during the June 1978-June 1979 year. 

 The nine species were snook, croaker, 

 sheepshead, mullet, black drum, weak- 

 fish, gafftopsail catfish, gray snap- 

 per and Irish pompano. The dis- 

 charges from the St. Lucie Canal over 

 the survey period amounted to 440,000 

 acre-feet as compared to the previous 

 21-year average of 561,000 acre-feet. 

 In addition, 91,000 acre-feet from 

 C-23 and 88,000 acre-feet from C-23A 

 also flowed into the estuary during 

 the survey period. 



We propose three theories for 

 the significant changes in catch 

 rates we found in response to the 

 moderate discharges: (1) movement 

 within the estuary, possibly due to 

 the enhanced food supply flushed 

 through with the freshwater; 

 (2) movement to adjacent areas within 

 the estuary that are initially less 

 influenced by the effects of the dis- 

 charges; and (3) movement to the far- 

 thest zone from the discharge 

 sources . 



Both snook and mullet moved 

 within the estuary in patterns that 

 would indicate movement in response 

 to food supply. However, snook being 

 carnivorous, exhibited a more in- 

 stantaneous reaction. Small fishes, 

 including juveniles and forage fishes 

 are often flushed through with the 

 discharges. As a result, it is a 

 well-known fact that snook move to 

 these structures during discharge 

 periods and are more easily caught. 

 Mullet, on the other hand, consume 

 primarily detritus and algae. Their 

 feeding response, evident after pro- 

 longed discharges, may be a side- 

 effect of the increased detrital ma- 

 terial made available. 



Mullet also demonstrated tempor- 

 ary movement into the South Fork in 

 response to North Fork discharges as 

 did gray snapper. Previous studies 

 indicate that a stratified layer of 



freshwater moves out of the South 

 Fork, into the main estuary, and out 

 the inlet following St. Lucie Canal 

 discharges (Haunert and Startzman 

 1980). If indeed waterflow from one 

 or the other fork results in minimum 

 mixing with the adjacent fork, then 

 the sheltering effect of the adjacent 

 fork, not only from salinity changes 

 but associated turbulence, is an 

 important consideration in fish move- 

 ment . 



It is evident that there may be 

 a threshold discharge amount beyond 

 which sheltering is no longer effec- 

 tive. For example, gray snapper catch 

 rates increased instantaneously down 

 the South Fork as a result of North 

 Fork discharges. However, as the 

 duration of North Fork discharges 

 lengthened, catch rates for snapper 

 increased in the inlet section. 

 Indications are that the first re- 

 sponse of gray snapper may be to move 

 to the South Fork as a result of 

 North Fork discharges, but soon 

 afterwards, as effects are felt in 

 the South Fork as well, movement is 

 toward the inlet section. 



Sheepshead, black drum, Irish 

 pompano or mojarra, weakfish, gaff- 

 topsail catfish, croaker and event- 

 ually gray snapper all showed a 

 tendency for increased catch rates in 

 the inlet section, the farthest zone 

 from the discharge sources. This 

 movement lagged 1-6 weeks behind the 

 discharge. Although several of these 

 species are considered to be salinity 

 limited, they were all affected simi- 

 larly by the discharges. 



An analysis was done to see how 

 the four inflow sources, rain, St. 

 Lucie discharges, C-23 and C-23A 

 discharges, affect the time fisher- 

 men spend on the estuary. Short-term 

 effects of the discharge sources on 

 fishing pressure indicate that 

 neither weekly cumulative rainfall 



280 





