Brendan Talwar

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New research directions and this year's American Elasmobranch Society meeting

Lab Note #20
Aug 27, 2015

Post-release survivorship of giant isopods

Giant isopods (Bathynomus giganteus) are common bycatch in a number of deep water fisheries around the world, and are often discarded due to low economic value. That may be changing as a group in Japan has started turning them into rice crackers... which is kind of disgusting to most people familiar with their smell - notice the part about the 'smell of their organs' being too pungent to crush them into a powder without any added flavor.

In any case, while they aren't quite as much fun to work with as deep sea sharks, the same questions we're asking about the Cuban dogfish can easily be applied to these isopods which are caught by the thousands in trap and gillnet fisheries as close as western Atlantic waters.

Photo- Kat Magnenat

What makes them so interesting in a fisheries context is that they exhibit a number of consistent reflexes that disappear as their condition declines... after observing 6 of these reflexes, we decided to answer the following questions:

1) Can we predict the post-release survivorship of deep sea isopods using reflex impairment?

2) What factors contribute to their mortality?

3) What are the possible sub-lethal effects of capture, as measured through locomotion and or feeding behavior in the 24-hrs post-capture?

Since we already had the gear to do the project, in large part thanks to everyone who contributed here on Experiment.com, we quickly got the project up and running in early summer and ran it to completion in early June until cage #2 was lost at sea during a trial. Sad day. On the bright side, we had a large enough sample size to analyze the data that we had already collected and we are now in the process of writing a manuscript about our findings. I'll be sure to post it on here when it (hopefully) gets published, but for now here is our most interesting result:

Of our six chosen reflexes, four were good at predicting whether an isopod would live or die after 'release' (as in dropped to the ocean floor at around 800m in the cage). These four reflexes were assessed very simply on the boat, as either positive or negative, right after hauling animals to the surface. We then took the sum of negative reflexes and called it the 'reflex impairment score'- so basically the higher the score, the worse off the isopod was doing on the boat and subsequently the higher the rate of 5-day mortality among individuals with that score. It seems obvious and straightforward- the less an animal responds to a given stimulus, the worse its condition... because it is!

What this means is that we've provided a baseline estimate of post-release mortality for this species as well as allowed researchers in the future to test the various effects of haul rate, light levels, temperature changes, deck times, etc on their mortality without the need for containment after release. Instead, they can use the reflexes we've identified and the curve we've created to examine how those factors affect the reflex impairment score and ultimately tease apart the reasons why a deep sea crustacean like an isopod might die after release- is it because of the thermal shock from ascent or from air exposure on the boat? Where should fisheries direct their attention in order to increase survivorship of bycatch species? Hopefully we've contributed a small piece to that puzzle.

American Elasmobranch Society Meeting, Reno, Nevada 2015

Presenting on the predictors of post-release mortality that we've found for Cuban dogfish, such as at-vessel blood lactate.

In July, I had the opportunity to travel to Reno, Nevada to present our research on Cuban dogfish and gulper sharks at the American Elasmobranch Society meeting held in conjunction with the Joint Meeting of Ichthyologists and Herpetologists. There were lots of interesting talks on a range of topics, from applied fisheries work to energetics and descriptions of new species (and even one talk on the rarely found pocket shark from the Gulf of Mexico, which was conveniently on display and worth checking out here). If you're interested in the work being conducted by members of the society, which focuses on sharks, rays, and skates, you can read about the research through the record of live-tweets compiled over the course of the conference here.

Our whole lab was in attendance and had a great time on top of all the interesting science. Big thanks to the Cape Eleuthera Institute Forrestal Travel Award, AES Student Travel Award, and FSU Congress of Graduate Students for providing the funding for me to attend.

Current and former members of the Grubbs lab at FSU.

I hope you're all doing well! Thanks as always for your interest. The next post will be about an upcoming trip to the New England Aquarium, where I'll be analyzing all of our blood samples in late September.

Best,

Brendan

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