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

Abstract

We report on the interaction between common bottlenose dolphins, Tursiopstruncatus (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, Mugilliza 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.

Diagnóstico ambiental do estuário do rio Tramandaí, litoral norte do Rio Grande do Sul, Brasil

“…As amostragens tiveram duração de seis horas: 8h -11h e 14h-17h. Nas amostragens, a cada 15 minutos, os espécimes capturados foram identificados até o nível de espécie e quantificados (Santos et al, 2018).…”

“…No litoral norte do Rio Grande do Sul, é comum a prática da pesca amadora como forma de lazer ou para complementar a renda (Wurdig & Freitas, 2009). O aumento do número de pescadores com tarrafas na época de reprodução da tainha está ligado à eficiência dessa arte de pesca (Santos et al, 2018). A competição entre a pesca artesanal profissional e a pesca amadora no estuário gera conflitos pelo uso do espaço e pela venda do pescado (Cotrim & Miguel, 2009).…”

Coastal zone use and migratory behaviour of the southern population of Mugil liza in Brazil

“…Since 2004, M. liza has been ranked as overexploited by Brazilian authorities (MMA, 2004) and there is strong evidence that the southern population is overfished, particularly due to fishing pressure during the reproductive migration period (Lemos et al, 2014;Santana et al, 2017;Santos et al, 2018). Brazilian authorities have increased regulations on fishing (MPA/MMA, 2014), but these measures have been largely ineffective, systematically transgressed and apparently inadequate for ensuring the maintenance of M. liza population abundance (Lemos et al, 2016;Santos et al, 2018).…”

Long-Term Spatiotemporal Variation in the Juvenile Fish Assemblage of the Tramandaí River Estuary (29°S) and Adjacent Coast in Southern Brazil

“…The system is permanently connected to the sea by an estuarine bar, which represents a transition zone (Silva, 1982) where juvenile marine-estuarine related fishes shelter and feed in shallow estuarine waters (Ramos and Vieira, 2001). Artisanal fishing is of social and economic importance for a large part of the local population, and the TRE attracts tourists during the summer vacation (Silva, 1982;Malabarba and Isaia, 1992;Santos et al, 2018). Other anthropogenic activities in the TRE, such as the petroleum industry, agriculture, forestry, rice cultivation, and sand extraction, have increased considerably in recent decades (Loitzenbauer and Mendes, 2012) and could adversely affect the functional and ecological execution of the system.…”

“…Currently, limited information has been compiled about the TRE and the adjacent coast (AC- Ramos and Vieira, 2001;Malabarba et al, 2013;Garcia et al, 2018b;Santos et al, 2018). However, the Laboratory of Ichthyology at the Federal University of Rio Grande (FURG) has a series of unpublished long-term data on the TRE.…”