Wilber: Influence of Apalachicola River flows on Callinectes sapidus 



181 



gulf waters near Apalachicola Bay to spawn and 

 that the larvae are distributed to the south by loop 

 currents (Oesterling and Evink, 1977). Evidence 

 supporting this hypothesis was examined in this 

 study. 



Methods 



Fisheries data 



Several aspects of the blue crab fishery may lead to 

 inaccurate fishery representation of adult stock 

 abundance. For example, unreported landings from 

 the recreational fishery and crab bycatch from the 

 shrimp fishery are potential sources of bias in blue 

 crab landings statistics (Perry, 1984). Although these 

 sources of error cannot be controlled, if they are 

 independent of river flow and account for a rela- 

 tively constant proportion of the total landings over 

 time, a valid, although perhaps conservative, rep- 

 resentation of environmental effects on the species 

 can be obtained. 



The Florida Department of Natural Resources 

 (FDNR) provided monthly landing data for blue 

 crabs from Franklin and Wakulla Counties for 1979- 

 90, monthly effort data (number of trips) for 1987- 

 90, annual landing data from Wakulla County from 

 1973 to 1990 (excluding 1977), and annual landing 

 data from the Florida west coast from 1960-1990. 

 Franklin County annual landing data from 1952 to 

 1979 were also obtained (Herbert et al., 1988 3 ). Sta- 

 tistical analyses (Wilkinson, 1990) were conducted 

 by using the full 39-year Franklin County dataset, 

 as well as a shorter (1973-90) dataset, which al- 

 lowed comparisons between Franklin and Wakulla 

 Counties that were not confounded by differences in 

 time periods. The limited amount of effort data pre- 

 cluded analyses of catch per unit of effort. 



Flow and rainfall data 



The Apalachicola River begins at the Florida state 

 line by the confluence of the Chattahoochee and 

 Flint Rivers. Apalachicola flow data were collected 

 at the United States Geological Service gauge at 

 Blountstown, Florida, which is the closest station to 

 the estuary (105 km upstream) with an adequate 

 period of record. This station is not immediately 

 adjacent to the estuary, therefore fresh water from 

 local inputs and storm events are not included. The 

 drainage area downstream from the Blountstown 

 gauge is less than 9% of the total area drained by 



the Apalachicola-Chattahoochee-Flint River system 

 (Leitman et al., 1983 4 ). 



Parameters examined included the highest and 

 lowest average flows for 7 and 120-consecutive days 

 each year (referred to as the 7- and 120-day maxi- 

 mum and minimum flows). Monthly minimum, 

 mean, and maximum values, and the mean monthly 

 flow during the growout period (September-May) 

 were also examined. By using these flow durations, 

 associations between landings and seasonal high 

 and low flows could be examined, which was not 

 possible when analyses included only mean annual 

 flow. The growout-flow time period was adapted 

 from a similar study correlating blue crab landings 

 in Georgia with river discharges (Rogers et al., 

 1990 5 ). 



Sufficient historical flow data were also available 

 for the Suwannee, Econfina, St. Marks, and 

 Ochlockonee Rivers (Fig. 1), thus permitting a re- 

 gional analysis of associations between flows and 

 blue crab landings. For each river, the annual one- 

 day minimum, one-day maximum, and annual mean 

 flows were used. One-day high and low flow magni- 

 tudes were used because of their availability and 

 because preliminary analyses which substituted 

 other flow durations (annual minimums and maxi- 

 mums) on the Apalachicola River did not change 

 results considerably. 



Statistical analyses 



Blue crab landings and flow data were tested for 

 monthly, seasonal, and inter-annual dependencies 

 through autocorrelations. Data were adjusted to 

 remove dependencies when autocorrelations were 

 significant. If autocorrelations between successive 

 months were present, data were replaced by the 

 difference between each month and the preceding 

 month. If seasonal autocorrelations were present, 

 the effects were removed by dividing each value by 

 a seasonal factor. For instance, if landings exhibited 

 a significant autocorrelation with a 12-month time 

 lag, which reflected a similarity in catches for the 

 same month among years, each monthly value was 

 divided by the month's mean value and replaced by 

 the quotient. Similar analyses were conducted with 

 seasonal (three-month averages) landings and flow 

 data following adjustments to remove significant 

 autocorrelations. Flow data were log 10 transformed. 



3 Herbert, T. A., and Associates. 1988. The Franklin County 

 Fisheries Options Report, 164 p. 



4 Leitman, H. M., J. E. Sohm, and M. A. Franklin. 1983. Wet- 

 land hydrology and tree distribution of the Apalachicola River 

 flood plain, Florida. U.S. Geological Survey Water-Supply Pa- 

 per 2196, 52 p. 



5 Rogers, S. G., J. D. Arrendondo, and S. N. Latham. 1990. As- 

 sessment of the effects of the environment on the Georgia blue 

 crab stock. Final Rep. Georgia Dep. Natl. Resources, 69 p. 



