Mucus: a new tissue fraction for rapid determination of fish diet switching using stable isotope analysis

Stable isotope analysis of diet switching by fishes often is hampered by slow turnover rates of the tissues analyzed (usually muscle or fins). We examined epidermal mucus as a potentially faster turnover tissue'' that might provide a more rapid assessment of diet switching. In a controlled hatchery experiment, we switched the diet of juvenile steel-head (sea-run rainbow trout, Oncorhynchus mykiss) from a plant-based feed with low delta(13)C and delta(15)N to a fish-meal-based diet with higher delta values. We found mucus to provide a significantly more rapid response to diet switching (half-life = 36 days for delta(15)N, 30 days for delta(13)C) than muscle tissue (half-life = 94 days for delta(15)N, 136 days for delta(13)C), even for growing juvenile fish. Mucus may provide a rapid turnover tissue'' for analysis of diet (or habitat) switching by fish. It has the additional advantage that it may be sampled nonlethally in some fishes, thereby avoiding problems in studying threatened or endangered species. This is the first report of the use of fish mucus in stable isotope analyses of fish tissues.