FISHERY BULLETIN: VOL 78, NO 4 



surements. The experiments were conducted in 

 the same tank in which the fish were maintained. 

 This removed two potential problems: first, the 

 possibility of injury to the fish as a result of fre- 

 quent handling; and second, the possibility of 

 stress resulting from recent handling and transfer 

 of the fish to an unfamiliar tank. Stress has been 

 shown to affect the respiration rate and swimming 

 performance of schooling fishes (i.e., Skazhina 

 1975; Hartwell and Otto 1978). 



One week before the experiments were begun, 

 one-half of the fish were removed for length and 

 weight determination, leaving 12 Atlantic 

 menhaden in the tank. We have found that 12-15 

 adult fish is an optimum number for a tank of this 

 size; fewer fish begin to show signs of stress, 

 whereas more fish begin to interfere with each 

 other during feeding. Experiments were carried 

 out between 26 July and 9 September 1977. At the 

 end of this period the fish were sacrificed for length 

 and weight determination. All fish appeared to be 

 in excellent condition thoughout the experimental 

 period, and showed no evidence of injury or dis- 

 ease. 



Experimental Procedure 



The experiments were carried out at 20°±1.0°C 

 and a salinity of 31%o. Prior to each experiment, 

 the bottom and walls of the tank were thoroughly 

 cleaned with a wire brush. The fish were fed their 

 normal ration, and then deprived of food for 36 h 

 until the beginning of the experiment to permit 

 evacuation of the intestine and to avoid any effect 

 of the previous meal on the metabolism of the fish. 

 During this 36-h period, the seawater inflow was 

 filtered through a GAF polypropylene bag filter of 

 5 fxm nominal pore size. Feces from the last meal 

 were periodically siphoned from the tank. On the 

 evening before the experiment, the tank walls 

 were again scrubbed and the tank rapidly flushed 

 several times with filtered seawater. 



Each experiment was begun at approximately 

 0630 h, with an initial baseline measurement of 

 respiration rate and voluntary swimming speed of 

 the unfed fish. Plankton was then added to the 

 tank at a constant rate during a 7-h period from 

 approximately 0800 to 1500 h. During each exper- 

 iment, respiration rates and voluntary swimming 

 speeds were measured on 10 occasions, termed 

 "measurements," which lasted for about 1 h when 

 the fish were feeding, and iy2-2 h when they were 

 not. These measurements correspond to the fol- 



lowing periods: no. 1, initial (unfed for 36 h); no. 

 2-4, feeding during the 7-h period of food input; no. 

 5, the transition from feeding to postfeeding; no. 

 6-8, during the first 10 h following feeding; and no. 

 9 and 10, the next morning, 15-20 h after feeding. 

 To prevent an excessive accumulation of am- 

 monia in the tank when the fish were fed the 

 larger rations, the tank was flushed briefly with 

 filtered water at the conclusion of feeding. 



Food 



The solitary diatom Ditylum brightwelli was 

 used as food in the experiments. These large cells 

 (—80 ixm. long) are readily eaten by menhaden. 

 Phytoplankton was raised in outdoor batch cul- 

 tures. Narragansett Bay water was filtered into 

 400 1 fiber glass tanks, using a series of four car- 

 tridge filters culminating in a Gelman 0.45 /nm 

 membrane filter, and then enriched to the level of 

 Guillard's F/2 (Guillard and Ryther 1962). 



Large volumes (up to 2,500 1) of culture were 

 raised for each experiment, and it was necessary 

 to concentrate the cells before feeding them to the 

 fish. Since the duration of the feeding period was 7 

 h, the culture was divided into seven batches of 

 equal volume. Each batch was concentrated by 

 gentle back filtration into a volume of 18 1, which 

 was then subsampled by filtering onto a precom- 

 busted glass fiber filter for determination of the C 

 and N concentration (Hewlett Packard Model 

 185B CHN Analyzer). On several occasions addi- 

 tional subsamples were centrifuged to form a pel- 

 let, from which the water was aspirated off, and 

 the C, N, ash (combustion at 475°C for 4 h), caloric 

 (Parr adiabatic bomb calorimeter), and Si (Durbin 

 1977) contents were determined. Each 18 1 batch of 

 food was slowly siphoned into the tank over a 1-h 

 period, to provide an approximately constant rate 

 of input of food. By changing the concentration of 

 plankton in these batches, different concentra- 

 tions of food in the tank and different ration sizes 

 could be obtained. The volume of water added with 

 the food was balanced by the volume removed dur- 

 ing sampling, and thus the volume in the tank 

 (1,400 1) remained approximately constant. The 

 chlorophyll a concentration in the tank was also 

 periodically determined by fluorometry (Yentsch 

 and Menzel 1963; Lorenzen 1966). 



Turbulence produced by the fish stirred the tank 

 and kept the plankton in suspension. The Atlantic 

 menhaden were estimated to filter 13-20% of the 

 tank volume per minute, removing the D. 



878 



