Norman Hall

Less variable than life history parameters (LHPs), it is life history ratios (LHRs) that define how taxa allocate energy between growth, maintenance and reproduction, and respond to fishing pressure. Limited by small samples, variable data quality, and a focus on LHP estimation, previous meta-analyses have failed to settle debate about the extent to which LHRs are relatively invariant across all taxa or characteristic of specific taxa. We collected de novo 1335 published studies and applying rigorous standardization and quality control procedures developed, and make available, a database of high-quality M/K and L-m/L-infinity estimates. We describe two parallel but independent meta-analyses: a cross-validation study of the predictability of M/K by taxonomic category and an evaluation of alternative relationships between the LHRs using Akaike information criteria. These analyses demonstrate that the LHRs are correlated and vary predictably by taxa, with aggregation to the level of family and genera having the most predictive power in our database. We postulate that the LHRs of taxa may relate to their stoichiometric niches, which could open up interesting lines for ecological research and provide new tools for predicting the LHRs of poorly studied taxa.

This study explored the extent to which the hermaphroditic and reproductive characteristics vary among five congeneric (Choerodon) and co-occurring species of the Labridae, a family in which hermaphroditism is widespread. Sampling was located in the large sub-tropical Shark Bay (26° S, 113° E), one of only twenty World Heritage Properties in Australia. Choerodon cephalotes, C. cyanodus, C. cauteroma and C. schoenleinii live within the main body of the embayment, which approaches the southern limit of their tropical, geographical ranges, whereas C. rubescens lives along its outer western boundary, close to the northern limit of its warm, temperate distribution. A combination of comprehensive length and age compositions, histological analyses of gonads and logistic regressions of lengths and ages at maturity and sex change are consistent with all five species being monandric protogynous hermaphrodites. Choerodon cephalotes, C. rubescens and C. schoenleinii are sexually dichromatic, with the lengths and ages of males at the extreme upper end of those of females and low proportions of males among adults (adult sex ratios, ASR: 11–33%). It is proposed that these species have polygynous mating systems, such as leks or harems. Although C. cauteroma has an ASR of essentially parity, its pronounced sexual dichromatism and the ages of its males typically lying at the upper end of those of its females indicate that this species also has a polygynous mating system. Limited sexual dichromatism and an ASR as high as 71% suggest that C. cyanodus is not polygynous. As the timing of the main spawning period of the four species within Shark Bay increases sequentially between early spring and late summer, the temperature trigger for spawning by those species also presumably increases progressively. This phasing of spawning, allied with some interspecific spatial variation, reduces the potential for competition among the larvae and early juveniles of the five Choerodon species. Choerodon rubescens spawns earlier in Shark Bay than in the cooler waters of the Abrolhos Islands, 230 km further south. The management implications of data for Choerodon species, and particularly of the size compositions and the ASRs of the two largest species, C. rubescens and C. schoenleinii, are discussed. The variations in the hermaphroditic and reproductive features of five congeneric species of labrid, in the region in which their distributions overlap, highlight the evolutionary/biological plasticity of this family and thereby contributes to its diversity and success.
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Identification of key drivers of population dynamics and prediction of rates of population recovery following stock decline are crucial aspects of fisheries management. The abundance of a blue swimmer crab population (Portunus armatus) in Cockburn Sound, Western Australia, which once supported the largest commercial fishery for this short-lived species in the State, declined markedly and has remained low despite two commercial fishery closures. This study employed state-space biomass dynamics models to explore evidence for potential factors contributing to the lack of stock recovery, including fishing pressure, changes in primary production (using chlorophyll-a as a proxy) and depensatory stock-recruitment dynamics. Likelihood ratio test results indicated better statistical fits for models with production functions modified to account for chlorophyll-a or both depensation and chlorophyll-a. Models incorporating both depensation and chlorophyll-a provided the most biologically-feasible results (e.g. estimated intrinsic increase, , not near zero) and the estimated biomass trajectories were less uncertain. For all models, estimated annual harvest rates peaked in the late 1990s, prior to the first major stock decline, and again in 2009−12, when the fishery was briefly re-opened with tight management restrictions. Results for models including both depensation and chlorophyll-a indicated that stock productivity is positively-related to chlorophyll-a. Thus, below-average chlorophyll-a concentrations in Cockburn Sound in recent years, in combination with some form(s) of depensation (e.g. associated with predation pressure), may be preventing stock recovery. Despite a sustained period of very limited recreational fishing and no commercial fishing, stock recovery appears highly uncertain under current environmental conditions. The results of this study highlight the value of incorporating environmental data and alternative stock-recruitment assumptions when fitting production models to explore key factors influencing population dynamics.

The characteristics of detached macroalgae (drift) in nearby highly eutrophic and mesotrophic estuaries in south-western Australia are compared to elucidate the magnitude and types of changes that occur in macroalgal drift when estuaries receive excessive nutrient input. Drift characteristics in the large basins of the microtidal, eutrophic Peel-Harvey and mesotrophic Swan-Canning, which is not subjected to large nutrient inputs directly from agricultural land, differed markedly. Biomass (dry weight) in mesotrophic estuary was dominated by rhodophytes (92%), particularly Laurencia and Hypnea, and in eutrophic estuary by opportunistic chlorophytes (68%), especially Chaetomorpha and Ulva. Prevalence and biomass of drift were far greater in the eutrophic estuary, particularly during summer and autumn when macroalgal growth rose sharply. Macroalgal biomass in the eutrophic estuary was positively related to salinity. These results facilitate predictions of how climatic and other anthropogenic changes influence extent of macroalgal growth and thus change the estuarine environment.

The giant ruby snapper, Etelis sp., attains the largest size of any lutjanid in the Indo-Pacific and is one of the most valuable species harvested from deep-water fisheries along the continental and insular shelf margins throughout its broad geographic distribution. Despite this species supporting important commercial, artisanal and subsistence fisheries, quantitative assessments of the status of stocks have been limited by an absence of biological information, unreliable catch and effort statistics, and until recently, misidentification with a cryptic congener. This study aimed, firstly, to describe and compare the age, growth and reproductive characteristics of Etelis sp. between the eastern Indian and western central Pacific Oceans; and secondly, to provide an age-based assessment of the stock in north-western Australia, the only stock for which available data were sufficient to quantify stock status. Although the growth of Etelis sp. differed significantly between sexes and oceans, longevity was similar with a maximum age of 56 years recorded in the Pacific Ocean. Spawning of this species occurred over five months during the austral summer to mid-autumn (i.e. December to April) in the Indian Ocean, but was not well defined in the Pacific Ocean. The estimated ages at 50 % maturity for females and males in the Indian Ocean were similar (i.e. 4–5 years), whereas lengths at 50 % maturity differed ( = 527 and 456 mm fork length, FL, respectively), but were consistent with corresponding differences in growth between sexes. Estimates of the relative female spawning potential ratio for Etelis sp. in north-western Australia suggest the status of this stock remained relatively unchanged from 1997 to 2011, at around 60 % of the unfished level. This assessment provides an example of the relative sustainable exploitation levels for this stock, and potentially other Eteline snappers that exhibit similar life history characteristics, particularly in locations where monitoring and assessments may be data and/or resource limited.

Long-term monitoring data and models have been used to explore the efficacy of restocking an estuarine-resident teleost species. Juveniles of Acanthopagrus butcheri were cultured from broodstock from the Blackwood River Estuary in 2002 and their otoliths stained with alizarin complexone. 150,000 marked juveniles were released into the Blackwood River Estuary at three months old in early 2003, with the stain in their otoliths subsequently remaining visible during the following 16 years. Samples of A. butcheri were obtained annually from a commercial gill net fisher and at intervals by fishery-independent methods. Commercial catch per unit effort was positively related to freshwater discharge. Indices of year class strength, derived from annual age-frequency distributions and assuming length-dependent natural mortality, demonstrate that between 1993 and 2012, natural recruitment to the population was only strong in 1999 and 2008 and was not significantly related to freshwater flow. The number of wild stock A. butcheri ≥ minimum legal length (250 mm total length), comprising mainly the 1999 year class, were estimated to have declined sequentially to only ∼12,000 fish in 2010. The restocked fish contributed between 61 and 73 % to the total number of fish in the commercial catches between 2007 and 2010 and their numbers remained at > 10 % of their number at release until 2011. The contribution of the 2008 year class, of which ∼50 % had been estimated as derived from restocked fish, increased from 20 % of the commercial catch in 2012 to between 50 and 80 % between 2013 and 2018. A progressive decline in growth of year-class groups of wild stock fish, from 1970-1998 to 2008-2011, is considered to reflect the detrimental effects of hypoxia. Restocked fish grew less rapidly than younger and older year-class groups of wild stock fish, suggesting that the growth of cultured fish may have been compromised by hatchery processes. The large contributions made by restocked fish to commercial catches over several years and to future generations highlight the efficacy of restocking for replenishing and sustaining a depleted population of an estuarine-resident species.

This study has elucidated the environmental factors associated with spawning and the ecology of early life cycle stages of the sparid Acanthopagrus butcheri, which is atypically long-lived and large for a species confined to its natal estuary. The prevalence of A. butcheri eggs in monthly plankton samples from the Blackwood River Estuary, south-western Australia, in 2014/15, allied with long-term gonadal data, imply that spawning in this estuary commences in mid-spring (October) as water temperatures increase to ~17 °C and peak at ~22 °C in early summer (December). Eggs were initially found only in deeper, downstream areas, where a salt wedge had started to penetrate following precipitous seasonal decline in freshwater discharge. Spawning occurred progressively further upstream during November/December as the salt wedge continued its intrusion of the upper estuary. The concentrations of eggs and yolk-sac and preflexion larvae, which were related positively to salinity and temperature and less so to oxygen concentration, were located predominantly below the halocline, implying that successful spawning typically requires salinities ≥15. Concentrations of eggs and yolk-sac and preflexion larvae peaked in December and of flexion larvae in January. As the concentrations of copepod nauplii exceeded by orders of magnitude those of larval A. butcheri, with which they co-occur and constitute their main prey, it is unlikely that restricted food resources accounted for the poor recruitment of the 2014/15 year class. In this context, the presence of many yolk-sac larvae and preflexion larvae in the low oxygen concentrations, which have become increasingly prevalent in microtidal estuaries of south-western Australia due to climate change, is likely to have led to considerable mortality. In most populations throughout its range in southern Australia, A. butcheri spawns in a restricted period between the austral mid-spring and mid-summer, with the synchrony achieved through differences in the temperatures that cue spawning at different latitudes, i.e. higher temperature cues at lower and thus warmer latitudes. Spawning occurs earlier, however, in atypical estuaries in which salinities and temperatures are high in late winter/spring. Despite experiencing episodic recruitment, A. butcheri sustains substantial populations through maturing early relative to its maximum age, and therefore producing numerous offspring in those few years when spawning conditions are optimal. Episodic recruitment could thus be related to subtle inter-annual differences in environmental conditions during the short spawning periods, with oxygen levels greater than usual a potential key factor. The adaptability of A. butcheri makes this sparid useful in assessing the degree, direction and scale of climate change effects.

A 26-year time series of monthly samples from the water intake of a power station has been used to analyse the trends exhibited by number of species, total abundance, and composition of the mysids and caridean decapods in the inner Bristol Channel. During this period, annual water temperatures, salinities and the North Atlantic Oscillation Index (NAOI) in winter did not change significantly, whereas annual NAOI declined. Annual mean monthly values for the number of species and total abundance both increased over the 26 years, but these changes were not correlated with any of the measured physico-chemical/climatic factors. As previous studies demonstrated that, during a similar period, metal concentrations in the Severn Estuary and Bristol Channel (into which that estuary discharges) declined and water quality increased, it is proposed that the above changes are due to an improved environment. The fauna was dominated by the mysids Mesopodopsis slabberi and Schistomysis spiritus, which collectively contributed 94% to total abundance. Both species, which were represented by juveniles, males, non-brooding females and brooding females, underwent statistically-indistinguishable patterns of change in abundance over the 26 years. When analysis was based on the abundances of the various species, the overall species composition differed significantly among years and changed serially with year. When abundances were converted to percentage compositions, this pattern of seriation broke down, demonstrating that changes in abundance and not percentage composition were responsible for the seriation. As with the number and abundance of species, changes in composition over the 26 years were not related to any of the physico-chemical/climatic factors tested. Species composition changed monthly in a pronounced cyclical manner throughout the year, due to statistically different time-staggered changes in the abundance of each species. This cyclicity was related most strongly to salinity.