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Tidal, diurnal, lunar and seasonal cycles of catchability of juvenile Penaeus merguiensis, P. esculentus and Metapenaeus endeavouri were investigated in the Embley River in the Gulf of Carpentaria, Australia, over a full year. After seasonal effects were removed, catches of all species were found to be influenced by day-night and tidal cycles. P. merguiensis catches were primarily influenced by the tidal cycle and secondarily by the day-night cycle. The largest catches were usually on the last stage of the ebb tide which had the greater range of the 2 daily tides. When the 2 ebb tides were of equal amplitude more prawns were caught during the night tide. Catches of M. endeavouri were most strongly influenced by the day-night cycle; virtually no prawns were caught during the day. P. esculentus catches were least influenced by the tide and day-night cycles although, in general, more were caught at night and towards low tide. Tide and light cycles appear to affect the basic activity patterns of each of the 3 species in a similar manner, although the relative strengths of the responses are different for each species. All species were more active near high tide and catches were lowest at this time. Towards low tide, activity decreased and catches increased. However, when water levels were very low, P. esculentus and A4 endeavouri buried in the substrate and, therefore, were not catchable by trawling. In contrast, P. merguiensis did not bury, but concentrated near the shallow water's edge at low tide, and catches were highest then. All species were more active at night than during the day and catches were higher at night. The relationship between activity and catchability was different for the day-night and tidal cycles. Catchability with respect to day-night increased when the prawns were more active but catchability with respect to tide increased when the prawns were less active The results of the study are applicable to many areas in Australia and the Indo-West Pacific where similar habitats are used by juvenile penaeid prawns.

Catchability and sampling of three species of juvenile penaeid prawns in the Embley River, Gulf of Carpentaria, Australia