BIOMASS-SIZE SPECTRA AND THE PREDICTION OF FISH BIOMASS IN LAKES

Biomass-size spectra of planktonic communities offer a promising appro ach to predict fish biomass from simple measurements of phytoplankton or zooplankton biomass. Theoretical models based on the transfer of en ergy between size-classes have predicted fish biomass in the Great Lak es within two fold of measured biomass. To test the applicability of o ne group of biomass-size spectrum models (U. Borgmann 1987. Can. J. Fi sh. Aquat. Sci. 44(Suppl. 2): 136-140) to lakes in general, we gathere d biomass and production data for phytoplankton, phytobenthos, zooplan kton, zoobenthos, and fisk from 24 lakes and reservoirs studied during the International Biological Programme. In planktonic communities, th e intercepts of biomass-size spectra are positively related to phylopl ankton productivity and to lake mean depth, and the slopes become more negative with increasing lake size. Despite these regularities, fish biomass is predicted poorly from Borgmann's biomass-size spectrum mode ls. Predictions of fish biomass were only weakly correlated to measure d fish biomass (r(2) = 0.19, n = 24). Fish biomass was overestimated o r underestimated by more than 50 kg . ha(2) in almost half the lakes w here the models were tested. The inclusion of benthic algae and benthi c invertebrates in the models does not improve the predictions of fish biomass.