A method to estimate sediment ingestion by fish

A method is proposed to use naturally occurring radionuclides of the 238U decay series as mass balance tracers to estimate sediment ingestion by benthic fish. The mass balance tracer method can take two forms: one that does not account for tracers that would be ingested in invertebrate food items (simple mass balance tracer method) and one that does (adjusted mass balance tracer method). The contents of the gastrointestinal (GI) tracts from 17 Shorthead Redhorse Suckers (Moxostoma macrolepidotum) were analyzed using the proposed method to determine the proportion of sediment in the fish gut. The proportion of sediment in the GI tract samples was also determined using a method developed by Beyer et al. (1994), where the percent soil in wildlife is estimated from the acid insoluble residue (AIR) content of their scat and an assumed level of digestibility of their food (AIR method). The mean mass of sediment in the GI tracts of the 17 fish sampled, calculated using the simple mass balance tracer method, was observed to be 1.14 g (standard deviation 0.99 g) or approximately 46% (standard deviation 16%) of the dry weight of total GI tract contents. A mean of 0.97 g (standard deviation 0.85 g), or approximately 38% (standard deviation 13%) of the dry weight of total GI tract contents, was observed when the mass of sediment in the GI tract was calculated using the adjusted mass balance tracer method. When using the AIR method, the mean percentage of sediment in the GI tracts was observed to be 30% (standard deviation 16%). The findings in this study support the assertion that direct ingestion of sediment by benthic fish may be an important pathway for contaminants in aquatic food webs.