Changes in the Foodweb Supporting Fishes of the Upper San Francisco Estuary: Why Species Matter

Distribution, behavior, feeding, and reproduction of a fish population depends first on the species of fish.  Likewise, the species and size composition of foodweb organisms can have a substantial role in the productivity of the foodweb and the support it provides to fish populations.  We have been examining changes in the foodweb of the upper San Francisco Estuary, with emphasis on the low-salinity summer-fall habitat of the endangered delta smelt, Hypomesus transpacificus, a planktivore throughout its life.  Grazing by an introduced clam and possibly nutrient effects were responsible for a sharp and permanent decrease  in 1987 not only in phytoplankton biomass and productivity, but also in the mean size of the cells. This change has shifted the primary consumer community from particle-feeding copepods to  microbes. In turn, the  formerly abundant calanoid copepods that provided much of the food for larval and juvenile fish are now largely confined to marginal habitats either in time (winter instead of summer) or space (freshwater rather than brackish).  Their place in the low-salinity zone has been taken by the small (<0.5mm) introduced cyclopoid copepod Limnoithona tetraspina, which is now the numerical dominant.  This copepod appears to be less vulnerable to predation by some fish species by virtue of its small size and cryptic behavior, and it is less suitable to support the growth of delta smelt.  Furthermore, this species feeds on motile prey such as ciliates rather than phytoplankton, and in turn is fed upon by an introduced predatory copepod that itself is an important prey for some fish species.  This puts small planktivorous fish about two trophic steps higher in the foodweb than they were before the change, and the efficiency of the foodweb is correspondingly reduced.  Although these changes were precipitated by a change at the base of the foodweb, the details of species and size composition throughout the foodweb are critical to understanding how much of that limited productivity is available to fish.