Composition of Chinook salmon faecal wastes with implications for environmental management

Intensive salmon aquaculture is now a major industry globally, with Chinook salmon (Oncorhynchus tshawytscha) the dominant culture species in New Zealand. Understanding the composition of Chinook salmon wastes is important for assessing environmental risks of coastal open-cage aquaculture, both in terms of nutrient inputs, and for evaluating potential characteristics that could be used as fish waste markers in the environment. The aim of our study was to describe the biochemical composition of faecal waste from Chinook salmon with a view to describing the characteristics most important for assessing waste inputs in the environment. A broad suite of biochemical characteristics, including nutrient, mineral and trace element concentrations, bulk carbon and ni-trogen stable isotope and compound specific 813C fatty acid and amino acid values, were measured during an experiment on fish kept in a recirculating aquaculture system (RAS) and fed a standard commercial pellet diet. We found that all biochemical characteristics of feed were conserved in faecal material, but some were conserved better than others. Fatty acid composition was similar in feed and faeces, but element profiles differed and showed high variability in faeces. Carbon and nitrogen bulk stable isotope values of faecal samples were generally lower than mean feed signatures, particularly for 815N. For compound specific stable isotopes, the 813C values in most fatty acids were similar to that of feed, however, 813C values in amino acids were lower (with the exception of phenylalanine and glutamic acid). Based on our findings, we suggest that it may be possible to predict biochemical composition of faeces for well-conserved, or predictably-altered indicators using composi-tion of feed samples alone. The differences between feed and faeces for some markers (e.g., elements, nutrient ratios, 815N and 813C in amino acids) may be useful for distinguishing between the presence of these two farm -derived wastes in the environment. Our results also suggest that the elemental composition of Chinook salmon waste is generally similar to that of other salmonid species, and supports the transfer of element composition data from other salmonids when setting mass balance model parameters for Chinook salmon.