Disentangling the response of fishes to recreational fishing over 30 years within a fringing coral reef reserve network

Few studies assess the effects of recreational fishing in isolation from commercial fishing. We used meta-analysis to synthesise 4444 samples from 30 years (1987–2017) of fish surveys inside and outside a large network of highly protected reserves in the Ningaloo Marine Park, Western Australia, where the major fishing activity is recreational. Data were collected by different agencies, using varied survey designs and sampling methods. We contrasted the relative abundance and biomass of target and non-target fish groups between fished and reserve locations. We considered the influence of, and possible interactions between, seven additional variables: age and size of reserve, one of two reserve network configurations, reef habitat type, recreational fishing activity, shore-based fishing regulations and survey method. Taxa responded differently: the abundance and biomass inside reserves relative to outside was higher for targeted lethrinids, while other targeted (and non-targeted) fish groups were indistinguishable. Reef habitat was important for explaining lethrinid response to protection, and this factor interacted with reserve size, such that larger reserves were demonstrably more effective in the back reef and lagoon habitats. There was little evidence of changes in relative abundance and biomass of fishes with reserve age, or after rezoning and expansion of the reserve network. Our study demonstrates the complexities in quantifying fishing effects, highlighting some of the key factors and interactions that likely underlie the varied results in reserve assessments that should be considered in future reserve design and assessment

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

Impact of cyclones and aquatic macrophytes on recruitment and landings of tiger prawns Penaeus esculentus in Exmouth Gulf, Western Australia

The cover of seagrasses and macroalgae, landings and fishery-independent measures of spawning stock and recruitment for brown tiger prawns, were monitored immediately following a major cyclone in Exmouth Gulf, Western Australia in March 1999. Anecdotal evidence on the extent of seagrass from 1990 to 1998 suggests that the cyclone caused a major, immediate disruption and loss of the seagrass/macroalgal beds (to ≤2% cover), the critical prawn nursery habitat, and mangroves in the shallow inshore waters of the system. Prawn landings and recruitment to the fishery were not affected in the year of the cyclone, but were markedly lower in the two years immediately afterwards and then increased as the cover of macrophytes increased to over 40% in 2003. Tiger prawn landings and catch rates were not affected in Shark Bay, a system 500 km south of Exmouth Gulf that did not experience cyclonic disturbance. Seagrasses in Exmouth Gulf showed a succession of species from small colonising species (Halophila ovalis and Halodule uninervis) to larger, broad-leaved species (Cymodocea serrulata, Syringodium isoetifolium) only two years after the cyclone. The recruitment and landings of tiger prawns were correlated significantly with the total cover of macroalgae and seagrass. The large loss of seagrass and macroalgae reduced the settling habitat for postlarvae and the nursery habitat for juvenile tiger prawns, probably leading to the lower recruitment to the fishery. These findings suggest that the extent of seagrass and macroalgae are some of the factors defining the productivity of the tiger prawn fishery in Exmouth Gulf

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)

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

Investigation of artifacts from chronographic tethering experiments—interactions between tethers and predators

We used a submersible chronographic tethering device to compare the survival time of tethered juvenile brown tiger prawns (Penaeus esculentus [Haswell, 1897]) in seagrass and bare plots at two different sites. At one site, more prawns survived and those that were eaten survived for longer among the seagrass than on the bare habitat. However, at the other site there was little difference between numbers of survivors and survival time in the seagrass compared to the bare substrate. Laboratory observations suggest that the difference in survival was due to an effect of the tethers on the prawn's ability to escape from blue-swimmer crabs (Portunus pelagicus [Linnaeus 1758]), and differences in the abundance of P. pelagicus at the two field sites. P. pelagicus rely on olfaction, rather than sight to locate their prey, and so the crabs' efficiency at locating tethered prawns is not affected by the structure that seagrass provides. Our results clearly demonstrate an example of an artifact caused by experimental intervention, and highlight the need to investigate the possible effects of these types of artifacts

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)

New directions for research in prawn (shrimp) stock enhancement and the use of models in providing directions for research

Despite attempts in many countries, large-scale prawn (shrimp) stock enhancement programmes have only been implemented and continue to operate in Japan (Penaeus japonicus) and China (Penaeus chinensis). In these countries, prawn stock enhancement has been possible because of the capacity to produce large numbers of small individuals for release as a result of aquaculture. While exploring the feasibility and bio-economics of tiger prawn (Penaeus esculentus) stock enhancement in Exmouth Gulf, Australia, a broad framework for stock enhancement research was developed. This framework included developing a bio-economic model and risk assessment for all components of a stock enhancement operation, i.e., hatchery, grow-out, harvest-transport-release, population dynamics of released and wild prawns, and monitoring the fishery. However, predictions from the bio-economic model were made with great uncertainty about the values for post-release mortality of prawns because there were no data from experiments on optimal release strategies. Consequently, data from field studies of growth, and laboratory studies of predation, were used to set values for growth and mortality in Monte-Carlo simulations of the potential success of P. esculentus releases in different habitats. The survival of 2 mm carapace length (CL) prawns to the size at emigration from the nursery habitat was ∼1.9 times higher for individuals associated with seagrass of high (∼100 g m-2) than for those in low (∼10 g m-2) plant biomass. Survival was 19 times higher for prawns in high biomass seagrass than for those on bare substratum. The differences in survival between habitats were reduced greatly when the releases of much larger prawns (10 mm CL) were simulated. These findings highlight the importance of research on developing optimal release strategies for prawns in stock enhancement programmes. In particular, the size, habitat, time and density at release needs to be identified to provide more reliable information for the bio-economic assessments of releasing hatchery-reared juveniles to augment production in stock enhancement programmes

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