MANZER: DISTRIBUTION AND FOOD OF STICKLEBACK 



these, 12 sets failed to catch stickleback. Most of 

 the sets which failed to catch stickleback (21 of 33) 

 were made in February and November. Arithme- 

 tic and geometric means of the numbers caught in 

 each survey are also provided. The latter are 

 included because of the skewness of the catch data 

 and were obtained from log (» + 1) transforma- 

 tion of the data where )i is the stickleback count in 

 each set. This transformation permitted utiliza- 

 tion of zero catches in the computations: in all 

 likelihood during the spring to fall months at least 

 one stickleback would have been caught had 

 fishing been repeated. 



The distribution and relative abundance of 

 stickleback and size composition of the catches 

 according to small (<30 mm), medium (30-59 mm), 

 and large (60+ mm) stickleback are illustrated in 

 Figure 2. (The size-groups were arbitrarily chosen 

 but in general approximate age-groups: <30 mm = 

 age; 30-59 = lyr old; 60-1- mm = 2 yr and older.) 

 Gear efficiency was assumed to be reasonably 

 constant, although a few sets were made under 

 conditions of relatively strong wind and current. 

 It was further assumed that after spawning (July 

 and later) stickleback were catchable regardless of 

 size. Abundance levels just prior to spawning may 

 have been higher than catches indicate because of 

 the decreased vulnerability of mature individuals, 

 especially males which repair to nesting areas. 



Some differences in survey dates, especially in 

 the early part of the year, and some changes in the 

 sampling sites in the 2 yr prevent a strict time and 

 place comparison of the data. Nevertheless some 

 general conclusions on distribution and relative 

 abundance can be made from Table 1 and Figure 2. 

 Seasonally, stickleback appeared along shore prior 

 to mid-April. Their abundance at this time was low 

 and appeared to vary between locations. Most 

 stickleback in almost all localities ranged in length 

 between 30 and 59 mm. A few larger individuals 

 were caught but none smaller. In both years it was 

 obvious that in all areas stickleback progressively 

 increased in numbers, from July through October, 

 although apparently they were less abundant off 

 rock slopes than on gentle sloping beaches covered 

 by either gravel or boulders. This increase is due to 

 the recruitment of fish of the year as evidenced by 

 the large proportion of fish less than 30 mm in July 

 and August. The average seasonal catch was 

 largest in October and consisted of stickleback 

 measuring between 80 and 59 mm long. Fish 

 belonging to the small and large size groups also 

 were present in considerable numbers, and in 



some areas small fish predominated (for example, 

 the central part of the north shore). The small or 

 zero catches made in November suggest that 

 stickleback prior to winter had abandoned the 

 shore areas. 



Observations on diel size variation in stick- 

 leback along the shore were made in conjunction 

 with diel feeding habits, which are described in a 

 later section. Paired samples taken 100 m and 15 

 min apart were collected at station 1 at 3-h inter- 

 vals between 0700 and 1900 h in October 1970 and 

 through the 24-h cycle in July 1971. Diel size 

 changes observed during each series are illustrat- 

 ed in Figure 3 using the graphic method developed 

 by Dice and Leraas (Simpson and Roe 1939). At 

 each site and date the size of stickleback decreased 

 from dawn to midday and then increased again by 

 dusk, suggesting that the large fish are less 

 available in the littoral area during the day. This 

 trend is most clearly shown by fish in July at site B. 

 Here, stickleback at midday are significantly 

 smaller than at either dawn or dusk. 



Stickleback virtually abandon the shore areas by 

 November, but their presence in numbers in the 

 pelagic zone of the lake during the winter could 

 not be established. Limited purse seining (four 

 sets) in February in the pelagic zone of the eastern 

 part of the lake failed to yield any stickleback. 

 Mid-water trawling in March, along transverse 

 and longitudinal axes of the lake over a lineal 

 distance of 22 km and at depths ranging from 10 to 

 100 m in the eastern half of the lake, resulted in 

 the capture of one stickleback; ice cover prevented 

 trawling in the western half of the lake. This 

 stickleback measured 37 mm long and could have 

 been caught at some depth down to 50 m. From the 

 results of these fishing operations stickleback 

 apparently either leave the lake or retreat to areas 

 where they cannot be caught for the winter 

 months, becoming available again between Feb- 

 ruary and April. 



Reliable estimates of the size of the stickleback 

 population could not be made from the available 

 catch data. Within any survey, catches varied 

 widely between locations. In addition, local var- 

 iance in the catches is not precisely known, al- 

 though judging from a few instances when two 

 sets were made in the same location the numbers 

 caught can vary greatly. The catch data are 

 considered more informative for the period 

 beginning in July when coverage was more com- 

 plete and stickleback availability increased. As- 

 suming that factors contributing to variability in 



651 



