King threadfin

Abstract: Proper management of marine fisheries requires an understanding of the spatial and temporal dynamics of marine populations, which can be obtained from genetic data. While numerous fisheries species have been surveyed for spatial genetic patterns, temporally sampled genetic data is not available for many species. We present a phylogeographic survey of the king threadfin Polydactylus macrochir across its species range in northern Australia and at a temporal scale of 1 and 10 yr. Spatially, the overall AMOVA fixation index was Omega(st) = 0.306 (F-st' = 0.838), p < 0.0001 and isolation by distance was strong and significant (r(2) = 0.45, p < 0.001). Temporally, genetic patterns were stable at a time scale of 10 yr. However, this did not hold true for samples from the eastern Gulf of Carpentaria, where populations showed a greater degree of temporal instability and lacked spatial genetic structure. Temporal but not spatial genetic structure in the Gulf indicates demographic interdependence but also indicates that fishing pressure may be high in this area. Generally, genetic patterns were similar to another co-distributed threadfin species Eleutheronema tetradactylum, which is ecologically similar. However, the historical demography of both species, evaluated herein, differed, with populations of P. macrochir being much younger. The data are consistent with an acute population bottleneck at the last glacio-eustatic low in sea level and indicate that the king threadfin may be sensitive to habitat disturbances.

Abstract: Based on light and scanning electron microscopical studies, the following nine species of Philometridae (Nemaoda: Dracunculoidea) are described from female worms parasitizing marine perciform fishes belonging to six families off the northern coast Australia (near Darwin): Philometra australiensis sp. n. from the swimbladder of the king threadfin Polydactylus macrochir (Gunther) (Polynemidae); P. epinepheli Dewi et Palm, 2013 from the operculum of the orange-spotted grouper Epinephelus coioides (Hamilton) (Serranidae); Philometra johnii Moravec et Ali, 2013 from the gonad of the croaker Johnius sp. (Sciaenidae); P. macrochiri sp. n. from the sensory fin of P. macrochir; P. zabidii sp. n. from the ovary of the ninespine batfish Zabidius novemaculatus (McCulloch) (Ephippidae); Philometra sp. 1 and Philometra sp. 2 from the ovary of the Spanish flag snapper Lutjanus carponotatus (Richardson) (Lutjanidae) and the silver grunt Pomadasys argenteus (Forsskal) (Haemulidae), respectively; Philometroides eleutheronemae Moravec et Manoharan, 2013 from the ovary of the fourfinger threadfin Eleutheronema tetradactylum (Shaw) (Polynemidae); and Spirophilometra endangae Dewi et Palm, 2013 from the pectoral fins of E. coioides. The new species P. australiensis is characterized mainly by the structure of the cephalic end, 14 minute cephalic papillae, absence of caudal projections and body length of gravid female (67 mm), P. macrochiri by the presence of a conspicuously large anterior oesophageal bulb, 14 very small cephalic papillae and the truncated posterior end of body without any caudal projections, whereas P. zabidii is characterized by the presence of distinct caudal projections, the number (14) and larger size and arrangement of cephalic papillae, a poorly developed anterior oesophageal inflation, the body length (114 mm) and the host family (Ephippidae). All above-mentioned species were recorded from Australian waters for the first time.

Abstract: The requirement for Queensland, Northern Territory and Western Australian jurisdictions to ensure sustainable harvest of fish resources relies on robust information on the resource status. In northern Australia management of inshore fisheries that target blue threadfin (Eleutheronema tetradactylum) and king threadfin (Polydactylus macrochir) is independent for each of these jurisdictions. However, the lack of information on the stock structure and biology of threadfins means that the appropriate spatial scale of management is not known and assessment of the resource status is not possible. Establishing the stock structure of blue and king threadfin would also immensely improve the relevance of future resource assessments for fishery management of threadfins across northern Australia. This highlighted the urgent need for stock structure information for this species. The impetus for this project came from unsuccessful attempts in Queensland to conduct stock assessments for the king and blue threadfin resource, research that indicated the potential for localised stock structure, and the assessment that blue and king threadfin in Western Australia were fully and over-exploited respectively. The project objectives were to determine the stock structure of blue and king threadfin across their northern Australian range, and use this information to define management units and their appropriate spatial scales. We used multiple techniques concurrently to determine the stock structure of each species, including: genetic analyses (mitochondrial DNA and microsatellite DNA), otolith (ear bones) stable isotope ratios, parasite abundances, and life history parameters (growth and size at sex change). This holistic approach to stock identification gave the advantage of using techniques that were informative about the fish's life history at different spatial and temporal scales, increasing the likelihood of detecting different stocks where they existed and providing greater certainty in the signals given by the data. Genetics can inform about the evolutionary patterns as well as rates of mixing of fish from adjacent areas, while parasites and otolith microchemistry are directly influenced by the environment and so will inform about the patterns of movement during the fishes lifetime. Life history characteristics are influenced by both genetic and environmental factors. We adopted a phased sampling approach whereby sampling was carried out at broad spatial scales in the first year at locations along the east coast, within the Gulf of Carpentaria (GoC), and the Western Australian coastline. Using each of the techniques to compare fish samples collected from each of these locations we tested the null hypothesis for each species that they were comprised of a single homogeneous population across northern Australia. The null hypothesis was rejected after the first year leading us to re-sample the first year locations to test for temporal stability in stock structure, and to assess stock structure at finer spatial scales by sampling at other locations as well. Blue threadfin showed strong site fidelity with localised stock structuring evident and adjacent stocks separated by only tens of kilometres. This was found even where continuous habitat was present along coastlines with no obvious barriers to mixing. This was shown by clear and consistent signals of differences between fish from different locations including genetic differences. Blue threadfins also show what is called 'isolation by distance' whereby the farther apart stocks are from one another the greater the genetic differences between them. There was also extreme variability found in the life history characteristics among the different stocks. Similarly, king threadfin also showed fine scale stock structure with limited mixing between adjacent stocks separated by tens to hundreds of kilometres. Where there was sufficient distances separating them, or bio-geographical barriers such as headlands separating adjacent stocks, king threadfin were also genetically distinct. King threadfin also exhibited 'isolation by distance' though the pattern was notas strong as in blue threadfin. King threadfins also show a high degree of variation in their life history characteristics among the different stocks identified. Further, in the eastern Gulf of Carpentaria evidence of overfishing of king threadfin was evident in the truncation of size and age structures compared with samples taken over a decade ago, and the presence of females much smaller than found elsewhere or reported from the same region previously. The management implications of these results indicate the need for management of threadfin fisheries in Australia to be carried out on regional scales much finer than are currently in place. Given the fine spatial scale stock structure evident for both threadfin species management at local scales may not be pragmatic. At the very least management should consider these spatial dynamics by implementing monitoring and assessment of threadfin fisheries guided by the stocks identified in this study, and by the likely spatial scale of stocks indicated by these results. We also encourage the assessment of the threadfin resource status for the major fishery region in northern Australia. We recommend that the signals of overfishing detected for king threadfin in the Gulf of Carpentaria need to be investigated to assess the status of the stocks present in that region.

Abstract: Temporal and spatial patterns in parasite assemblages were examined to evaluate the degree of movement and connectivity of post-recruitment life-history stages of a large, non-diadromous tropical estuarine teleost, king threadfin Polydactylus macrochir, collected from 18 locations across northern Australia. Ten parasites types (juvenile stages of two nematodes and seven cestodes, and adults of an acanthocephalan) were deemed to be suitable for use as biological tags, in that they were considered to have a long residence time in the fish, were relatively easy to find and were morphologically very different to each other which aided discrimination. Univariate and discriminant function analysis of these parasites revealed little difference in temporal replicates collected from five locations, suggesting that the parasite communities were stable over the timeframes explored. Univariate, discriminant function, and BrayCurtis similarity analyses indicated significant spatial heterogeneity, with BrayCurtis classification accuracies ranging from 55 to 100% for locations in north-western and northern Australia, 24 to 88% in the Gulf of Carpentaria, and 39 to 88% on the east coast of Queensland. Few differences were observed among locations separated by <200 km. The observed patterns of parasite infection are in agreement with concurrent studies of movement and connectivity of P. macrochir in that they indicate a complex population structure across northern Australia. These results should be considered when reviewing the management arrangements for this species.