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

Abstract

By-catch and target species taken in 25 consecutive nights of research trawling off Duyfken Point (Weipa) in the north-eastern Gulf of Carpentaria during October–November 1995 were counted and weighed. The 24 most abundant species (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 analyses. To extrapolate trends in catch over the whole Gulf fishery over the same 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 rates for all species combined were dominated by the high abundance of Leiognathus splendens over the 25 nights. Plots of catch rates for individual species 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), Metapenaeus ensis (P<0.001), Penaeus esculentus (P=0.007), Sardinella albella (P<0.001), Sillago sihama (P=0.005), Torquigener whitleyi (P<0.001), Trachypenaeus spp. (P<0.001) had strong significant differences in mean catch rates between lunar phases, but the patterns differed among groups of species. The most common pattern was a distinct peak during the first-quarter moon (Sardinella albella, G. filamentosus, J. amblycephalus and L. splendens); next most common pattern was a distinct last-quarter peak (Torquigener whitleyi, L. moretoniensis and P. esculentus). The commercial penaeid Metapenaeus ensis had first-quarter and full-moon peaks, while Trachypenaeus spp. showed new- and full-moon peaks. Data obtained from the commercial fleet for endeavour prawns (Metapenaeus endeavouri and M. ensis) and tiger prawns (P. esculentus and P. semisulcatus) at the same time 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 prawns had a last-quarter peak (P<0.001), although the latter was not significant in the research sampling. The prawn catch rates from different areas of the Gulf showed significant differences with moon phase: P<0.001 for endeavour 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-catch species may need to be excluded from monitoring programmes where no account of moon phase is possible. The corollary is that the majority of by-catch species studied showed no significant changes in catch rates with moon phase and would therefore be ideal species for any sampling programme that cannot exclude the effect of moon phase.