Table 2. Logistic Regression Results for Fish Tested with Wild Plankton Assemblages

ERDC TN-DOER-E16

September 2004

concentration (Table 2). A higher percentage of menhaden fed on wild plankton (31 percent)

than on brine shrimp (20 percent). Spot also had a greater tendency to feed upon wild plankton

(74 percent) than on brine shrimp (65 percent). The model parameter estimates were relatively

close to those observed for the brine shrimp logistic regression models with prey concentration

being more influential than turbidity. A lower percentage of flounders (31 percent) fed on wild

plankton than on the brine shrimp (39 percent), however, the parameter estimates were similar

for the two logistic regression models for this fish taxon.

Table 2

Logistic Regression Results for Fish Tested with Wild Plankton Assemblages

Species

Fed

p-value TSS

p-value Prey

St. Est. TSS

St. Est. Prey

Menhaden

108

0.0001

0.0717

-0.86

0.26

Spot

102

0.0698

0.0001

-0.26

0.67

Flounders

108

0.0001

-096

0.76

CONCLUSIONS: Results of these experiments demonstrate that prey and suspended sediment

concentrations affect estuarine fish foraging success differently for different species. Of the fish

taxa tested, paralicthid flounder exhibited the greatest sensitivity to variations in the combined

variables. Flounder fed when TSS concentrations were low and prey concentrations were high

and did not feed when these conditions were reversed (Figure 5). Menhaden exhibited a similar

response profile (Figure 1). However, menhaden did consume some prey at the highest turbidity

levels. Pinfish, spot, and croaker did not curtail feeding as much in response to increasing TSS

concentrations (Figures 2-4) and were able to feed at the highest TSS concentrations when prey

concentrations were also high.

The two highest TSS concentrations (2,000 mg/L and 20,000 mg/L) used in these experiments

greatly exceed suspended sediment concentrations that are commonly associated with most

dredging projects (Wilber and Clarke 2001), except within short distances from the source (e.g.,

bucket transversing the water column or immediately adjacent to pipeline discharges or hopper

barge dumps). Other studies of larval estuarine fish responses to suspended sediments also report

adverse impacts at very high TSS concentrations. For instance, larval Pacific herring Clupea

harengus pallasi, reduce feeding when held at a TSS concentration of 2,000 mg/l for one day

(Boehlert and Morgan 1985) and larval Atlantic herring larvae exposed to TSS concentrations of

19,000 mg/l for 48 hr suffered 100 percent mortality (Messieh et al. 1981). Results of this study

indicate prey availability interacts with TSS concentration to affect fish feeding success on a

species-by-species basis. When predicting the potential impact of elevated turbidity levels,

consideration must be given to how different fish taxa detect and capture prey. For instance, it is

not evident whether reduced visual acuity or physical contact with suspended particles, which

may clog gill tissue, was responsible for reduced feeding rates.

ACKNOWLEDGEMENTS: This research was supported by the U.S. Army Engineer District,

Mobile, as a component of the National Thin-Layer Disposal Demonstration Project. All

experimentation was conducted at the National Marine Fisheries Service Laboratory in Beaufort,

NC.