No mullet, no gain: cooperation between dolphins and cast net fishermen in southern Brazil

ABSTRACT We report on the interaction between common bottlenose dolphins, Tursiops truncatus (Montagu, 1821) and cast net fishermen in southern coast of Brazil. The fishery was monitored in the mouth of the Tramandaí River Estuary to investigate the seasonality of catches and their relationships with a set of variables: presence/absence and number of bottlenose dolphins, fishing area, temperature, salinity, wind and water flow direction in the channel. The mullet, Mugil liza Valenciennes, 1836 is the target species and was the dominant fish in the catches (77% of total catch; 50% in frequency; 0.2 ind. x f-1). The use of GLM models helped to reveal that the number of the bottlenose dolphins, time of year (months) and spatial variation of fishing activity were the main factors explaining the presence and abundance of mullet in the fishermen’s catches. The presences of bottlenose dolphins in the fishing area raise the probability of fishermen catch larger number of mullets with smaller fishing effort. However, the size of the mullet is influenced basically by seasonality. The mullets are the “currency” of bottlenose dolphins and fishermen interaction. There are reasons for concern about the sustainability of the southern Brazilian M. liza stock, once the decrease of this fishing resource can lead this rare and traditional fishery to the extinction.摘要 本研究报道了巴西南部海岸普通宽吻海豚（*Tursiops truncatus* (Montagu, 1821)）与撒网渔民之间的种间互动关系。研究团队于特拉曼达伊河河口河道开展渔业监测工作，旨在探究渔获的季节性分布特征及其与多组变量的关联，涉及的变量包括：宽吻海豚的出现情况与种群数量、捕捞作业区域、水温、盐度、风向及河道水流方向。本研究的目标捕捞物种为巴西鲻（*Mugil liza* Valenciennes, 1836），其为本次渔获中的优势类群：占总渔获量的77%，出现频率达50%，单位捕捞努力量渔获量为0.2尾·捕捞次⁻¹。借助广义线性模型（Generalized Linear Model，GLM）开展分析后发现，宽吻海豚的种群数量、季节（以月份表征）及捕捞活动的空间异质性，是解释渔民渔获中巴西鲻的存在与否与丰度水平的核心影响因子。当宽吻海豚出现在捕捞作业区域时，可提升渔民以更低捕捞努力量获取更多巴西鲻的概率，但巴西鲻的体型大小主要受季节性波动调控。巴西鲻是宽吻海豚与渔民之间互动关系的核心纽带。当前巴西南部海域的巴西鲻种群可持续性亟待关注，该渔业资源的衰退或将导致这种兼具传统特色的珍稀渔业走向灭绝。