Posted by Melanie Hutchinson
9 February 2014
Melanie Hutchinson is a Ph.D. student at the University of Hawaii’s Hawaii Institute of Marine Biology. She began her career in fisheries as a longline fisheries observer in Hawaii. Since then she has been interested in developing and testing ways to reduce the impact that fishing has on shark populations.
Shark populations are in decline in every ocean, mostly due to overfishing (Worm et al., 2013; Dulvy et al., 2014). There is now increased attention on shark-fishery interactions and the effects such interactions are having on shark populations.
Understanding post release mortality rates
A specific area of concern that is increasingly investigated by scientists is the post release mortality rates of sharks captured and discarded in commercial fisheries. Yet obtaining accurate estimates of such rates is a challenge. Often, on-board observers record sharks that are alive upon release as released ‘alive’, with the assumption that the animal survives the fishing interaction and subsequently thrives. This is not necessarily the case. The physiological disturbances that an animal may incur during the fishing interaction can range from minor changes to the blood chemistry from stress hormones and increased ventilation rates to more prolonged physiological effects and/or physical traumas – from hooks, gaff, or nets, for example – that can result in death. Failing to recognize these occurrences leads to underestimates of the fishing mortality parameter used in stock assessments and, in turn, can impair the ability to model the integrity of shark populations worldwide.
Researchers are conducting several studies to investigate the post release mortality rates that incidental sharks may suffer. Specifically, scientists are now looking at the degree of physiological disruption in blood chemistry and validating whether or not an animal survived the fishing interaction through the use of Pop-up Satellite Archival Tags (PSATs). Several studies have shown that certain blood chemistry parameters – elevated lactate, adrenaline, glucose levels, and low pH, for example – correlate to mortality in many shark species. Additionally, these studies are also finding that responses to the stress of capture are species specific. Many of the deep diving, wide ranging, faster swimming shark species with higher metabolic demands, such as hammerhead and thresher sharks, exhibit higher post release mortality rates than slower, coastal species such sandbar and tiger sharks (Heberer et al. 2009; Marshall et al. 2012; Gallagher et al. 2014). These results have major implications for stock assessments and fishery management plans.
ISSF sponsored study
Due to concerns over the declining silky shark populations in the Western and Central Pacific Ocean, the ISSF Bycatch Project sponsored a study to look at the post release survival rates of juvenile silky sharks incidentally captured in purse seine nets when fishing for tuna around drifting fish aggregating devices (FADs). Researchers from the University of Hawaii participated in a 44-day cruise on a commercial purse seine vessel and conducted several experiments on juvenile silky sharks captured as bycatch.
We identified three main research questions addressing the shark survival component of the Bycatch Project:
To get at these questions we drew blood from 90 sharks that were captured at different stages of the fishing operation. We also captured a control group of sharks that were caught using handlines from a small work skiff, inside the net while it was still open enough for the animals to swim freely. Some of the sharks that had been blood sampled were also satellite tagged to validate survival or mortality with the blood chemistry parameters.
Our results indicated that lactate and pH are the best predictors of post release fate. We found that lactate levels higher than 11.31 mmol/L and pH levels below 6.789 were indicative of a dead animal. We also found that sharks that were released from inside the net while they were still free-swimming had 100% post release survival rates. Furthermore, animals that had become entangled in the net had 68.4% post release survival rates, whereas animals that were brought on board during standard loading procedures had low survival rates 16 – 6%. We attributed this to the fact that the sharks become confined in what is termed the ‘sack’ at the end of the net hauling procedure and drown because of their inability to swim water across their gills to breath and become crushed from the weight of the fish on top of them.
Essentially, juvenile silky sharks that are captured in purse seine fishing nets have very low post release survival rates – ~15% overall. The best means of reducing the impact this fishery has on silky shark populations may be 1.) continued efforts to avoid shark capture all together and 2.) releasing the sharks from net while the net is large enough for the animals to swim, as through a release panel in the net.
Building on this research, the ISSF Bycatch Project has laid plans to continue testing release panels in the Atlantic, Western Central and Eastern Tropical Pacific in the coming year. Additionally, we will be looking for ways to better identify the species composition in a school of fish for possible avoidance strategies of non-target species.