Nerka at parent population densities approach- 

 ing 7,000 spawners per square kilometer of 

 nursery area (fig. 4) suggests decreased spawn- 

 ing efficiency or increased mortality of young at 

 high population densities. There is some evidence 

 of both. 



The variation in the relations of abundance of 

 juveniles to number of parent spawners per unit 

 lake area in lakes of the Wood system is probably 

 due to a combination of several factors : (1) er- 

 rors in estimating numbers of spawners, (2) 

 changes in survival from egg to juveniles avail- 

 able to the tow nets, and (3) random error in 

 tow net sampling. Catches with the tow net 

 indicate the same general abundance of juvenile 

 salmon as do echo soundings recorded at the time 

 of towing. 



Evidence that the capacity of the nursery area 

 may control the growth of fish in the Wood sys- 

 tem is provided by data on growth of age sock- 

 eye salmon in each of the four main Wood system 

 lakes in 1958-62. The mean length of age juve- 

 niles on September 1 is plotted against numbers 

 of parent spawners per unit lake area for each 

 lake (fig. 5). The inverse relation indicates that 



2,000 4,000 6,000 



^ PARENT SPAWNERS PER SQUARE KILOMETER 



OF SURFACE (NUMBER) 



Figure 5. — Relation between mean fork length of age 

 sockeye salmon captured in tow nets, 1958-62, and 

 abundance of parent spawners in lakes of the Wood 

 system. The weighted mean fork length of the age 

 fish in each sample was adjusted to September 1 by 

 applying a mean growth of 0.3.3 mm. per day, com- 

 puted from growth data. The fish were measured after 

 at least 24 hours in 10 percent Formalin. '° 



'■■' Trade names referred to in this publication do not imply endorse- 

 ment of commercial products. 



the progeny of large spawning populations grow 

 slowly. Although the trend of the relation is 

 obvious, the true shape of the line is probably 

 obscured by five factors: (1) minor differences 

 in growth potential (factors such as food avail- 

 ability and temperature) between lakes, (2) 

 competition with variable numbers of yearling 

 sockeye salmon from the previous year, (3) 

 variability in the abundance of competitor spe- 

 cies, (4) annual differences in climate that could 

 affect the time of emergence, the length of grow- 

 ing season, and food supply, and (5) inaccura- 

 cies in estimates of numbers of spawners. 



Differences are frequently observed in size of 

 juvenile sockeye salmon between various parts 

 of nursery lakes. These differences may be due to 

 many factors including (1) real differences in 

 growth rates, (2) differences in length of time 

 the young have been in the lake, (3) differences 

 in size of fry at hatching, and (4) differential 

 rates of dispersion of faster and slower growing 

 individuals. We have, therefore, used the term 

 "apparent growth rate" in discussions of differ- 

 ences in size of juvenile sockeye salmon. 



The apparent growth of young sockeye salmon 

 in each of the three single-basin lakes in the 

 Wood system is greatest in the east third and 

 least in the west third of each lake (table 19). 

 This relation holds for years of high and low 

 population densities. Although the difference in 

 apparent growth suggests basic differences in 

 the growth potential of lake areas, progeny are 

 smaller in the end of the lake that is closer to the 

 major spawning grounds ; and the differences in 

 size within each lake may be the combined result 

 of the longer lake residence of the early emerg- 

 ing fry and a more rapid dispersion of the larger 

 fish. 



Table li). — Mea7i fork lengths^ of age sockeye salmon on 

 September 1^ in three single-basin lakes of the Wood system, 

 1958-62 



Lake Aleknagik Lake Be verley Lake Kulik 



Year West Middle East West Middle East West Middle East 



third third third third third th ird third third third 



Mm. Mm. Mm. Mm. Mm. Mm. Mm. Mm. Mm. 



1958 59.0 59.0 63 4 59 60 58.8 59 8 60.0 



1959 57.5 62.7 60 8 50 5 57.3 61.2 61.5 60.6 58.8 



1960 53.3 53 7 54 3 42 44.5 46 4 50.3 61.7 52.3 



1961 52.2 57.6 57.5 51.1 52.2 59,2 51.7 52.8 65.1 



1962 47.8 54.2 55.2 58.6 58.9 67.3 57.6 59.5 60.1 



Mean 54.0 57.4 58.2 52.2 54 6 59.8 56 56,9 57 3 



' After at least 24 hours in 10 percent Formalin. 



• Lengths were adjusted to September 1 by applying a mean growth 

 of 0.33 mm. per day computed from growth data. 



SOCKEYE SALMON IN MAJOR RIVER SYSTEMS IN SOUTHWESTERN ALASKA 



431 



