SELECTIVE INGESTION OF DETRITUS BY A NORTH TEMPERATE OMNIVOROUS FISH,THE JUVENILE WHITE SUCKER, CATOSTOMUS-COMMERSONI

The capacity of the juvenile white sucker, Catostomus commersoni, to f eed selectively on detritus was examined by analyzing gut contents of fish feeding in a northern Michigan pond and in laboratory aquaria. Su ckers selected fine particle detritus. The median length of detritus p articles consumed by pond suckers ranged from 26-38 mu m for all seaso ns sampled. Although suckers in the pond ingested invertebrates > 215 mu m they never ingested detritus particles > 215 mu m Suckers fed pon d detritus in laboratory aquaria selectively ingested the fine particl e fraction (Kolmogorov-Smirnov test, p < 0.0001). Suckers excluded par ticles > 220 mu m and ingested a median particle size of 54 mu m Amorp hous detritus particles comprised 100% of the detritus particles in 93 % of the foreguts analyzed. Suckers fed fine particle (< 45 mu m) pond detritus in laboratory aquaria rejected the inorganic fraction and se lectively ingested organic material with the highest protein and energ y content. Ash free dry mass (AFDM) as a percent of dry-mass was 2.6 t imes greater in the foregut contents than in the detritus (p < 0.001). The amino acid concentration and energy density of the ingested organ ic matter were 1.9 times higher (p < 0.044) and 1.3 times higher (p < 0.025) than the respective amino acid and energy content of the organi c fraction of the detritus. These results indicate that suckers have t he capacity to consume selectively fine particle detritus with the hig hest nutritional value.