Factors related to gizzard shad and threadfin shad occurrence and abundance in Florida lakes

Gizzard shad Dorosoma cepedianum were collected in 23 and threadfin shad D. petenense were collected in 22 of the 60 Florida lakes sampled. Logistic r egression equations were 94% effective for predicting gizzard shad occurren ce from chlorophyll and lake surface area, and 84% effective for predicting threadfin shad occurrence from lake surface area and lake volume inhabited (PVI). Occurrence of both shad species was related positively to lake size . In lakes where gizzard shad or threadfin shad were collected, shad densit y and biomass of both shad species were related positively to chlorophyll. Gizzard shad populations were generally vulnerable to predation in lakes, w ith the percent of gizzard shad less than or equal to 200mm L-gamma values exceeding 60% with few exceptions. Effects of gizzard shad and threadfin sh ad on fish community dynamics may be confined to relatively large (> 100 ha ) and fertile (chlorophyll > 20-30 mu g 1(-1)) Florida lakes. (C) 2000 The Fisheries Society of the British Isles.