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Notes: Abstract 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).

Notes: Abstract The population dynamics of small tiger prawns (Penaeus esculentus and P. semisulcatus) were studied at three sites around north-western Groote Eylandt, Gulf of Carpentaria, Australia, between August 1983 and August 1984. Seagrasses typical of open-coastline, reef-flat and river-mouth communities were found in the shallow depths (≤2.5 m) at these sites. The temperature and salinity of the bottom waters did not differ among the shallowest depths of the three sites and mean values at night ranged from 21.9 to 32.0 °C, and from 30.1 to 37.5% S. Data from fortnightly sampling with beam trawls showed that virtually all post-larvae (∼90%) were caught in the intertidal and shallow subtidal waters (≥2.0 m deep). At one site, where the relationship between seagrass biomass, catches and depth could be studied in detail, high catches were confined to seagrass in shallow water, within 200 m of the high-water mark. This was despite the fact that seagrass beds of high biomass (〉100 g m-2 between August and February) were found nearby, in only slightly deeper water (2.5 m). It is likely, therefore, that only the seagrass beds in shallow waters of the Gulf of Carpentaria act as important settlement and nursery areas for tiger prawns. In general, catches of tiger prawn postlarvae (both P. esculentus and P. semisulcatus) and juvenile P. esculentus on the seagrass in the shallowest waters at each site were higher in the tropical prewet (October–December) and wet (January–March) seasons than at other times of the year. Juvenile P. semisulcatus catches were highest in the pre-wet season. While seasonal differences accounted for the highest proportion of variation in catches of tiger prawn postlarvae and juvenile P. semisulcatus, site was the most important factor for juvenile P. esculentus. In each season, catches of juvenile P. esculentus were highest in the shallow, open-coastline seagrass, where the biomass of seagrass was highest. The fact that the type of seagrass community appears to be more important to juvenile P. esculentus than to postlarvae, suggests that characteristics of the seagrass community may affect the survival or emigration of postlarval tiger prawns. Few prawns (〈10%) from the seagrass communities in shallow waters exceeded 10.5 mm in carapace length. Despite the intensive sampling, growth was difficult to estimate because postlarvae recruited to the seagrass beds over a long period, and the residence times of juveniles in the sampling area were relatively short (∼8 wk).

Notes: Abstract Postlarval and juvenile Penaeus merguiensis de Man from the Embley River estuary on the north-eastern Gulf of Carpentaria were sampled every 2 wk from September 1986 until August 1989, using a small beam trawl. Settlement of planktonic postlarvae peaked during the pre-wet season (October to December), and declined through the wet season (January to March). Using length-frequency analysis between 12 and 14 cohorts of juvenile prawns were identified each year. Length-frequency analysis and modal progression were used to derive growth rates during the estuarine phase of the life cycle. Growth rates, which could be described by a linear model, ranged from 0.63 to 1.65 mm CL (carapace length) wk-1. Growth rates were positively influenced by water temperature and negatively influenced by prawn density. Salinity had no effect on growth rates. Prawns spent between 6 and 20 wk in the Embley River before emigrating offshore from the estuary. Weekly instantaneous rates of natural mortality (M) ranged from 0.23 to 0.94, and in general were lowest during the dry season (July to September) and highest during the pre-wet and wet seasons. Only temperature significantly influenced mortality rates, with mortality rates increasing with temperature. By projecting juvenile growth rates forward through time, we established which cohorts contributed to the offshore fishery each year. In 1987 and 1988 the April fishery consisted of prawns which had settled in the river before the end of January each year. Slow growth rates during the pre-wet season of 1988 meant that only cohorts that were settled before early December 1988 contributed to the fishery in April 1989. Whether a cohort contributes to the fishery depends on the settlement date, water temperature and prawn density.

Notes: Juvenile Monocanthus chinensis digested most of the flesh of post-larval Penaeus monodon within 60 min, and most exoskeletal components within 180 min. Rapid cooling of predator samples should improve estimates of consumption of young prawns.