A BIOENERGETIC APPROACH ON FOOD-CONSUMPTION OF ROACH (RUTILUS-RUTILUS(L)) IN A EUTROPHIC LAKE

A bioenergetics model was fitted for roach to estimate their food cons umption in L. Vesijarvi, southern Finland. When tested with independen t data the consumption estimates by the bioenergetics model differed l ess than 10 % from those obtained with other methods. Sensitivity anal ysis showed that the model estimates are most sensitive to changes in the respiration parameters, the relative change in the consumption est imate being of the same magnitude as that in the value of the paramete r. In L. Vesijarvi the bioenergetics model underestimated the seasonal growth of roach age groups 3 and 5 in July and overestimated the grow th of older roach, which was due to differences in food availability. The decline in the proportion of maximum ration consumed (the P-values of the model) in late summer was steepest in the age groups 3 and 5 d ue to the the collapse of the cladoceran density, while older roach we re able to complement their diets with plants and detritus. The daily consumption rates were highest in July-August reaching 1.2-2.5 g ww in the different age groups. It is suggested that the growth rate of roa ch is at present limited by the long capture and handling times of the small-sized cladocerans dominant in the lake. Older age groups can co mplement their diets with other food items but they are also forced to carry out energy-consuming migrations and their growth rate remains l ow.