Evaluating the effect of slot size and environmental variables on the passage of estuarine nekton through a water control structure

Water control structures (WCSs) installed to regulate water levels can alter both the hydrology and ecology of salt marshes. WCSs are thought to limit nekton ingress into, and egress from, managed marshes. Slots (vertical openings that span most of the water column) incorporated into WCSs are thought to facilitate nekton passage through structures, but little research has directly examined how slot size affects passage rates. We used dual-frequency identification sonar (DIDSON) acoustic imaging to examine the effect of slot width (10, 15, 30, or 60 cm), tidal cycle, diel period, and season on nekton passage at a WCS located in a tidal salt marsh canal. Few individuals (total numbers and relative percentages) used the slots for passage through the structure during any stage of the tidal cycle, day or night, or seasonally. The number and size of migrants were similar for all four slot sizes examined. Nekton used the slots most often on flood tides to access the managed marsh (i.e., swim inside), primarily at night. Individuals entering the managed marsh were larger than those observed leaving the managed marsh. Whereas the majority of migrants were observed during winter months, season did not affect nekton passage in our study. Acoustic imaging allowed a unique and comprehensive evaluation of nekton passage by permitting an examination of factors such as swimming direction and proportion of migrants that are unobservable with other sampling techniques.