Irruptions of gizzard shad, Dorosoma cepedianum (Le Sueur), in North Twin Lake, Iowa in 1954 and 1955 provided opportunities for investigating certain basic ecological relationships of this species. Of the approximately 358 plankton species and varieties found in North Twin Lake, 260 were encountered in shad digestive tracts. On a volume basis, phytoplankton (primarily Cyanophyta and Chlorophyta) constituted the bulk of young-of-the-year and yearling gizzard shad food. Crustacean zooplankton were important food elements of recently hatched individuals. Rotifers and crustaceans were about equally abundant in the plankton and in gizzard shad digestive tracts, and a fair degree of relationship was found between the amounts of green algae and diatoms ingested by age 0 shad and the proportions of the same forms in the plankton. Yearling gizzard shad apparently select green algae. Blue-green algae were apparently not consumed in proportion to their relative abundance in the environment. Where the relatively scarce Microcystis aeruginosa appeared to be a selected item, the most abundant plankter, Aphanocapsa delicatissima, constituted comparatively small portions of gizzard shad foregut contents. Gizzard shad are capable of consuming relatively minute plankters, some commonly ingested forms being no larger than 20 microns in greatest dimension. Circumstantial evidence suggests that large shad populations may play a significant role in suppressing irruptions of objectionable algae. Where hatching periods overlap, feeding activities of newly hatched shad may adversely affect survival of the young game species through direct competition for crustacean plankton which both groups seem to require during early growth stages.
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J.H. Kutkuhn (1958) studied this question.