Watershed Effects on the Value of Marshes to Fisheries

by Roger J. Zimmerman, Natl Marine Fisheries Service, Galveston, United States,
Thomas J. Minello, Natl Marine Fisheries Service, Galveston, United States,



Document Type: Proceeding Paper

Part of: Coastal Zone '93

Abstract: Watershed size and rainfall patterns are among the factors that influence the extent to which fishery species use estuarine marshes as nurseries. We examined three Texas bays with different watershed characteristics to determine how marsh use by fishery species was affected. In each bay, we measured densities of aquatic fauna on marsh surfaces at sites located along the salinity gradient during seasonal periods of varying rainfall. Fishery species collected were transient juveniles of brown shrimp, white shrimp, spotted seatrout, southern flounder, red drum, and others. The largest bay with the largest watershed was Galveston Bay where, due to a relatively high river inflow rate and a large inlet from the Gulf of Mexico, a steep salinity gradient always persisted. During periods of high or low rainfall, the gradient was translocated but it was never eliminated. Mesohaline and polyhaline marshes of the middle and lower Galveston Bay were continuously used by fishery juveniles, whereas the oiligohaline marshes of the upper bay were only intermittently used. By comparison, San Antonio Bay is small in size and has a large watershed. High rainfall often caused the salinity gradient in the bay to be virtually eliminated. Marsh use by most of the fishery species in San Antonio Bay was inversely related to the river flow and directly related to salinity. Gulf menhaden was the only exception where utilization was positively related with river flow. Lavaca Bay was also small in size and possessed a small watershed. A dam situated about 10 km above the river mouth further restricted river inflow. High salinities in this bay often extended onto the delta and high rainfall periods had little lasting effect on lowering salinity. The fishery species in Lavaca Bay used marshes the other bays, extended intervals of mesohaline and polyhaline salinities promoted the use of marshes by fishery animals. Persistent oligohaline conditions, on the other hand, depressed the use of marshes by fishery species in all three systems: We conclude that fishery production is least stable in small bays with large watersheds and that large bays with large mesohaline zones are most productive.

Subject Headings: Watersheds | Bays | Fish management | Salinity | Salt water | Rainfall | Estuaries | Rivers and streams | North America | Texas | United States | San Antonio | Gulf of Mexico

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