Larval Fish Feeding Responses to Variable Suspended Sediment and Planktonic Prey Concentrations

ERDC TN-DOER-E16

September 2004

Larval Fish Feeding Responses to Variable

Suspended Sediment and Planktonic Prey

Concentrations

PURPOSE: Understanding fish feeding responses under conditions that simulate turbidity

plumes and variations in prey availability enhances the ability to predict ecological impacts from

dredging projects. For example, in the context of nearshore placement of mixed sediments,

concerns have been raised that winnowing of fine sediments from deposited dredged material

may elevate turbidity and have a detrimental impact on early life history stages of fishes. The

feeding responses of larval fishes to varying concentrations of suspended sediments and

planktonic prey are reported herein.

BACKGROUND: There is little empirical information concerning how turbidity influences the

behavior of estuarine and marine fishes under conditions that are typically encountered during

dredging projects (Wilber and Clarke 2001). Suspended sediments cause turbidity by attenuating

light through both particulate scattering and absorption (Wilber 1970). Reduced water clarity

may affect fishes' abilities to forage, which if sufficiently curtailed, could prevent satisfaction of

food requirements for their metabolism and growth. Fishes need sufficient light at depth to be

able to detect food particles, which for larval fishes are viewed laterally and not vertically.

Suspended sediments can mask or block light reflected horizontally, thereby reducing water

clarity. Turbidity may also affect a predator's ability to detect and capture larval fish. Increased

light attenuation caused by turbidity shortens the depth of the photic zone, which in turn affects

primary production. Thus, turbidity may influence the productivity of fish stocks through a

number of mechanisms. High turbidity can reduce feeding in adult Atlantic croaker

(Micropogonias undulatus) and pinfish (Lagodon rhomboides)(Minello et al. 1987); however,

potential impacts on larval fishes are not well understood.

Turbidity within a waterbody varies due to changes in soil composition, land development,

agricultural practices, weather, and a host of other factors in the watershed. Turbidity at a given

location can vary over time by orders of magnitude due to either natural (e.g., tidal re-

suspension, storm runoff) or anthropogenic (e.g., logging, dredging) disturbances. Likewise,

plankton are patchily distributed, with concentrations varying over orders of magnitude within a

few meters. In this study, the feeding success of five estuarine fish taxa exposed to varying

turbidity and prey concentrations is examined. These experimental factors are varied over orders

of magnitude so as to maximize opportunities to detect taxonomic differences in response to the

two factors.

METHODS: Experimentally exposing fish larvae to controlled turbidities requires methods that

compensate for the tendency of clay and silt particles to floculate and settle out of suspension,

making it difficult to maintain a specified turbidity for the duration of a test interval. Maintaining

uniform prey densities also poses a challenge, because many planktonic organisms exhibit strong

phototaxis. Therefore, in a static system, prey may aggregate with respect to an existing light