Interactions between gillnet fisheries and small cetaceans in northern Rio de Janeiro, Brazil: 2001-2002

Abstract

In northern Rio de Janeiro (21°18'S-22°25'S), Brazil, gillnets are responsible for the by-catch of a number of small cetaceans. Franciscanas (Pontoporia blainvillei) and marine tucuxis (Sotalia fluviatilis) are the most impacted species. From November 2001 to October 2002 data on fisheries and mortality of small cetaceans were obtained through interviews with fishermen of 20% of the gillnet fleet. Seasons were grouped into spring-summer and autumn-winter and the region was divided into two fishing areas: Area I (Barra do Itabapoana to São Tomé Cape) and Area II (São Tomé Cape to Macaé). A total of 374 gillnet operations were recorded between 0.02-42.1n.miles from shore and in waters 5-67m deep. Gillnets were 3.1m in length and of 120mm mesh size. The fishing effort for the entire fleet (n=50 boats) was estimated at 7,161.8km of net and the total number of small cetaceans caught was 225. The CPUE was 0.031 animal × (km of net × day)^-1. Fisheries and by-catches did not show seasonal differences. Fishing trips were more prevalent in Area I, whereas the by-catch of franciscanas and marine tucuxis was higher in Area II. Fisheries were distributed uniformly in areas close to shore (first 10n.miles) and beyond. In Area I they occurred predominantly within the 30-m isobath. The by-catch of franciscanas and marine tucuxis also occurred within these limits, suggesting that 30-m isobath may constitute the offshore limit of their distribution along the coast. The variability in the width of the continental shelf in Areas I and II may explain the differences in the magnitude of the by-catch of small cetaceans in these areas. Gillnet fisheries may adversely impact the coastal populations of small cetaceans. In northern Rio de Janeiro, particularly between 22°00'S and 22°25'S where the 30-m isobath is found close to shore, fisheries operating within the first 10n.miles from shore can pose substantial threat to dolphins. Due to the distribution of their preferred prey, franciscanas may be at greater risk of by-catch in areas near the Paraíba do Sul river mouth. It is thus proposed that these areas be closed to gillnet fisheries year round, so that the impact on coastal dolphins is minimised.