Release protected or species of concern alive after capture
Overall effectiveness category Awaiting assessment
Number of studies: 6
Background information and definitions
Releasing protected or other threatened fish species alive after incidental capture should, in theory, protect the released individuals and wider populations, particularly weaker ones, from fishing mortality and overexploitation. However, the effectiveness of live release relies on a range of factors affecting the survival of fish post-capture, including: the extent of damage from the gear, fishing duration, catch size, exposure to the air and temperature changes, depth of capture and behavioural impairments that increase the chance of predation after release (Davis 2002). In some countries where commercial fish catches are managed by quota limits, there are policies that state fish cannot be returned to the sea unless certain conditions are met, such as evidence of the species having a high likelihood of survival (Ellis et al. 2016). Survival may depend on multiple factors including species vulnerability, capture method (and depth of capture), duration of time exposed to the air and handling techniques.
For related interventions, see ‘Handling of catch’.
Davis M.W. (2002) Key principles for understanding fish bycatch discard mortality. Canadian Journal of Fisheries & Aquatic Sciences, 59, 1834–1843.
Ellis J.R., McCully Phillips S.R. & Poisson, F. (2016) A review of capture and post-release mortality of elasmobranchs. Journal of Fish Biology, 90, 653–722.
Supporting evidence from individual studies
A replicated study in 2000–2001 of bottom fishing grounds in the Cantabrian Sea, Spain (Rodríguez-Cabello et al. 2005) reported that a large proportion of unwanted (discarded) small-spotted catshark Scyliorhinus canicula survived after capture by commercial trawl nets. In commercial trawl catches, average catshark survival was 78% (range 47–91%). In addition, increases in sorting time during sorting of catches had a weak effect on survival (i.e. slightly decreased it), however there was no influence of haul duration (3–6 h; data reported as graphical analysis). In all but one haul there was no difference in survival between male and female catsharks (data reported as statistical results). Data were collected from otter trawl deployments during commercial fishing trips (16 hauls). For each deployment, groups of twenty catshark (10 of each sex) were selected and were exposed to air for roughly six different intervals between 18–85 min. After each interval, catshark were transferred to onboard tanks and survival was assessed after 1 h.Study and other actions tested
A replicated study in 2009–2011 on an area of sandy and muddy seabed in the Gulf of Maine, north Atlantic Ocean, USA (Mandelman et al. 2013) reported that a large proportion of unwanted (prohibited) thorny skate Amblyraja radiata, but less than half of unwanted smooth skate Malacoraja senta, survived for three days after incidental capture in otter trawls targeting little skate Leucoraja erinacea and winter skate Leucoraja ocellata. Across all tow durations, overall survival of unwanted thorny and smooth skates after 72 h was 81% and 41% respectively. For the two commercial skates, overall survival was 92% (winter) and 86% (little). In addition, for all skates combined, species was the only significant factor affecting survival (out of other factors include catch biomass, temperature, tow duration). Individually, there were no significant predictors of 72 h mortality detected for thorny skate (data reported as statistical results; insufficient data for smooth skate). Data were collected from skates caught in 71 otter trawl deployments. Immediate mortality of skates was assessed before randomly selected live skates were transferred to deck tanks, then to sea pens that were lowered to the seafloor. Sea pens were retrieved after 72 h to assess mortality and all live skates released.Study and other actions tested
A replicated study (year not stated) of two areas of rocky reef and gravel in the Coral Sea off Australia (Campbell et al. 2014) found that the majority of pearl perch Glaucosoma scapulare released alive after capture by hook and line survived for up to three days, and survival was influenced by hook location and signs of barotrauma (effects of capture at depth). The overall short-term (3 day) post-release survival rate of perch was 92%. In addition, survival rates were higher for perch hooked in the lip or mouth (93–100%) than those hooked in the throat (36%), and for those with no obvious signs of barotrauma (93%) compared to fish observed with swollen or everted stomachs (63–69%). Data were collected for 183 pearl perch (19–61 cm total length) caught during four field trips (dates/year of sampling unspecified) using conventional baited rod and reel, in one shallow (<80 m) and one deep (>80 m) area off the coast of Queensland, 50 nm apart. Hook location was recorded before the captured perch were tagged and placed either into onboard holding tanks or vertical enclosures anchored to the seabed. Post-release survival was assessed after three days.Study and other actions tested
A replicated study in 2004 in bottom fishing grounds in the Atlantic Ocean off Canada (Grant & Hiscock 2014) found that Atlantic wolffish Anarhichas lupus (protected species) caught in the yellowtail flounder Limanda ferruginea trawl fishery typically survived for five days after capture and release. Overall post-capture survival of 41 wolffish (either kept in holding tanks for 48 h or held in tanks for 10 h before being placed in sea cages on the seabed for 2 d) ranged from 0–100%. However, survival varied with duration of air exposure after capture, with most individuals exposed to air for periods <2 h surviving (<30 min: 100%, 30 min–1 h: 88%, 1 h–1.5 h: 100%, 1.5–2 h: 90%), while none of the 4 fish exposed for >2 h survived. In autumn 2004, post-capture survival of wolffish captured in yellowtail flounder trawl net deployments was assessed. A total of 23 wolffish were held in temporary holding tanks and survival assesses up to 48 h, and 18 wolffish were held in tanks for 10 h before being moved to holding cages deployed on the seafloor where survival was monitored for up to 2 d.Study and other actions tested
A replicated study in 2012–2013 in two areas of pelagic water in the Pacific Ocean, off Oregon, USA (Hannah et al. 2014) found that almost all protected yelloweye rockfish Sebastes ruberrimus survived for four days after capture and release, but post-release survival of protected canary rockfish Sebastes pinniger decreased with capture depth. Overall post-release survival of yelloweye rockfish Sebastes ruberrimus was 95% (77 of 81 fish survived), while survival of canary rockfish Sebastes pinniger was 78% (42 of 54 fish survived). Estimates (to compensate for small sample sizes) of the proportion of yelloweye rockfish surviving with capture depth were similar (83–93%) across all five depth ranges sampled. For canary rockfish, estimated survival decreased with increasing depth of capture; >75% at depths of 46–84 m, decreasing to 25% at depths below 135 m. In coastal waters of the USA, non-retention rules for several species of Pacific rockfishes formed part of the Pacific Coast Fishery Management Plan of 2012. Sampling was done between September 2012 and October 2013 at two areas near Stonewall Bank, off Newport. A total of 81 yelloweye and 54 canary rockfish were caught with rod and reel from five depth zones across a total depth range of 46–175 m. After capture, each fish was placed in a specifically designed cage that was then deployed in the sea at a depth similar to the depth at fish capture. Cages were retrieved after 44–96 h and survival recorded.Study and other actions tested
A replicated study in 2013–2015 in coastal pelagic waters of the Tasman Sea, off northern New Zealand (Francis & Jones 2017) found that some individuals of the protected and endangered spinetail devilray Mobula japonica survived release after accidental capture in purse-seine nets and normal handling practices, but survival was decreased in rays brought aboard entangled in the net. Three of seven data tagged devilrays survived capture and release, two over monitoring lengths of 30 days and one with a monitoring length of 82 days. Four rays died within 1–2 days of release, indicated by rapid descents in the data to ~1,800 m. All three rays that survived had been brought onboard from the purse-seine in a convey net, or brailer, and released directly from the convey net or using a rope and winch. All rays that died had been entangled in a part of the net (bunt) and hauled onboard in the final section of the main purse-seine net. Rays were tagged by observers aboard commercial skipjack purse-seine vessels after being lifted on deck. Only rays hauled onboard were tagged as those released whilst in the water were all expected to survive. Satellite-archival tags were anchored in the central part of the wing musculature using an umbrella anchor with eight plastic barbs. Anchors were attached to tags by 10-11 cm monofilament nylon or stainless-steel tethers. Tags were secondarily attached to the wing with a numbered conventional plastic tag and had a release device that triggered at 1,700–1,800 m.Study and other actions tested