R.A. Kenyon

Habitat type and light affect sheltering behaviour of juvenile tiger prawns (Penaeus esculentus Haswell) and success rates of their fish predators

The burying and sheltering behaviour of two different sizes of juvenile tiger prawns (shrimp) Penaeus esculentus Haswell was studied during light and dark periods, in three habitats: bare silt substratum; short, thin-leaved seagrass (Halodule uninervis Aschers.); and tall, broad-leaved seagrass (Cymodocea serrulata Ashers & Magnus). Relative predation rates by the sand bass Psammoperca waigiensis Cuvier were also studied in these three habitats and in an additional seagrass with tall, thin leaves (Syringodium isoetifolium Ashers). Burying behaviour differed with prawn size, seagrass habitat and exposure to light. Small prawns (2.5 to 3.5 mm carapace length [CL]) rarely buried, even on bare substratum, whereas large prawns (11 to 13 mm CL) buried often and in all habitat types. However, large prawns buried in the substratum less often in tall, broad-leaved seagrass than in short, thin-leaved seagrass. Small prawns remained emerged in both the light and dark, whereas large prawns spent more time above the substratum in the dark than in the light. Small prawns perched on all types of seagrass leaves, whereas large prawns perched only on the broad-leaved seagrass. Fish detected juvenile P. esculentus (4 to 5 mm CL) more quickly in narrow-leaved seagrasses than in broad-leaved seagrass, and more quickly and more often, on bare substratum than in narrow-leaved seagrasses. More juvenile prawns were detected in bare substratum and the thin-leaved seagrasses, S. isoetifolium and H. uninervis, than in the broader-leaved C. serrulata. More prawns were caught and eaten on bare substratum than in the broad-leaved seagrass. These results indicated that small juvenile tiger prawns would suffer higher rates of predation in short, thin seagrass and unvegetated habitats, and this would lead to a decrease in abundance in these habitats. In the field, observations of the abundance of prawns in seagrass habitats of different structural complexity suggests post-settlement processes such as predation may explain prawn distribution.

The influence of seagrass type on the distribution and abundance of postlarval and juvenile tiger prawns (Penaeus esculentus and P. semisulcatus) in the western Gulf of Carpentaria, Australia

Postlarval and juvenile tiger prawns (Penaeus esculentus and P. semisulcatus) were sampled by beam trawls in different seagrass communities of the western Gulf of Carpentaria to test the influence of seagrass characteristics on their distribution and abundance. Two sampling regimes were carried out: fortnightly sampling over a 3 month period during the time of peak recruitment to investigate the importance of different seagrass types to tiger prawns; and at one time to investigate the wider geographic patterns of distribution and abundance of tiger prawns. The results from both studies showed that the pattern of distribution and abundance of 1–1.9 mm carapace length (CL) postlarvae differed from those for the 2–2.9 mm CL postlarvae and juvenile tiger prawns (≥3 mm CL). The 1–1.9 mm CL size class consists of individuals that are about to settle and those that have recently settled: this size class was more widely distributed than the larger postlarvae and juveniles and was sometimes found on substrates where no seagrass was present. Larger tiger prawns were found in higher numbers on seagrass beds where the tall, broad-leaved seagrass Enhalus acoroides was the dominant species. Although seagrass biomass was not a consistent linear predictor of juvenile tiger prawn numbers, mean catches of both the 2–2.9 mm CL postlarvae and juvenile P. esculentus were highest when the biomass of seagrass exceeded 100 g m−2. However, these high biomass seagrass beds contribute only 6% to the total extent of seagrasses in the shallow waters (<2.5 m deep) of the Gulf of Carpentaria. Although the numbers of juvenile tiger prawns were lower in the low-biomass seagrass beds, because of their extent, these seagrass beds are the main nurseries for sustaining the production of the valuable Northern Prawn Fishery in Australia.

Population dynamics of juvenile tiger prawns (Penaeus esculentus and P. semisulcatus) in seagrass habitats of the western Gulf of Carpentaria, Australia

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).

Assessing techniques for estimating the extent of mangroves : topographic maps, aerial photographs and Landsat TM images

Coastal habitats are critical to the sustained production of many fisheries. It is important, therefore, that fishery managers obtain accurate estimates of the extent of these habitats. This study investigated three methods of estimating the linear extent and area of mangroves (commercially available topographic data, aerial photographs and Landsat Thematic Mapper satellite imagery) in two regions in northern Australia: the Joseph Bonaparte Gulf (in particular, the Berkeley and Lyne Rivers) with typically narrow fringes of mangroves (<50 m wide), and the Embley River on Cape York Peninsula, with much broader mangrove stands (50-1000 m wide). Ground-truthing verified that aerial photographs provided the most accurate estimates of extents of mangroves in all rivers, because of their high spatial resolution (2 m). Landsat Thematic Mapper imagery gave good estimates of the area of habitats, but, because of the 30 m pixel resolution, it underestimated the linear extent in places where the mangrove fringe was narrow. Topographic data gave good estimates of the extent of mangroves where the forests were more extensive and less linear in shape, but were very poor otherwise at this scale. These findings have implications for the use of remote sensing techniques in ecological studies in these regions.

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.