480 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



The majority of Gulf oyster communities fall 

 into one of four distinctive categories which I 

 classify on the basis of arbitrarily selected average 

 salinity levels. Intergrades between the catego- 

 ries and exceptions to the generalizations are nec- 

 essarily common, but they do not seriously alter 

 the over-all description. Permanent communities 

 establish themselves and flourish within a salinity 

 range of 10 to 30 parts per thousand. In years 

 of drought or of excessive precipitation populations 

 may appear sporadically in areas normally having 

 salinity levels too low or high to support a 

 population. 



Oyster reefs located near the head of a typical 

 estuary comprise the first category. In such re- 

 gions the salinity ranges annually from 15 parts 

 per thousand to and averages near 10 parts per 

 thousand or below. The sparse population reflects 

 the marginal nature of this environment. Oysters 

 are mostly small and rounded, with smooth, whit- 

 ish shells. The rate of spatfaU is low, and al- 

 though young oysters grow well their first season, 

 growth of older oysters is relatively slow. Average 

 annual mortality rates are high, and the population 

 is periodically decimated by excessive fresh water 

 in years of flood. Primarily, such reefs are of 

 commercial use as seed areas. In the occasional 

 years of drought these populations may show, 

 however, an excellent growth and jdeld a good 

 harvest. Typically, these oysters are free from 

 most fouling organisms, and there are few preda- 

 tors or parasites present. These oysters have 

 many characteristics in common with populations 

 growing at or above mean low water regardless of 

 salinity level. 



The next definite community type is found 

 where salinity levels fluctuate between 10 and 20 

 parts per thousand with a yearly average near 15 

 parts per thousand. The population density of 

 oysters on these reefs reaches a maximum because 

 of high reproductive ability, availability of cultch, 

 and a relatively low concentration of predators. 

 The growth of individual oysters is moderately 

 good and quite uniform so that the population 

 forms rather definite year classes. Oyster valves 

 are usually smooth and dense, although in some 

 areas they may have moderate infections of the 

 boring sponge aud clam. The oysters form large 

 or small interlocking clusters, depending on the 

 nature of the bottom. Their narrow shape makes 

 them diflicult to handle commercially except as 



canning stock. When cultivated they yield clus- 

 ters of two or three better shaped oysters that are 

 suitable for the raw trade. The meats are of high 

 nutritional quality but are frequently insipid to 

 the taste. The increased number of associated 

 animals on these reefs leads to intense competi- 

 tion for both food and space. Barnacles and 

 mussels are frequently present in such numbers 

 as to interfere with oyster culture. While these 

 communities prosper biologically in most years, 

 ia times of drought they may experience severe 

 losses due to predation by the drill. In flood years 

 the growth of the population may be retarded for 

 several months. 



The third category of oyster reefs is found 

 nearer the mouth of a typical estuary, in areas 

 having an average salinity level of 25 parts per 

 thousand. Annual fluctuations ia salt content of 

 water range from a low of 10 to 12 parts per thou- 

 sand in the spring to about 30 parts per thousaud 

 in the dry season. The community here is char- 

 acterized by unusually good growth, although this 

 factor may be masked by the large amount of 

 shell erosion and predation. The variety of ani- 

 mals living within the oyster community reaches 

 a maximum at this saliuity level. The reproduc- 

 tive potential of the oysters probably reaches a 

 maximum here too, but the high population den- 

 sity found in commimities at lower salinity levels 

 does not occur here because of the large numbers 

 of parasites and predators. Mortality rates of the 

 very young oysters are relatively high. Having 

 survived its first season the oyster's chances for 

 survival are good. The shells have a massive 

 appearance and may be greatly eroded. The 

 valves show heavy concentric ridges, indicating 

 periods of fast growth rates. These communities 

 are consistent producers of large market oysters 

 of excellent quality. But the environment has 

 its greatest value when used as a bedding ground. 

 Medium-sized seed planted here quickly matures 

 and may be harvested before the usual predators 

 and parasites are established. The decreased spat 

 sm-vival prevents the transplanted oyster from 

 becoming "wrapped up" in young growth. The 

 periodic complete harvesting of these areas when 

 used as bedding groimds prevents or greatly re- 

 tards the accumulation of xmdesirable commensals. 



The fourth type of oyster community is foimd 

 at the junction of the typical estuary with the 

 waters of the Gulf where salinity levels are con- 



