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

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 ran- domly conducted in coastal waters 4 to 17 m deep. Seventy-five percent of positive catches con- sisted of the capture of a single loggerhead sea turtle with up to 10 loggerhead sea turtles cap- tured 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 tur- tle capture in any given trawling event was significantly greater following the capture of a logger- head sea turtle in the previous trawling event, but twice as great within hotspots (0.53) as else- where (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-occur- rence. 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.