Bottom-trawl fisheries may supply us with much of the tasty fish we like to enjoy, but it does come with its problems.  Also known as ‘dragging’, bottom trawling essentially involves dragging a large net, held open either with a beam (beam trawling) or large metal/wooden ‘doors’ (otter trawling) along the sea bed, or just above it.  It is used to catch a range of commercial species like cod, shrimp, flounder, and halibut.  One of the problems of trawling is that it is not a very selective form of fishing.  Other species are caught in the process, and this bycatch can include at risk species such as skates, rays and sharks.  As well as ecological implications, bycatch can be bad for fishers, who often end up throwing away bycatch either because it isn’t worth anything, or because they are not allowed to land it.  Bycatch reduction is a win-win for fishers and for the marine life caught. Reducing bycatch of sharks, rays, and skates (collectively known as elasmobranchs) in bottom trawls is one of the many fishery-related issues on the mind of scientists at Marine Scotland Science.  As this piece of research from the Marine Scotland Science team shows, one possible solution (though not perfect) may not be all that tricky to implement.

Focusing on the fishing gear

Once elasmobranchs end up inside a trawl net, they have little chance to escape.  This is particularly the case for skates and rays whose large flat bodies limit escape options from things such as square mesh panels or bycatch grills that are put into the nets to allow smaller fish to escape.  The researchers reason that prevention is better than cure, so looked to how they ended up in the nets in the first place.  They focused on something called a tickler chain.  These optional chains are fitted in front of the mouth of the trawl under the footrope, startling fish in front of the net, causing them to flee from the seabed, eventually ending up in the net.  You can see a short video of one of these chains in action.  Among this fleeing fish is the endangered elasmobranchs.  Remove the ticklers, the researchers hypothesised, and you could reduce skate and ray bycatch.  To test this, the researchers set up their own trawl gear, some with ticklers and some without.  They used underwater observation to see what was going up beneath the waves, sensors that gave information on the net itself, and of course checked the hauls to see the differences in species composition.  They also set up “groundgear bags” behind the net opening where the tickler are set, which would give them a conservative estimate of escape levels underneath the gear.

Tickler removal makes a big difference

The research produced some statistically significant findings.  Use of the tickler increased the catch of skates, rays, and sharks.  3.6 times more skates and rays were caught in the trawl net when the tickler was used than without.  Shark catch rates were not as high, but still significant – 1.6 times higher with ticklers than without.  Mirroring these results, the ground bags which were used to estimate escape levels, suggested that a greater portion of elasmobranchs escape the trawl when ticklers are removed.

What about the fisher’s catch?

The whole point of the tickler is to improve catches on smooth seabeds (the chains are removed for rough ground as they are more likely to snag), so removing them could have an impact on the very fish the fishers are targeting.  Removing the tickler significantly lowered catch of anglerfish, a species with high commercial value.  Catches of other species – haddock and whiting, as well as flatfishes – weren’t compromised.  The researchers reason that this means ticklers could be removed when targeting haddock and whiting without causing hardship on the fisher.  However, when fishers want to catch anglerfish, tickler removal may be a bigger problem for the fisher.  To find a middle ground between conservation and fisheries objectives, they suggest combining information on species distribution and abundance to manage where (and likely when) ticklers should be removed.  Under this scenario, fishers would remove ticklers where elasmobranchs were known to be.

What happens to the skates and rays that the trawl passes over?

If an animal doesn’t end up in the net and remains on/just under the sea bed surface, then the trawl will pass over it.  The researchers note that there is evidence that common skates at least may survive, but this is an area that needs further exploration.  They do make one very good point…that “overall, the likelihood of survival after being passed-over by a trawl is likely to be higher than when discarded after having been towed, brought to the surface and handled”. 

Read the research for yourself

The paper was published in the ICES Journal of Marine Science.  The authors have paid to make it open access so why not have a read of the research yourself http://dx.doi.org/10.1093/icesjms/fsv037.