feature of estuarine systems that 

 makes them so productive. The same 

 could be said of the importance in 

 this regard of salt marshes or estu- 

 arine circulation patterns, both of 

 which are features which vary widely 

 among estuaries (Nixon 1980; Kjerfve 

 et al. 1978). 



Secondary Production 



It is reasonable to ask if the 

 similarity of estuarine primary pro- 

 duction is reflected in a relative- 

 ly small range in secondary produc- 

 tion. Unfortunately, the great 

 difficulty of obtaining measure- 

 ments of the rate of production of 

 higher trophic levels has made it 

 impractical to answer this question 

 directly. The best we may be able 

 to do is to estimate the relative 

 production of animal biomass in 

 estuaries through the use of fish- 

 ery yields. It is always risky to 

 use landings data because of sam- 

 pling problems and a number of other 

 difficulties. Nevertheless, these 

 data represent the best comparative 

 information available and, after 

 reviewing the fisheries yield data 

 for a large number of lakes, Ryder 

 (1965) concluded "that catch is a 

 reliable estimate of fish produc- 

 tion despite the variables affect- 

 ing it." As far as I am aware, 

 however, it is not known how good 

 fish production is as an indicator 

 of total secondary production. 



There are at least two ways to 

 address the problem. First, the 

 yield of a given estuary can be 

 compared with its freshwater input 

 as they vary from year-to-year and, 

 second, the yields of various es- 

 tuaries with different levels of 

 freshwater input can be compared. 

 In looking at variations over time 

 in the Gulf of St. Lawrence region, 

 Sutcliffe (1972, 1973) was able to 

 find a strong positive correlation 



between river discharge and the catch 

 (after appropriate time lags) of var- 

 ious species. A similar positive re- 

 lationship was reported by Turner 

 (1979) for oyster landings in Mobile 

 Bay, Alabama. However, an analysis 

 of five years of total commerical 

 landings data from five estuaries on 

 the Texas coast provided more ambig- 

 uous results which Armstrong (1980) 

 interpreted as showing a curvilinear 

 relationship with an optimum rate of 

 freshwater input. In the case of 

 shrimp, on the other hand, it appears 

 that there is a strong negative lin- 

 ear correlation between freshwater 

 input and production if one compares 

 the mean annual salinity of Lake 

 Pontchartrain with the Louisiana 

 landings (Turner 1979). A similar 

 relationship appears to be evident, 

 particularly over the past 30 years, 

 between Mississippi River discharge 

 and Louisiana shrimp landings (Bar- 

 rett and Gillespie 1973; White and 

 Boudreaux 1977), though there are 

 other, longer-term cycles and trends 

 in the shrimp data as well (Figure 

 6). But there appears to be little, 

 if any, relationship between the dis- 

 charge of the Mississippi and the 

 production of the other major com- 

 mercial species in coastal Louisiana 

 (Figure 7). A cursory examination of 

 landing records from Chesapeake Bay 

 also failed to show any simple rela- 

 tionship with discharge from the Sus- 

 quehanna River, the major freshwater 

 input to that system. 



Regional Variation 



If different estuaries are com- 

 pared, it does not seem to me that 

 the results show any relationship 

 between yield and freshwater input 

 (Table 3). An earlier comparison 

 of seven Texas estuaries by Chap- 

 man (1966) reached the opposite con- 

 clusion, and he may be correct for 

 the special conditions along the 

 south Texas coast. However, there 



43 



