— RIVER 



OCEAN 



Figure 366. — Estuarine structure and net (i.e., nontidal or residual motion of water). Dotted line marks the boundary 

 between the upper and lower net movement; the boundary is the level of "no net motion." From Pritchard, 1951, 

 fig. 3. 



in which larvae move from the spawning grounds 

 at the mouth of the river up into the seed oyster 

 bed area. Bousfield (1955) applied Pritchard's 

 ideas to an analysis of distribution of barnacle 

 larvae in the Miramichi estuary, New Brunswick, 

 Canada. The retention of larval populations in 

 this estuary was found to be due to a combination 

 of two factors: changes in the average vertical 

 distribution of successive larval stages, and the 

 strength and direction of transport by residual 

 drift at different depths. This theory of larval 

 retention and the mechanism of transport is not 

 apphcable to bodies of water in which there is no 

 saUnity stratification or where the residual upriver 

 drift is insignificant. 



Least favorable in the life of an oyster com- 

 munity are occasional turbulent currents of high 

 velocities which may dislodge and cany away 

 young and even adult oysters not attached to the 

 bottom. Oysters attached to rocks and other 

 structures are not destroyed by strong currents, 

 but their valves are injured by small pebbles and 

 sand acting as an abrasive material. Live 

 oysters with shells damaged by abrasion can be 

 found in the Sheepscot and other tidal rivers 

 along the coast of Maine. 



Continuous renewal of sea water running over 

 a bottom in a nonturbulent flow is the most 

 desirable condition for a flourishing oyster com- 

 munity. On the basis of experimental studies 

 discussed in chapter IX, p. 195, it may be assumed 

 that under optimal conditions of temperature 

 and salinity an average adult C. virginica trans- 

 ports water at the rate of 15 1. per hour. With 

 250 large oysters to a bushel and 1,000 bushels 

 per acre, an oyster bed of that size would require 

 3.75 million liters of water per hour. My 

 observations show that under the best of condi- 



tions oysters can take in water only from a 

 distance not exceeding 2 inches from the shell. 

 It is, therefore, necessary to know the rate of 

 water exchange within the narrow layer adjacent 

 to the oyster bottom. The situation may be 

 dift'erent in the case of vertical mixing of water 

 due to turbulent flow. 



The amount of water available to an oyster 

 population can be calculated if the number of 

 individuals on the bottom and the rate of water 

 movements are known. In the case of a turbulent 

 flow, vertical mixmg of water depends on the 

 degree of turbulency. If the mixing extends to 

 the height of 1 foot above the mud line, the total 

 volume of water in which the oyster population 

 lives in our example is 1 acre-foot or 325,851 

 gallons (1.25 million 1.). It follows that the 

 current velocity must be strong enough to renew 

 the volume of the layer above three times (3.75-^ 

 1.25) every hour or 72 times in 24 hours. In 

 cases of greater population density the current 

 requirements are proportionally higher. Due 

 allowance should be made, of course, for the 

 presence of other water-filtering animals which 

 compete with the oyster for food. It is clear, 

 therefore, that great concentrations of water- 

 filtering organisms are possible only where there 

 is sufficient renewal of water. The oyster reef 

 in the Altamaha Sound, Ga., (fig. 367) is a good 

 example of this condition. Such concentrations 

 of live oysters crowded over a limited space 

 cannot exist in sluggish water and are found only 

 in rapid tidal streams. 



The water movement factor can be evaluated 

 by determining whether the rate of renewal of 

 water over the bottom is sufficient for the needs 

 of the population and whether the pattern of 

 circulation is such that a certain percentage of 



FACTORS AFFECTING OYSTER POPULATIONS 



403 



