Zooplankton Food Web Connections to Fish

Many Hudson fish depend on zooplankton as a food resource. This is especially true for the early life history stages of resident and anadromous fishes. Striped bass, for example, consume zooplankton during their larval stage. Copepods and Bosmina are primary prey, but the smallest striped bass larvae also consume rotifers, copepod nauplii, and even zebra mussel veligers while larger larvae switch to larger zooplankton such as Leptodora.Se-lectivity is positive for larger copepods. Limburg etal. (1997) observedstripedbasslarvaewithcope-pod eggs in their guts, suggesting consumption of ovigerous females was important. Striped bass switch to benthic and epibenthic prey such as am-phipods and shrimp as they develop from the larval to juvenile stage (Hurst and Conover, 2001). Juveniles still prey on animals that are partially planktonic such as Neomysis americana. Atlantic tomcod (Microgadus tomcod) also exhibit a similar ontogenetic shift in prey items, switching from copepods such as Eurytemora affinis to mysids and amphipods (Grabe, 1978). In addition to fish that utilize zooplankton primarily during early life stages, some species are largely planktivorous including American shad, alewife, blueback herring, gizzard shad, bay anchovy, menhaden, and Atlantic silverside (Grabe, 1996). For these organisms, zooplankton are a primary resource during their residence in the Hudson.

For the smallest of some fish larvae, ciliates may be an important food resource. In the laboratory, small larval menhaden fed preferentially on tintin-nids compared to copepod nauplii while larger larvae preferred nauplii (Stoecker and Govoni, 1984). To date, however, little is known about the importance of ciliates in larval diets of fish in the Hudson River. This maybe due, in part, to the use of gut content analysis of preserved specimens as the primary method of determining fish feeding preferences. Although fish have been known to consume cili-ates, rapid digestion or cell disintegration due to preservation may explain the absence of these organisms in gut contents. Chemical and immuno-logical approaches could help in determining if Hudson fish utilize ciliates and other microbes as food resources.

Overall, exploitation of zooplankton as prey is a critical feature of the Hudson ecosystem. Reductions in zooplankton would presumably affect planktivorous fish unless these fish can compensate by some mechanism (e.g., increased feeding or switching to alternate resources). The zebra mussel invasion provides a means of testing the dependence of fish on zooplankton with the provision that the most selected prey of fish (that is, cope-pods) underwent little change in abundance. The long-term data on fish populations in the Hudson resulting from monitoring studies by utilities and state agencies indicate planktivorous fish exhibited changes in abundance, growth, and distribution in association with the zebramusselinvasion (Strayer, Hattala, and Kahnle, 2004).

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