Some ecological aspects of commercially important decapod crustaceans in low salinity marshes.
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From March 1966 through May 1968, semi-monthly hydrological and biological samples were taken in a low salinity marsh bordering Trinity Bay in the Galveston Bay Estuarine System, Texas. The study area is the site of the proposed Wallisville reservoir and this study sought to determine to what extent the area was utilized as a nursery ground for commercially important decapod crustaceans. Trawl, seine, and marsh net samples were used to determine the seasonal abundance and distribution of the brown shrimp (Penaeus aztecus Ives), the white shrimp (Penaeus setiferus Linn.), and the blue crab (Callinectes sapidus Rathbun). Salinities within the study area are regulated by the discharge of the Trinity River. During the late spring of 1966 and in the spring of 1968, high Trinity River flow reduced salinities throughout the study area to less than 0.5%. The highest salinity recorded in the study area was 15.5% during a period of reduced Trinity River flow. Postlarval brown shrimp appeared in the samples during April in 1966, but were absent from samples taken during and after heavy flooding of the Trinity River in May 1966. In the spring of 1967, flood conditions did not exist and postlarval brown shrimp were first collected in March and peak recruitment of brown shrimp occurred in April. In the spring of 1968, flood conditions existed again and no brown shrimp were taken. White shrimp were first collected during July in 1966 and during May in 1967, and were most abundant during September and October of both years. The decline in abundance of white shrimp in the study area during both 1966 and 1967 coincided with water temperatures falling below 20 degrees Celsius. Blue crabs of 20 mm width or less were taken during all months of the year, but were most abundant during the winter. There were no distinct patterns of distribution and abundance for crabs larger than 20 mm width. During the study period, 515 adult males and only 26 adult female crabs were taken. Growth rates of both brown and white shrimp varied seasonally and with water temperature. White shrimp grew at a rate of approximately 0.5 mm per day during October through December 1966 (average water temperature=19.5 degrees Celsius) and at the rate of approximately 0.3 mm per day during the same period in 1967 (average water temperature=16.9 degrees Celsius), but at a rate of approximately 1.0 mm per day during June through August 1967 (average water temperature=29 degrees Celsius). Brown shrimp grew at a rate of approximately 0.7 mm per day during November through December 1966 (average water temperature=18.3 degrees Celsius) and at the rate of approximately 1.3 mm per day during April through May 1967 (average water temperature=24.7 degrees Celsius). A width-weight relationship for males blue crabs was determined to be: Log weight= 2.73956, Log width=3.66107 and that for females was: weight=2.618324 Log width=3.45805. A coefficient of condition was determined for male blue crabs using the formula K=W/Lb x 10 exp 5 mean monthly condition factors for these male blue crabs were compared for the period; the highest mean condition coefficients occurred during the summer months. The construction of the Wallisville Reservoir will destroy approximately 12,500 acres of nursery grounds behind the proposed dam by isolation and freshwater innundation. Detrimental effects to the nursery grounds below the dam site are indicated.