Are dolphins whales?

Figure 1.
Harbour porpoise entangled in a gill net

There are more than 80 cetacean species worldwide. Cetaceans are divided into baleen and toothed whales, with small toothed whale species being commonly referred to as dolphins or porpoises. This means that dolphins and porpoises are indeed whales. About 40 species of cetaceans inhabit Japanese waters, each managing to keep their niches by living in different habitats and exploiting different prey. However, very large scale commercial fishing operations are catching fish that constitute the prey of cetaceans, and laying fishing nets in waters where cetaceans are swimming. The harbour porpoise, one of the smallest toothed whales, inhabit coastal waters, and are particularly prone to getting inside fishing nets to eat the fish trapped in them. As a result, they often get caught together with the fish, and sometimes become entangled or drown and die. This unintended capture, known as bycatch, is an unfortunate outcome for both the porpoises and fishers. Cetacean stands at the top of the marine ecosystem. The coexistence between cetaceans and fisheries is perhaps symbolic of the challenge facing humanity to continue to survive on the face of this planet alongside a myriad of other species.

 

Rescuing porpoises

Figure 2.
Porpoise rehabilitation in a makeshift tank

In my laboratory, we are working with the Hokkaido University Cetacean Research Group, a student body, to study bycatch and think about ways of preventing it. Various types of research are being carried out at the Hokkaido University Usujiri Fisheries Station, Hakodate with the cooperation of local fishers. My lab rescues harbour porpoises trapped in fishers’ nets and rehabilitates them. Every year during the month of April, we go out to the fishery operation with the fishers every day. If any porpoises are caught in the nets, we take them up carefully and take them back to the station to rehabilitate in a tank. Some die soon after rescue, but about 70% start swimming again once they have been set free in the tank. During the week or so that we keep rescued porpoises before returning them to the sea, we record body length, gender and other biological information, and observe their echolocation sounds and behaviour in the tank.

 

What do we learn?

Figure 3. Migration rout of a harbor
porpoise obtained from a satellite tagging


Figure 4. Deterrent experiments
at Otaru Aquarium

When the porpoises are rehabilitated, sometimes we attach little devices that transmit radio waves to satellites so that we can track their migration after release. We have attached such transmitters to eight individual porpoises, and have succeeded in mapping their migrations from Usujiri to the Sea of Okhotsk at furthest, as a collaboration with Kyushu University. Before our research, the migration routes of harbour porpoise was estimated only from sightings, bycatch and corpses washed ashore, but now we have the much clearer picture of those migrations.
We have also kept porpoises at Otaru Aquarium on a long-term basis with their collaboration, and we have carried out more detailed experiments in the aquarium. For example, we placed food on the other side of a narrow passageway in a pool to investigate the minimum width that porpoises can pass through, and found that they are capable of navigating a space that is only slightly wider than their bodies. We are also trying to find effective sounds for acoustic deterrents of harbour porpoises from approaching fishing nets. We expose various sounds to the aquarium porpoises, and observe the behaviour.
Through such research, we are slowly broadening our knowledge about the mechanisms behind porpoise bycatch and possible ways of preventing it.

 

Can bycatch be reduced?

It would be difficult to prevent bycatch completely, but I think that we can reduce it somewhat. Our three strategies are —barring entry, allowing escape, and keeping porpoises away. If we can devise ways of preventing porpoises from entering nets, allowing those that have got into nets to escape, and keeping porpoises away from approaching nets, I think we can reduce porpoise bycatch. To do this, we are also investigating the characteristics of harbour porpoises from various directions. For example, through stranding survey (survey of corpses of cetaceans that have become stranded on seashores), we can learn about their diet, distribution, growth, breeding locations and other aspects of their lives. Moreover by using underwater sound recording devices, we can gather timing and frequency data of their appearance, net entering, and escape of the porpoises around nets. These data will be important hits to find whether they enter nets by mistake or with the clear intention of stealing the fish inside.
Hokkaido University’s School of Fisheries Sciences is the only undergraduate school of fisheries sciences in one of the former Imperial Universities in Japan, which means it is the top school in the fishery science field in Japan. In this school, many students have a desire to study cetaceans. However, studying these creatures is very tough, both physically and emotionally. Sometimes a porpoise will die in front of your eyes. If you want to study cetaceans at our school, the first thing you should do is to join the Cetacean Research Group and try to find what cetacean research is. If you do so, hopefully you will be able to find something that you yourself can do to help people to coexist with cetaceans.