A statistical analysis of trends in Florida’s loggerhead nest counts with data through 2015.
A detailed analysis of Florida's long-term loggerhead nesting data (1989-2015) revealed three distinct annual trends (Figure 1). Following a 24 percent increase between 1989 and 1998, nest counts declined sharply over nearly a decade (1999-2007). However, annual nest counts showed a strong increase (74 percent) over the last eight years (2008-2015). Examining only the period between the high-count nesting season in 1998 and the most recent nesting season (2015), researchers found a slight but nonsignificant increase, indicating a reversal of the post-1998 decline. The overall change in counts from 1989 to 2015 was significantly positive (38 percent); however, it should be noted that wide confidence intervals are associated with this complex data set.
Figure 1. The 27 years of nest counts recorded a pronounced increase, then decline, then increase in nesting. Scientists do not yet understand what caused these changes. This trend line describing annual loggerhead nest counts was estimated by fitting a 4-knot restricted cubic spline (RCS) curve to the total counts via negative binomial regression. The 4-knot RCS model had the greatest support and appeared to be the best choice from the available set.
Scientists estimated (corrected) nest-count points in Figure 1 based on small variations in missing count data among years. These estimated counts differed slightly from the observed nest counts reported in the index nesting beach survey. The curved line describing the annual trend in loggerhead nest counts was estimated using methods similar to the one described in the 2009 publication: Decreasing Annual Nest Counts in a Globally Important Loggerhead Sea Turtle Population.
Hundreds of participants surveyed index beaches and counted turtle nests to collect data for this analysis. All participants hold a Florida marine turtle permit and are trained in sea turtle nest identification. Scientists at the Fish and Wildlife Research Institute coordinate the nest counts according to a rigorous protocol to ensure that they reveal unbiased trends.
In contrast to the loggerhead nesting trend, nest counts for green turtles and leatherback turtles in Florida have increased dramatically over the 27-year period. These two species nest on many of the same beaches in Florida as the loggerhead, but in smaller numbers.
Florida beaches are of worldwide importance to loggerhead sea turtles. Approximately 80 percent of the global loggerhead population nests either on Florida beaches or in Oman, a country on the Arabian Peninsula. Florida accounts for approximately 90 percent of U.S. loggerhead nesting.
View the article Index Nesting Beach Survey Totals (1989-2015) for more information on how scientists at the FWC’s Fish and Wildlife Research Institute collect nesting-trend data and for nesting trends of green and leatherback turtles.
Loggerheads Face Wide-Ranging Threats to Their Survival
In Florida, coastal development and high vessel traffic present challenges for sea turtle protection. On developed beaches, coastal armoring meant to protect buildings from erosion has eliminated nesting habitat where natural dunes once stood. Artificial lighting on nesting beaches causes hatchlings from nests to crawl inland rather than toward the water. Throughout the state's waters, collisions with boats are the most common identifiable cause of trauma in sea turtles that wash up dead on Florida beaches.
The FWC's conservation work includes assisting the rescue and rehabilitation of sick and injured sea turtles and expanded efforts to guide coastal construction practices and commercial fishing activities. The agency also works to correct errant lighting that could lead sea turtle hatchlings to their death on developed beaches.
Additional threats to Florida's loggerheads occur far from the state's waters and beaches, throughout the Atlantic Ocean basin. During the approximately 30 years that it takes a loggerhead sea turtle to mature, it will travel widely through international waters and many national jurisdictions. Many drown in shrimp trawls; others get hooked or entangled by open-ocean longlines set to catch reef fish, sharks, tuna and swordfish. Occasionally, mass strandings of dead or sick loggerheads occur without clear evidence of what disease, toxin or event was responsible.
To help protect sea turtles outside Florida waters, the FWC provides nesting data to the federal agencies with management oversight beyond Florida's jurisdiction: the National Oceanic and Atmospheric Administration (NOAA) Fisheries Service and the U.S. Fish and Wildlife Service. The U.S. Fish and Wildlife Service and the state’s sea turtle license plate provide funding for FWC's sea turtle nest monitoring programs.
Learn more about sea turtles and threats to their survival by visiting the Sea Turtles section of this website.