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Fishery Bulletin 92(1), 1994 



River flows. Statistically significant correlations 

 were positive and primarily restricted to a time lag 

 of one year, indicating higher flows were associated 

 with higher blue crab landings the following year 

 and lower flows with poorer landings the next year. 

 The mean flow during the growout period (Septem- 

 ber through May) of the previous year was the most 

 highly correlated flow parameter with blue crab 

 landings in both counties. 



A number of explanations are consistent with the 

 observation that more fresh water (within a certain 

 range) was associated with higher blue crab land- 

 ings the following year. Greater freshwater inflows 

 reduce estuarine salinities, thereby increasing the 

 area of suitable habitat in the middle, and perhaps 

 lower, estuary where juvenile blue crabs can forage 

 and develop (Livingston et al., 1976; Perry, 1984). 

 Increases in low salinity habitat may reduce preda- 

 tion by marine species on juvenile blue crabs. 

 Greater freshwater flows may also broaden an 

 estuary's signal to offshore female migrants and/or 

 megalopae, thus increasing the potential recruit- 

 ment population base (Perry and Stuck, 1982; 

 Mense and Wenner, 1989). In addition, higher in- 

 flows carry more detrital and nutrient matter 

 (Mattraw and Elder, 1982), which may either di- 

 rectly or indirectly enhance food availability. 



In both Franklin and Wakulla counties, flows be- 

 low approximately 600 m 3 /sec appear more closely 

 related to the following year's landings than higher 

 growout flows, i.e., the regression equation fits the 

 data better at the low end of the flow spectrum (Fig. 

 3). Several factors may explain this phenomenon. 

 Food availability may limit blue crab production at 

 flows below a certain level but may not be limiting 

 at flows above this level and, therefore, crab produc- 

 tivity is not influenced by further increases in flow. 

 Prey limitation at low flows may also lead to canni- 

 balism, further limiting blue crab population size 

 (Lipcius and Van Engel, 1990). 



The finding that more recent years produce a 

 stronger correlation between blue crab landings and 

 river flows was also observed in Georgia (Rogers et 

 al., 1990 5 ). Total discharges from September to May 

 (growout period) of five Georgia rivers were posi- 

 tively correlated with landings (r 2 >0.8). Shorter time 

 periods (the most recent 14 and 19 years of land- 

 ings statistics) produced better correlations with 

 flow than the full period of record (37 years). The 

 authors concluded increased fishing pressure in 

 more recent years resulted in only one year class 

 being fished, and, thus, environmental effects were 

 more obvious on a single year class in the shorter 

 dataset. Similarly, that more recent landings for 

 Franklin County were more highly correlated with 



Apalachicola River flows than landings for the 

 longer 39-year period may reflect a trend toward 

 harvesting a single year class. 



The significant 1- and 12-month time lags in 

 Franklin and Wakulla County reflect similarities in 

 catches between successive months and a seasonal 

 component, respectively. The 12-month auto- 

 correlation indicates that trends in landings occur 

 at the same time of year (e.g., summer peaks) and 

 should not be confused with an annual auto- 

 correlation, which is indicative of a similarity in 

 harvests between entire years. The positive corre- 

 lations between unadjusted monthly flow and land- 

 ings data correspond to the summer peak in blue 

 crab landings following 3-5 months after the spring 

 peak in flows, and low winter landings following low 

 late-summer and fall flows. The negative correla- 

 tions with 2-4 month time lags reflect fall low flows 

 following peak summer harvests and high spring 

 flows occurring after low winter harvests. The ab- 

 sence of significant correlations between monthly 

 landings and flows, once these data were adjusted 

 to remove seasonal autocorrelations, indicates that 

 residual (non-seasonal) variation in monthly flows 

 is unrelated to the non-seasonal variation in mon- 

 thly landings. 



Livingston (1991) found a positive contemporane- 

 ous correlation between monthly Apalachicola River 

 flows and blue crab abundances in trawl surveys 

 conducted from 1972 to 1985. This finding corre- 

 sponds to high juvenile abundances during high-flow 

 months. The positive correlation in the present 

 study between monthly flows and blue crab landings 

 3-5 months later may reflect the maturation of ju- 

 veniles into adults in the summer, and thus the 

 observed time lag in the correlation. 



The majority of the Apalachicola-Chattahoochee- 

 Flint basin is in Georgia and is subject to different 

 climatic conditions than are the other north Florida 

 rivers examined, which may explain the relatively 

 small correlations between Apalachicola River flows 

 and flows on these other rivers. Georgia rainfall is 

 more strongly correlated with Apalachicola River 

 flows than Florida rainfall (Meeter et al., 1979). A 

 consistent and important finding of the multivari- 

 ate regression analyses was that Apalachicola flows 

 were more highly correlated with Franklin, Wakulla, 

 and Florida west coast landings of the next year 

 than any other river drainage tested. Regressions 

 comparing Apalachicola flows to west coast landings, 

 after Franklin and Wakulla County landings were 

 removed, were not significant, suggesting the influ- 

 ence of the Apalachicola drainage is restricted pri- 

 marily to Franklin and neighboring Wakulla County. 

 Thus, there was no evidence supporting the hypoth- 



