D.J. Vance

Seagrass and algal beds as nursery habitats for tiger prawns (Penaeus semisulcatus and P. esculentus) in a tropical Australian estuary

We evaluated the importance of seagrass and algae to two species of tiger prawns (Penaeus semisulcatus and P. esculentus) by detailed sampling at four sites (two seagrass, two algae) in the Embley River estuary, and through sampling 26 sites in 7 adjacent estuaries at one time of year. Samples of tiger prawns were collected in the Embley River estuary with a small beam trawl at night every 2 wk from September to May for 2 yr (1990 to 1992). The two seagrass sites, which were 11 and 13 km from the river mouth, showed less seasonal variation in salinity than the two algal sites, which were 15 and 20 km from the river mouth. The algal beds at the two upstream sites almost disappeared during the wet season, but the biomass of seagrass did not change significantly between the wet and dry seasons. The grooved tiger prawn (P. semisulcatus), the main species at all sites, comprised 88% of the total tiger prawn catch over the two years. They were found at all sites during the pre-wet season, but after the onset of the wet season, they disappeared along with the algae, from the upstream sites. The brown tiger prawn (P. esculentus) was found almost exclusively (97% of the total catch) on the seagrass sites downstream. In the study of several estuaries, juvenile P. semisulcatus were caught at all 26 sites, and P. esculentus were caught in much smaller numbers, at 16 sites. Approximately equal numbers of P. semisulcatus were caught in seagrass and algal beds in the pre-wet season. Very few individuals >10 mm carapace length of either species, were caught. The results from this study highlight the importance of algal beds during the pre-wet season as nursery areas for one species of tiger prawn (P. semisulcatus).

Use of a mangrove estuary as a nursery area by postlarval and juvenile banana prawns,Penaeus merguiensis de Man, in Northern Australia

In the Embley River, Gulf of Carpentaria, Australia, the largest catches of the commercially important banana prawns, Penaeus merguiensis , were made on mangrove-lined, steeply sloping mud banks. The upstream limit of distribution of P. merguiensis was found to coincide with the distribution of broad bands of fringing mangrove forests but, except in the wet season, was not related to salinity levels. Although some postlarval P. merguiensis settled on all habitat types in the estuary, large catches were only taken on the mangrove-lined banks. Catches of both postlarvae and juveniles in the upstream reaches of a small creek were almost five times higher than those in the river near the creek mouth. Moreover, prawns in the 2 to 4 mm carapace length (CL) size class were poorly represented in the river but were abundant in catches in the small creek. This suggests that either the survival rate of postlarvae is highest in the upper reaches of the small creeks, or that the small prawns are migrating from the main river into the creek. As prawns increase in size above 5 mm CL it appears that they take part in daily tidal migrations from small creeks to the river and begin a gradual migration from the creeks to the river.

Abundance of fish and crustacean postlarvae on portable artificial seagrass units: daily sampling provides quantitative estimates of the settlement of new recruits

Artificial collectors and seagrass units have mainly provided qualitative samples of epifaunal abundance or have been difficult and time-consuming to sample. Consequently, they are useful for distinguishing temporal or spatial trends in abundance or they are deployed for several weeks and, as a result, the quantitative samples are cumulative. We developed a portable artificial seagrass unit (ASU) with buoyant plastic artificial seagrass (47 cm long by 15 mm wide strips) that can be retrieved, harvested and re-deployed with 98% catch efficiency in about 5 min by two people from a small boat. They can quickly and easily quantify settlement of crustacean and fish postlarvae over tidal or deil periods. When set for 24 h, postlarvae settled from the plankton during the night and their abundance is the result of a distinct settlement event. When set for longer periods, the numbers of postlarvae may represent several settlement events and post-settlement activities. Crustacean and fish postlarvae and juveniles used ASUs deployed within seagrass in a similar way to natural seagrass. Estimates of juvenile tiger prawn abundance from beam-trawl catches showed similar densities in natural seagrass (2.93 Penaeus semisulcatus de Haan m−2 day) to those in the ASUs (2.40 P. semisulcatus m−2 day−1); their density was significantly lower on bare trays (0.48 P. semisulcatus m−2 day−1). When deployed on bare areas, more epifaunal crustacean postlarvae were collected from the ASUs (e.g. Portunus pelagicus Linnaeus, 1.21 m−2 day−1; caridean shrimp 4.03 m−2 day−1) than from the bare trays (e.g. P. pelagicus, 0.46 m−2 day−1; caridean shrimp 0.78 m−2 day−1). However, greater abundances of the postlarvae of other crustacean taxa were collected from the bare trays than the ASUs (e.g. Sergestes spp. 1.21 and 0.31 m−2 day−1; tiger prawn postlarvae 0.15 and 0.06 m−2 day−1, respectively). Sampling with portable ASUs allows settlement to be assessed temporally (e.g., daily or tidally) or spatially (e.g., distinct areas affected by different current regimes). The strength of settlement can be used to evaluate the productivity of nursery habitat for fishery populations.

Microhabitat distribution of juvenile Penaeus merguiensis de Man and other epibenthic crustaceans within a mangrove forest in subtropical Australia

The distribution of juvenile Penaeus (Fenneropenaeus) merguiensis de Man and other epibenthic crustaceans in different microhabitats within a riverine mangrove forest was examined in subtropical eastern Australia. Catches in vegetated and cleared microhabitats were compared at sites located in mangroves on the creek edge (1-2 m into the forest) and the inner forest (a further 14-22 m into the forest). Crustaceans were sampled using 3 x 3 m lift nets that were activated at the top of spring flood 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. A large size range of P. merguiensis was caught in the mangrove forest (2-13.5 mm carapace length) and the maximum density recorded was 1 prawn m-2. Catches of P. merguiensis were significantly higher at the creek edge than at the inner forest sites. Catches did not differ significantly between vegetated and cleared microhabitats, but catches were highly variable within microhabitats. This result was attributed to the very high activity levels of P. merguiensis and suggests that detecting differences between microhabitats used by prawns in the field would require a large number of samples. Substantial numbers of Acetes sibogae australis (Hansen), Macrobrachium novaehollandiae (de Man) and Metapenaeus bennettae (Racek and Dall) also entered the forest and catches of these species followed a similar pattern to those for P. merguiensis, i.e. catches were higher at the creek edge than the inner forest, did not differ between microhabitats and were highly variable within a microhabitat.

Seasonal Variation and Environmental Influences on Juvenile Banana Prawn (Penaeus Merguiensis) Abundance in a Subtropical Estuary (Logan River) of Eastern Australia

Although the life history-environment relationship for juvenile Penaeus merguiensis in tropical regions has been well documented, little is known about the species towards the subtropical limits of its range. Seasonal patterns of abundances of juvenile and postlarval P. merguiensis were studied in the Logan River, a subtropical estuary in eastern Australia from January 1998 to July 2000. Prawns were sampled using a small beam trawl at fortnightly intervals when they were abundant and monthly intervals at other times. The prawns ranged in size from 1.8 to 22 mm in carapace length (CL), with a mean size ( ± 1 SE) of 6.7 ± 0.1 mm CL and were caught at densities of up to 14.2 ± 3.3 prawns m-2. Postlarvae were caught from January to July, with peaks in April and May; and juveniles were abundant from December to June, although this varied between years. The numbers of postlarvae, temperature and rainfall explained 23% of the overall variation in juvenile catches. Temperature and rainfall also explained some of the variation in postlarval catches (9.9%). Recruitment varied considerably between years, and was the highest in years when rainfall was low (1998 and 2000). In general, both the seasonal patterns of catches of postlarval and juvenile P. merguiensis, and environmental processes affecting their abundance were similar to tropical areas. However, one notable difference was that recruitment occurred over a more restricted time period in the Logan River than in tropical regions.