Spatial Analysis of Incidental Mortality as a Threat for Franciscana Dolphins (Pontoporia Blainvillei)

Article excerpt

Fisheries bycatch is considered one of the main threats for most large marine vertebrates, such as seabirds, marine mammals, turtles and sharks (review, among others, by Shaughnessy et al. 2003, Read et al. 2006, Zydelis et al. 2009). Incidental entanglement is a direct human threat with a straightforward effect on species mortality. Three problems arise when results from bycatch research are translated into wildlife managment actions. First, predictions of population trends are normally weak because large-scale estimates of demographic parameters are difficult to generate (e.g., Moore and Read 2008). Second, bycatch impact is rarely compared to other threats, such as depletion of prey base, ecosystem changes, habitat degradation or disease. The effects of these stressors are not always as conspicuous as bycatch, and may be more difficult to evaluate (Taylor et al. 2007). Third, fisheries can offer short-term benefits to some marine vertebrates that overshadow the long-term costs of this threat.

An emerging method since the development of GIS and spatial statistics is the comparison between distributions of species and threats that are formalized in habitat suitability models or ecological-niche models. These models relate presence-absence or abundance observations based on random or stratified field sampling with stressors and other environmental variables (Guisan and Thuilier 2005, Sims et al. 2008).

Franciscana dolphins (Pontoporia blainvillei) are small cetaceans restricted to shallow waters of the South Atlantic Ocean, from southeastern Brazil (18[degrees]25'S) to northern Patagonia (42[degrees]-10'S) (Crespo et al. 1998). It has been classified as 'vulnerable' by the IUCN (Reeves et al 2008). There is no current abundance estimate for the species as a whole.

Incidental entanglement or bycatch of Franciscana dolphins was reported repeatedly: Di Beneditto et al. (2001), Kinas (2002), Dans et al. (2003), Freitas Netto and Barbosa (2003), Secchi (2003), Bordino and Albareda (2005), Cappozzo et al. (in press). IUCN defines incidental mortality in gillnet fisheries as the main threats to this species (Reeves et al. 2008). Secchi et al. (2001) first estimated bycatch in the overall Franciscana population via population abundances based on aerial surveys. They calculated that removal by gillnets ranged between 1.1% and 3.5% per year. The Scientific Committee of the International Whaling Commission has noted that incidental mortality of 1% is sufficient for concern about the conservation future of coastal dolphins. However the various assumptions and parameters of these estimations are difficult to test. For example, Secchi et al. (2001) counted an average of 4.25 individuals per flight and extrapolated an overall abundance of 42,000 Franciscana dolphins (Reeves et al. 2008). It is expected that slight changes in parameters assumptions could produce significant changes to these results.

I used a spatial approach to analyze the role of incidental mortality as a threat to Franciscana dolphins. I compared information from different areas of their distribution with different levels of fishery activity. I followed the main assumption of most habitat models, i.e., that distribution of population abundance is positively related to habitat qualities (Guisan and Thullier 2005). If incidental entanglement is a main mortality factor, then areas with higher incidental mortality should have lower population densities.

Based on the genetic structure of the species, Secchi et al. (2003) proposed four management units (MU) (Fig. 1). MUI= coastal waters of Espirito Santo and Rio de Janeiro states, Brazil; MU2=waters off Sao Pablo, Parana and Santa Catarina states, Brazil; MU3= waters off Rio Grande Sul State (Brazil) and Uruguay; MU4= waters off Argentina. Two studies have estimated abundance via aerial surveys for MU3 (Secchi et al. 2001) and MU4 (Crespo et al. 2009). In MU3, 8 flights of equal length (185. …