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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



fresh water to the area only in times of flood. As 

 a result of these changes in the drainage system 

 the inner bays and lakes lying some distance 

 from the coast, formerly entirely fresh, have now 

 become saline enough to support large populations 

 of oysters. Connecting bays lying between them 

 and the Gulf have, in turn, become more salty 

 The increased salt content here encouraged the 

 survival of the conch which has decimated or 

 destroyed entirely many large reefs (Moore and 

 Pope 1910). A secondary result from this chan- 

 neling has been an increase in erosion along the 

 shore. The annual load of silt brought down by 

 the rivers or bayous no longer reaches the coastal 

 areas, where in the past it counter-balanced 

 shore erosion and created a fairly stable coast line. 

 Shore erosion, now uncompensated, has destroyed 

 some of the barrier reefs; formerly protected bays 

 are now open to the Gulf and cannot support 

 oyster populations. Conversely, some bays which 

 formerly had connecting channels to the Gulf have 

 become landlocked by sand spits, and their 

 oyster communities have largely disappeared. 

 There probably has been no great net change in 

 the amount of bottom available for oyster culture, 

 but the reefs have migrated inland away from the 

 sea and have become more susceptible to the 

 effects of seasonal floods in their new location. 

 There are some places along the coast where man 

 has opened up channels and deposited spoil banks 

 in the construction of inland waterways. These 

 operations have frequently changed salinity levels 

 permitting an increased survival of oyster 

 predators and, in some cases, have biu"ied reefs in 

 mud. 



Extensive changes in oyster communities have 

 also resulted from harvesting methods. Redfish 

 Reef in Galveston Bay, which was more than 5 

 miles in length, produced a large annual harvest 

 throughout the nineteenth century. In 1890, 

 by the use of a tug and two power dredges, this 

 reef was eradicated in a single year (Baughman 

 1947, unpublished ms.). Overharvesting without 

 replacement of shell cultch inevitably causes the 

 deterioration of a reef. In places where over- 

 harvesting has continued until the dead shells 

 supporting the reef as well as the live oysters 

 have been removed, it has been a matter of but 

 a few years before the area deteriorated to such 

 an extent that even nature could not rehabili- 

 tate it. 



Oyster reefs are normally established in posi- 

 tions protected during average storm conditions. 

 The occasional hurricanes experienced in the Gulf " 

 area may destroy large numbers of oysters in 

 shallow water areas and on sandy bottoms. In 

 1947, for example, many miles of reef along the 

 north edge of Mississippi Sound were covered 

 with mud by wave action and destroyed (Engle 

 1948). This area has shown no significant natural 

 rehabilitation in the past 5 years. 



We have mentioned some of the factors which 



m 



may permanently eradicate the oyster reef. 

 There are other items which are of greater or 

 lesser importance in affecting the continuity of 

 the populations. 



Crevasses. — Disastrous but relatively temporary 

 changes result from exceptional floods from the 

 river basins. In 1890 the flood of the Nita 

 crevasse extended east from the Mississippi River 

 and affected oyster reefs 180 miles away in Mis- 

 sissippi Sound. Forty miles of oyster reefs lying 

 between Lake Borgne and Biloxi were seriously 

 damaged at that time. The history of floods of the 

 lower Mississippi River shows that, although they 

 may wipe out huge oyster populations in a short 

 time, these populations are quickly reestablished, 

 often at a more luxuriant level than prior to the 

 crevasse. The annual spring floods occurring in 

 most river basins cause some mortality in the 

 oyster population by lowering the salinity of the 

 water, but in most years these losses are in- 

 significant. 



Temperature. — The oyster is extremely versatile 

 in adapting itself to changes in temperature and is 

 commonly found where the annual range is from 

 — 2° to +30° C. The Gulf oyster is less ac- 

 customed to severe cold weather than its Atlantic 

 coast relatives, and occasionally large numbers of 

 oysters growing at mean low-water level are 

 destroyed during sudden winter freezes. 



Pollution. — Pollution has constituted a serious 

 factor in the continuity of oyster populations 

 during the past 50 years, but due to more effective 

 control measures, it is becoming of less importance. 

 Oysters have been destroyed in several areas by 

 the effluent from paper mills, for example, and 

 other industrial wastes are reputed to have 

 damaged large oyster-growing areas. The control 

 of industrial wastes no longer constitutes an 

 unsolvable problem. Pollution resulting from 



