Carlos Lozano, University of Texas

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Abstract:

The diets of larval and age-0 juvenile striped bass (Morone saxatilis) were evaluated with respect to the estuarine turbidity maximum zone (ETM) in upper Chesapeake Bay. Stomachs of larval and juvenile M. saxatilis collected on NSF sponsored BITMAX cruises (2001-2003) were analyzed. The three years in which samples were collected had experienced different hydrological conditions that affect hydrography of the upper Bay and its ETM. Precipitation and freshwater flow were near average in 2001, low in 2002, but well above average in 2003. Prey items in striped bass stomachs were quantified and identified to the lowest feasible taxonomic level. Prey compositions and amounts consumed were compared among the three years as well as among locations relative to the ETM (above or within). Feeding incidence, fullness index ([Stomach Content Weight] + [Stomach Weight + Content Weight] X 100), and weight of prey items per fish stomach were used as measures of feeding success. Ontogenetic shifts in feeding and feeding success were determined. Feeding and feeding success were analyzed with respect to salinity gradients and turbidity that define the ETM. Diets of larval striped bass in upper Chesapeake Bay consisted mainly of copepods (Eurytemora affinis and other calanoid copepods) and cladocerans (Bosmina longirostris). The analysis revealed that larval striped bass fed more successfully in the wet year 2003 than in the average year 2001. The most common prey items in juvenile striped bass diets were amphipods in three genera (Gammarus, Monoculodes, and Corophium) and shrimps in two genera (Neomysis and Crangon). Juveniles fed better in the dry year 2002, when their numbers were low, than in either 2001 or 2003 when they were more abundant. Spatial feeding success differed between years. In 2001 juvenile striped bass fed more successfully above the ETM whereas in 2002 and 2003 they fed better within the ETM. The ontogenetic analysis revealed that larval striped bass became increasingly successful feeders as they grew. Complex combinations of physical and biological factors, including retention of prey and its visibility under variable turbidity and salinity conditions in the ETM region, can control prey consumption, growth and survival of young striped bass in estuarine nurseries.