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沿岸普通宽吻海豚（Tursiops truncatus, Montagu, 1821）与撒网渔民之间的互动行为。研究于特拉曼达伊河河口湾入海口区域对该渔业作业开展监测，旨在探究渔获物的季节变化规律及其与多项变量的关联：宽吻海豚的有无及数量、作业海域、水温、盐度、航道内风向与水流方向。本次捕捞的目标物种为鲻鱼（Mugil liza, Valenciennes, 1836），其为优势渔获物种，占总渔获量的77%、出现频率达50%，单位捕捞努力量的个体丰度为0.2尾/作业次。通过广义线性模型（Generalized Linear Model, GLM）分析可知，宽吻海豚数量、年度时间（月份）以及捕捞活动的空间差异，是解释渔民渔获中鲻鱼出现与否及种群丰度的核心影响因子。作业海域内出现宽吻海豚，可提升渔民以更低捕捞努力量获取更多鲻鱼的概率。但鲻鱼的个体规格主要受季节变化调控。鲻鱼是宽吻海豚与渔民之间互利互动的核心媒介。巴西南部海域的鲻鱼种群可持续性堪忧，一旦该渔业资源量下降，这种稀有且传承已久的捕捞作业将濒临消亡。