Excretion by Adfluvial Fishes: Interspecific Variation and Potential Impacts

Freshwater fish migrations frequently link different aquatic habitats, and can produce strong effects in recipient habitats by introducing otherwise unavailable materials and influencing the distribution and processing of existing resources.  For example, many streams are linked to lakes via migrations by spawning fish, and such streams may receive inputs of lake-derived nutrients (LDN) from these adfluvial migrations.  One pathway by which migratory fishes may introduce LDN to recipient systems is excretion, which supplements pools of dissolved inorganic nutrients available for biological uptake.  However, excretion measurements from freshwater migrants have rarely been placed in the context of stream ecosystem nutrient dynamics.  Moreover, nutrient excretion is likely to vary as a function of body size and stoichiometric constraints, both of which exhibit interspecific variation.  Consequently, the potential impact of excreted LDN in tributary streams remains poorly understood, as does the role played by migrant identity.  To address these knowledge gaps, we initiated a study of nutrient translocation by migratory spawners in the Utah Lake system in central Utah.  Several species of adfluvial spawners are found within the lake, including white bass (Morone chrysops), common carp (Cyprinus carpio), and the endemic June sucker (Chasmistes liorus).  Measurements of NH₄-N (N) and PO₄-P (P) excretion rates were made, and variation among species was examined.  Preliminary analyses suggest that excretion rates of both N and P differ among species, and that excretion by adfluvial migrants may be capable of meeting a substantial (>50%) portion of stream nutrient demand.  Furthermore, the fact that different species exhibit variation with respect to nutrient excretion rates has the potential to mediate the impact of adfluvial fishes in the Utah Lake system, where different tributaries attract disparate migratory assemblages and encompass a range of physical and chemical conditions.  Future work will include parameterization of population-level excretion models with system-specific data on spawner population densities and stream ecosystem demand for inorganic nutrients, examining the temporal dynamics of stream food web response to the introduction of LDN, and determining the effect of interactions among biological and physicochemical conditions on impacts associated with LDN.  Collectively, these components should enhance the ability of natural resource managers to predict when and where ecosystem-level effects of migratory fishes will be most pronounced.