EFFECT OF CROWDING ON RELATION BETWEEN EXPLOITATION 

 AND YIELD IN TIL API A MACROCEPHALA 



Ralph P. Silliman^ 



ABSTRACT 



An experiment was performed to assess the effect of crowding on yield in Tilapia ma- 

 crocephala. Populations of nearly equal number and weight were started in control 

 (155.2 liter) and test (77.6 liter) tanks. Food amounts and environmental attributes 

 other than space were the same for both tanks. Each of the two populations was ex- 

 ploited — first at a bimonthly rate of 10% for 14 months, then at a bimonthly rate of 25% 

 for 10 months. 



Equilibrium yields at each rate for each tank (four points) were fitted with a Fox 

 exponential surplus-yield model. Deviations from this general population curve showed 

 that yields were greater for the larger tank at the 25% rate and greater for the smaller 

 tank at the 10%^ rate. This is believed to result from the fact that the entire yield came 

 from growth at the lower rate whereas part of the yield came from recruitment at the 

 higher rate. A low rate of conversion of food to fish (18%) is believed to be due to the 

 large proportion of liver in the diet. 



Current interest in aquaculture suggests an im- 

 minent increase in the holding of fish in restrict- 

 ed enclosures. Such holding, particularly at high 

 densities, provides the fish with a drastically al- 

 tered environment as compared with their native 

 habitat. Some of the effects are undesirable, as 

 is only too well known to hatchery men. Meta- 

 bolic wastes accumulate rapidly and some com- 

 municable diseases spread easily among enclosed 

 populations. There may also be undesirable ef- 

 fects due to reduced area for spawning — for 

 example, spawning fish that are close together 

 may use more energy defending their territory 

 against the intrusion of other fish than spawners 

 that are farther apart. Effects of crowding can 

 also be beneficial, such as increased growth rate 

 with less expenditure of energy in swimming. 



The purpose of the experiment described here 

 was to investigate one class of results from 

 crowding — those related to the yield in self-sus- 

 taining populations. Of many possible exper- 



^ National Marine Fisheries Service, Northwest Fish- 

 eries Center, 2725 Montlake Boulevard East, Seattle, WA 

 98112. 



Manuscript accepted November 1971. 



FISHERY BULLETIN: VOL. 70, NO. 3, 1972. 



imental animals, Tilapia ynacrocephala belongs 

 to a genus which is already important in pond 

 culture. This species is small enough to raise 

 in laboratory tanks and has reasonably rapid re- 

 production and growth. A conventional control- 

 test design was adopted in which the control pop- 

 ulation was in a tank that had been demon- 

 strated as being of suitable size for T. macro- 

 cephala. The test population was in a tank ex- 

 actly one-half the size of the control tank. Food, 

 light, temperature, and initial populations were 

 kept as nearly identical as possible. 



MATERIALS AND METHODS 



Two conventional glass-wall aquarium tanks 

 were used. The water mass in the control (larg- 

 er) tank (L) had a volume of 155.2 liters, with 

 approximate dimensions 39 by 90 by 44 cm. 

 Volume of the test (smaller) tank (S) was one- 

 half that of L, or 77.6 liters. To keep propor- 

 tions the same, linear dimensions of S and its 

 equipment were 1/2^''^ those of L. 



693 



