The effect of habitat and environmental history on otolith chemistry of barramundi Lates calcarifer in estuarine populations of a regulated tropical river

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Abstract

We examine the microchemistry of otoliths of cohorts of a fished population of the large catadromous fish, barramundi Lates calcarifer from the estuary of a large tropical river. Barramundi from the estuary of the large, heavily regulated Fitzroy River, north-eastern Australia were analysed by making transects of 87Sr/86Sr isotope and trace metal/Ca ratios from the core to the outer edge. Firstly, we examined the Sr/Ca, Ba/Ca, Mg/Ca and Mn/Ca and 87Sr/86Sr isotope ratios in otoliths of barramundi tagged in either freshwater or estuarine habitats that were caught by the commercial fishery in the estuary. We used 87Sr/86Sr isotope ratios to identify periods of freshwater residency and assess whether trace metal/Ca ratios varied between habitats. Only Sr/Ca consistently varied between known periods of estuarine or freshwater residency. The relationships between trace metal/Ca and river flow, salinity, temperature were examined in fish tagged and recaptured in the estuary. We found weak and inconsistent patterns in relationships between these variables in the majority of fish. These results suggest that both individual movement history within the estuary and the scale of environmental monitoring were reducing our ability to detect any patterns. Finally, we examined fish in the estuary from two dominant age cohorts (4 and 7 year old) before and after a large flood in 2003 to ascertain if the flood had enabled fish from freshwater habitats to migrate to the estuary. There was no difference in the proportion of fish in the estuary that had accessed freshwater after the flood. Instead, we found that larger individuals with of each age cohort were more likely to have spent a period in freshwater. This highlights the need to maintain freshwater flows in rivers. About half the fish examined had accessed freshwater habitats before capture. Of these, all had spent at least their first two months in marine salinity waters before entering freshwater and some did not enter freshwater until four years of age. This contrasts with the results of several previous studies in other parts of the range that found that access to freshwater swamps by larval barramundi was important for enhanced population productivity and recruitment.

Introduction

Water extraction for human activities has a major impact globally on the productivity of rivers, their estuaries and associated wetlands (Gopal and Sah, 1993, Jensen, 2001, Gillanders and Kingsford, 2002, Hillman and Brierley, 2002). To reduce the scale and severity of these impacts, governments worldwide are increasingly moving from an engineering-based to an environmental management approach to water allocation (Finlayson and Brigza, 2000). The recognition of the need for environmental flows has been incorporated in water management policy in over 30 countries (Hillman and Brierley, 2002).

To develop effective water management that includes freshwater flows for riverine and estuarine organisms, we need a greater understanding of the causal mechanisms involved (Staunton-Smith et al., 2004, Robins et al., 2005). Most studies to date have focused on comparing catch rates of commercial fisheries with freshwater flows (Sutcliffe et al., 1977, Lloret et al., 2001, Quiñones and Montes, 2001, Robins et al., 2005). These studies have found significant covariation between flow and catch of many marine and estuarine species. They suggest that freshwater flows can influence fish spawning, survival and growth during their first year of life (Drinkwater and Frank, 1994).

Mechanisms by which freshwater flow enhances estuarine fish populations will vary between species and depend on their life history (Robins et al., 2005). For anadromous and catadromous fish species, freshwater flow is required to maintain natural migrations around dams and barrages. Catadromous fish species such as centropomids may have enhanced survival and growth in years when larvae and juveniles from coastal spawning grounds can access freshwater habitats (Staunton-Smith et al., 2004).

The large catadromous centropomid, barramundi (Lates calcarifer) is an important commercial and recreational species of estuaries and freshwater wetlands throughout southern and southeast Asia, Papua New Guinea and northern Australia. In northern Australia, it reaches up to 30 kg and occurs widely in most habitats in both the estuaries and accessible freshwater reaches of most rivers. In many rivers, barrages and dams have limited access by barramundi (and other species) to the extent that they no longer occur in freshwater reaches where they were previously abundant (Hogan and Vallance, 2005). To help overcome the disruption to natural migration, there has been a stocking program for barramundi in many natural and artificial water bodies in northeastern Australia (Queensland).

Many river systems in northeastern Queensland, such as the large Fitzroy River have numerous barrages that restrict or even halt flows during the Austral winter when rainfall is minimal. Barramundi stocked in these barrages can only move downstream during floods that cause sufficient flow to fill all the barrages and allow connectivity. Thus, there needs to be a minimum flow in these regulated rivers before fish can migrate.

Besides movement of freshwater-resident fishes, moderate to large floods also fill temporary coastal nursery swamps and provide large amounts of additional habitat for larval and juvenile barramundi (Moore, 1982, Davis, 1987). These temporary coastal freshwater swamps are believed to enhance both survival (Russell and Garrett, 1983, Russell and Garrett, 1985) and growth (Robins et al., 2006) of juvenile barramundi. In contrast, a recent study of otolith chemistry of barramundi in Papua New Guinea found little evidence that fish were spending extended periods in freshwater during the first year of their life (Milton and Chenery, 2005). This study used both Sr/Ca ratios and 87Sr/86Sr isotope ratios to infer fish movements between freshwater and coastal habitats. This combined approach is more sensitive to changes in water chemistry as 87Sr/86Sr isotope ratios are not subject to the same potential confounding effects often detected for trace element chemistry (Bath et al., 2000, Elsdon and Gillanders, 2002, Elsdon and Gillanders, 2003).

Otolith chemistry methods are being increasingly used to improve understanding of fish habitat use and movements. This is largely due to changes in water chemistry being detectable in otolith concentrations of several metals and their isotopes (Kennedy et al., 1997, Campana, 1999, Milton and Chenery, 2001). Otoliths are primarily aragonitic calcium carbonate and the concentrations of common metals such as Sr, Ba, Mn and Mg change in response to environmental conditions (Campana, 1999, Gillanders, 2005). Strontium has been the most widely used trace metal to infer movements between freshwater and marine habitats (Secor and Rooker, 2000). Temperature, growth and stress have also been shown to influence Sr concentrations in fish otoliths (Kalish, 1992, Sadovy and Severin, 1994).

87Sr/86Sr isotope ratios in otoliths of freshwater fish have been shown to represent the underlying geology of the parent rock in the catchments (Kennedy et al., 1997, Outridge et al., 2002, Bacon et al., 2004). The 87Sr/86Sr isotope ratios in the host rocks vary with Rb and Sr concentration and their age as 87Sr is the daughter of 87Rb (Dickin, 1995). Once dissolved from the rock during weathering, 87Sr/86Sr isotope ratios in water and the food chain remain stable without further fractionation (Blum et al., 2000, Kennedy et al., 2000). Thus, 87Sr/86Sr isotope ratios in otoliths can provide a reliable marker of the catchment history of each fish (Milton and Chenery, 2003).

The aims of this study are: (1) to examine 87Sr/86Sr isotope ratios and trace element concentrations in tagged fish to verify that otolith chemistry can detect differences in fish from freshwater, estuarine and marine habitats; (2) determine the scale of variation in trace metal concentrations between habitats; (3) examine any linkage with temperature, salinity, growth and freshwater flow volumes in the Fitzroy River in order to (4) assess the role of freshwater flows for providing connectivity for freshwater barramundi that may have returned to the estuary after a large flood event.

Section snippets

Description of study area

Barramundi were collected from several locations within the large Fitzroy River basin ∼142,537 km2 (Fig. 1). The region is near the Tropic of Capricorn (23° 30′ S) near the southern extent of barramundi distribution in Australia. The river is typical of tropical Australian systems and has a strongly seasonal flow regime with high summer flows (November–April) (Erskine et al., 2005) and low or zero winter flows (Staunton-Smith et al., 2004, Robins et al., 2005). The river has a mean annual

Results

A total of 29 tagged adult fish and 20 juveniles were examined for trace metals and 14 of these were reanalysed for Sr isotopes (Table 1). Adult fish were mostly chosen from two distinct groups, those caught and tagged in freshwater and subsequently caught in the estuary (10) and those that were tagged and recaptured in the estuary (10). Other fish analysed had been tagged and recaptured in freshwater impoundments and were known to be originally stocked (7 fish) or caught in hyper-saline water

Validation of metal/Ca ratios

The marked variation in 87Sr/86Sr isotopes in most freshwater bodies from the stable ratio in estuarine/marine waters (0.70918) offers a powerful tool to identify periods of freshwater and estuarine residency by fish (Kennedy et al., 1997). We have used these differences in the Fitzroy River to identify periods when tagged barramundi were in estuarine or fresh waters. This has then enabled us to assess the value of the trace metal/Ca ratios of fish for identifying fish habitat history. Our

Acknowledgments

We thank the Bob Packett from the Coastal Cooperative Research Centre for providing the temperature and salinity data. Marc Norman from ANU Research School of Earth Sciences helped with LA-ICPMS analysis. Phillip Ford and Grant Douglas from CSIRO Land and Water provided flood sediment Sr isotope data and 2003 flood water samples. Drs Stephen Swearer, Ron Thresher, Charles Bacon and Robyn Hannigan reviewed the manuscript and provided constructive comments on earlier versions. This study was

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