Literatured Cited 



DeLacy, a. C, B. S. Miller, and S. F. Borton. 



1972. Checklist of Puget Sound fishes. Univ. Wash. Sea 

 Grant Publ. 72-3, 43 p. 



Grinols, R. B. 



1965. Check-list of the offshore marine fishes occurring in 

 the northeastern Pacific Ocean, principally off the coasts 

 of British Columbia, Washington, and Oregon. M.S. 

 Thesis, Univ. Washington, Seattle, 217 p. 



Hart, J. L. 



1973. Pacific fishes of Canada. Fish. Res. Board Can., Bull. 

 180, 740 p. 



HuBBS, C. L., and K. F. Lagler. 



1958. Fishes of the Greak Lakes region. Revised ed. 



Cranbrook Inst. Sci., Bull. 26, 213 p. 

 Makushok, V. M. 



1958. The morphology and classification of the northern 



blennioid fishes (Stichaeoidae, Blennioidei, Pisces). Proc. 



Zool. Inst., Akad. Nauk SSSR Tr. Zool. Inst. 25:3- 



129. (Translated by A. R. Gosline, Ichthyol. Lab., U.S. 



Fish Wildl. Serv., 1959, 59 p.; U.S. Natl. Mus., Wash., D.C.) 

 Pearcy, W. G., and S. S. Myers. 



1974. Larval fishes of Yaquina Bay, Oregon: A nursery 

 ground for marine fishes? Fish Bull., U.S. 72:201-213. 



QuAST, J. C, AND E. L. Hall. 



1972. List of fishes of Alaska and adjacent waters with a 

 guide to some of their literature. U.S. Dep. Commer., 

 NOAA Tech. Rep. NMFS SSRF-658, 47 p. 



Richardson, S. L. 



1973. Abundance and distribution of larval fishes in waters 

 off Oregon, May-October 1969, with special emphasis on 

 the northern anchovy, Engraulis mordax. Fish. Bull., 

 U.S. 71:697-711. 



Walker, E. T. 



1953. Records of uncommon fishes from Puget Sound. 

 Copeia 1953:239. 



Sally L. Richardson 

 Douglas A. DeHart 



School of Oceanography 

 Oregon State University 

 Carvallis, OR 97331 



EFFECT OF CROWDING ON STOCK AND 

 CATCH IN TILAPIA MOSSAMBICA 



In a previous report (Silliman 1972) I described the 

 effect of crowding on the relation between 

 exploitation and yield in Tilapia macrocephala. 

 Subsequently I performed a similar experiment 

 with T. mossambica. Since the results were 

 somewhat different for the latter species and 

 because of its wide use in pond culture, a brief 

 report of the second experiment seems justifiable. 



Apparatus and Procedures 

 Most of the procedures and apparatus were 



identical with those reported by Silliman (1972). 

 Essentially the approach was to raise the popula- 

 tions in two conventional aquariums, one (L) with 

 a volume of 155.2 liters and the other (S) with 77.6 

 liters so that S had exactly one-half the capacity of 

 L. Aeration was by airstones and illumination by 

 overhead fluorescent lamps. Rectangular spaces at 

 the ends of the aquariums were fenced off with 

 rods placed 3 mm apart, providing refuges for the 

 young. Further shelter was provided by floats with 

 suspended cords and by fiber brush shelters. 

 Covering part of the aquarium walls with black 

 plastic furnished shaded areas for spawning. 

 Water condition was maintained by filtration and 

 weekly partial water changes. Water temperature 

 was 24° + 2°C to month 5.7 and 30° ± 2°C 

 thereafter. Feeding details are given in Table 1. 



Populations were counted and weighed at 

 approximately 2-mo intervals. Since T. mossam- 

 bica is a mouthbreeder, it was desirable not to 

 handle the fish more often than this. The 2-mo 

 period includes 1.0 to 2.6 of the brood intervals 

 reported by various authors (Kelly 1957, 30-40 

 days; Swingle 1960, 30-40 days; Uchida and King 

 1962, 23-61 days). Exploitation consisted of 

 removing each 10th fish. In weighing, fish were 

 drained in a net and placed in a previously 

 weighed container of water; fish weight was total 

 weight less the tare. 



Table 1.— Food (in g) placed in tanks. 



iCommercial makes of dry food. 



2This was combined witti the Friday A.M. feeding in 35 out of 

 131 wk and with the Sunday feeding once. 



Results and Conclusions 



The two populations were started 10 July 1970 

 (Table 2, Figure 1). Recruitment (estimated from 

 counts as in Silliman 1972) occurred after the 

 temperature increase at month 5.7 and readjust- 

 ment of the sex ratios at month 6.9 (Table 2). As 

 was true for T. macrocephala, recruitment was 

 greater in tank L (62) than in tank S (20). Some 



685 



