Does water depth influence size composition of estuary-associated fish? Distributions revealed using mobile acoustic-camera transects along the channel of a small shallow estuary

Quantifying the abundance and distribution of fish is fundamental to gaining an understanding of how habitat type, water depth or abiotic conditions influence fish assemblages throughout estuarine systems. Such investigations are inherently difficult because estuaries typically contain a range of habitats across varying depth strata, and data usually consist of replicate samples that cover only a small portion of an entire estuary. We used replicate acoustic-camera (DIDSON) transects along the entire length of a small South African estuary to determine the distributions of different size cohorts of fish. Each size cohort was distributed heterogeneously along the estuary, with abundances peaking in discreet sections of the system. By comparing the abundance of fish to the bathymetry, we found correlations between depth and abundance for two of three size classes. Large fish (>401 mm) were more abundant in deep holes (<3 m), whereas there were more small fish (100-250 mm) in shallow (>1.5 m) upper reaches of the estuary. This investigation demonstrated the usefulness of extended DIDSON transects to rapidly collect data on the distributional abundance of estuarine fish from the estuary mouth to the head, showing that bathymetry of a system may be an important factor in determining these patterns.