ZOOPLANKTON, LARVAL FISH, AND INVESTIGATIVE METHODS IN THE CHOPTANK RIVER

Abstract

Accurate predictions of recruitment are vital for stock assessments and fisheries management, however recruitment of fish larvae to adult populations is highly variable and the driving forces underlying it are complex. Physical environmental conditions such as temperature, salinity, and dissolved oxygen indirectly impact fish recruitment by altering the distribution of zooplankton prey. Studies of zooplankton and larval fish ecology have previously relied mainly on taxonomic identification via morphological features. This method comes with drawbacks that tissue metabarcoding of the gene cytochrome oxidase 1 (CO1) shows promise in overcoming. This dissertation will investigate the use of a powerful genetic tool, use it to characterize larval fish diets, and from that knowledge examine abundances of important prey species and zooplankton community structure across three decades. The study location is the Choptank River, a tidal tributary of the Chesapeake Bay and an important spawning and nursery ground for striped bass (Morone saxatilis), white perch (Morone americana), and bay anchovy (Anchoa mitchilli). The Choptank has a long history of monitoring for physical conditions, zooplankton, and juvenile fish, making it an ideal study site. Metabarcoding of the CO1 gene shows promise in detecting rare organisms and expanding our knowledge of zooplankton diversity; it is hindered by a lack of standardization and metrics directly comparable to counts. CO1 metabarcoding also shows promise as a tool to investigate larval fish diets, revealing a higher diversity of prey and ontogenetic changes in feeding strategy. The abundance of zooplankton taxa found to be important as prey for larval fish was significantly different in the study years (2018-2019) from that expected based on the decadal trends revealed in the Chesapeake Bay Program zooplankton monitoring data (1984-2002). This makes a strong case for continued monitoring to capture changes in zooplankton abundance and determine factors driving the observed patterns. Changes in the abundance of key zooplankton prey, associated with changes in environmental conditions, may contribute to observed declines in recruitment of larval fish.