Bycatch and discard mortality in commercially caught blue sharks Prionace glauca assessed using archival satellite pop-up tags

Abstract

Blue sharks Prionace glauca are the most frequently discarded fish species during commercial pelagic longline fishing operations worldwide, yet their post-release mortality rate has never been measured. A generalized linear model of 12 404 blue sharks observed during the Canadian Atlantic pelagic longline swordfishery suggested a hooking mortality of 12 to 13%, yet scientific examination of 902 of these sharks indicated that hooking mortality was actually higher. A random sample of 40 of these blue sharks were tagged with satellite pop-up archival transmission (PAT) tags, then monitored for periods of up to 6 mo after release. All of the surviving sharks exhibited a depthholding recovery behaviour for a period of 2 to 7 d after release. All healthy sharks survived, while 33% of those that were badly injured or gut hooked subsequently died. Overall blue shark bycatch mortality in the pelagic longline fishery was estimated at 35%, while the estimated discard mortality for sharks that were released alive was 19%. Survival time models indicated that 95% of the mortality occurred within 11 d of release, indicative of death by trauma rather than starvation. The annual blue shark catch in the North Atlantic was estimated at about 84 000 t, of which 57 000 t is discarded. A preliminary estimate of 20 000 t of annual dead discards for North Atlantic blue sharks is similar to that of the reported nominal catch, and could substantially change the perception of population health if incorporated into a population-level stock assessment. KEY WORDS: Discard mortality · Hooking mortality · Tagging · Bycatch · Satellite tags Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 387: [241][242][243][244][245][246][247][248][249][250][251][252][253] 2009 ronmental conditions at the time of release (Davis 2002). The difficulty in quantifying post-release mortality is due to the scarcity and/or expense of methods for tracking released fish in the wild over periods of time of up to several months. Most studies have attempted to avoid this issue by holding fish in cages or pens for several days after capture (e.g. Neilson et al. 1989). However, holding pens provide a clearly artificial and spatially constrained environment, and thus have the potential to introduce (or avoid) sources of mortality that would not be present under natural, free-swimming conditions. As a result, some sort of tag-recapture or telemetry program is required to properly estimate the post-release mortality rate of discarded fish (Davis 2002, Pollock & Pine 2007, Skomal 2007. Such a program would be well suited for monitoring released fish in the wild for extended periods of time, and has been successfully applied in estimating short-term discard mortality rates in large pelagic fish such as striped marlin (Tetrapturus audax) (Domeier et al. 2003). An additional advantage of such studies is that evidence of physical trauma or stress indicators from blood chemistry can ultimately be linked to the subsequent s...

An overview of the hooking mortality of elasmobranchs caught in a swordfish pelagic longline fishery in the Atlantic Ocean

“…The smooth hammerhead seems to be a particularly vulnerable species in this respect, as 71% of the specimens are captured already dead. Campana et al (2009) about blue sharks caught by the Canadian longline fishery in the northwest Atlantic Ocean, concluded that short-term hooking mortality was in the 12−13% range (measured by fishery observers), which is very close to our study (14%). However, these authors stated that hooking mortality might be underestimated: it could be around 20% in blue sharks in the Canadian fishery.…”

“…For pelagic elasmobranchs captured in longline fisheries, previous studies addressing hooking mortality have focused mainly on the blue shark (Prionace glauca). Campana et al (2009) carried out a comprehensive study of blue shark caught in the NW Atlantic (Canadian fishery), including both the short-term hooking mortality recorded at haulback and the post-release longterm mortality recorded by satellite telemetry. Also in the NW Atlantic, Diaz and Serafy (2005) analysed factors that could affect the numbers of blue sharks in good enough condition for live release, using data from the U.S. Atlantic pelagic fishery observer program.…”

Transboundary movements, unmonitored fishing mortality, and ineffective international fisheries management pose risks for pelagic sharks in the Northwest Atlantic

“…Lewison et al (2004) used a rigorous grid approach to estimate 206 million hooks in the North Atlantic, but noted that more of the effort was directed at tuna rather than swordfish (swordfish-directed sets tend to catch more blue sharks than tuna-directed sets). Campana et al (2009) reported a similar value for North Atlantic effort of 212 million hooks. ICCAT (2012) reported 135 million hooks for the North Atlantic in 2006; however, this value of effort came only from vessels that reported catch-effort data (T2 data) and ignored vessels that reported only catch (T1 data).…”

Ocean-wide tracking of pelagic sharks reveals extent of overlap with longline fishing hotspots

“…Although the implementation of these measures can often result in higher discards, postrelease survival rates of pelagic sharks are relatively high (65%; ref. 33), emphasizing that catch quotas/size limits may well be the simplest option to regulate/limit pelagic shark catches in international waters. Therefore, greater international efforts are needed to approach implementation of regulations aimed at limiting the catches of pelagic sharks by longlining fishing vessels that we have found to overlap their preferred habitats almost entirely.…”