The percentage of time available to an oyster 

 population for growth and "fattening", or for re- 

 production, can be used in evaluating the effect 

 of the temperature factor on the productivity of 

 an oyster bottom. A distinction should be made 

 between the reproductive capability of the popu- 

 lation and its growth and "fattening". In the 

 practice of oyster culture the areas of bottom 

 most suitable for setting are not considered de- 

 sirable for the rapid growth and conditioning of 

 oysters for market and vice versa. 



FOOD 



The quantities of food available to water- 

 filtering animals may be determined by taking 

 plankton and nannoplankton samples and by 

 noting the food requirements of a given species. 

 It has been shown by J0rgensen (1952) and 

 J0rgensen and Goldberg (1953) that the oyster 

 (C. virginica) and the ascidians {Ciona intestinalis 

 and Molgula manhattensis) filter about 10 to 20 1. 

 of water for each ml. of oxygen consumed, and 

 that about two-thirds of the energy absorbed by 

 them can be used for growth. The actual food 

 requirements of the animals studied by J0rgensen 

 probably do not exceed 0.15 mg. of utiUzable or- 

 ganic matter per liter of water used. Determina- 

 tions of phytoplankton in American coastal waters 

 made by Riley (1941), Riley, Stommel, and 

 Bumpus (1949), and Riley and Gorgy (1948), 

 show that the organic matter of the phytoplankton 

 in their samples ranged from 0.17 to 2.8 mg. per 

 liter. These waters contain enough material to 

 supply the energy requirements of C. virginica 

 which, according to my determinations, differ 

 from those made by J0rgensen (see p. 210 in ch. IX) ; 

 under normal conditions at 24° to 25° C. C. vir- 

 ginica uses from 3 to 4 mg. of oxygen per hour. 



Quantitative samples of plankton and micro- 

 plankton taken throughout the year from the 

 water over a thriving oyster population can be 

 compared with samples collected in the plankton- 

 poor waters of the tropics. The water should be 

 pumped from the bottom zone, with care being 

 taken not to stir the sediment. Vertical hauls are 

 useless since the water a few inches above the 

 oysters does not come in contact with them ex- 

 cept in the case of strong vertical mixing. 



Seasonal changes in the volume of plankton and 

 microplankton of water over a commercially pro- 

 ductive oyster bed in Long Ishind Sound are shown 

 in figure 369. Both types of samples were collected 



S N D 



Figure 369. — Seasonal changes in the volume of plankton 

 (upper curve) in cm.' per 50 1. and microplankton 

 (nannoplankton) in cu. cm. per 1 m.' (lower curve) in 

 Long Island Sound in 1932 and 1933. 



at the same time. For the plankton study 50 1. 

 of water were filtered through No. 20 bolting silk. 

 The collected material was preserved in 2 percent 

 formalin and transferred into tall glass cylinders, 

 and its volume read 24 hours later after the mate- 

 rial had settled at the bottom; the results are ex- 

 pressed in cm.'. For microplankton determina- 

 tion a 1 liter sample was taken from the bottom 

 and the water passed through a high-speed Foerst- 

 Juday type centrifuge rotating at 20,000 r.p.m. 

 The centrifugate was transferred to a 15 mm. 

 diameter tube and centrifuged for 5 minutes in a 

 clinical centrifuge at 14,000 r.p.m. and its volume 

 measured. Since the waters of Long Island Sound 

 are relatively free of silt, the amounts of detritus 



408 



FISH AND WILDLIFE SERVICE 



