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NORTHERN RECOVERY UNIT - NESTING FEMALE ABUNDANCE AND POPULATION TRENDS

Presented by Mark Dodd

The Northern Recovery Unit is the second largest loggerhead nesting aggregation in the western north Atlantic. Annual nest totals from northern beaches ranged from 3,629 to 6,642 between 1989 and 1998, representing approximately 1,287 nesting females per year (adapted from TEWG 2000 - the Northeast Florida data reported by TEWG 2000 was revised to include only those nesting activities reported from Amelia Island north to the Florida-Georgia border). In this summary, loggerhead nest counts from standardized daily beach surveys and aerial surveys conducted by South Carolina Department of Natural Resources were used to assess population trends for Northern Recovery Unit beaches.

Standardized Nest Surveys

Historically, survey effort on Northern Recovery Unit beaches has been variable making it difficult to assess nesting trends. In order to standardize the data used in our analysis, we included only annual nest totals from beaches that met the following criteria: (1) surveys began between May 1-May 15 and continued through the end of the nesting season (August 31), (2) surveys were conducted daily although 1-2 days may have been missed due to logistical difficulties, and (3) survey area was standardized throughout the length of the study (+ 1 kilometer for changes in beach profile).

Two time-series were examined for trends, 30 and 21 years. For the 30-year time-series, only three beaches (Wassaw, Blackbeard, and Little Cumberland) met the criteria mentioned above for analysis. Figure 1 shows the summed nest counts from these three beaches from 1973-2002. A regression of log-transformed nest totals showed a significant (P=0.0363) annual decrease of 1.2 percent in loggerhead nesting. However, this trend must be viewed with caution because the datasets used for this analysis represented only 6 percent of the Northern Recovery Unit nesting and may not be representative of overall population trends.

Figure 1. Summed annual loggerhead nest counts from three Northern Recovery Unit beaches (Wassaw, Blackbeard, and Little Cumberland), 1973-2002.

In order to gain a more representative sample, we examined a shorter time-series (21 years). Figure 2 shows the summed nest totals from eight Northern Recovery Unit beaches (South, Cape, Edisto Beach State Park, Fripp, Wassaw, Ossabaw, Blackbeard, and Little Cumberland) from 1982-2002. These totals represent approximately 31 percent of Northern Recovery Unit annual nesting. A regression of log-transformed nest totals showed no trend (P=0.16). According to power analysis conducted using the TRENDS program (Gerrodette 1993), 29 years of data is required to detect a 2.0 percent annual change in nests (alpha=.20, power=.90). By using a shorter time-series, we obtain a more representative sample of the population, but lose power to detect nesting trends.

Figure 2. Summed annual loggerhead nest counts from eight Northern Recovery Unit beaches (South, Cape, Edisto Beach State Park, Fripp, Wassaw, Ossabaw, Blackbeard, and Little Cumberland), 1982-2002.

In an attempt to utilize all available nesting data, we also used a statistical technique called Route Regression to assess nesting trends on Northern Recovery Unit beaches. Route Regression was developed by Geissler and Sauer (1990) to assess regional trends in breeding bird counts. A regional trend is calculated as a weighted average of the individual route (beach) trends. The dataset was analyzed using the program ESTEQN (Collins 1997). This technique was attractive because we could use all available data including datasets with missing years. The route-regression analysis showed no significant trend in loggerhead nesting (P=0.55) over the last 38 years. Future work will include power analysis to estimate the percent annual change detectable using this technique.

Aerial Nest Surveys

Standardized aerial nest surveys conducted by the South Carolina Department of Natural Resources represent another dataset for assessing Northern Recovery Unit nesting trends. Beginning in 1980, nests were surveyed from the side window of a fixed wing aircraft. Twelve surveys were conducted biweekly during June and July each year for the entire South Carolina coast with the exception of Myrtle Beach. Surveys were conducted for three consecutive years followed by two years of "spot check" surveys. An annual nest total was derived by estimating the percent nesting represented by the 12 flight days (after adjusting for bias from ground truth beaches) and then extrapolating out to an overall total for the season (from a summary composite curve of nesting). Figure 3 shows loggerhead nest estimates from South Carolina aerial surveys, 1980-2002. A regression of log-transformed nest totals showed a significant (P=0.0009) annual decrease of 3.1 percent in loggerhead nesting. The South Carolina data represents approximately 59 percent of Northern Recovery Unit nesting totals.

Figure 3. Loggerhead nest estimates for South Carolina from aerial surveys, 1980-2002.

Summary

Estimates of loggerhead nesting trends from standardized daily beach surveys ranged from no trend to a 1.2 percent annual decrease in nesting. Because of annual variability in loggerhead nesting, 28 to 30 years of standardized survey data are necessary to detect +2 percent annual change in nesting. Nest totals from aerial surveys conducted by South Carolina Department of Natural Resources showed a 3.1 percent annual decline in nesting since 1980.

Literature Cited

Collins, B.T. 1997. ESTEQN: A program to analyze trend data. Canadian Wildlife Service, National Wildlife Research Center, Hull, Province of Quebec, Canada.

Geissler, P.H. and J.R. Sauer. 1990. Topics in route-regression analysis. Pages 54-57 in J.R. Sauer and S. Droege (editors). Survey Designs and Statistical Methods for the Estimation of Avian Population Trends. U.S. Fish and Wildlife Service, Biological Report 90(1).

Gerrodette, T. 1993. Trends: Software for a power analysis of linear regression. Wildlife Society Bulletin 21:515-516.

Turtle Expert Working Group. 2000. Assessment update for the Kemp's ridley and loggerhead sea turtle populations in the western North Atlantic. NOAA Technical Memorandum NMFS-SEFSC-444.

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