Longline fisheries are not a shark's best friend

Longline fisheries provide us with some of our most popular seafood dishes. Longlines are what they sound like – long lines with many many baited hooks out of the backs of boats, lying either on or near the surface of the ocean, or even submerged deeper in the water column down to the sea floor. One of the issues with longline fishing is that they are somewhat in discriminate in what they catch, often attracting – and hooking – species that the fishers don’t necessarily want to catch. Bycatch – individuals caught by a fishery that weren’t the target – is a global issue, and one that spans all fisheries and not just longlines. Bycatch is a collateral damage of fishing. It contributes to overfishing, and it impacts both commercially caught and non-commercial ocean dwellers. Some of these non-commercial species include endangered species, like the Atlantic yellow-nosed albatross (Thalassarche chlororhynchos), the loggerhead sea turtle (Caretta caretta), and the Scalloped Hammerhead (Sphyrna lewini) .

Back in 2009, a meeting between tuna Regional Fishery Management Organizations (RFMOs) noted that bycatch was one of the most pervasive threats to the shark group, and there is growing evidence that bycatch is reducing some populations of sharks. There are all sorts of measures one can put in place to reduce the likelihood of bycatch or any species. Each technique is particular to the type of fishery and the species that you want to try reduce the bycatch of. Still, sharks do get caught out, but as we know from recreational catch-and-release fishers who target sharks, a hooked shark is not necessarily a dead shark. So you pull up your longline, and you find a live shark on a hook. You successfully get it off the hook and back in the water. That certainly meets the regulations many fishery managers put in place for threatened species. Well what if that shark you pulled up wasn’t alive? You could throw it back into the water, but a dead shark certainly isn’t going to meet any conservation objectives. If a particular shark species is more likely to die on a hook that survive, then that is a big problem for their long term persistence.

Austin Gallagher and his colleagues from The University of Miami and the National Marine Fisheries Service (NMFS) took a look 17 years of data from the US pelagic longline fishery that operates in the western Atlantic Ocean and the Gulf of Mexico. This fishery focuses its effort on three main pelagic predatory fish - swordfish (Xiphius gladius) , yellowfin tuna (Thunnus albacares) , and bigeye tuna (Thunnus obesus) . Accurate data on bycatch is always a tricky thing to obtain, but the data provided to the team from the NMFS and the Pelagic Observers Program (POP) was pretty good. The data included information on the time and place the lines were deployed, estimated hook depth, what bait was used, the species and size of the sharks caught, and if they were alive, dead, or if it had extensive injuries (damaged – this was regardless of whether the shark was alive or dead). Using all this data, some modelling, and some statistical analysis for 12 species of shark, the team were able to figure out not just survival rates of these sharks, but also the impact of different operational techniques (like the depth and length of time hooks are in the water) on survival.

Whilst just 1% of the sharks recorded in the dataset came up as having extensive injuries, it seems that post-release survival is highly variable across shark species. Focusing on the swordfish fisheries, out of the twelve species examined, tiger sharks (Galeocerdo cuvie) showed the highest survival rates (~97% of hooked individuals) whilst the lesser known night shark (Carcharhinus signatus) showed the lowest (~22%). The situation was a little different for the tuna fisheries. Tiger sharks still came out with the highest rate of survival (~97%) but the longfin mako (Isurus paucus) came out bottom, with ~37% of hooked individuals coming up alive.

So why the difference? The researchers put this down to the different styles of fishing undertaken for the two target species. For example, swordfish longlining typically takes place at sunset, near the surface – just the sort of time and place where a night shark likes to go to catch it’s dinner. Blue ( Prionace glauca) , scalloped hammerhead (Sphyrna lewin) , and night shark survival increased with hook depth, possibly because the cooler waters found at depth limit oxygen deficits when they are captured (the colder the water, the more dissolved oxygen is held). The soak time – the time that the hooks were left in the ocean for – also seems to have an impact on survival rates, with blue, porbeagle (Lamna nasus) , and silky ( Carcharhinus falciformis) shark survival decreasing with soak time. The researchers put this down to asphyxiation – these three species all need to keep moving to breath (ram-ventilation).

Even more concerning, the species that lay at the lower end of the survival rate scale also the slow growing ones, meaning that they take a long time to reach maturity and thus able to reproduce. Declines in species where individuals are slow-growing species is often more concerning that for faster-growing ones because it takes means it would take a long time for the population to recover.

So what does this mean for sharks? Well to help increase survival, additional techniques like reducing soak time, avoiding fishing when waters are warmer can help. But because the survival rate in several of the assessed species is so low, if we really want to reduce our impact on shark populations, then we really do have to reduce the likelihood of our fishing gear interacting with them in the first place.

The paper is published in the open acess journal Global Ecology and Conservation. You can have a read of it here http://dx.doi.org/10.1016/j.gecco.2014.06.003

Image: A dead Silky shark (Carcharhinus falciformis) on a longline hook. Credit Jürgen Freund / WWF-Canon. You can see more of Jürgen's work here http://juergenfreund.com

#science #marinescience #sciencesunday #shark #fishing
Shared publiclyView activity