Fisheries biology and movements of mangrove red snapper, Lutjanus argentimaculatus (Forsskål 1775), in New South Wales

Toby P Piddocke

The mangrove red snapper Lutjanus argentimaculatus is a medium to large tropical snapper (family Lutjanidae) found throughout the tropical and subtropical Indo-West Pacific. Lutjanus argentimaculatus biology has been thoroughly studied in tropical waters, where an ontogenetic migration from inshore juvenile habitats to offshore environments coincides approximately with sexual maturity. Tropical studies have also identified age-based demographic traits that render the species vulnerable to overexploitation, including longevity >50 years, low instantaneous natural mortality rates (M) and attainment of sexual maturity relatively late in life. However, the extent to which life-history traits described for tropical waters persist at the species’ cooler range limits, in the state of New South Wales (NSW) on the east coast of Australia, was unclear. Lutjanus argentimaculatus in NSW is not directly targeted by any commercial fisheries, but is prized by recreational line and spear fishers. Few species-specific regulatory measures have been set in the fishery and the NSW Saltwater Recreational Fishing Trust had identified the fishery biology of L. argentimaculatus as a researchable knowledge gap, providing an impetus for this project. The primary aim of the thesis was to provide biological and ecological information to inform management of this fishery. A secondary aim was to critically assess lutjanid age validation research. These aims are addressed in four major chapters contextualised by an Introduction and Synthesis. Chapter 2 critiques lutjanid age validation research through a literature review, and Chapter 3 presents an age and growth study using sectioned otoliths. The age structure described in Chapter 3 is consistent with ontogenetic migration from estuaries to offshore. Chapter 4 therefore seeks further evidence of ontogenetic migration by comparing gonadal development of estuarine and offshore L. argentimaculatus. Chapter 5 is an acoustic telemetry study that provides an additional line of evidence for ontogenetic migration, and also investigates L. argentimaculatus movements in three northern NSW estuaries. Chapter 2 demonstrates that age validation studies have uniformly supported annual increment formation in lutjanid otoliths, including those of L. argentimaculatus. The four main techniques used to validate lutjanid ages are marginal increment analysis (MIA), chemical marking, bomb radiocarbon dating and radiometric dating using the uranium pair 210Pb:226Ra (lead-radium dating). Bomb radiocarbon and lead-radium dating provide absolute age estimates, which can validate increment counts in calcified structures. However, bomb radiocarbon dating cannot accurately age fish older than ~55 years, an age some lutjanids meet or exceed. In contrast, lead-radium dating is applicable to fish aged up to approximately 100 years. Technical advances and empirical verification of key assumptions over the past two decades have established the lead-radium chronometer’s accuracy and validity, but precision varies with several factors including otolith mass and 226Ra activity. Chemical marking using fluorochromes clarifies changes in increment appearance and periodicity as fish age, but lutjanid recapture rates are often poor, while captive rearing may affect influence increment deposition. Marginal increment analysis (MIA) is cost-effective and conceptually simple, but is susceptible to bias and misinterpretation if not rigorously applied. Chapters 3–5 provide several lines of evidence suggesting that the ontogenetic migration from estuaries to offshore habitats reported for tropical L. argentimaculatus populations persists at the species’ southern range limits. In the age and growth study using sectioned otoliths (Chapter 3), all 16 fish of age ≥20 years were from offshore environments, whereas the oldest estuarine fish was 8+ years and 584 mm fork length (LF). The species’ age-based demography at its cooler range limits is broadly similar to that described for tropical waters. The oldest fish aged was 57+ years, which is the greatest age yet recorded for a lutjanid. Consistent with this longevity, instantaneous natural mortality rates (M) were low, ranging from 0.072–0.25. Growth was described by the von Bertalanffy growth function with parameters L∞ = 874.92 mm, K = 0.087 year-1 and t0 = -2.76 years. The reproductive study (Chapter 4) involved macroscopic staging of 119 L. argentimaculatus gonads, with a subset (n = 17) examined microscopically after histological preparation. There was no evidence of reproductive maturity or previous spawning in any estuarine fish (n = 67). All spawning capable fish (n = 16) were captured offshore. The smallest such male and female were 657 mm LF and 757 mm LF respectively. In the telemetry study (Chapter 5), thirteen fish were detected leaving the study estuaries for the open sea, and six of these were subsequently recaptured or detected by offshore acoustic receivers after movements of up to 230 km. Estuarine movements were characterised by high site fidelity, although this varied between estuaries and individuals. Duration of continuous detection periods (‘residence events’) provided an index of fish activity at spatial scales ≥ tens of metres, and indicated that fish in the two smallest study estuaries were most active during spring or summer. Seasonal variation in activity was less marked in the third, much larger estuary, possibly reflecting a different spatial arrangement of habitat types. Across all three estuaries, short residence events measured by the receivers indicated that fish were consistently more active at night than during daylight. Overall, the age-related demography and spatial ecology of L. argentimaculatus suggest susceptibility to overexploitation. Exploitation of immature estuarine individuals is possible because angling for the species is commonplace in estuaries and may be problematic for population persistence, as may excessive harvesting of very large fish on offshore reefs. However, data on recreational catch and effort are lacking, as are objective measures of abundance. The species’ current status is therefore unclear, but precautionary management measures, such as low individual catch quotas (‘bag limits’) are advisable. Rapid recovery from capture and surgical implantation of acoustic transmitters in this project suggests that L. argentimaculatus is a good candidate for catch-and-release fishing. The species’ capacity for long-distance dispersal following departure from estuaries indicates that cross-jurisdictional management approaches may be required.

Fisheries biology and movements of mangrove red snapper, Lutjanus argentimaculatus (Forsskål 1775), in New South Wales

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Fisheries biology and movements of mangrove red snapper, Lutjanus argentimaculatus (Forsskål 1775), in New South Wales

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