THREESPINE STICKLEBACK OF KODIAK ISLAND 



545 



30 



Bare Lake 



Karluk Lake 



MAY 

 30 



JUNE JUNE 

 10 20 



JUNE 

 30 



JULY JULY 

 10 20 



1956 



JULY 

 30 



AUG. 

 10 



AUG. 

 20 



AUG. 

 30 



Figure 4. — Water temperatures, surface, summer of 1956. Upper curve, Bare Lake. Lower curve, Karluk Lake. 



Curves smoothed slightly. 



or those preserved in .alcohol, can be used, as 

 formalin destroys the otoliths in only a few 

 weeks. 



The standard length in millimeters, and the 

 sex and state of maturity are recorded for each 

 fish. Then, working under a low-power micro- 

 scope, the top of the skull is sliced off, and the 

 otoliths are extracted by sharp-pointed forceps. 

 Only the sagitta, the largest of the three otoliths 

 to be found in each side of the skull, is used. 

 Both sagittae from one fish are treated and 

 mounted together. 



The otoliths (sagittae) are placed in 65 per- 

 cent alcohol, where they are cleaned of bits of 

 tissue. Then they are put into pure creosote for 

 from 5 to 15 minutes for clearing. Finally, they 

 are mounted in Canada balsam or some other 

 clear mounting fluid. They are read under a 

 magnification of about 40 diameters, using a 

 strong light reflected from the surface of the 

 otolith, with a black mat background. 



For a description of the appearance of the 

 otolith and an interpretation of its markings, we 

 quote Jones and Hynes. 



The sagitta first appears when the fish is about 6 mm. 

 long, i. e., a few days after its emergence from the nest ; 

 the otolith is then a small body either uniformly trans- 

 parent or more or less opaque. By the end of June the 

 center is usually completely formed. Otoliths in this 

 state or in an earlier state of development were desig- 



nated S— . During June or early July the first trans- 

 parent (S) ring appears, followed by the opaque zone 

 (_|-) which is visible in some fish in July and in all by 

 September. At first the opaque zone is narrow, but it 

 rapidly increases in width during the summer. From 

 September until the following June the otoliths of all fish 

 in their first year read S+, i. e., they consist of center, 

 one transparent ring and an opaque zone. Thereafter 

 the S ring becomes evident in July, not June or July as 

 in young fish : This may be due to the fact that it is not 

 so easy to see the S ring on the edge of the larger 

 otoliths. The opaque zone ( + ) begins to be visible in 

 August or September, and is present in all fish by October. 



This delineation appears to hold true for the 

 Kodiak Island sticklebacks with minor varia- 

 tions. In the otoliths from the Bare Lake and 

 Karluk Lake fish, the transparent rings appar- 

 ently form during a rather short period in June 

 and early July. In about early August, the ma- 

 terial which gives cloudiness starts to be deposited, 

 Seemingly (although it is difficult to be certain), 

 this material accumulates to some extent in the 

 outer portion of the previously clear zone, as well 

 as in a newly formed zone. Thus the result is 

 that the apparent clear bands are narrower than 

 the cloudy bands. 



At any rate, in age determination each opaque 

 ring (outside of the centrum and first transparent 

 ring) is read as one winter (although the opaque 

 band actually starts to form in late summer or 

 early fall). For example, the otolith of a fish 



