INTERACTION OF FOOD LEVEL AND EXPLOITATION IN EXPERIMENTAL 



FISH POPULATIONS 



By Ralph P. Silliman, Fishery Biologist, Bureau of Commercial Fisheries Biological Laboratory, 



Seattle, Wash. 98102 



ABSTRACT 



Nine populations of guppies (Lebistes reticulatus) 

 were establislied in separate aquariums. Food supply 

 was constant for groups of three populations in ratios of 

 0.5, 1.0, and 1.5 to a "standard" diet. Temperature, 

 light, and space were constant and the same for all 

 populations, .\fter 28 weeks, populations had reached 

 near-asymptotic levels, and mean numbers and weights 

 for each group of three were in the same rank as their 

 food levels. 



Twenty-five percent, 33 percent, and 50 percent of the 

 fish were removed per 3-week brood interval for each 

 food-level group of three populations, thus providing 



nine combinations. Continuation of exploitation at 

 these rates led to relatively stable yields during weeks 

 59 to 72, after initial declines due to readjustment of 

 populations. Yield curves for each food level revealed 

 relation of yield to exploitation rate and biomass to be 

 independent of amount of food consumed. Maximum 

 yields occurred near the 0.33 (33 percent) exploitation 

 rate for all food levels and represented about 25 percent 

 of the food consumed. Results suggest that if commer- 

 cially fished populations behave as the experimental 

 ones did, management strategies may be applied 

 independently of amount of food organisms available. 



The ])urposes of laboratory fish-population e.\- 

 l)ermients and their relation to other work in 

 fishery dynamics have been set forth rather fully 

 by Sillunan (1948) and Silliman and Outsell 

 (1958). Briefly, the purposes are to provide ex- 

 perimental measurement of the eft'eet of exploi- 

 tation on stocks of fish, under as fully controlled 

 environmental conditions as possible. The above 

 authors also pointed out the advantages of the 

 o'uppy (Lebistes reticulatus) as an experimental 

 animal: rapid gi'owth and reproductive rates, 

 small size, and hardiness. 



Food supply and exploitation rate must l)e 

 among the most important factors that determine 

 biomass and yield in exi)loited fish populations. 

 The resjjonse of poi)ulations to exploitation is 

 \\ ell known, as set forth in such works as Beverton 

 and Holt (1957). The importance of food supply, 

 although not as fully dociuuented, is well recog- 

 nized. For example, Zheltenko\'a (1961) adduced 

 data indicating that a decreased supi)ly of food 

 reduced the rate of growth and catches of bream 



Published February 19G«. 



FlSHf:RY ISt-J.LETIX : VOLUME 6. XO. 



in the Sea of Azov. She also reported a number 

 of qualitative examples in another work (1958) 

 that, although lacking numerical estimates of 

 food amounts, tended to support the thesis that 

 food supply is important in determining yield 

 and rate of growth of several fishes in the U.S.S.R. 

 These examples indicate not only the importance 

 of food level at any given time but also the im- 

 portance of the great fluctuations in this le\el 

 that occur from one time to another. 



Quantitative supjjort for the idea that fluctua- 

 tions in food supply would modify fluctuations 

 in fish stocks resulting from other causes was 

 provided early by Jensen (1928). His data on 

 measm-ed amounts of bottom food in certain 

 Danish waters in the fall were significantly cor- 

 related with catches of plaice. 



Because yield is related to both food supply 

 and rate of exploitation, the interaction of these 

 two is of obvious interest to the fishery manager. 

 Might it be possible, for instance, to harvest a 

 greater percentage of the stock when food supply 

 and abundance are higli than when they are low? 

 The experhuents described in this report were 



425 



