The microhabitat distribution of juvenile banana prawns, Penaeus merguiensis and processes influencing their distribution and abundance were investigated using field and laboratory experiments. The field experiments were based in the Logan River, a subtropical estuary of eastern Australia. Seasonal patterns of abundance of juvenile and postlarval P. merguiensis and the influence of environmental variation on P. merguiensis abundance were also examined.

Prawns were sampled from January 1998 to July 2000 using a small beam trawl at fortnightly intervals when they were abundant and monthly intervals at other times. Recruitment of postlarval P. merguiensis varied considerably between years and was the highest in years when rainfall was low (1998 and 2000). Both the seasonal patterns of catches of postlarvae and juveniles, and environmental processes affecting their abundance were similar to tropical areas. One notable difference was that postlarval recruitment occurred over a more restricted time period in the Logan River than in tropical regions.

Penaeus merguiensis and other nekton were sampled from microhabitats within the mangrove forest using 3 m x 3 m lift nets that were activated at the top of the flood tide on spring tides between March and May 2000. The abundance of P. merguiensis in the adjacent creek was also monitored during both high and low tides using a small beam trawl. At high tide, P. merguiensis moved into the mangrove forest, with more prawns found at the mangrove-creek edge (5 - 8 metres into the forest) than a further 25 metres into the mangrove forest. The reasons for this are uncertain, but food availability or tidal inundation are suggested. Differences in predation were unlikely to have affected the catches of P. merguiensis as few predators were caught within the mangrove forest.

Most fishes in the mangrove forest ate very few juvenile prawns; they either ate other macrofaunal prey, such as amphipods, brachyurans and gastropods, or zooplankton (calanoid copepods or brachyuran zoea). Such alternative prey items to penaeid prawns may be more available to predators.

Microhabitats within the mangrove forest were cleared of vegetation to test the hypothesis that prawns select mangrove structure over bare substrata (see Figures 2 and 3). No differences in catches of P. merguiensis between vegetated or cleared habitats were detected, but catches were highly variable. This variation was attributed to the active behaviour of P. merguiensis and high rates of movement between microhabitats.

The processes of predation and habitat selection were further investigated in the laboratory, where the potentially confounding effects of food availability and tidal inundation were removed (Figure 4). Selection for artificial mangrove structures by three size classes of P. merguiensis (3 to 6, 6 to 10 and 10 to 14 mm carapace length), in the presence and absence of predatory fish (Arius graeffei and Lates calcarifer), and in both light and dark was examined. Juvenile prawns of each size class selected structurally complex habitats, in both light and dark conditions. Selection for structure was the strongest when predation risk was the highest. However, predation rates did not differ significantly between habitat structures and most attacks occurred on swimming prawns.

The hypothesis that elevated turbidity at low tide reduces predation on juvenile P. merguiensis was investigated. Predation by two predators (A. graeffei and L. calcarifer) on juvenile P. merguiensis was assessed for a range of turbidities. Turbid water reduced the predation rate for both A. graeffei and L. calcarifer. However, both predators were able to use non-visual methods to detect and consume prey in highly turbid water. In highly turbid estuaries, turbidity is suggested to act primarily at the community level by reducing the overall abundance of predatory fishes in the estuary.

It is hypothesised that at low tides, the inactive behaviour of prawns and several aspects of their habitat reduce predation; and at high tides mangrove forests provide suitable habitats for foraging with minimal predation risk (Figure 5). Further experiments on the use of shallow water as a predation refuge, and on the role of access to mangrove forests on the growth and survival of P. merguiensis are recommended.

Meager, J.J. (2003). The microhabitat distribution of juvenile banana prawns, Penaeus merguiensis de Man in subtropical eastern Australia and processes affecting their distribution and abundance. Ph.D Thesis. School of Natural Resource Sciences, Queensland University of Technology, Brisbane. 221 pp.

[Email me for a PDF version of my Ph.D thesis]

Further details are also available in the following articles:

Meager, J.J.; Williamson, I.; Loneragan, N.R. and Vance, D.J. (2005). Habitat selection of juvenile banana prawns, Penaeus merguiensis (de Man): testing the roles of habitat structure, predators, light phase and prawn size Penaeus merguiensis. Journal of Experimental Marine Biology and Ecology 324 (2): 89-98.

Meager, J.J.; Vance, D.J ; Williamson, I. and Loneragan, N.R. (2003). Microhabitat distribution of juvenile Penaeus merguiensis de Man and other epibenthic crustaceans within a mangrove forest in subtropical Australia. Journal of Experimental Marine Biology and Ecology 294: 127-144

Meager, J.J.; Vance, D.J ; Williamson, I. and Loneragan, N.R. (2003). Seasonal variation, and environmental influences on juvenile banana prawn (Penaeus merguiensis) abundance in a subtropical estuary (Logan River) of eastern Australia. Estuarine, Coastal and Shelf Science 56: 1-8.

SUPERVISORS

Dr. Ian Willliamson: School of Natural Resource Sciences, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia

Prof. Neil Loneragan, Director, Centre for Fish and Fisheries Research, Division of Science and Engineering, Murdoch University, WA, Australia

David Vance: CSIRO Division of Marine Research, PO Box 120, Cleveland, Queensland 4163, Australia

FUNDING

Funded by the Australian Research Council and QUT, with support from CSIRO

FIGURES

Figure 3: Bottom profile of study site (MHWS, Mean High Water Spring Tide; MHWN, Mean High Water Neap Tide; MSL, Mean Sea Level). The numbers of banana prawns caught at each location (mangrove-creek edge and inner mangrove forest) are given for the dates when both locations were inundated. [Back to top]

Figure 4: Photograph of the experimental tank, showing the diameter, habitat structures and predator.[Back to top]

Figure 5 (left): Conceptual model for the distribution of juvenile banana prawns over the tidal cycle (low, flood, high and ebb tides). At low tide, P. merguiensis are relatively inactive and concentrated on shallow intertidal mudbanks or amongst submerged structure, in turbid water. As the tide floods, the depth in these habitats increases substantially, the turbidity decreases and P. merguiensis migrate into the mangrove forest. This reduction in turbidity occurs because the tidal current is slowed by the vegetation within mangrove forests. I hypothesised that P. merguiensis tidally migrate into mangrove forests as it represents a suitable habitat for foraging with minimal predation risk. My thesis, and research to date, suggests that banana prawns reduce predation risk at low tide by remaining inactive in shallow, highly turbid water, and at high tide by moving amongst mangrove structure. My results also suggested that the presence of other macrofaunal prey items reduces predation risk to prawns within mangrove forests. Foraging at low tide was suggested to be limited by the risk of predation, rather than the availability of food items.