THREESPINE STICKLEBACK OF KODIAK ISLAND 



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STANDARD LENGTH (CLASS INTERVALS) 



Figure 6. — Length-frequency histograms, sticklebacks 

 from Bare Lake, 1954. Each diagram represents a 

 sampling of 100-400 fish on a given date. Vertical 

 ordinate, percent of total sample. Horizontal ordinate, 

 standard length group. 



placed the lifespan of the species at from 1 to 4 

 years. Some have said that the fish breeds only 

 once, others that it may breed in each of 2 or 3 

 successive years. No doubt there is considerable 

 variation, which is due to differences in race or 

 strain or to environmental differences. 



In the Birket River sticklebacks (Gasterosteus 

 aculeatus), Jones and Hynes found the maximum 

 lifespan to be 3 years plus. The fish at the end 

 of the first summer of life (i. e., when a few 

 months old) averaged about 24 mm., standard 

 length. At the end of the second summer, the 

 average was about 34 mm., and at the end of the 

 third summer about 40 mm. Most of the growth 

 occurred during the summer. Maximum size 

 achieved was about 65 mm. 



Assuming that our interpretation of the oto- 

 liths is correct, the sticklebacks in Karluk and 

 Bare Lakes have a lifespan of about two and one- 

 fourth years. Most of the 2-year-old fish die 

 shortly after spawning, as indicated by the suc- 

 cessively smaller numbers of large fish in the sam- 

 ples as the summer progresses. A very few oto- 

 liths appeared to show an extra winter ring. It 

 may be that an occasional fish survives the third 

 winter. 



In the samples from Bare Lake in the summer 

 of 1954 (fig. 6), at the time of the first sampling 

 on May 23, very few young-of-the-year fish were 

 present. The frequency mode at about 35 mm. 

 apparently represented 1-year-old fish, the 2-year- 

 old fish being much fewer and showing a broad 

 frequency mode at about 45 mm. 



In the next sample, taken on June 7, the small 

 fish predominate, with a mode at about 25 mm. 

 The fish of the other two size groups (age 

 classes) have fallen off in comparative numbers. 

 Successive samples throughout the remainder of 

 the summer show: (1) a fairly rapid increase 

 in size of the fish of the year; (2) a somewhat 

 slower but steady growth of the 1 -year-old and 

 2-year-old fish; and (3) a reduction in numbers 

 of the older fish, especially of the 2-year-old 

 group, probably caused by their death and dis- 

 appearance from the population. 



The same picture holds good for the summers 

 of 1951 and 1953, with certain variations, prob- 

 ably caused in part by differences in spawning 

 dates and hence in the growth achieved in the 

 first summer. In the 1953 samples the 1-year-old 

 group appeared in weak numbers throughout 

 the summer, possibly indicating poor survival 

 through the preceding winter. 



In Karluk Lake in 1949 (fig. 7), the first sam- 

 ple of the summer was taken on June 23. The 

 young-of-the-year group was almost or entirely 



