GULF OF MEXICO 



483 



production. The number of spat produced under 

 these circumstances is always quite small. The 

 most reasonable explanation for this situation is 

 that oysters on the mud flats are induced to spawn 

 sporadically in the wintertime because of the 

 sharp elevations in temperature which may take 

 place in small, poorly circulating bodies of water. 

 Water temperatures of 10° to 20° C. in the main 

 bays and estuaries during the winter are not high 

 enough to permit spawning but may permit the 

 growth and setting of the larvae produced in iso- 

 lated bayous and marsh ponds. 



Growth rates of oysters in the Gulf are reputed 

 to be astounding when compared to the rates 

 along the Atlantic coast. This reputation is based 

 on impressions of oystermen, a few fortuitous 

 natural experiments, and a very meager amount 

 of scientific data based on exceptional conditions. 

 It is certain that during its first year the oyster 

 grows to a length of two or more inches on the 

 average. Under special circumstances and under 

 experimental conditions at Pensacola a growth of 

 .3 and 4 inches has been obtained in 12 months. 

 It should be noted that a growth of 2}^ inches 

 during the first season is not at all unknown in 

 the Chesapeake area, and July spat have been 

 reported growing to a length of 2% inches before 

 their first hibernation period in Virginia (Mackin 

 1946). Since the growth of oysters in the Gulf is 

 not interrupted by a hibernation period, it would 

 be more surprising if they did not grow up to 3 

 inches during their first year. In those cases 

 where oysters grow 5 to 6 inches in 24 to 36 months, 

 and there are many such instances, the oysters are 

 usually of the inferior "coon" type. Oj'sters grow- 

 ing in crowded positions can increase in size only 

 in one direction, and so they soon become unu- 

 sually long. Oystei-s growing as "singles" can 

 increase in size in all directions, and single oysters 

 in the Gulf do not exhibit unusual growth rates 

 after their first season (Moore and Pope 1910). 

 Cultivation procedures developed since 1900 by 

 oystermen in Louisiana involve the transplanta- 

 tion of year-old seed to better growing areas and 

 the marketing of the oysters from 18 to 24 or more 

 months later. Good-sized market oysters pro- 

 duced along the Gulf coast probably average three 

 or more j'ears in age. In the Gulf area major in- 

 creases in growth, measured by length and width, 

 take place in the months of November through 



2595340— .54 32 



March when water temperatures range from 10° 

 to 20° C. The major increases in size in New 

 England oj'sters take place in the summer months 

 at similar temperature levels (Loosanoff and No- 

 mejko 1949). 



The time required for a Gulf oyster to reach 

 sexual maturity is significantly shorter than in 

 northern waters (Menzel 1951). Approximately 

 one-third of the oysters setting in the early part 

 of the summer become sexually mature by the 

 time they are a month old and still less than an 

 inch in diameter. It is probable that a majority 

 of the spat attain sexual maturity during their 

 first season and make a significant contribution 

 to the larval population of late summer. In other 

 words, it is a normal event for two generations of 

 oysters to be produced in this area each summer. 



During the past century there have been many 

 changes of a temporary or permanent nature in 

 the continuity of oyster communities and in the 

 physical location of the reefs. Man has caused or 

 accelerated many of these changes. Other changes 

 due to the natural succession of events in an 

 estuarine environment are presumably of no 

 greater importance today than they were in pre- 

 historic times. 



The many rivers draining into the Gulf annu- 

 ally deposit an enormous load of silt which pro- 

 duces multiple effects in the estuary. The major 

 silt load deposited in the delta gradually pushes 

 the head of fresh water seaward. In the past 50 

 years the Colorado River has filled in more than 

 6,000 acres of upper Matagorda Bay, and delta 

 mud now lies on top of once productive oyster 

 reefs (Baughman 1947, unpublished ms.). In its 

 progress to the sea the water carries much silt 

 with it, and this has additional effects on the 

 oyster population. It d icases the penetration 

 of sunlight into the water, thus limiting the pro- 

 duction of plankton, the oyster's food supply. 

 Fine silt, by coating the old shells on the bottom, 

 makes them no longer available as cultch for the 

 young oysters (Butler 1951). 



An entirely different type of change has been 

 brought about by man in the oyster areas west of 

 the Mississippi River. In this region a number of 

 bayous in the past contributed a regular supply of 

 fresh water to the bays and inlets along the coast. 

 Man has channeled these bayous and changed 

 their exits to the sea so that they contribute 



