Seasonal Distributions and Abundances of Loggerhead and Leatherback Sea Turtles in Waters of the Northeastern United States

Abstract

Seasonal distributions and abundances of loggerhead (Caretta caretta) and leath- erback (Dermochelys coriacea) sea turtles in continental shelf waters off the coast of the northeastern United States were derived from more than three years of aerial and shipboard surveys. There were 3460 sea turtle sightings, including 2841 loggerheads, 128 leatherbacks, and 491 unidentified. Relative abundance patterns, corrected for uneven distribution of survey effort, demonstrated an extensive area of loggerhead distribution from near Long Island, New York, along the mid-shelf to near Cape Hatteras, North Carolina. Areas of high relative abundance of leatherbacks were more scattered, with clusters south of Long Island and in the central and eastern Gulf of Maine. Loggerheads occurred significantly farther south (x = 38?20' N) than leatherbacks (x = 40005' N), and in significantly warmer waters (mean surface temperatures of 22.2 C and 20.4 C, respectively). The two species did not differ significantly in water depth or bottom slope at sighting; the modal depth interval for both was 21-40 m. Patterns of distribution and concentration of sea turtles differed greatly from nearly all species of marine mammals, suggesting little overlap in resource utilization. Both relative and absolute density estimates were much higher for loggerheads. Overall mean relative densities were 21.6 loggerheads per 1000 km of survey track, and 6.85 leatherbacks per 1000 km. Absolute density estimates, derived from 454 special aerial surveys, ranged from 1.64 x 10-3 to 5.10 x 10-1 loggerheads km-2 and 2.09 x 10-3 to 2.16 x 10-2 leatherbacks km-2. The maximum densities for both species were higher than values reported for the Gulf of Mexico and off the eastern Florida shore. Total study area populations during the summer were estimated at 2200-11,000 loggerheads and 100-900 leatherbacks. These estimates are minimal because they are based on observations of turtles at the surface, and may represent much greater abundances. The patterns of relative abundance derived from this analysis could be used as a basis for the designation of critical sea turtle habitat off the northeastern U.S.