Field observations of moult cycle, feeding behaviour, and diet of small juvenile tiger prawns Penaeus semisulcatus in the Embley River, Australia

Juvenile tiger prawns Penaeus semisulcatus (2.5 to 5.5 mm carapace length) were collected on an intertidal seagrass bed in the Embley River estuary in the Gulf of Carpentaria, Australia. Prawns were sampled every 2 h over one 24 h period in the pre-wet (early October) and over one 24 h period in the late-wet season (late March) and the moult cycles and foregut contents determined. Premoult prawns were found mostly between 12:00 and 22:00 h and ecdysis occurred mostly between 20:00 and 0:00 h. A mean moult period of 3.0 d was estimated from the percentage of the prawns in premoult and ecdysis stages in early evening samples. On a scale of 0 (empty) to 10 (full), the mean foregut fullness of small juveniles was greater than 4.5 during both the day and night in each season. Copepods, filamentous algae, diatoms and unidentified material were the most common prey in both seasons. Insect larvae were more common in the pre-wet and nematodes and ostracods were more common in the late-wet. Little diel variation in diet was noted. Rapid foregut passage rates under field starvation conditions showed that between 55 and 60% of foregut contents was cleared in 1 h. This result, and the values for mean foregut fullness over 24 h, suggested that small prawns fed continuously through the day and night. Further field experiments in the presence of food showed that both hard and soft prey items were cleared from the foregut within 2 h and also that moult stage affected the feeding behaviour of prawns. Intermoult prawns maintained means of foregut fullness greater than 7, while premoult prawns ate less

Predation of juvenile tiger prawns in a tropical Australian estuary

We investigated predation of juvenile penaeids, in particular the grooved tiger prawn Penaeus semisulcatus and the brown tiger prawn P. esculentus, by sampling prawns and their potential fish predators on 2 intertidal seagrass beds in the Embley River, Queensland, Australia. Despite large differences in above-ground biomass of seagrass, these beds are both critical nursery areas for postlarval [=3 mm CL) tiger prawns. Thirty-seven species of fish were found to eat juvenile penaeids, but 76% of P. semisulcatus and 43% of P. esculentus (numbers) were found in the stomachs of 1 species of fish (Scomberoides commersonianus). Predation levels at the 2 seagrass beds did not appear to be related to the amount of seagrass present; rather the numbers of Penaeus semisulcatus eaten by fish increased with the numbers of P. semisulcatus on the seagrass beds. Compared to the size distributions of tiger prawns resident on the seagrass, postlarval and small juvenile tiger prawns were under-represented in the stomach contents of fish and only 17 of the 287 P. semisulcatus found in fish stomachs were <=4 mm CL. Although we were unable to quantify the component of natural mortality attributable to predation by fish, we have identified the major fish predators of juvenile tiger prawns and provide evidence suggesting that fish are not significant predators of postlarval and small juvenile tiger prawns

Seasonal, diel and tidal variation in beam-trawl catches of juvenile grooved tiger prawns, Penaeus semisulcatus (Decapoda:Penaeidae), in the Embley River, north-eastern Gulf of Carpentaria, Australia

Seasonal, diel and tidal cycles of beam-trawl catches of juvenile tiger prawns, Penaeus semisulcatus, from an intertidal seagrass bed in the Embley River, Gulf of Carpentaria, were studied at three-week intervals between August 1981 and August 1982. Two seasonal peaks in catches of P. semisulcatus were recorded around November and April. Split-plot analysis of variance showed that after seasonal variation was removed, catches were influenced by day-night and tidal cycles. Few P. semisulcatus juveniles were caught during the day, and more were caught around low tide than around high tide. Results from drop-trap sampling at low tide showed that juvenile P. semisulcatus remained in the seagrass bed even when it became exposed and the water temperature exceeded 35°C. These findings, and practical considerations, suggest that to study long-term changes in relative abundance of juvenile P. semisulcatus with beam trawls, sampling should be completed at night and during the early part of the flood tide

Distribution of juvenile penaeid prawns in mangrove forests in a tropical Australian estuary, with particular reference to Penaeus merguiensis

Several species of prawns, including juveniles of Penaeus merguiensis, will move into mangrove forests when the forests are inundated by flood tides. However, we do not know how extensively the prawns use the forests or whether some parts of the forests are more valuable to the prawns than others. We assessed the distribution of juvenile prawns in 3 different mangrove communities in intertidal forests adjacent to a small creek and a river in northern Australia between December 1993 and February 1995. The 3 mangrove communities were dominated, respectively, by the structurally complex Rhizophora stylosa, and the more open Ceriops tagal and Avicennia marina. We used stake nets (100 m long, 2 mm mesh) to sample discrete areas of the mangrove forests, and fyke nets (2 mm mesh) to sample prawns moving through the forests. The area of each stake net site ranged from 430 to 650 m2 and the distance of the midpoint of each site inland from the creek or river mangrove fringe ranged from 13 to 225 m. A large size range of juvenile P. merguiensis (2 to 21 mm carapace length, CL) was caught in the mangroves and prawns were caught as far as 200 m into the forests. In the creek forest, there was no clear relationship between mangrove community type and prawn catches. The highest densities of P. merguiensis were recorded in the creek and were 28.1 and 27.6 prawns 100m-2 in Rhizophora sp. and Ceriops sp. forest respectively. The highest mean catches were taken 59 m from the creek mangrove fringe. In contrast to the creek, in the river mangrove forest, there was a clear pattern of catches: the number of P. merguiensis caught decreased with distance into the mangroves and at shallower water depths. We have hypothesised that the distribution of juvenile P. merguiensis inside the mangroves depends largely on the local topography and pattern of water currents within each forest. Large numbers of unidentified Metapenaeus spp. and smaller numbers of M. ensis and P. monodon were also recorded from the samples inside the mangrove forests

How far do prawns and fish move into mangroves? Distribution of juvenile banana prawns Penaeus merguiensis and fish in a tropical mangrove forest in northern Australia

It is accepted that mangroves are important nursery areas for prawns and fish, including some of major commercial importance, but little is known about how these mobile animals use the mangrove forests. We recorded the distribution of juvenile banana prawns Penaeusmerguiensis and of fish in an intertidal mangrove forest adjacent to a small creek in northern Australia in November 1992 and in March 1993. Four discrete areas of the forest were enclosed with a 100 m long, 2 mm mesh stake net: 2 at the creek mangrove fringe and 2 at further distances into the mangroves. The mean distance of each sampling site inland from the creek mangrove fringe ranged from 13 to 59 m and the area of the sites ranged from 480 to 640 m2. Two mangrove communities, one dominated by the structurally complex Rhizophorastylosa, the other by the more open Ceriopstagal were sampled. A large size range of juvenile prawns and small fish moved at least 43 m into the mangroves at high tide, and the density of prawns near the creek mangrove fringe was inversely related to the maximum tide height. The highest density of juvenile P. merguiensis recorded in the mangroves in November was 18.3 prawns 100 m-2 and in March was 334.5 prawns 100 m-2. Mean fish density over all samples was 83.0 fish 100 m-2 and mean fish biomass was 3.9 g m-2; 55 species of fish were caught during the sampling. P.merguiensis showed no apparent preference for either of the 2 mangrove communities sampled; however, more fish (101 fish 100 m-2) and more fish species (26) were caught at the creek mangrove fringe site than at the other more inland sites; the lowest numbers of fish (27 fish m-2) and species (13) were caught at the inland site (Ceriops). On average, fish caught at the fringe site were also longer and heavier than fish caught at the other sites. By moving well into the mangrove forest, prawns and small fish are probably less vulnerable to predation by larger fish

Seasonal and annual variation in abundance of postlarval and juvenile banana prawns Penaeus merguiensis and environmental variation in two estuaries in tropical northeastern Australia:a six year study

We studied the fortnightly, seasonal and annual variation in abundance of postlarval and juvenile Penaeus merguiensis in the Embley and Mission River estuaries (northeastern Australia) and of juveniles emigrating from the rivers between September 1986 and March 1992. The climate is tropical with distinct wet and dry seasons, and the pattern of salinity and temperature changes in the estuaries closely reflects this seasonal variation. Postlarval and juvenile prawns were caught for up to 8 mo of the year, from before the wet season to just after the wet season (October to May). There was a weak bimodal distribution of catches during this period but the pattern of variation was quite variable from year to year. There were also subtle differences in the seasonal patterns of abundance between the Embley and Mission Rivers, probably due to different impacts of wet season rainfall on the 2 estuaries. In the rivers, environmental variation explained very little of the variation in catches of planktonic postlarvae, benthic postlarvae and juveniles, but increased emigration of juveniles from the rivers was significantly correlated with increased rainfall. The main factor determining the abundance of juvenile P. merguiensis in the estuaries was the supply and successful settlement of postlarvae from offshore areas. The highest densities of prawns caught within the estuaries were near the upper reaches of small creeks rather than in the main rivers. It is likely that these differences are due to differential settlement patterns of postlarvae between the creek and the main river. Although the largest emigration of juvenile prawns from the estuaries occurred during the wet season, lower levels of migration of prawns from the creeks to the main rivers occurred throughout the year. Over the 6 yr of this project, the annual offshore commercial catch of adult P. merguiensis was significantly correlated with catches of prawns emigrating from the Embley River during the wet season

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

Estimating the influence of prawn stocking density and seagrass type on the growth of juvenile tiger prawns ( Penaeus semisulcatus ): results from field experiments in small enclosures

Fine mesh enclosures (0.9 m 2 in basal area, 1 m high, with 100 μm mesh) and a jet-net retrieval system were developed to test the influence of juvenile prawn stocking density on growth rates in (1) different months (April and October/November) and (2) different types of intertidal seagrass beds in the Embley River estuary of tropical Australia. Small juvenile tiger prawns (3-6 mm in carapace length, CL) were stocked in enclosures at densities of 4-32 prawns per enclosure (4.4-35.5 prawns m -2) on a high biomass seagrass bed (about 70 g m -2 of mostly Enhalus acoroides) and one with low biomass (about 10 g m -2 of mostly Halodule uninervis). After 2-3 weeks in the enclosures, recovery rates, and hence possibly survival, were greater on the high biomass Enhalus than on the low biomass Halodule. However, not all fish and crustaceans could be excluded from the enclosures. Growth rates were twice as fast on the high biomass Enhalus than on the low biomass Halodule. It is likely that the high biomass Enhalus, with its greater surface area, supported more epiphytic flora and fauna and reduced the potential for interference competition between prawns, compared with the low biomass Halodule. Growth rates on Enhalus were significantly faster at a stocking density of 4 prawns per enclosure (1.3 mm CL week -1) than at a stocking density of 16 and 32 prawns per enclosure (both 0.8 mm CL week -1), and did not differ significantly between April and October/November (temperatures were about 30°C at both times). The mean growth rate at 8 prawns per enclosure (1.1 mm CL week -1) did not differ significantly from those at 4, 16 and 32 prawns per enclosure. These results from two seagrass beds suggest that the carrying capacity for juvenile tiger prawns was greater in the high biomass Enhalus than the low biomass Halodule bed

Estimating the efficiency of a small beam trawl for sampling tiger prawns Penaeus esculentus and P. semisulcatus in seagrass by removal experiments

The efficiency with which a small beam trawl (1 x 0.5 m mouth) sampled postlarvae and juveniles of tiger prawns Penaeus esculentus and P. semisulcatus at night was estimated in 3 tropical seagrass communities (dominated by Thalassia hemprichii, Syringodium isoetifolium and Enhalus acoroides, respectively) in the shallow waters of the Gulf of Carpentaria in northern Australia. An area of seagrass (40 x 3 m) was enclosed by a net and the beam trawl was repeatedly hand-hauled over the substrate. Net efficiency (q) was calculated using 4 methods: the unweighted Leslie, weighted Leslie, DeLury and Maximum-likelihood (ML) methods. The Maximum-likelihood is the preferred method for estimating efficiency because it makes the fewest assumptions and is not affected by zero catches. The major difference in net efficiencies was between postlarvae (mean ML q +/- 95% confidence limits = 0.66 +/- 0.16) and juveniles of both species (mean q for juveniles in water <=1.0 m deep = 0.47 +/- 0.05), i.e. the beam trawl was more efficient at capturing postlarvae than juveniles. There was little difference in net efficiency for P. esculentus between seagrass types (T. hemprichii versus S. isoetifolium), even though the biomass and morphologies of seagrass in these communities differed greatly (biomasses were 54 and 204 g m-2, respectively). The efficiency of the net appeared to be the same for juveniles of the 2 species in shallow water, but was lower for juvenile P. semisulcatus at high tide when the water was deeper (1.6 to 1.9 m) (0.35 +/- 0.08). The lower efficiency near the time of high tide is possibly because the prawns are more active at high than low tide, and can also escape above the net. Factors affecting net efficiency and alternative methods of estimating net efficiency are discussed