Fishing activities pose the most serious, immediate and obvious threat to Taiwan’s critically endangered humpback dolphins (Sousa chinensis), but can be addressed most easily, according to researchers.
In a study published in the journal Endangered Species Research, Dr. Elizabeth Slooten and colleagues say that the impact of fishing gear on the small population of dolphins, also known as the Eastern Taiwan Strait (ETS) humpback dolphins, must be reduced in order to avert their extinction.
Taking action to switch to more dolphin-friendly fishing equipment would require short-term investment, but the benefits both for the dolphins and for the fishing industry could be significant and lasting.
There are estimated to be fewer than 75 dolphins in the population, and more than 30% show signs of having been caught in or injured by fishing gear. Some can be seen swimming with lines still attached to their fins and around their bodies, while others bear deep, lasting scars from previous entanglement.
Surveys of fishing activities within their habitat reveal widespread use of the kinds of fishing gear most likely to cause humpback dolphin ‘bycatch’ - death from fisheries interactions. Of most concern are the thousands of gillnets along the west coast of the island, which are designed to catch fish by their gills but are also known to kill humpback dolphins, as well as most other kinds of cetaceans.
One kind of gillnet, called a trammel net, consists of multiple (usually three) layers of netting, some more slack than others, which makes it particularly easy for marine wildlife to become entangled. This is the most prevalent kind of gillnet in the dolphins’ range.
Trawling, another kind of fishing that can cause Sousa bycatch, has been banned in much of the dolphins’ near-shore habitat, but continues illegally nevertheless, often in plain sight.
The study shows that, in order for the population to survive and recover, levels of bycatch must be reduced to less than one dolphin every seven years. It is not known how many dolphins are killed or have a reduced lifespan due to fisheries injuries, but photographic evidence and bycatch reports suggest that the mortality rate is higher than this.
To prevent the extinction of this unique population, the researchers recommend halting the use of gillnets and trawling within the humpback dolphins’ habitat. This would include strict enforcement of the existing trawling ban, and adopting alternative fishing methods which are more selective, and less damaging to cetaceans and other marine life and their habitat.
Converting to these methods will cost money in the short term. But in addition to giving the dolphins a good chance of recovery, the benefits for the fisheries themselves could be significant as populations of higher value fish species recover and grow, the entire ecosystem improves, and the fishing industry becomes more sustainable.
The study notes that bycatch is only one of five major threats to the dolphins: habitat loss from land reclamation, pollution, loss of freshwater flow from rivers, and noise and other disturbance, are also contributing to their decline, and all need to be addressed. But in the short term, argue the researchers, because of the immediacy and seriousness of fishing impacts, stopping the use of certain fishing practices would be the ‘single most effective conservation measure for ETS Sousa.’