The horizontal migration of hammerhead sharks along the southern Brazilian coast, based on their exploitation pattern and considerations about the impact of anchored gillnets activities on these species.

Abstract

Between 1995 and 2009 hammerhead sharks were sampled from the landings of the industrial fleets based in the harbours of Itajaí and Navegantes, Santa Catarina State, and Ubatuba, São Paulo State, Brazil. In this case, fishing boats which operated with gillnets longlines and trawls along the southern Brazilian Economic Exclusive Zone and international adjacent waters were targeted. A total of 2483 and 353 S. lewini and S. zygaena carcasses respectively were sexed, measured and converted to total lengths (L_T). Additionally information about, year, season, latitude/longitude and local depth (m) from the catches, by fishing category, were obtained. During the considered period, intense fishing mortality over pups was caused by gillnets and trawls operating on shallow waters (≤ 20 m) and over juveniles along the continental shelf (> 20 m and ≤ 200 m). Additionally, adults were exploited by driftnets and longlines along the shelf border and slope (> 200 m). Therefore, both hammerhead species are exploited at all life-stages and throughout their migratory circuit. This includes during their inshore-offshore migration while they are growing from pups to juveniles and as the offshore-inshore migration of pregnant females to pupping areas in shallower waters. This apparently unsustainable exploitation pattern, over different size classes (newborns-juveniles-adults), and the economic pressure caused by the international fin market, is one of the reasons for population declining of these two species in southern Brazil. Non fishing zones for the hammerheads, protecting their migratory circuit, which is driven by their growth pattern and reproduction, are necessary. Additionally, fishing effort reduction and a control over the international fin market are recommended.