FISHERY BULLETIN: VOL. 74, NO. 3 



Table 7.-0ccurrence (percent) of different organisms in the diet 

 of threespine sticklel)ack of four size groups- <30 mm group 

 contained 100 fish; 30-49 mm, 34 fish; 50-69 mm, 60 fish; and 70-1- 

 mm, 11 fish. Based on samples taken on 22 July and 5 August 

 1970. 



Table 8.- Relative importance (percent) of food organisms of 

 different bulk units in the diet of threespine stickleback of four 

 size groups- <30 mm group contained 100 fish; 30-49 mm, 34 fish; 

 50-69 mm, 60 fish; and 70-1- mm, 11 fish. Based on samples taken 

 on 22 July and 5 August 1970. 



larvae were, the largest, a cottid, measuring 14 

 mm. It is reasonably clear that a positive rela- 

 tionship exists between food size and stickleback 

 size. This relationship is also apparent when for 

 each stickleback size-group the different food 

 organisms, especially the common items (namely, 

 Bosmina, Holopedium, and Epischura), are ex- 

 pressed as a percent of the total stomach content 

 for that group (Table 8). 



Diet in Relation to Sexual Maturity 



Mature males showed a higher incidence of 

 feeding (90%) than did gravid females (61%) 

 (Table 9), the difference being statistically sig- 

 nificant, (x" = 13.811, n = 2,P= <0.01). 



Nongravid females, gravid females, and mature 

 males fed on a variety of similar kinds of organ- 

 isms (Table 9) and, except for Epischufa, none of 

 the items were of great importance as food. Since 

 Epischura is the largest planktonic form, its 

 predominance in the diet of large individuals is not 

 unexpected. Epischura formed more than 90% of 

 the bulk units and the mean number ingested was 

 very much higher than for any other single item. 

 In contrast to 54% of gravid females which had 

 eaten this item, its occurrence in nongravid 

 females and in males was considerably less, 18 and 

 16%, respectively. Planktonic crustaceans, insects. 



Rotifera 92T ___ 10 4T 

 Cladocera: 



Holopedium 23 6 1 32 75 7 17 13 1 



Bosmina — — — 4TT 61T 



Alona 51T ___ 22T 

 Copepoda: 



Epischura 18 141 94 54 268 92 16 230 94 



Diaptomus — — — — — — ITT 



Cyclops 14 3 T 11 T T 22 11 T 



Harpacticoid 5TT — — — 4TT 



Copepodids 52T — — — 91T 

 Insecta: 



Chironomidae L 27 4 1 4 T T 17 1 T 



Chironomidae P 18 T 1 11 T T 15 2 3 



Coleoptera 5TT — — — ITT 



Ceratopogonidae 14 1 2 — — — 11 T T 



Other 18 T T 14 T T 15 T T 



Araneida — — — — — — ITT 



Acari ___ ___ 9TT 



Ostracoda 5TT ___ 5TT 



Pelecypoda — — — — — — 15 TT 



Isopoda — — — — — — ITT 



Amphipoda 5TT 4TT 7TT 

 Eggs: 



Zooplankton 9 5 T 14 13 T 19 8 T 



Stickleback ___ 4TT 92T 



Detritus — — — 1— — 13 — — 



'Percentage of stomachs vi^ith item. 



2f^ean number of items per stomach examined. 



'Item = percent of total bulk units. T = Trace = < 1 organism 



or <1%. 



eggs of zooplankton and stickleback, and other 

 miscellaneous taxonomic groups, some of which 

 are littoral in habitat, made up most of the 

 remainder of the stomach contents. Males ate 

 more benthic and epibenthic forms, as well as 

 detritus (mainly sand and twigs), than did 

 females. Detritus in individual male stomachs 

 made up from 10 to 100% of the contents and was 



660 



