FOOD-HABITS OF DIFFERENT PHENOTYPES OF THREESPINE STICKLEBACK IN PAXTON LAKE, BRITISH-COLUMBIA

We investigated relationships between the diet and morphology of what appeared to be two undescribed species and intermediate morphs of the threespine stickleback Gasterosteus aculeatus complex living in a clos ed-basin, coastal lake in British Columbia during 1969 and 1970. One ' 'species'' was without a pelvic girdle and lateral plates and the othe r had a complete pelvic girdle, pelvic spines, and five or more latera l plates per side. For convenience, the former species was referred to as ''benthic'' because of its association with the lake bottom and th e other as ''limnetic'' because of its pelagic distribution. The two s pecies and intermediate morphs were divided into four groups based on girdle condition (none, partial, complete with incomplete pelvic spine development, and complete with long pelvic spines). Each group was di vided into four subgroups based on lateral plate number (0, 1-2, 3-4, and 5 or more) on one side. Plateless morphs of each group fed mostly on bottom prey. Consumption of pelagic prey was associated with an inc rease in plate number for all groups in 1969. For a given plate number , however, the percentage of pelagic food in the diets was higher as g irdle condition increased. In 1970, when there was intense predation o n the threespine sticklebacks by introduced coho salmon Oncorhynchus k isutch only individuals in subgroups with complete girdles continued t he trend of greater consumption of pelagic prey with an increase in pl ate number. Although the sample sizes were very small for some subgrou ps, the results suggested that higher numbers of lateral plates, in co mbination with girdle condition, were associated with pelagic feeding behavior of all groups in absence of coho salmon predation in 1969. In 1970, the pelvic girdle and lateral plate armor were important in mai ntaining pelagic feeding behavior in the presence of coho salmon preda tion.