1. Bycatch is the most significant threat to marine megafauna (sea turtles, marine mammals, elasmobranchs, seabirds) worldwide, and the leading cause of the decline of several cetacean species. The bycatch issue in the Indian Ocean is poorly understood, but high bycatch levels in gillnet fisheries have been documented for the past two decades, in both small-scale and semi-industrial fisheries. Unfortunately, methods to reduce bycatch are often unavailable, financially non-viable or socially unacceptable to fishermen.
2. Using a network of trained boat captains in the tuna drift gillnet fishery in the Arabian Sea, targeted catch and bycatch data were collected from 2013 to 2017 off the coast of Pakistan (northern Indian Ocean). Two fishing methods using multifilament gillnets were used: surface deployment and subsurface deployment (i.e. headline of net set below 2 m depth).
3. Predicted catch rates for targeted species did not differ significantly between the two fishing practices, although a drop in tuna (6.2%) and tuna-like (10.9%) species captures was recorded in subsurface sets. The probability of cetacean bycatch, however, was 78.5% lower in subsurface than in surface sets.
4. Cetacean bycatch in tuna drift gillnet fisheries has the potential to be significantly reduced at a relatively low cost for fishers. However, further research with an appropriate sampling design and a large sample size is required to confirm the efficacy of the proposed mitigation method. The acceptability and adoption of subsurface setting by fishers also needs to be further investigated. Despite some limitations, this preliminary study also highlights the importance of crew-based observer data as an alternative source of data when observers cannot be deployed on fishing vessels.

中文翻译：

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将网放低：减少流刺网渔业中鲸类兼捕的潜在低成本缓解方法

1. 兼捕是世界范围内海洋巨型动物（海龟、海洋哺乳动物、弹鳃类动物、海鸟）的最重大威胁，也是导致多种鲸类物种减少的主要原因。印度洋的副渔获物问题知之甚少，但在过去的二十年中，小型和半工业化渔业中刺网渔业的高副渔获物水平已被记录在案。不幸的是，渔民通常无法获得减少兼捕的方法、经济上不可行或社会上不可接受的方法。
2. 使用阿拉伯海金枪鱼漂流刺网渔业训练有素的船长网络，收集了 2013 年至 2017 年巴基斯坦海岸（印度洋北部）的目标捕获量和兼捕数据。使用了两种使用复丝刺网的捕鱼方法：水面部署和水下部署（即网的标题设置在 2 m 以下）。
3. 目标物种的预测捕获率在两种捕捞方式之间没有显着差异，尽管在地下集合中记录的金枪鱼 (6.2%) 和类金枪鱼 (10.9%) 物种捕获量下降。然而，在地下，鲸类兼捕的概率比在地表中低 78.5%。
4. 金枪鱼漂流刺网渔业中的鲸类兼捕有可能以相对较低的成本显着减少渔民。然而，需要通过适当的抽样设计和大样本量进行进一步研究，以确认所提议的缓解方法的有效性。渔民对地下环境的接受和采用也需要进一步调查。尽管存在一些局限性，但这项初步研究还强调了当观察员无法部署在渔船上时，基于船员的观察员数据作为替代数据来源的重要性。