Affiliation:
1. California Department of Fish and Wildlife
Abstract
The density and quality of zooplankton prey affect the feeding success of larval and juvenile fishes and thus can drive growth, survival, and recruitment. As part of a larger effort investigating potential causes of a pelagic fish decline, we examined regional feeding success (food presence/absence, stomach fullness), diet composition and prey selection of young Longfin Smelt (Spirinchus thaleichthys) in spring and summer as a function of fish size and prey availability in the San Francisco Estuary. We conducted our sampling during two wet and two dry years, because weather and river flow influence prey community composition and location. Larval and juvenile fish showed evidence of food limitation: high proportions of empty stomachs (≤ 70%) and stomach contents totaling <10% of maximum stomach content volume. The total weight of prey consumed increased with fish length, and in most regions and years this resulted from fish consuming larger prey as they grew; however, in many regions during dry years, fish consumed greater numbers of prey instead of larger prey as they grew. Larval fish preferentially consumed Eurytemora spp. except when rotifers or barnacle nauplii occurred in extremely high densities. Juvenile fish consumed a greater diversity of prey yet relied on mysids in most regions and years. Adult calanoid copepods (Pseudodiaptomus spp., Eurytemora spp., and Acartia spp.) were regionally important in juvenile diets and were positively selected when mysids were in low densities, mostly in eastern regions during dry years. This switch from much larger mysids to smaller calanoid copepods explains the increase in prey number (instead of prey size) consumed in these regions. These results, coupled with food limitation in most regions, suggest that the current densities and quality of zooplankton in the San Francisco Estuary are limiting feeding success and potentially growth and survival of young Longfin Smelt.
Publisher
California Fish and Wildlife Journal, California Department of Fish and Wildlife
Reference127 articles.
1. Alpine, A. E., and J. E. Cloern. 1992. Trophic interactions and direct physical effects control phytoplankton biomass and production in an estuary. Limnology and Oceanography 37(5):946–955.
2. Anderson, E. 1974. Trophic interactions among ctenophores and copepods in St. Margaret's Bay, Nova Scotia. Dissertation, Dalhousie University, Halifax, Novia Scotia, Canada.
3. Avila, M., and R. Hartman. 2020. San Francisco Estuary mysid abundance in the fall, and the potential for competitive advantage of Hyperacanthomysis longirostris over Neomysis mercedis. California Fish and Wildlife 106:19–38.
4. Barnhisel, D. R. 1991. Zooplankton spine induces aversion in small fish predators. Oecologia 88(3):444–450.
5. Barros, A., J. Hobbs, M. Willmes, C. Parker, M. Bisson, N. Fangue, A. Rypel, and L. Lewis. 2022. Spatial heterogeneity in nursery value of wetlands for a threatened estuarine smelt: Prey availability, feeding success, and dietary preference. Estuaries and Coasts. Available from: https://doi.org/10.1007/s12237-021-01024-y