Movement, connectivity and population structure of a large, non-diadromous, tropical estuarine teleost

Moore, Bradley Roland (2011) Movement, connectivity and population structure of a large, non-diadromous, tropical estuarine teleost. PhD thesis, James Cook University.

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Understanding the degree of exchange between groups of a species, or connectivity, is fundamental to the effective management and conservation of aquatic species and ecosystems, yet remains poorly understood for tropical estuarine fishes. With anthropogenic pressures in the form of increased fishing pressure, habitat modification and climate change on this group projected to increase, understanding patterns of connectivity becomes increasingly important so that effective management can be implemented. This thesis sought to provide one of the first empirical assessments of connectivity of a tropical, non-diadromous estuarine teleost, using the king threadfin, Polydactylus macrochir, as a focal species. Uniquely, this thesis provides the first use of a multidisciplinary approach to discern patterns of connectivity of a tropical estuarine teleost, incorporating data from multiple complementary techniques to assess connectivity at a range of spatial, temporal and ontogenetic scales across the species' Australian distribution.

In Chapter 3, life history parameters of P. macrochir were examined to provide preliminary information on the connectivity and population structure of the study species across northern Australia. Specifically, the timing of annuli deposition and spawning, and patterns in growth, mortality, length and age at maturity, and length and age at sex change of P. macrochir were examined at up to 18 locations. Considerable variation in life history parameters was observed among locations. Both unconstrained and constrained (t0=0) estimates of von Bertalanffy growth function parameters differed significantly among all neighboring locations, with the exception of two locations in Queensland's east coast and two in Queensland's Gulf of Carpentaria waters, respectively. Comparisons of back-calculated length-at-age 2 provided additional evidence for growth differences among some locations but were not significantly different among locations in the south-eastern Gulf of Carpentaria or on Queensland's east coast. Total mortality rates varied among locations, and were highest for estuaries within the Gulf of Carpentaria. The length and age at sex change differed markedly among locations, with fish from the east coast of Australia changing sex from males to females at significantly greater lengths and ages than elsewhere. Sex change occurred earliest at locations within Queensland's Gulf of Carpentaria, where a large proportion of small, young females were recorded. While it is unclear whether the spatial differences reflect genetic relationships, or result from differing environmental conditions amongst locations, the differences in life history parameters indicate limited mixing of at least post-larval fish, suggesting the probable existence of a number of spatially distinct groups of adult P. macrochir assemblages across northern Australia. These results suggest that future studies examining connectivity and geographic population structure of estuarine fishes will likely benefit from the inclusion of comparisons of life history parameters.

In Chapter 4, temporal and spatial patterns in parasite assemblages were examined to further assess the degree of movement and connectivity of P. macrochir collected from the same 18 locations examined in Chapter 3. 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. 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 Bray-Curtis similarity analyses indicated significant spatial heterogeneity, with classification accuracies ranging from 55–100% for locations in north-western and northern Australia, 24–88% in the Gulf of Carpentaria, and 39–88% on the east coast of Queensland. Few differences were observed among locations separated by < 200 km. The observed patterns of parasite infection support earlier examination of life history and suggest a complex population structure of P. macrochir in northern Australia, with post-larval populations generally undergoing limited movement and connectivity.

In Chapter 5, age-related trends in otolith elemental chemistry were examined to provide an indication of the degree of larval dispersal of P. macrochir, and to provide an additional measure of connectivity in post-larval fish. Elemental signatures (⁷Li, 24²⁴Mg, ⁵⁵Mn, ⁵⁹Co, ⁸⁸Sr and ¹³⁸Ba) of transverse sections of otoliths of 3+ year fish from the 2005 year class collected from 17 of the 18 locations examined in Chapters 3 and 4 were sampled using laser ablation inductively coupled plasma mass spectrometry, providing age-related elemental profiles from the otolith core through the first three years of a fish's life. Univariate and discriminant function analyses demonstrated little similarity in age-related average elemental concentrations among locations. Elemental signatures of the otolith core appeared different among most locations, although some similarities were evident among locations in the eastern Gulf of Carpentaria and among two neighbouring locations in Western Australia. Differences were evident in elemental signatures of post-larval otolith material among locations separated by as little as 50 km, suggesting fine-scale spatial structuring of juvenile and adult assemblages of P. macrochir. In the analyses, average ¹³⁸Ba concentration provided the most discrimination among locations. The spatial structuring evident here is largely consistent with examination of parasite assemblages and life history data and suggests that P. macrochir populations are highly susceptible to local depletion in most locations, with limited opportunity for replenishment from neighbouring populations. In addition, the results add to the growing body of literature that demonstrates limited connectivity of estuarine species and suggest that the age-related approach adopted here provides a viable, albeit indirect, alternative to assessing patterns of connectivity, particularly for studies in which collection of larvae or juveniles is not feasible.

Finally, in Chapter 6, data from 3,718 P. macrochir tagged by members of the Australian National Sportfishing Association and state fisheries researchers were analysed to provide additional information on the movement and connectivity of juvenile and adult fish in Queensland's east coast and Gulf of Carpentaria waters. Recapture information was available for 182 individuals tagged on the east coast and 40 individuals tagged in the Gulf of Carpentaria. No difference was observed between recapture rates of dart and anchor tagged fish in either region, suggesting the performance of these tags were similar. Connectivity among estuaries on the east coast of Queensland was limited, with 96% of all recaptures occurring in the same estuary in which fish were tagged. Movements outside of tagging estuaries on Queensland's east coast ranged from 1 km to 23 km. In the Gulf of Carpentaria, 70% of all recaptures occurred in the same estuary in which fish were tagged. Twelve individuals were recaptured outside of their tagging estuaries in this region. While four of these individuals were recaptured within 10 km from the mouth of their tagging estuary, eight individuals moved greater than 80 km outside of their tagging estuaries, including one individual that moved approximately 570 km. The limited connectivity among spatially-distinct estuaries is consistent with results of previous chapters and indicates that P. macrochir form a number of demographically-isolated populations in Queensland waters.

The findings of this study have a number of important implications for the management of P. macrochir, and for future research into the connectivity of estuarine fishes and fish populations in general. The limited connectivity evident for P. macrochir suggests that populations are susceptible to over-fishing, with limited opportunity of replenishment from neighbouring populations. These results indicate that P. macrochir, and other tropical estuarine species that exhibit similar patterns of restricted connectivity, should ideally be managed on a local scale to avoid localised depletion, and may be suitable candidates for protection by marine protected areas. More broadly, the findings of this thesis reinforce that a number of complementary techniques are required to accurately determine connectivity of fish populations and suggest that future studies examining connectivity of tropical estuarine fishes, and fishes in general, will greatly benefit from the inclusion of multiple, complementary approaches. Given the paucity of studies examining connectivity of non-diadromous tropical estuarine fishes, the findings of this thesis provide fundamental information from which more specific hypotheses of connectivity, population structure and gene flow of such species can be tested.

Item ID: 29751
Item Type: Thesis (PhD)
Keywords: population connectivity; fisheries management; estuarine fishes; life history; tropics; Polydactylus macrochir; king threadfin; Gulf of Carpentaria; eastern Queensland
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 3. Moore, Bradley R., Simpfendorfer, Colin A., Newman, Stephen J., Stapley, Jason M., Allsop, Quentin, Sellin, Michelle J., and Welch, David J. (2012) Spatial variation in life history reveals insight into connectivity and geographic population structure of a tropical estuarine teleost: king threadfin, Polydactylus macrochir. Fisheries Research, 125-126 . pp. 214-224.

Chapter 3. Moore, B.R., Welch, D.J., and Simpfendorfer, C.A. (2011) Spatial patterns in the demography of a large estuarine teleost: king threadfin, Polydactylus macrochir. Marine and Freshwater Research, 62 (8). pp. 937-951.

Chapter 4. Moore, B.R., Welch, D.J., Newman, S.J., and Lester, R.J.G. (2012) Parasites as indicators of movement and population connectivity of a non-diadromous, tropical estuarine teleost: king threadfin Polydactylus macrochir. Journal of Fish Biology, 81 (1). pp. 230-252.

Date Deposited: 14 Oct 2013 01:43
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 50%
07 AGRICULTURAL AND VETERINARY SCIENCES > 0704 Fisheries Sciences > 070403 Fisheries Management @ 50%
SEO Codes: 83 ANIMAL PRODUCTION AND ANIMAL PRIMARY PRODUCTS > 8302 Fisheries - Wild Caught > 830201 Fisheries Recreational @ 33%
83 ANIMAL PRODUCTION AND ANIMAL PRIMARY PRODUCTS > 8302 Fisheries - Wild Caught > 830204 Wild Caught Fin Fish (excl. Tuna) @ 34%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 33%
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