When your food is easier to catch, how much risk would you take to get some?
Researchers in Australia have found that certain dolphins repeatedly brave the dangers of actively fishing trawls to get to the prize… with a potentially lethal result.
The dolphins of the Pilbara fishing grounds are known to frequently interact with the fishery, following vessels in hopes of an easy dinner. Up to 50 dolphins can be seen hanging around the stern as the catch is winched up. But most observations of interactions to date come from sightings at the surface by observers placed on the vessels. So what goes on in the water where we can’t see?
To get to the bottom of this question, Murdoch University Honours student Vanessa Jaiteh, with the help of Murdoch’s Cetacean Research Unit and the University of WA placed video cameras inside trawl nets in the Pilbara Trawl Fishery, a finfish fishery off North-Western Australia that earlier this year was brought under scrutiny because of its dolphin bycatch.
What they found was a much higher rate of interactions than previously suspected.
Dolphins are known to associate with all kinds of fisheries around the world, from gill nets to fish farms, and are regularly seen feeding on trawl discards after a catch has been winched up. But what we now know is they also take advantage of the active fishing gear to grab an easy meal too.
Jaiteh et al.‘s study found that “Dolphins were present outside the net in 94% of all assessed trawls and for up to 99% of the duration of an individual trawl. [Dolphins also] entered the net during 81% of all trawls and were present inside the net for up to 98% of the trawl duration.”
It didn’t matter how many dolphins were in the net (although they did like to be the ‘first ones in’): inside, individuals were mostly foraging… or chasing other dolphins. The trawl nets provided an easy fishing ground for the opportunistic individual, whereby large numbers of potential prey could be found in one area, and also because the “net’s surface provides a barrier against which dolphins can chase and catch fish”. Outside, some fish escaped the trawl by swimming underneath it, but other dolphins were clever enough to find these prey too.
The cameras also captured some dolphins exhibiting a strange behaviour, ‘trampolining’, in which individuals repeatedly bounced against the net, turning and twisting their bodies and sometimes rubbing against it deliberately. The researchers think this could be a way for the dolphins to remove old skin or parasites… or maybe just playful investigation. Either way, it shows an additional use the dolphins make of the nets, which only increases the concern about their interactions.
Up to 50 dolphins were seen around the trawl vessels when the catch was hauled in, which is likely to represent only a small part of the Pilbara local population. It may be that there is a small, specialised community within the population that targets trawlers – something that has been shown in Moreton Bay, Queensland. In addition, only 29 individuals were identified foraging inside the nets, and 22 of these did so several times during the trawl. This suggests that foraging inside trawl nets may be a specialised behaviour exhibited by a limited number of individuals – and often only when the opportunity arises.
Whilst the dolphin bycatch rate is relatively low in this fishery (less than 1 dolphin per 100 trawls), this extrapolates to an alarming 17-50 dolphins per year… With potentially many more unobserved deaths. Foraging opportunity is made risky by the potential for entanglement in the gear. In fact, “fishing-related mortality is considered the most severe and immediate threat to populations of small cetaceans worldwide”.
The current Bycatch Reduction Devices (BRDs) installed in the Pilbara Trawl Fishery consist of a grid designed to deflect dolphins towards an exit hole in the bottom of the net. But this means that a) (according to the camera observations) dolphins are actually more likely to swim forward and upward when they come into contact with the grid, thereby not being able to locate the escape hatch, and b) that dolphins that die in the nets may fall out of the bottom without being detected by surface observers at all.
The study’s authors recommended that “[the fishery] should focus on preventing dolphins from becoming caught, injured or killed in the gear, rather than attempting to prevent these individuals from interacting with the nets”. This could be achieved by developing more effective BRDs – something which the Department of Fisheries is supposed to be trialling at the moment.
For those daredevil dolphins in the Pilbara, a reduced risk of death from dinner could only be a good thing.
The original paper was published in the online journal Marine Mammal Science: Jaiteh VF, Allen SJ, Meeuwig JJ & Loneragan NR 2012. Subsurface behaviour of bottlenose dolphins (Tursiops truncatus) interacting with fish trawl nets in north-western Australia. Marine Mammal Science DOI: 10.1111/j.1748-7692.2012.
For a comment from Murdoch University, see this article: Dolphins Filmed Fishing In Trawler Nets.