Spatial clustering of loggerhead sea turtles in coastal waters of the NW Atlantic Ocean: implications for management surveys

Abstract

A regional (29.9 to 33.1° N) trawl survey was conducted from 2000 to 2003 and 2008 to 2011 to assess the relative abundance of sea turtles on an important foraging ground. A total of 1461 loggerhead sea turtles Caretta caretta were captured in 23% of 4756 trawling events randomly conducted in coastal waters 4 to 17 m deep. Seventy-five percent of positive catches consisted of the capture of a single loggerhead sea turtle with up to 10 loggerhead sea turtles captured per event. Loggerhead sea turtle capture locations were significantly clustered throughout the survey area. Nine percent of sampling events (446) occurred in spatial ‘hotspots’ and captured 23% of loggerhead sea turtles (339). Four percent of sampling events (193) occurred in spatial ‘coldspots’ and captured 1% of loggerhead sea turtles (18). The probability of loggerhead sea turtle capture in any given trawling event was significantly greater following the capture of a loggerhead sea turtle in the previous trawling event, but twice as great within hotspots (0.53) as elsewhere (0.25). Hot- and coldspots were not explained by carapace length, turtle sex, genetic haplotype, 25 biotic and abiotic attributes associated with trawling events, or bycatch co-occurrence. Because of the universal application of these standardized and relatively easy to compute metrics, we recommend their inclusion in future studies to account for discrepancies in spatial distribution patterns.