Temporal trends in mortality and effects of by-catch on common bottlenose dolphins,Tursiops truncatus, in southern Brazil

Abstract

The mortality of the bottlenose dolphin, Tursiops truncatus, on the southern portion of Rio Grande do Sul State coast was investigated based on 914 beach surveys conducted between 1969 and 2006. A total of 188 stranded bottlenose dolphins were recorded during this period, indicating a 1.8M:1F sex-ratio of those animals sexed (N = 79). Mortality was low in calves, high in juveniles and sub-adults and slightly lower than in adults. The overall mortality was clearly seasonal overlapping with higher fishing efforts in the Patos Lagoon Estuary and adjacent coastal areas, where most individuals washed ashore. Analysis of a continuous 14-year long subset (1993–2006) of the data indicated relatively low levels of mortality between 1995 and 2000 and a marked increase between 2002 and 2005 followed by an apparent drop in 2006. By-catch was responsible for at least 43% of the recorded mortality between 2002 and 2006. Juvenile males were more susceptible to incidental catches. Among females, by-catch of adults represented 75%. Results of a potential biological removal analysis suggest that current levels of fishing-related mortality are unsustainable for the small resident population of bottlenose dolphins that inhabits the Patos Lagoon Estuary, and that this population may be declining.

Report of the Working Group on Population Parameters and Demography of Tursiops truncatus in the Southwest Atlantic Ocean

“…In contrast, in the southeastern region, stranded animals were mostly older dolphins, irrespective of sex. (Fruet et al, 2012). These results should, however, be taken as preliminary, particularly because the dataset from both regions contains a mix of individuals that died of natural and fisheries-related causes; the sample size in the southeastern area is small and therefore stochasticity may account for much of this pattern.…”

“…An approach using the northeast Atlantic inshore population as a reference for estimating the length of sexual maturity of stranded dolphins in southern Brazil has been undertaken (Fruet et al, 2012). It was estimated that males and females longer than 318cm and 278cm, respectively, are sexually mature.…”

“…Applying closed-population MR models for photoidentification data, Dalla Rosa (1999) (Fruet et al, 2012) and the impacts of such deterministic pressure in this very small population should be looked closely. Future successive abundance estimations will help to elucidate population trends over large periods.…”

First estimate of common bottlenose dolphin (Tursiops truncatus) (Cetacea, Delphinidae) abundance off Uruguayan Atlantic coast

“…(Praderi, 1985), because of the minimal overlap between the distribution of dolphin and artisanal coastal (Franco-Trecu et al, 2009;Zappes et al, 2014) and industrial offshore fishing activities (Domingo et al, 2006). However, bycatch occurs in neighbour areas of southern Brazil (Fruet et al, 2012). Considering the high mobility of this species along the Uruguayan coast (Laporta, 2009) and a potential demographic and genetic connection between dolphins from Uruguay and southern Brazil (e.g.…”

“…Bastida et al, 2007;Lodi et al, 2016 this volume), but resident coastal populations are particularly found between southern Brazil (Itajaí River, 26°54'S, 48°38'W) and Argentina (San Antonio Bay, 40°50'S, 64°50'W; Lodi et al, 2016 this volume). Along this area, abundance and other life history traits have been estimated for some of the populations inhabiting protected environments as in Patos Lagoon Estuary (Fruet et al, 2012;2015a, b) and Santo Antonio Lagoon, southern Brazil (Daura-Jorge et al, 2013) and San Matias Gulf, Patagonia, Argentina (Vermeulen and Cammareri, 2009). On the other hand, studies are scarce for bottlenose dolphins occuring in open coastal environments along the SWAO.…”