Lunar periodicity of prawns and by-catch in trawls from the Gulf of Carpentaria, northern Australia

By-catch and target species taken in 25 consecutive nights of research traw ling off Duyfken Point (Weipa) in the north-eastern Gulf of Carpentaria dur ing October-November 1995 were counted and weighed. The 24 most abundant sp ecies (by numbers per hour) caught in at least 34 of the 36 trawls, plus 2 less abundant penaeids that occurred in all trawls, were used in the analys es. To extrapolate trends in catch over the whole Gulf fishery over the sam e time, the prawn catch rates of the research trawls and of the commercial fishery in the same and other areas around the Gulf were compared. Catch ra tes for all species combined were dominated by the high abundance of Leiogn athus splendens over the 25 nights. Plots of catch rates for individual spe cies indicated some lunar periodicity in the patterns of abundance for many species. Ten species, Gerres filamentosus (P=0.012), Johnius amblycephalus (P=0.012), Leiognathus moretoniensis (P <0.001), L. splendens (P=0.007), M etapenaeus ensis (P <0.001), Penaeus esculentus (P=0.007), Sardinella albel la (P <0.001), Sillago sihama (P=0.005), Torquigener whitleyi (P <0.001), T rachypenneus spp. (P <0.001) had strong significant differences in mean cat ch rates between lunar phases, but the patterns differed among groups of sp ecies. The most common pattern was a distinct peak during the first-quarter moon (Sardinella albella, G, filamentosus, J, amblycephalus and L, splende ns); next most common pattern was a distinct last-quarter peak (Torquigener whitleyi, L. moretoniensis and ensis had first-quarter and full-moon peaks , while Trachypenaeus spp, showed new- and full-moon peaks. Data obtained f rom the commercial fleet for endeavour prawns (Metapenaeus endeavouri and M , ensis) and tiger prawns (P, esculentus and P. semisulcatus) at the same t ime and area clearly showed the same patterns as in the research sampling. Endeavour prawns had a significant full-moon peak (P <0.001), and tiger pra wns had a last-quarter peak (P <0.001), although the latter was not signifi cant in the research sampling. The prawn catch rates from different areas o f the Gulf showed significant differences with moon phase: P <0.001 for end eavour and tiger prawns at North Groote; P <0.001 for endeavour prawns and P=0.028 for tiger prawns at East Vanderlins; P <0.015 for endeavour prawns at West Mornington, although the peak catch rates did not occur at the same moon phase as in Weipa. The results of this study suggest that some by-cat ch species may need to be excluded from monitoring programmes where no acco unt of moon phase is possible. The corollary is that the majority of by-cat ch species studied showed no significant changes in catch rates with moon p hase and would therefore be ideal species for any sampling programme that c annot exclude the effect of moon phase.