Texas; (c) suggest new perspectives; 

 (d) establish the rationale for an 

 interdisciplinary approach; and (e) 

 present a research outline. 



HISTORICAL OVERVIEW 



Activities of man that affect 

 the quantity and/or timing of the 

 flow of fresh water to estuaries in- 

 clude: (1) dams for irrigation and 

 power; (2) diversions; (3) canals in 

 uplands; (4) deforestation; (5) 

 clearcutting; (6) grazing; (7) road 

 construction; and (8) paving (as in 

 urban development). Ways in which 

 these activities affect flow are 

 shown in Table 1. All activities 

 that increase peak flow and decrease 

 dry season flow change the timing of 

 flow by decreasing the lag between 

 rainfall and runoff. 



Studies examining the potential 

 effects of such changes fall into 

 several types: 



(1) laboratory studies relating 

 growth rates and mortalities of spe- 

 cific organisms to salinity and tem- 

 perature ; 



(2) field studies determining the 

 frequencies and abundances of cer- 

 tain organisms at various salinities 

 and temperatures; 



(3) statistical studies determining 

 estuarine food chains; 



(4) statistical studies of the re- 

 lationships between landings and 

 prior salinities or river discharge 

 rates (effects on recruitment). 



Findings of these and related studies 

 are summarized as follows: 



(1) Salinity-occurrence ranges, tol- 

 erance ranges, and optima have been 

 determined for a number of estuarine 

 species . 



(2) Certain synergistic effects of 

 salinity and temperature have been 

 established. 



(3) Many organisms occurring in es- 

 tuaries can withstand wide fluctu- 

 ations in salinity. High calcium 

 concentrations improve the capabil- 

 ity of estuarine organisms to toler- 

 ate near-freshwater conditions. 



(4) Estuaries are nursery grounds of 

 many fish and invertebrate species 

 that primarily occur, spawn, and are 

 harvested offshore. 



(5) Salinity gradients partition 

 estuarine habitat between different 

 species and (possibly) different co- 

 horts of the same species. 



(6) Low salinities (and possibly 

 high salinities) reduce predation and 

 parasitism on American oysters. Al- 

 though difficult to demonstrate, ju- 

 venile fishes and invertebrates may 

 be protected from predation by sa- 

 linity extremes. 



(7) Terrestrial detritus flushed in- 

 to estuaries by the flow of fresh wa- 

 ter forms the base of a major estu- 

 arine food chain. 



(8) Significant relationships be- 

 tween fishery yields and freshwater 

 flow have been demonstrated by cor- 

 relation-regression techniques for a 

 number of estuaries and species. The 

 correlations have been positive in 

 some estuaries and negative in others 

 and have been positive for some spe- 

 cies while negative for others. 



404 



