MANZER: DISTRIBUTION AND FOOD OF STICKLEBACK 



RESULTS 



Feeding Relationships 

 Seasonal Variations in Diet 



Data on size and stomach contents of stick- 

 leback examined in 1970 and 1971 are summarized 

 by survey in Tables 5 and 6. The predominant 

 features regarding seasonal change in diet are 

 depicted in Figure 4. Observations for 1970, except 

 for August when almost all stations were sampled, 

 are based mainly on samples taken from the 

 eastern part of the lake. Observations for 1971 are 

 based on samples from most of the key sampling 

 stations except in November when fishing was 

 confined to the eastern end of the lake. 



Although the numbers of stickleback examined 

 differed by survey, a similar seasonal trend in the 

 proportion of fish with empty stomachs was ob- 

 served for the 2 yr: low in the spring and early 

 summer, highest in midsummer, and again low in 

 the fall. The mean weight of stomach contents 



fluctuated in each year but generally was higher in 

 the spring and early summer. The higher mean 

 values in the early part of the year are probably 

 related to fish size. On the average, stickleback 

 were larger in the spring and early summer than 

 in the late summer and fall. The relatively high 

 proportion of fish with empty stomachs in mid- 

 season can be explained by feeding behavioral 

 diff"erences associated with sexual maturity. 



In each of the 2 yr stickleback had a wide but 

 rather similar diet. They predominately fed on five 

 species of organisms: two cladocerans 

 {Holopedium gibberum, Bosmina coregoni), two 

 copepods {Epischura nevadensis, Diaptomus 

 oregonensis), and a cyclopoid copepod {Cifclops 

 biciispidatus). Larvae and pupae of the family 

 Chironomidae were also of some importance. The 

 distinction between zooplankton eggs and fish 

 eggs in 1971 represents a qualitative refinement in 

 analysis of the data, rather than any difference in 

 diet. Other kinds of organisms consumed at var- 

 ious times but of minor importance were harpac- 

 ticoid copepods, insects, pelecypods, ostracods. 



Table 6.-Seasonal change in the diet of threespine stickleback in Great Central Lake, 1971. 



Date 12, 20 May 10, 17 June 9 July 10 Aug. 14 Oct. 30 Nov. 



No. fish examined 74 110 130 105 99 26 



% empty 2.7 4.5 18.5 19.1 8.1 3.8 



Size range (mm) 29-86 33-82 15-86 14-80 23-77 24-78 



Mean size (mm) 54 54 58 33 38 34 



Mean content wt (mg) 26.1 45.3 28.0 16.5 18.6 13.9 



Organism 1122 3^1 2 3 123 123 1 23 123 



Rotifer _ _ _ 10 1 T 40 15 T 44 27 T 74 35 T 31 12 T 



Cladocera: 



Holopedium 1 T T 34 14 1 59 47 16 57 22 28 89 163 67 65 20 35 



Bosmina 47 10 2 37 2 T 19 T T 40 10 4 89 85 12 69 43 25 



Daphnia — — — — — — — — — — — — 19 1 T — — — 



Alona 5 T T 4 1 T 2 3 T 54 17 14 20 1 T 23 1 1 



Copepoda: 



Epischura 1 7 19 56 286 95 50 55 68 6 6 28 19 1 1 — — — 



Diaptomus — — — 10 2 1— — — 5 2 2 71 28 8 54 10 12 



Cyclops 40 75 19 49 68 2 35 4 T 4 T T 65 20 3 42 6 3 



Copepodids 40 13 3 23 8 T 40 12 1 6 T T 48 52 7 50 38 22 



Nauplii 32 22T— — — 13 6T— — — 28 2 1— — — 



Harpactlcoid 10 1T9 13T3TT8TT1 TT4TT 



Insecta: 



Chironomid larvae 19 IT 6 TT9TT11TT 1 TT4T — 



Chironomid pupae 23 3 38 11 TT 7 1 6 2TT — — — — — — 



Other 12 1 13 9 TT 16 16 51 21 3 TT4TT 



Eggs - zooplankton 5 1 T 19 3 T 39 22 2 1 T T 54 15 2 31 3 2 



Fish ITT 3 TT 4TT ITT — — — — — — 



Other: 



Amphipoda 4TT 2 TT 2TT— — — — — — 4TT 



Pelecypoda 8 1 2 3 TT TTT — — — — — — — — — 



Ostracoda — — — 2 TT4TT17 1T — — — 8TT 



Acari 8TT4 TT— TT22 12 — — — — — — 



Aranelda 1 T T — — — — — — — — — — — — — — — 



Fish ____________ 1 TT— — — 



Coleoptera 2 T T — — — — — — — — — — — — — — — 



Ceratopogonidae 11 T T 3 T T — — — — — — — — — — — — 



Isopoda 1 T T — — — — — — — — — — — — — — — 



Unidentifiable % 51 47 38 24 36 34 



'% stomachs with item. 



^Mean no. items per stomach examined. 



3|tem = % of total bulk units. T = Trace =<1 organism or <1%. 



657 



