ESTIMATING DENSITIES OF SMALL FISHES AND DECAPOD CRUSTACEANS IN SHALLOW ESTUARINE HABITATS - A REVIEW OF SAMPLING DESIGN WITH FOCUS ON GEARSELECTION

Shallow estuarine habitats often support large populations of small ne kton (fishes and decapod crustaceans), but unique characteristics of t hese habitats make sampling these nekton populations difficult. We dis cuss development of sampling designs and evaluate some commonly used d evices for quantitatively sampling nekton populations. Important consi derations of the sampling design include the size and number of sample s, their distribution in time and space, and control of tide level. Hi gh, stable catch efficiency should be the most important gear characte ristic considered when selecting a sampling device to quantify nekton densities. However, the most commonly used gears in studies of estuari ne habitats (trawls and seines) have low, variable catch efficiency. P roblems with consistently low catch efficiency can be corrected, but l arge unpredictable variations in this gear characteristic pose a much more difficult challenge. Study results may be biased if the variabili ty in catch efficiency is related to the treatments or habitat charact eristics being measured in the sampling design. Enclosure devices, suc h as throw traps and drop samplers, have fewer variables influencing c atch efficiency than do towed nets (i.e., trawls and seines); and the catch efficiency of these enclosure samplers does not appear to vary s ubstantially with habitat characteristics typical of shallow estuarine areas (e.g., presence of vegetation). The area enclosed by these samp lers is often small, but increasing the sample number can generally co mpensate for this limitation. We recommend using enclosure samplers fo r estimating densities of small nekton in shallow estuarine habitats b ecause these samplers provide the most reliable quantitative data, and the results of studies using these samplers should be comparable. Man y kinds of enclosure samplers are now available, and specific requirem ents of a project will dictate which gear should be selected.