Encalhe de cetáceos, prospecções sísmicas e fatores ambientais no nordeste do Brasil

Abstract

Cetaceans are highly dependent on sound for vital activities and have been negatively affected by anthropogenic noise, which can cause from behavioral changes to physical damage. Mass strandings have been linked to active sonars, which seem to mainly affect deep diving species. Another worrying source of noise is seismic surveys (SS), which uses airguns to generate high intensity impulsive sound waves. However, a causal effect of seismic surveys on the mortality and stranding of cetaceans is still controversial. We examined the relationship between cetacean strandings and 3D seismic surveys in the equatorial northeastern Brazil, accounting for some oceanographic variables. We gathered the strandings registered between 2000 and 2019 in the states of Ceará and Rio Grande do Norte and separated the species into 5 ecological groups according to their occurrence, size, and usual diving pattern. Then, we characterized the spatiotemporal occurrence of the strandings and selected two areas where the search effort was more consistent. For these two areas, we analyzed whether SS and wind and wave conditions affected the occurrence and number of strandings of each ecological group. We also simulated the drift of mimic cetaceans considering a constant mortality rate inside seismic operations´ limits and compared the spatial distribution of these simulated landings to the observed strandings for the same period. In the 20 years considered, 351 strandings were recorded, including 22 species and 5 families. Stranding records were higher in summer, when tourist seasonality increases the surveillance of beaches, and increased along the years, which may reflect an increase in monitoring effort in response to environmental constraints from 2010 onwards. Seismic surveys also increased in coverage area and duration compared to operations of the first decade of compiled data. From the 65 strandings that occurred up to one week after an SS operation, 5 had pathological alterations compatible with gas embolism syndrome, which might be associated with exposure to seismic surveys. SS occurrence resulted in increase of strandings of oceanic species with shallow dive, despite oceanographical factors. The number of strandings of deep diving species as well as all species combined was also affected by SS occurrence, when considering specific wave states. Furthermore, 43% of the observed strandings positions overlapped with landings from simulated mortality inside seismic operational areas. Drift simulations showed that animals that could have died in SS of Ceará may beach in other states of Brazil and would go undetected by the current monitoring efforts. Also, areas with highest landing probabilities for carcasses coming passively from seismic operational areas reached well beyond the extension of beach monitoring programs required by the environmental regulation institution. We recommend that previous drift simulations are carried out as a requirement of the licensing process to determine which portion(s) of the coast should be monitored in beach monitoring projects. Our results revealed that a portion of the animals reported were probably affected by seismic surveys while another large portion of affected animals would not be detected because of oceanic drift. Given this effect, the impact seen here is likely an underestimate of the actual impact of seismic surveys on the mortality and strandings of cetaceans in the region.