Variance from seawater concentra- 

 tion, rather than low salinities per 

 se, may provide favorable habitat for 

 the euryhaline young of many species 

 that utilize estuaries as nursery 

 grounds. The whole idea of recruit- 

 ment being correlated with area of 

 favorable habitat is entirely con- 

 sistent with Gunter's hypothesis 

 (Gunter 1961) that euryhaline young 

 are protected from stenohaline preda- 

 tors by salinity gradients in estu- 

 aries . 



The simple input-output model in 

 Figure 2 depicts freshwater flow as 

 the input that determines area of 

 favorable habitat, which in turn de- 

 termines the output, fishery recruit- 

 ment. Factors such as temperatures 

 during critical periods, freezes, and 

 hurricanes also affect recruitment, 

 creating "noise" in the relationship 

 between area and recruitment. Bar- 

 ring substantial interference from 

 "noise", production of fish and 

 shellfish recruits in estuaries may 

 correlate with area of favorable hab- 

 itat during the nursery season, al- 

 though not necessarily with fresh- 

 water flow during this time. Because 

 area of favorable habitat is a func- 

 tion of complex interactions between 

 freshwater flow, tidal prism, and 

 estuarine geomorphology , the rela- 

 tionship between fishery production 

 and freshwater flow is not likely to 

 be linear over more than a restricted 

 range, differing for each estuary and 

 each species. For any estuary there 

 is a rate of freshwater flow that 

 will push the band of favorable sa- 

 linities beyond estuarine boundaries 

 into open waters, eliminating favor- 

 able habitat entirely. Likewise, for 

 every estuary there is a freshwater 

 flow so low that the band of favor- 

 able salinities retreats upriver 

 where the area of favorable habitat 

 is small. 



A complication is that factors 

 other than freshwater flow could 

 change the area of favorable habitat. 

 Seasonal and supra-annual variation 

 in tidal stages can vary the area 

 covered by water. For instance, 

 Allen et al. (1980) observed that 

 the greatest area of tideland in the 

 Florida Bay region is flooded by an 

 annual rise in sea level that occurs 

 from August to December. They further 

 noted that the annual peak in abun- 

 dance of juvenile pink shrimp occurs 

 at that time. They quote Collier and 

 Hedgepeth (1950) as saying, "Some of 

 the principal fisheries are dependent 

 upon the young gaining the protection 

 and nourishment offered by tidal 

 flats during the early days of their 

 lives. The degree to which a given 

 year-class is successful might de- 

 pend upon the extent, both spatial 

 and temporal, to which the tidal 

 flats and low marshlands are flood- 

 ed." Allen et al. (1980) suggest 

 that cyclic variations in height of 

 Florida sea level may have a marked 

 influence on the annual abundance of 

 juvenile pink shrimp and, ultimately, 

 the offshore commercial shrimp catch. 



Most fishery biologists conduct- 

 ing research in estuaries seem to be 

 unaware of the nonlinear effects of 

 freshwater flow on mixing or the po- 

 tential importance of mixing condi- 

 tions to the survival and growth of 

 juvenile and sessile estuarine spec- 

 ies. On the other hand, although 

 ocean engineers and physical oceano- 

 graphers study currents and mixing 

 patterns in estuaries, they seem un- 

 aware of the questions that need to 

 be answered to relate fishery re- 

 cruitment to freshwater flow rates. 

 An inter-disciplinary effort is the 

 best way to insure that a quantitive 

 understanding of the relationship 

 between freshwater flow and fishery 

 production is developed. 



421 



