opening had been open and the particular slipping event. These results provide important information for future science-based development of welfare friendly slipping practises.

Physiological response and survival of Atlantic mackerel exposed to simulated purse seine crowding and release

“…To further reduce the negative effects of crowding, fishers could target smaller schools to (i) reduce the likelihood of excess catch and therefore the need to slip and (ii) maintain low densities in the net. Good catch welfare practices during slipping should also ensure crowding levels do not exceed the bare minimum required to encourage fish to exit the net ( Anders et al , 2019a ).…”

“…Three trials (‘Low’, ‘Moderate’ and ‘High’) investigated physiological response and survival by crowding schools for ~15 minutes and then allowing the cage to sink back to maximum volume under its own weight (analogous to a typical, regulation compliant, crowding and slipping event in the fishery; Anders et al , 2019a ). These trials were monitored for post-crowding survival for either 8 or 20 days ( Table 1 ).…”

Physiological response and survival of Atlantic mackerel exposed to simulated purse seine crowding and release

“…To further reduce the negative effects of crowding, fishers could target smaller schools to (i) reduce the likelihood of excess catch and therefore the need to slip and (ii) maintain low densities in the net. Good catch welfare practices during slipping should also ensure crowding levels do not exceed the bare minimum required to encourage fish to exit the net ( Anders et al , 2019a ).…”

“…Three trials (‘Low’, ‘Moderate’ and ‘High’) investigated physiological response and survival by crowding schools for ~15 minutes and then allowing the cage to sink back to maximum volume under its own weight (analogous to a typical, regulation compliant, crowding and slipping event in the fishery; Anders et al , 2019a ). These trials were monitored for post-crowding survival for either 8 or 20 days ( Table 1 ).…”

Atlantic mackerel (Scomber scombrus) change skin colour in response to crowding stress

“…The applied durations were intended to simulate typical fishery crowding durations for either released catches ( c . 15 min, Anders, Breen, et al ., 2019 ) or retained catches ( c . 60 min, during which catches are pumped onboard; Digre et al ., 2016 ).…”

“…Of the species targeted by the extensive and commercially important purse seine fisheries of Europe (ICES, 2020 ), Atlantic mackerel ( Scomber scombrus L.) are particularly vulnerable to crowding stress. For instance, crowding of this species has been associated with dermal injury (Anders et al ., 2021 ; Lockwood et al ., 1983 ), physiological disturbance (Anders et al ., 2020 , 2021 ; Lockwood et al ., 1983 ; Swift, 1983 ), behavioural impairment (Anders, Breen, et al ., 2019 ; Anders, Howarth, et al ., 2019 ; Handegard et al ., 2017 ), reductions in flesh quality (Anders et al ., 2020 ; Digre et al ., 2016 ) and excessively high rates of delayed mortality in released catches (Huse & Vold, 2010 ).…”

Physiological response and survival of Atlantic mackerel exposed to simulated purse seine crowding and release

“…To further reduce the negative effects of crowding, fishers could target smaller schools to (i) reduce the likelihood of excess catch and therefore the need to slip and (ii) maintain low densities in the net. Good catch welfare practices during slipping should also ensure crowding levels do not exceed the bare minimum required to encourage fish to exit the net ( Anders et al , 2019a ).…”

“…Three trials (‘Low’, ‘Moderate’ and ‘High’) investigated physiological response and survival by crowding schools for ~15 minutes and then allowing the cage to sink back to maximum volume under its own weight (analogous to a typical, regulation compliant, crowding and slipping event in the fishery; Anders et al , 2019a ). These trials were monitored for post-crowding survival for either 8 or 20 days ( Table 1 ).…”

Atlantic mackerel (Scomber scombrus) change skin colour in response to crowding stress

“…The applied durations were intended to simulate typical fishery crowding durations for either released catches ( c . 15 min, Anders, Breen, et al ., 2019 ) or retained catches ( c . 60 min, during which catches are pumped onboard; Digre et al ., 2016 ).…”

“…Of the species targeted by the extensive and commercially important purse seine fisheries of Europe (ICES, 2020 ), Atlantic mackerel ( Scomber scombrus L.) are particularly vulnerable to crowding stress. For instance, crowding of this species has been associated with dermal injury (Anders et al ., 2021 ; Lockwood et al ., 1983 ), physiological disturbance (Anders et al ., 2020 , 2021 ; Lockwood et al ., 1983 ; Swift, 1983 ), behavioural impairment (Anders, Breen, et al ., 2019 ; Anders, Howarth, et al ., 2019 ; Handegard et al ., 2017 ), reductions in flesh quality (Anders et al ., 2020 ; Digre et al ., 2016 ) and excessively high rates of delayed mortality in released catches (Huse & Vold, 2010 ).…”

Physiological response and survival of Atlantic mackerel exposed to simulated purse seine crowding and release

“…To further reduce the negative effects of crowding, fishers could target smaller schools to (i) reduce the likelihood of excess catch and therefore the need to slip and (ii) maintain low densities in the net. Good catch welfare practices during slipping should also ensure crowding levels do not exceed the bare minimum required to encourage fish to exit the net ( Anders et al , 2019a ).…”

“…Three trials (‘Low’, ‘Moderate’ and ‘High’) investigated physiological response and survival by crowding schools for ~15 minutes and then allowing the cage to sink back to maximum volume under its own weight (analogous to a typical, regulation compliant, crowding and slipping event in the fishery; Anders et al , 2019a ). These trials were monitored for post-crowding survival for either 8 or 20 days ( Table 1 ).…”

Atlantic mackerel (Scomber scombrus) change skin colour in response to crowding stress

“…The applied durations were intended to simulate typical fishery crowding durations for either released catches ( c . 15 min, Anders, Breen, et al ., 2019 ) or retained catches ( c . 60 min, during which catches are pumped onboard; Digre et al ., 2016 ).…”

“…Of the species targeted by the extensive and commercially important purse seine fisheries of Europe (ICES, 2020 ), Atlantic mackerel ( Scomber scombrus L.) are particularly vulnerable to crowding stress. For instance, crowding of this species has been associated with dermal injury (Anders et al ., 2021 ; Lockwood et al ., 1983 ), physiological disturbance (Anders et al ., 2020 , 2021 ; Lockwood et al ., 1983 ; Swift, 1983 ), behavioural impairment (Anders, Breen, et al ., 2019 ; Anders, Howarth, et al ., 2019 ; Handegard et al ., 2017 ), reductions in flesh quality (Anders et al ., 2020 ; Digre et al ., 2016 ) and excessively high rates of delayed mortality in released catches (Huse & Vold, 2010 ).…”