Gannon R.,University of New South Wales |
Gannon R.,Sydney Institute of Marine Science |
Taylor M.D.,University of New South Wales |
Taylor M.D.,Port Stephens Fisheries Institute |
And 9 more authors.
Journal of Experimental Biology | Year: 2014
Theoretical and laboratory studies generally show that ectotherm performance increases with temperature to an optimum, and subsequently declines. Several physiological mechanisms probably shape thermal performance curves, but responses of free-ranging animals to temperature variation will represent a compromise between these mechanisms and ecological constraints. Thermal performance data from wild animals balancing physiology and ecology are rare, and this represents a hindrance for predicting population impacts of future temperature change. We used internally implanted accelerometers near the middle of a species' geographical distribution and gill-net catch data near the species' latitudinal extremes to quantify temperature-related activity levels of a wild predatory fish (Platycephalus fuscus). We examined our data in the context of established models of thermal performance, and the relationship between thermal performance thresholds and biogeography. Acceleration data approximated a thermal performance curve, with activity peaking at 23° C but declining rapidly at higher temperatures. Gill-net catch data displayed a similar trend, with a temperature-associated increase and decrease in catch rates in temperate and tropical regions, respectively. Extrapolated estimates of zero activity (CTmin and CTmax) from the accelerometers were similar to the minimum and maximum mean monthly water temperatures experienced at the southern and northern (respectively) limits of the species distribution, consistent with performance-limited biogeography in this species. These data highlight the fundamental influence of temperature on ectotherm performance, and how thermal performance limits may shape biogeography. Biologging approaches are rarely used to examine thermal performance curves in freeranging animals, but these may be central to understanding the tradeoffs between physiology and ecology that constrain species' biogeographies and determine the susceptibility of ectotherms to future increases in temperature. © 2014. Published by The Company of Biologists Ltd.
PubMed | Charles Sturt University, WildFish Research, Australian Department of Primary Industries and Fisheries and IC Independent Consulting
Type: Journal Article | Journal: PloS one | Year: 2016
Information about spatial and temporal variability in the distribution and abundance of shark-populations are required for their conservation, management and to update measures designed to mitigate human-shark interactions. However, because some species of sharks are mobile, migratory and occur in relatively small numbers, estimating their patterns of distribution and abundance can be very difficult. In this study, we used a hierarchical sampling design to examine differences in the composition of species, size- and sex-structures of sharks sampled with bottom-set longlines in three different areas with increasing distance from the entrance of Sydney Harbour, a large urbanised estuary. During two years of sampling, we obtained data for four species of sharks (Port Jackson, Heterodontus portusjacksoni; wobbegong, Orectolobus maculatus; dusky whaler, Carcharhinus obscurus and bull shark, Carcharhinus leucas). Only a few O. maculatus and C. obscurus were caught, all in the area closest to the entrance of the Harbour. O. maculatus were caught in all seasons, except summer, while C. obscurus was only caught in summer. Heterodontus portusjacksoni were the most abundant species, caught in the entrance location mostly between July to November, when water temperature was below 21.5C. This pattern was consistent across both years. C. leucas, the second most abundant species, were captured in all areas of Sydney Harbour but only in summer and autumn when water temperatures were above 23C. This study quantified, for this first time, how different species utilise different areas of Sydney Harbour, at different times of the year. This information has implications for the management of human-shark interactions, by enabling creation of education programs to modify human behaviour in times of increased risk of potentially dangerous sharks.
Gray C.A.,WildFish Research |
Journal of Applied Ichthyology | Year: 2015
The dusky flathead (Platycephalus fuscus) is an important teleost harvested by recreational and commercial fishers throughout its endemic distribution along eastern Australia. This study indicates that the species has an extended spawning period throughout the austral summer, with females in spawning condition occurring in lower estuarine and coastal waters. Total length (L50) and age (A50) at which 50% (±1 SE) of the population was mature was 31.72 (±1.08) cm TL and 1.22 (±0.44) years for males and 56.75 (±0.60) cm TL and 4.55 (±0.13) years for females. The von Bertalanffy growth parameters differed significantly between sexes; females grew faster and attained a greater maximum TL and age than males. The largest female was 98.5 cm TL (7.5 kg), and the oldest 16 years, whereas the largest male was 61.5 cm TL (1.58 kg) and 11 years of age. A tag-and-release study identified the exchange of sub-adult and mature-sized individuals among estuaries. Determinations of length-based management regulations for the species are compounded by the large gender-based differences in growth and length-at-maturity. Current minimum legal lengths of 30-40 cm TL protect approximately 3-9% of the female spawning population. Alternative management options, including harvest slot sizes, need to be investigated and tested. © 2015 Blackwell Verlag GmbH.
Stocks J.R.,University of New South Wales |
Stocks J.R.,Batemans Bay Fisheries Center |
Gray C.A.,University of New South Wales |
Gray C.A.,WildFish Research |
And 2 more authors.
Journal of Sea Research | Year: 2014
Spatial and temporal variation in the growth of a widely distributed temperate marine herbivore, Girella elevata, was examined using length-at-age data and multi-decadal otolith increment growth chronologies. In total 927 G. elevata were collected from three regions of the Australian south-east coast, extending 780. km and covering the majority of the East Australian Current, a poleward-flowing western boundary current of the Southern Pacific Gyre and climate change hotspot. A validated ageing method using sectioned sagittal otoliths was developed to enumerate both daily (juvenile fish) and annual otolith increments. G. elevata exhibited great longevity with a maximum recorded age of 45. +. yrs. Spatial variation in growth from length-at-age data was observed with the highest growth rates within the centre of the species distribution. Analysis of otolith growth chronologies of 33. yrs showed a positive relationship with the Southern Oscillation Index. Identifying links between life-history characteristics and variation in oceanographic conditions across latitudinal gradients may shed light on potential impacts of expected climate shifts on fish productivity. © 2014 Elsevier B.V.
Gray C.A.,WildFish Research
Journal of Experimental Marine Biology and Ecology | Year: 2016
Scales of spatio-temporal variability in the densities and sizes of organisms need to be quantified so that the relative importance of different ecological processes that structure assemblages can be determined and future sampling strategies optimized. This study examined variability in the densities and size compositions of the beach clam, Donax deltoides, across tide stages and various hierarchical scales of time (day, week, month, season) and space (patch, area, beach). Densities of clams in the swash zone were most often least at high tide, which may be a consequence of their restricted shoreward movements and upper beach distributions. Regardless, the size compositions of sampled clams were the same across all tide stages. Future quantitative population sampling could be done across a six-hour window around low tide, providing ample time to access and sequentially sample many locations along a beach. The components of variation in densities of clams were in all but a couple of cases, greatest at the smallest temporal and spatial scales examined; among days and among replicate samples within a patch. Variation in size compositions was also greatest at the smallest spatial scale examined (among patches), but this was not the case for time. Size compositions were relatively stable across days and the influences of other temporal scales were inconsistent, suggesting that ecological processes influencing size compositions operate at different scales to those influencing densities. Nevertheless, the general predominance of small-scale variation was probably a consequence of fine-tuned, local-scale responses of clams to the variable and dynamic physical and biogenic features of the swash zone habitat. These results concur with studies in other habitats that show small-scale temporal and spatial variability is a dominant feature of aquatic benthic organisms and challenges the paradigm that ocean beach ecology is primarily driven by large-scale hydrodynamic forces. Small-scale patchiness of clams in time and space needs to be taken into consideration in future sampling strategies for population and impact assessments. Moreover, the ecological processes that drive small-scale temporal and spatial structuring of organisms on ocean beaches require greater investigation. © 2015 Elsevier B.V.
Gray C.A.,WildFish Research
Global Ecology and Conservation | Year: 2016
Spatial fishing closures are typically implemented for conservation and fisheries benefits, but the effects of such initiatives are often not tested. This study examined whether the densities and size compositions of beach clams differed between commercially fished and non-fished zones on beaches. Sampling of clams was stratified across two habitats (swash and dry sand) on two commercially fished beaches, before and during (early and late) the 6-month harvesting period. Two beaches that had no commercial fishing were also sampled the same way and acted as external controls. Differences in densities, but not size compositions, of clams were evident between zones on the commercially fished and control beaches, but they were mostly apparent only across short (day and week) periods before, early and late harvesting, and thus were most likely pulse responses of clams to stochastic, non-fishing related events that acted independently across the different zones on each beach. The potential movements of clams along and across beaches as well as current restrictions on commercial fishing probably dampened detection of longer-term fishing-related impacts and demographic differences in clams between commercially fished and non-fished zones. Direct fishing-related impacts on clams may only be discernable in the immediate vicinity of, and persist for a short period following, an actual fishing event on a beach. Nevertheless, the zones closed to commercial fishing may provide valuable protection to a portion of clams on each beach and alleviate beach-wide harvesting impacts. The broader value of these closed fishing zones requires knowledge of the impacts of fishing on other beach organisms and ecosystem functioning. Further experimentation that tests other aspects of management arrangements of beach clams may help determine their global applicability for sustainable harvesting, and contribute to the overall conservation management of sandy beach ecosystems. © 2015 The Author.
Gray C.A.,WildFish Research
Fisheries Research | Year: 2016
This study examined industry logbooks, and beach- and port-based fishery-dependent data sources for monitoring and assessing catch, effort, catch-per-unit-of-effort (CPUE) and size compositions of beach clams in a small-scale fishery in eastern Australia. The study was conducted across the six-month fishing season and encompassed two management regions, serving as a model for elsewhere. In general, values of catch, effort and CPUE did not differ significantly between logbooks and beach sampling, and spatial and temporal trends in examined indices were similar across both data sources. Beach sampling captured additional data that included the partitioning of fishing effort into search and dig time, and also the number and location of sites fished each day, which could be useful in addressing uncertainty in CPUE-clam density relationships and potential fishing impacts, and assist in spatial management of fishing across beaches. These data could be future sourced from industry and provided on modified logbooks. Compared to port sampling, the beach-based size composition data appeared to be biased and influenced by fisher behaviour. Cost-effective future monitoring of the fishery could be conducted using a combination of logbooks for catch and effort that includes strategic periodic validations using beach-observers, and port sampling for size compositions. The success of such a strategy is reliant on strong fisher cooperation that requires open co-management arrangements. Future assessments of the beach clam resources need to account for inherent differences in populations across individual beaches, including non-fished (control) beaches. © 2016 Elsevier B.V.
Gray C.A.,WildFish Research
PLoS ONE | Year: 2016
Management responses to reconcile declining fisheries typically include closed areas and times to fishing. This study evaluated this strategy for a beach clam fishery by testing the hypothesis that changes in the densities and size compositions of clams from before to during harvesting would differ between commercially fished and non-fished beaches. Sampling was spatially stratified across the swash and dry sand habitats on each of two commercially fished and two non-fished beaches, and temporally stratified across three six-week blocks: before, early and late harvesting. Small-scale spatio-Temporal variability in the densities and sizes of clams was prevalent across both habitats and the components of variation were generally greatest at the lowest levels examined. Despite this, differences in the densities and sizes of clams among individual beaches were evident, but there were few significant differences across the commercially fished versus non-fished beaches from before to during harvesting. There was no evidence of reduced densities or truncated size compositions of clams on fished compared to non-fished beaches, contrasting reports of some other organisms in protected areas. This was probably due to a combination of factors, including the current levels of commercial harvests, the movements and other local-scale responses of clams to ecological processes acting independently across individual beaches. The results identify the difficulties in detecting fishing-related impacts against inherent levels of variability in clam populations. Nevertheless, continued experimental studies that test alternate management arrangements may help refine and determine the most suitable strategies for the sustainable harvesting of beach clams, ultimately enhancing the management of sandy beaches. © 2016 Charles A. Gray.This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Gray C.A.,WildFish Research |
Gray C.A.,University of New South Wales
Aquatic Biology | Year: 2015
Length- and age-based information is used to assess the exploitation status of harvested populations of fish worldwide. This study examined variability in length-at-age, length and age compositions and derived demographic parameters of Acanthopagrus australis (Sparidae) captured in beach seines in 3 estuaries across 5 yr. The method of ageing fish by counting opaque growth zones on sagittal otoliths was validated by field and aquaria studies. Length-at-age relationships varied according to gender, age class and estuary. Females older than 3 yr had a significantly greater mean length-at-age than males, and this was consistent across all estuaries. For both genders, mean length-at-age of fish older than 4 yr was greatest in the southernmost estuary, and least in the northernmost estuary, suggesting differential growth dynamics among estuaries. Observed longevity differed among estuaries, ranging from 14 to 22 yr. Variations in length composition of retained catches were subtler than those for age composition. The average age of fish in catches varied among estuaries from 4.2−5.1 yr to 6.8−8.2 yr even though fish within 5 cm of the minimum legal length dominated catches in all estuaries. Estimates of total and fishing mortality varied 3-fold among estuaries, with fishing mortality exceeding natural mortality. Variability in demographic characteristics was generally greater among estuaries than among years within each estuary. These results show that the population demographics and ensuing assessments should not be extrapolated across estuaries or from one estuary to the entire A. australis population. Length is a poor predictor of age and future sampling of populations must be age-based and stratified to account for estuary-specific variation in demography. Failure to account for such varia - bility could confound assessments and management deliberations necessary for determining the most appropriate harvest and conservation strategies for such species. © The author 2015.
PubMed | WildFish Research
Type: Journal Article | Journal: PloS one | Year: 2016
Management responses to reconcile declining fisheries typically include closed areas and times to fishing. This study evaluated this strategy for a beach clam fishery by testing the hypothesis that changes in the densities and size compositions of clams from before to during harvesting would differ between commercially fished and non-fished beaches. Sampling was spatially stratified across the swash and dry sand habitats on each of two commercially fished and two non-fished beaches, and temporally stratified across three six-week blocks: before, early and late harvesting. Small-scale spatio-temporal variability in the densities and sizes of clams was prevalent across both habitats and the components of variation were generally greatest at the lowest levels examined. Despite this, differences in the densities and sizes of clams among individual beaches were evident, but there were few significant differences across the commercially fished versus non-fished beaches from before to during harvesting. There was no evidence of reduced densities or truncated size compositions of clams on fished compared to non-fished beaches, contrasting reports of some other organisms in protected areas. This was probably due to a combination of factors, including the current levels of commercial harvests, the movements and other local-scale responses of clams to ecological processes acting independently across individual beaches. The results identify the difficulties in detecting fishing-related impacts against inherent levels of variability in clam populations. Nevertheless, continued experimental studies that test alternate management arrangements may help refine and determine the most suitable strategies for the sustainable harvesting of beach clams, ultimately enhancing the management of sandy beaches.