Long-Term Seasonal and Interannual Patterns of Marine Mammal Strandings in Subtropical Western South Atlantic

Abstract

Understanding temporal patterns of marine mammal occurrence is useful for establishing conservation strategies. We used a 38 yr-long dataset spanning 1976 to 2013 to describe temporal patterns and trends in marine mammal strandings along a subtropical stretch of the east coast of South America. This region is influenced by a transitional zone between tropical and temperate waters and is considered an important fishing ground off Brazil. Generalized Additive Models were used to evaluate the temporal stranding patterns of the most frequently stranded species. Forty species were documented in 12,540 stranding events. Franciscana (n = 4,574), South American fur seal, (n = 3,419), South American sea lion (n = 2,049), bottlenose dolphins (n = 293) and subantarctic fur seal (n = 219) were the most frequently stranded marine mammals. The seasonality of strandings of franciscana and bottlenose dolphin coincided with periods of higher fishing effort and strandings of South American and subantarctic fur seals with post-reproductive dispersal. For South American sea lion the seasonality of strandings is associated with both fishing effort and post-reproductive dispersal. Some clear seasonal patterns were associated with occurrence of cold- (e.g. subantarctic fur seal) and warm-water (e.g. rough-toothed dolphin) species in winter and summer, respectively. Inter-annual increases in stranding rate were observed for franciscana and South American fur seal and these are likely related to increased fishing effort and population growth, respectively. For subantarctic fur seal the stranding rate showed a slight decline while for bottlenose dolphin it remained steady. No significant year to year variation in stranding rate was observed for South American sea lion. The slight decrease in frequency of temperate/polar marine mammals and the increased occurrence of subtropical/tropical species since the late 1990s might be associated with environmental changes linked to climate change. This long-term study indicates that temporal stranding patterns of marine mammals might be explained by either fishing-related or environmental factors.

High mortality of adult female Magellanic penguins by gillnet fisheries in southern Brazil

“…Some gillnets killed penguins inside the no‐fishing area and some nets exceeded the allowed length of 16 km (Figure ). Furthermore, a study showed that the current mortality of the Franciscana dolphin remains similar to that before the implementation of this novel regulation (Prado et al, ). Our data suggest that current fisheries regulations in the region, including closures and gear restrictions, are ineffective at preventing Magellanic penguin mortality.…”

“…During the last decade, the number of penguin strandings along the Brazilian coast has increased (García‐Borboroglu et al, , ; Mäder, Sander, & Casa, ). Concomitantly, the increase in fishing effort (net size and number of boats) by the gillnet fishery in southern Brazil has increased the incidental mortality of various marine megafauna, especially the Franciscana dolphins ( Pontoporia blainvillei ) and green turtles ( Chelonia mydas ) (Monteiro et al, ; Prado, Mattos, Silva, & Secchi, ; Secchi, Kinas, & Muelbert, ). Similar to these megafauna species, penguins are long‐lived and low‐fecundity animals with delayed maturity (Boersma, ).…”

Long‐term changes in fishery resources of an estuary in southwestern Atlantic according to local ecological knowledge

“…The current situation of overfishing for several artisanal fishing resources in the PLE, as demonstrated by this study, and for industrial resources in the coastal region as demonstrated by Haimovici and Cardoso (), requires the attention of fishery managers. However, there is a clear lack of proper management measures or enforcement in southern Brazil, as several threatened marine megafauna species are killed annually in high numbers and most of the fish resources are overexploited (Fogliarini, Bugoni, Haimovici, Secchi, & Cardoso, ; Haimovici & Cardoso, ; Prado, Mattos, Silva, & Secchi, , this study). From a governance context of lack of enforcement or ineffective management or conservation strategies, Dowling et al () suggested that the most effective measures would be spatial and temporal approaches, as for example, the establishment of no‐fishing zones or seasonal closures.…”

The ecological value of long-term studies of birds and mammals in Central America, South America and Antarctica

“…Prado et al [85] used 38 years of data on marine mammal stranding along the east coast of South America to analyze shifts in distributions and mortality rates in marine mammals. Five species made up 97% of all strandings: Franciscana dolphins (Pontoporia blainvillei), bottlenose dolphins (Tursiops truncatus), South American fur seals (Arctocephalus australis), subantarctic fur seals (Arctocephalus tropicalis), and South American sea lions (Otaria flavescens).…”

“…= However, the authors note that overfishing of other species could lead to negative effects on fur seals. Marine mammals may be at risk due to fishing mortality [85], as well as indirect effects caused by fishing such as increased competition for food [86]. These long-term studies will be critical for the monitoring of such effects.…”