BARRACLOUGH and ROBINSON: LAKE FERTILIZATION. III. 



ever recorded from the weir were taken, ranging 

 from 90 to 95 mm. In May, 1971 the average 

 length of 1,582 yearling smolts was 71 mm and 

 a weight of 3.7 g. This average length and 

 weight was the same as that of all yearling smolts 

 sampled in 1970. In June, 1971, 3,012 yearlings 

 were sampled and the smolts averaged 66 mm in 

 length and 3.3 g in weight. It has been noted 

 already that the smallest yearling smolt ever 

 caught at Robertson Creek weir was 55 mm. 



In conclusion, from the data presented here 

 it is apparent that the total lake population of 

 young sockeye salmon took advantage of the 

 extra zooplankton ration (mainly Epischura) 

 and that the average weight of individual fish 

 of the in-lake population was 30% larger than 

 in the previous year. However, the mechanism 

 whereby this overall average increase in growth 

 was observed is complex. The explanation of- 

 fered is that the first fry to enter the epilimnion 

 of the lake in April and May could take the 

 greatest advantage of the increased zooplankton 

 standing stock in June, when the average surface 

 temperature was ca. 15°C. The following year 

 (1971) this resulted in the migration of a group 

 of 1-year-old smolts which were the largest 

 (90-95 mm) ever observed from the lake. Fry 

 which entered the lake in June and July could 

 not take the same advantage of the zooplankton 

 standing stock because the surface temperature 

 was >16°C, and this reduced their feeding effi- 

 ciency (Foerster, 1968) . Thus increases in tem- 

 perature of the epilimnion through the long 

 period of fry emergence decreased the apparent 

 benefit to late-hatching fish. However, in spite 

 of this, most of the fry hatching later in the year 

 achieved a length greater than 55 mm and mi- 

 grated from the lake in June 1971; under normal 

 conditions it is believed that these fry would not 

 have reached 55 mm and would have migrated 

 the following year as 2-year-olds. Prelimi- 

 nary examination of the scales from juvenile 

 sockeye in 1971 reveals an absence of a winter 

 check on many scales which suggests that the 

 high concentrations of zooplankton persisting 

 through the winter enabled many fish to smoltify 

 and leave the lake. The combination of an early 

 run of very large 1-year-old smolts, combined 

 with this later run of much smaller smolts, 



tended to reduce the overall apparent effective- 

 ness of lake fertilization. Thus the real effects 

 of fertilization seem likely to be greater than 

 would be judged from considering only changes 

 in overall mean size of smolts. 



LITERATURE CITED 



Bams, R. A. 



1969. Adaptations of sockeye salmon associated 

 with incubation in stream gravels. In Symposium 

 on salmon and trout in streams, p. 71-87. H. R. 

 MacMillan Lectures in Fisheries, Univ. B. C, 

 Inst. Fish., Vancouver, B.C. 

 Brett, J. R., J. E. Shelbourn, and C. T. Shoop. 



1969. Growth rate and body composition of finger- 

 ling sockeye salmon, Oncorhynchus nerka, in re- 

 lation to temperature and ration size. J. Fish. 

 Res. Board Can. 26: 2363-2394. 

 Burgner, R. L. 



1962. Studies of red salmon smolts from the Wood 

 River Lakes, Alaska. Univ. Wash. Publ. Fish., 

 New Ser. 1: 247-314. 

 Foerster, R. E. 



1968. The sockeye salmon, Oncorhynchus nerka. 

 Fish. Res. Board Can. Bull. 162, 422 p. 

 Goldman, C. R. 



1960. Primary productivity and limiting factors 

 in three lakes of the Alaska Peninsula. Ecol. 

 Monogr. 30: 207-230. 



1964. Primary productivity and micro-nutrient lim- 

 iting factors in some North American and New 

 Zealand Lakes. Int. Ver. Theor. Angew. Limnol., 

 Verhandl. 15:365-374. 

 Johnson, W. E. ^ 



1961. Aspects of the ecology of a pelagic, zooplank- 

 ton-eating fish. Verh. int. Ver. Limnol. 14:727- 

 731. 



Krokhin, E. M. 



1959. (On the effect of the number of spawned-out 

 sockeye salmon (Oncorhynchus nerka) in a lake 

 on its supply of biogenic elements.) Dokl. Akad. 

 Nauk SSST 128(3) :626-627. (Fish. Res. Board 

 Can. Transl. Ser. 417.) 



1967. Influence on the intensity of passage of the 

 sockeye salmon (Oncorhynchus nerka Walb.) on 

 the phosphate content of spawning lakes. Izdatel'- 

 stvo "Nauka" Leningrad 15:26-31. (Fish. Res. 

 Board Can. Transl. Ser. 1273.) 

 LeBrasseur, R. J., W. E. Barraclough, 0. D. Kennedy 

 AND T. R. Parsons. 



1969. Production studies in the Strait of Georgia. 

 Part III. Observations on the food of larval and 

 juvenile fish in the Eraser River plume, February 

 to May, 1967. J. Exp. Mar. Biol. Ecol. 3:51-61. 



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