Graduate School of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido, Japan.
Rausu Fisheries Cooperative Association, Rausu-cho, Hokkaido, Japan.
PLoS One. 2021 Feb 11;16(2):e0246838. doi: 10.1371/journal.pone.0246838. eCollection 2021.
Bycatch of harbour porpoises (Phocoena phocoena) by gillnets is a recognised threat to populations. To develop effective mitigation measures, understanding the mechanics of bycatch is essential. Previous studies in experimental conditions suggested foraging activity is an important factor influencing porpoises' reaction to gillnets. We acoustically observed the behaviour of wild harbour porpoises around a bottom-gillnet set-up in a commercial fishing ground, especially foraging activity. Passive acoustic event recorders (A-tags) were fixed to the ends of the gillnet, and recorded for 1 392 hours. Although harbour porpoises frequently and repeatedly appeared around the net each day, incidental bycatch occurred only three times during the observations. The stomach contents of two individuals contained mainly Ammodytes sp., which were observable around the bottom-gillnet but not targeted by the fishery. A total of 276 foraging incidents were acoustically detected, and 78.2% of the foraging activity was in the bottom layer (deeper than 25 m). Porpoises appeared around the net with more frequency on the day of a bycatch incident than on the days without bycatch. These results suggest that the harbour porpoises appeared around the bottom-gillnet to forage on fish distributed in the fishing ground, but not captured by this bottom-gillnet. Thus, porpoises face the risk of becoming entangled when foraging near a gillnet, with the probability of bycatch simply increasing with the length of time spent near the net. Bycatch mitigation measures are discussed.
误捕港湾鼠海豚（Phocoena phocoena）是对种群的一种公认威胁。为了制定有效的缓解措施，了解误捕的机制至关重要。之前在实验条件下的研究表明，觅食活动是影响海豚对刺网反应的一个重要因素。我们在商业渔场内的底层刺网设置周围，使用声学方法观察野生港湾鼠海豚的行为，特别是觅食活动。将被动声学事件记录器（A 标签）固定在刺网的两端，记录时间为 1392 小时。尽管港湾鼠海豚每天都会频繁且反复地出现在网周围，但在观察期间仅发生了三次偶然的误捕事件。两个人的胃内容物主要包含 Ammodytes sp.，这种鱼在底层刺网周围可见，但不是渔业的目标。总共检测到 276 次觅食事件，其中 78.2%的觅食活动发生在底层（深于 25 米）。在发生误捕事件的当天，鼠海豚出现在网周围的频率比没有误捕事件的日子更高。这些结果表明，鼠海豚出现在底层刺网周围觅食分布在渔场内的鱼类，但不会被这种底层刺网捕获。因此，当海豚在刺网附近觅食时，它们面临着被缠住的风险，而误捕的可能性仅仅随着在网附近停留的时间增加而增加。讨论了误捕缓解措施。
