Larval and juvenile Longfin Smelt diets as a function of fish size and prey density in the San Francisco Estuary

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.

Years of Drought and Salt: Decreasing Flows Determine the Distribution of Zooplankton Resources in the San Francisco Estuary

“…Hyperacanthomysis longirostris abundance is usually highest in the summer (June-August), with lower abundances in fall, winter, and spring (Barros 2021). This species is relatively understudied in the estuary; however, it has been shown to play an important role in fish diets, particularly Longfin Smelt (Lojakovic Burris et al 2022).…”

Long-Term Trends in Seasonality and Abundance of Three Key Zooplankters in the Upper San Francisco Estuary

“…https://doi.org/10.15447/sfews.2023v21iss3art1 Smelt (Appendix A) (Jungbluth et al 2021;Lojkovic Burris et al 2022), likely because of a mismatch in the optimal salinities of Bosmina spp. and Longfin Smelt.…”

“…Acanthocyclops spp. is an important component of fish diets in this region, particularly for Longfin Smelt (see Appendix A; Hobbs et al 2006;Lojkovic Burris et al 2022) and larval Pacific Herring (Clupea pallasii) (Jungbluth et al 2021), with Acanthocyclops spp. being the most abundant cyclopoid detected in larval Longfin Smelt gut contents by molecular sequencing (Jungbluth et al 2021).…”

“…One of these species, Delta Smelt (Hypomesus transpacificus), is endemic to the estuary and relies on zooplankton for its full life cycle (Brown et al 2016). Another fish, Longfin Smelt (Spirinchus thaleichthys), also relies on zooplankton throughout its life cycle (Chigbu and Sibley 1998a;Chigbu and Sibley 1998b;Barros et al 2022;Lojkovic Burris et al 2022). A number of fish species, including Delta Smelt and Longfin Smelt, declined sharply in abundance in the early 2000s, during the "pelagic organism decline (POD)" (Feyrer et al 2007;Sommer et al 2007).…”