Spatial gradient in the distribution of whaler sharks (carcharhinidae) in Moreton Bay, southeastern Queensland

Abstract

Experimental gillnetting and setlining provided a detailed account of shark and ray composition at three shallow water sites in Moreton Bay between 2004 and 2007 (n=350 elasmobranchs). The species composition of elasmobranchs significantly differed between sites and shark abundance was highest at the western site (St Helena Island, Waterloo Bay). Juvenile Dusky (Carcharhinus obscurus) and Pigeye Sharks (C. amboinensis) were more abundant at the western site and appear to be rare in the eastern bay. Approximately 8% of the 206 tagged sharks were recaptured, 60% within two kilometres from their release position, with time at liberty ranging from four to 402 days. The results suggest that the documented east-west gradient in teleost diversity in Moreton Bay also extends to the Carcharhinidae. Further research is recommended to determine whether the diversity patterns observed from the three sites are broadly representative of each of these regions. Setlining and rod and line fishing for sharks in a deeper part of the bay between 1978 and 1992 (n=440 elasmobranchs) revealed a different species composition. The Spottail Shark (C. sorrah) and the Spinner Shark (C. brevipinna) comprised 50% and 39% of the catch in this deeper site, respectively, but were rarely caught in shallow regions of the bay, suggesting that the species composition is also partitioned by depth. Western fringes of the bay have been heavily modified by anthropogenic activities and the importance of this area to juvenile whaler sharks needs to be considered. Future sampling at the same fixed locations may provide the opportunity to examine whether recent re-zoning of the Marine Bay Marine Park in 2009, or other factors such as changes in commercial or recreational fishing, have influenced the species composition and abundance of sharks.