An ecological study of an oyster population, including selected associated organisms in West Bay, Galveston, Texas.




Gillard, R.M.

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Texas A&M University.


From June 1967 to June 1968 a study of a population of oysters, Crassostrea virginica (Gmelin) was made on Eight Mile Reef, located in West Bay, Galveston, Texas. A general description of the area is reported. Two mortality peaks between March and June and between October and November 1967 occurred when temperatures averaged approximately 21 to 26 C and salinities 26 to 28 o/oo. After the peaks, mortality decreased but continued until the end of the study when less that one percent of the original population remained. When salinities dropped below 20 o/oo during the last three months, no massive mortality comparable to the previous spring occurred. Predators contributed but could not have caused mortalities of the magnitude that occurred. High temperature coincident with air exposure is virtually eliminated as an important primary cause during the mortality peaks. The massive mortalities are considered to have been primarily caused by disease. Both peaks were seasonal and occurred shortly after the maximum population density. Recovery of the population was slow, survivors were in fairly good condition, and the community members suffered far less mortality that did the oysters. The suspected disease is an undescribed species of Labyrinthomyxa which was present in oysters from January to May, was apparently present during the mortality peaks, but was absent or in low abundance in June 1968. The average length and condition index of the oyster population is reviewed. Condition index was lowest in summer, increased but remained constant in winter, and peaked in spring during April. Because of high summer salinity and Crassostrea mortality, Ostrea Equestris became the dominant oyster of the community during winter, constituting 55 to 75 percent of the oysters. After salinities dropped below 20 o/oo during the spring, Ostrea remained numerous but suffered mortality subtidally. Intertidally, it suffered high mortality during the same period. Thirteen living molluscs (four gastropods and nine pelecypods), two barnacles, six crabs and four dominant filamentous algae were collected on the reefs. the observed seasonal and vertical abundance of the associates is discussed. (AA)


136 p., Thesis


oysters, marine molluscs, Crassostrea virginica, population dynamics, mortality, Labyrinthomyxa sp., fungal diseases, abundance, Ostrea equestris