Deep-sea sharks: do they survive? As it turns out, it is a matter of perspective.
The short version
So do deep-sea sharks survive after longline capture and release? Well, it kinds of depends on whether you are a glass half empty or glass half full kind of person. As it turns out, we documented a mortality rate of around 50% within 24 hours after release for the species that we studied most.
What does this mean for fisheries?
Well, that kind of depends as well. In my opinion, this glass is half empty and here is why:
The lowest post-release mortality (PRM) rate that we documented was for a small dogfish shark called Squalus cubensis at about 50%. For other species that we studied, it was higher... sometimes much higher. For the gulper shark, it was close to 100%. Granted, our sample sizes for these other species were very low. But keep in mind that we stopped fishing for gulper sharks because literally none of them survived, so the sample size that we ended up with was by design. We only discovered after thoroughly analyzing our at-depth video footage that one individual was technically ventilating after 24 h, then fell out of the cage when we hauled it. So it was assigned 'survivor' status, but likely died later.
I believe that the rates that we observed likely underestimate true post-release mortality rates for these and related species caught in tropical / sub-tropical fisheries. My main reason for thinking this way is that the enclosures that we used prevented predators from eating these little sharks. We'll never know what post-release predation rates are for every species released from fisheries gear - it will depend on way too many factors. But two things we can reasonably assume (and our data support) are that 1) predators are drawn to fishing activity and 2) the swimming behavior of small deep-sea sharks is generally impaired to some degree after being hauled up from depth. We know this is a problem for all kinds of fish in shallow water, where they need to swim maybe just a few meters to reach some kind of safety (like mangrove roots, for instance). But these sharks don't have it quite so easy - they have to swim hundred of meters down before they aren't silhouettes against the surface and are no longer easy targets for predators below. And, as we know all too well, even reaching the bottom doesn't guarantee safety (see video below).
Another reason these values might be underestimated is that fishermen need to sort their catch quickly. So instead of being really careful with these little sharks, which have two very sharp spines, fishermen will sometimes pull the hook out of their mouth to save time and avoid injury. In doing so they might break the shark's jaw, which is actually quite weak compared to those of shallower-dwelling species. I think it is fair to assume that a shark with a broken jaw has a much reduced chance of survival.
Generally, research on PRM rates lasts longer than 24 h after release because animals can live for a few days (or even weeks) before they die from capture-related trauma. We only looked for mortality for 24 h because our sampling protocols were logistically challenging and constrained by lots of factors - namely that our observations took place at >500 m in depth! Maybe more animals would have died after 24 h than what we recorded, but I personally don't think that this would inflate our PRM rates by a whole lot based on the recovery that we saw in the cages. It is reasonable, though, to assume that at least some animals would die later.
So, for those main reasons, I think that the PRM rate that we can expect for Squalus cubensis (caught using similar methods under similar temperature regimes) is 50% at a minimum.
But... now that we know this, maybe the glass is also half full :)
In the EU, something called the Landing Obligation / Discard Ban seeks to phase out discarding (the practice of throwing back unwanted catch). A part of this plan says that if there is evidence for high survivorship of a given species, it can be thrown back. If not, it has to be retained. While our results aren't directly applicable to fisheries there (because they won't be catching Cuban dogfish anywhere around the EU, we assume), they do suggest at least a starting point for PRM rates for deep-sea sharks, which were previously quite limited. And based on our data (and if you believe my thoughts above), it is probably best to retain small deep-sea sharks after longline capture. This may seem counterintuitive - that not throwing a fish back can be a good thing - but if they are going to die anyways, this is a much better option! It forces fishermen and consumers to find a use for the species that would otherwise be wasted during routine fishing. On top of that, the retained catch quickly fills up freezers on the boats and can result in reduced fish mortality with the same amount of fish coming to dock. From my point of view, that is a very good thing!
In the US, these results are more informative from a 'future use' perspective as deep longline fisheries haven't begun on a large scale. But, if they do, we'll at least have PRM rates for some of the species that might be encountered most often.
The long version
Feel free to make up your own mind based on our results - you can read the abstract below or the full paper from Marine Ecology Progress Series here. Also feel free to email me for a PDF copy (talwar.brendan@gmail.com) if you don't have access.
STRESS, POST-RELEASE MORTALITY, AND RECOVERY OF COMMONLY DISCARDED DEEP-SEA SHARKS CAUGHT ON LONGLINES
Abstract
Bycatch interactions with deep-sea elasmobranchs are increasingly common and can lead to dramatic declines in abundance over short time scales. Sharks hooked in the deep sea could face a higher likelihood of severe physiological disturbance, at-vessel mortality, and post-release mortality (PRM) than their shallower counterparts. Unfortunately, robust PRM rates have not yet been estimated for longline-caught deep-sea sharks, and as such are not currently incorporated into total fishery mortality estimates or bycatch assessments, limiting the effectiveness of current conservation or management initiatives. We empirically estimated PRM for 2 focal taxa of deep-sea shark, the Cuban dogfish Squalus cubensis and the gulper shark Centrophorus sp., using post-release enclosures deployed at-depth. We calculated 24 h PRM rates of 49.7 ± 8.5% (mean ± SE) for S. cubensis and 83 ± 16% for Centrophorus sp. and identified blood lactate, total length, glucose, and vitality scores as predictors of PRM in S. cubensis. We also observed all 24 h PRM within 11 h post-capture and demonstrated the effects of recovery depth and at-vessel blood chemistry metrics on post-release behavior. Our results suggest that PRM rates of deep-sea sharks are high and highlight the need for filling in this gap in fishery mortality estimates for other common discards in the future.
Photos of Cuban dogfish and gulper sharks in the post-release enclosure at ~600 m. In (D), you can see the isopod attack! If you haven't seen the video yet, despite me playing it during every presentation ever, here it is:
The end!
Thank you so much to everyone who helped make this project a reality and of course to those of you who donated to this little experiment of ours! I'm hopeful that it has been entertaining for you to keep track of our research over the years and that you have developed an appreciation for sharks, research, or maybe just murderous giant isopods. Research like this is important and necessary as we try to sustainably manage marine resources in the years to come, and so I encourage you to continue your support of science, education, outreach, and policy-making that work together to conserve ocean ecosystems and the communities that depend on them.
With that, I'll officially retire from this project and start some more. If you're interested in working together, please let me know! It looks like I'll be starting a PhD early next year while conducting research and teaching in The Bahamas (surprising, I know) and am excited to form new partnerships with folks there and around the world. There is so much more work to do.
Best,
Brendan