Sea turtle catch rates

Research surveys, shipping channels

Trawl surveys to collect sea turtles in shipping entrance channels characterize the majority of efforts expended in the Southeast U.S. since the 1970's. Trawl surveys in shipping channels have been conducted for various reasons, but principally to identify periods when channel dredging could be conducted with minimal negative impact to sea turtles. Because of the economic importance of ports in the Southeast U.S., a considerable amount of historic data sets regarding sea turtle catch rates are associated with trawling in shipping channels. As such, these locations represent important index study sites for monitoring long-term trends.

The first SCDNR fishery-independent trawling effort to monitor sea turtle catch rates in a shipping channel began in September 1990 in the Charleston, SC shipping channel [12]. Trawling with a 60' headrope mongoose net (towed for 15 to 20 min) was conducted at 12 stations each sampled twice monthly (once during the day and once at night) for a year prior to dredging, after which monthly monitoring was continued for four months. In total, 53 loggerhead and 1 Kemp’s ridley sea turtle were collected.

Trawl Survey Map Modification of Figure 1 from Van Dolah and Maier (1993) showing the locations where trawling for sea turtles was conducted in the Charleston, SC shipping channel in 1990 and 1991 and again during 2004 through 2007 (blue circles).

Trawling at the same locations within the Charleston, SC shipping channel was repeated between May and August 2004 to 2007 [13] to evaluate changes in catch rates since the early 1990s, a period of time corresponding to mandated use of TEDs in coastal shrimp fisheries. During 432 trawling events conducted in the Charleston, SC shipping channel between 2004 and 2007, 220 loggerheads, 2 Kemp's ridleys, and 1 juvenile green sea turtle were collected. Loggerhead sea turtle captures in the Charleston, SC were significantly greater during 2004–07 than in the early 1990s.

Bar Graph Similar to observations for juvenile loggerhead sea turtles in estuarine waters in Florida and North Carolina, catch rates in the Charleston, SC shipping channel increased significantly between the early 1990s and the mid 2000s. This observation bodes well for a future recovery of this species in the Northwest Atlantic provided that trends reported for this one coastal location are indicative of a larger population pattern and provided that annual survival rates are high.

During April 2006 and 2007, personnel from the SCDNR MRD and the University of Georgia Marine Extension Service conducted trawling in the Port Canaveral, FL shipping channel. Because the focus of this effort was to capture adult males associated with a presumed breeding aggregation, sampling effort was not extensive. However, it is worth nothing that 158 loggerhead sea turtles were collected during 58 sampling events during this effort. Thirty-three percent were adults (82% of which were adult males) and 67% were juveniles. Compared to trawling efforts at this location in 1992, capture rates for adult male loggerheads were similar but capture rates for adult females and juveniles were much greater in 2006–07.

12 Van Dolah, R.F. and P.P. Maier. 1993. The distribution of loggerhead turtles (Caretta caretta) in the entrance channel of Charleston harbor, South Carolina, U.S.A. Journal of Coastal Research 9:1004-1012.

13 Arendt, M. D., J. A. Schwenter, A. L. Segars, J. I. Byrd, P. P. Maier, J. D. Whitaker, D. W. Owens, G. Blanvillain, J. M. Quattro, and M. A. Roberts. 2012. Catch rates and demographics of loggerhead sea turtles (Caretta caretta) captured from the Charleston, South Carolina, shipping channel during the era of mandatory use of turtle excluder devices (TEDs). Fishery Bulletin 110(1):98–109.