Hideo Soeda

Studies on Echolocation of Porpoises Taken in Salmon Gillnet Fisheries

In the Bering Sea, marine mammals, particularly Dall’s porpoises, Phocoenoides dalli, get incidentally entangled in gillnets of the Japanese mothership salmon fishery. The maximum total number of Dall’s porpoises to be caught in a year was fixed by U.S. law. In order to develop techniques and devices to reduce the incidental catch in the gillnets, it is necessary to learn why and how they get entangled.

The Rate with which a Harbor Porpoise Uses Echolocation at Night

Apart from by vision, a Harbor porpoise (Phocoena phocoena) detects fish and obstructions by echolocation. But the porpoise sometimes fails to detect a gill net or judges it to be not dangerous and thus becomes entangled. Hatakeyama et al. (1988) reported that a porpoise could hear an echo from the netting of the gill net at a distance of 2m when it used echolocation. Au and Jones (1991) also reported that predicted biosonar detection of gill net by a Trusiops truncatus was 14 m. Despite that fact, unless it is very careful, a porpoise easily becomes entangled in the netting of the gill net at night (Akamatsu et al., 1991).

Forced oscillations of fish reef blocks hung in the water.

As fish reef block hung in sea water is oscillated by rolling or pitching of derrick barge, theproblem is arising about the safety and accuracy of setting fish reef blocks. In this study, weprepared cubes and fish reef block models and were swung by string from a point which wasoscillated horizontally in still water. By measuring horizontal displacement of model, we calculated velocity and acceleration. Assumed that drag coefficient CD and added mass coefficientCA were not varied with a short period Δt elapsed, we obtained continuous values of CD and CA for each time from the solution of equation of oscillation.The values of CD and CA during oscillation varied widely with time elapsed and were different from the values obtained in steady flow. Therefore, equation of motion expressed well the oscillation of models by substituting CD and CA obtained from displacement of oscillation into this equation.

The catch of cuttlefish gillnet depend on soaking time.

This paper dealt with the relation between the fishing effect and soaking time of cuttlefish gillnet.The results were as follows:1) The catch of one unit gear (one tan), as soaking time of gear increased, generally changed in 3 steps, this is, increase, decrease and notable increase. But the trend of step varied with fishing ground and/or fishing season.2) Within the soaking time, 300-1100 minutes, the correlation coefficient between CPUE and soaking time indicated a negative figure, the fishing efficiency decreaced as soaking time of grar increased. Regression coefficient varied with fishing ground and/or fishing season.