The biology of the Dhufish, Glaucosoma hebraicum, in offshore waters on the lower west coast of Australia

Samples of Dhufish, Glaucosoma hebraicum, were collected in each month between May 1996 and May 1997 from the catches of commercial and recreational wetline fishers and commercial trawlers. These catches were taken in the offshore waters of Western Australia near Geraldton (ca 28°S) and Perth (ca 33°S). Comparisons were made between the number of circuli on scales and the number of translucent zones on sectioned otoliths, and between the number of translucent zones on sectioned and whole otoliths. The number of circuli on scales was often either greater or less than the number of translucent zones on sectioned otoliths of the same fish. The number of translucent zones observed on whole otoliths were the same as on sectioned otoliths for fish with otoliths that have up to six translucent zones. However, the prevalence of underestimates, when using whole as opposed to sectioned otoliths, subsequently increased progressively as the number of translucent zones increased. The mean monthly marginal increments for sectioned otoliths showed a pronounced decline in spring and then a progressive rise during summer and mid-autumn, before levelling off in winter. These trends provide strong evidence that the translucent zones on the otoliths of G. hebraicum are formed annually and that their numbers in sectioned otoliths can be used to age this species. The von Bertalanffy growth parameters, L∞, K and t0, derived for the growth curves of G.hebraicum from length-at-age data were 1038, 0.108 and -0.172, respectively, for females and 1087, 0.109 and -0.219, respectively, for males. Males grew slightly faster than females, attaining total lengths of 234, 448, 688, 832 and 921mm after 2, 5, 10, 15 and 20 years, compared with 212,405, 626, 792 and 865mm for females at the same corresponding ages. Females and males reached the legal minimum (total) length (LML) for capture of 500mm at ca seven and six years, respectively. The maximum ages recorded for females and males were 29 and 35 years, respectively, and the maximum lengths for females and males were 976 and 1120mm, respectively. Although the growth curves of both females and males of G. hebraicum caught in waters near Geraldton were shown by a maximum likelihood ratio test to differ from those of fish caught in waters near Perth, the differences in the von Bertalanffy growth parameters for fish in these two regions were not pronounced. Macroscopic and microscopic examination of gonads showed that female and male G. hebraicum first reach sexual maturity at total lengths of between 250 to 300mm and 350 to 400mm, respectively. Sexual maturity was first attained by females and males at the end of their fifth and eighth years of life, respectively. Histological sections showed that some mature ovaries contained post-ovulatory follicles as well as granular and hydrated oocytes during the spawning period. This provides strong evidence that this species is a multiple spawner. On the basis of the monthly trends exhibited by gonadosomatic indices, gonad maturity stages and different stages in oocyte development, G. hebraicum is considered to breed between December and April, with spawning reaching a peak in late January/early February. For this reason, G. hebraicum was accorded a birth date of the first of February. The nematode parasite Philometra_sp. was found to infect the gonads of G. hebraicum. Preliminary studies have shown that the prevalence of infection is far higher in females than males, and that it increases with fish size and age. At the completion of spawning, this parasite can occupy 50% of the volume of the ovary. Thus, since philometrid worms are known to feed on blood, it is possible that the parasite has a deleterious effect on the reproduction of G. hebraicum. However, histological sections of ovaries failed to reveal visible signs of egg destruction by the parasite, and nor were there any signs of disruption of normal gonadal development of G. hebraicum. Observations of the life cycle stages of Philometra sp. throughout the year indicate that the life cycle of the parasite is closely synchronised with the pattern of reproductive development of its host. No fish were infected under the size at which sexual maturity is first reached and there was evidence that infection by the parasite occurs when G. hebraicum aggregates during the spawning period, when older, infected fish meet younger, uninfected fish. Since female and male G. hebraicum reach first sexual maturity at total lengths of between 250 to 300mm and 350 to 400mm, respectively, this means that the majority of both female and male G. hebraicum have had the opportunity to spawn at least once before they reach the LMLof 500mm. However because almost all Dhufish caught in waters >30m die upon release after capture, this species is subject to fishing mortality before they reach the LML. Therefore, in terms of fisheries management, there would be little value in maintaining a legal size limit for this species. Future management strategies for maintaining stocks of this recreationally and commercially important and heavily-fished species could include reducing fishing pressure in heavily-fished areas and closing from fishing certain areas of known high population density of G. hebraicum. However, for conservation purposes, further research is needed to determine locations of high population densities of Dhufish that could be restricted from fishing, and the migratory patterns of this species. Future studies involve the collection of a greater number of small G. hebraicum (i.e. <300mm), to provide more points for the commencement of the growth curve, more data for determining marginal increment trends in fish with otoliths containing one or two translucent zones, a more precise estimate of the size and age at which females and males reach sexual maturity, and the habitats of these small fish. An histological study of spermatogenesis would improve and consolidate the reproductive data already obtained for males and provide additional information for aiding the aquaculture studies presently being carried out on this species. Further investigations of Philometra sp. infection in G. hebraicum, including a comparison of the fecundity of parasitised fish and unparasitised fish, may provide information on the effects of this parasite on the reproduction of Dhufish

Status of inshore demersal scalefish stocks on the south coast of Western Australia. NRM Project 12034 Final Report

Inshore demersal scalefish in waters of 20-250 m depth in the South Coast Bioregion (SCB) are an important resource targeted by commercial, recreational and charter fishing sectors

Potential Eradication and Control Methods for the Management of the Ascidian Didemnum perlucidum in Western Australia

Assessments of the key indicator species for the West Coast Demersal Scalefish Resource (WCDSR; West Australian dhufish Glaucosoma hebraicum, Snapper Pagrus auratus and Baldchin groper Choerodon rubescens) in 2007 and 2009 demonstrated that the stocks were experiencing overfishing. Thus, between late 2007 and early 2010, substantial changes were made to the management of the commercial and recreational fisheries that exploit the WCDSR. These were designed to reduce catches of the entire suite of demersal scalefish species (and of each indicator species) by both the commercial and recreational sectors in the West Coast Bioregion (WCB) by at least 50 % of the 2005/06 levels (the catch benchmark), to allow stocks to recover

2021 assessment of the status of the West Coast Demersal Scalefish Resource

A recovery program for the West Coast Demersal Scalefish Resource was introduced between late 2007 and early 2010, based on the maintenance of retained catches of demersal species (overall suite and each indicator species) by both the commercial and recreational sectors below 50% of the catches reported in 2005/06 (original catch recovery benchmarks)

Biology of two species of sparid on the west coast of Australia

Various aspects of the biology of the tarwhine Rhabdosargus sarba and western yellowfin bream Acanthopagrus latus were studied. The studies on R. sarba have focused on populations in temperate coastal marine waters at ca 32 degrees S and the lower reaches of an estuary (Swan River Estuary) located at the same latitude and in a subtropical embayment (Shark Bay) at ca 26 degrees S, while those on A. latus were conducted on the population in the latter embayment. A combination of a macroscopic and histological examination of the gonads demonstrated that R. sarba is typically a rudimentary hermaphrodite in Western Australian waters, i.e. the juveniles develop into either a male or female in which the ovarian and testicular zones of the gonads, respectively, are macroscopically undetectable. This contrasts with the situation in the waters off Hong Kong and South Africa, in which R. sarba is reported to be a protandrous hermaphrodite. However, it is possible that a few of the fish that are above the size at first maturity and possess, during the spawning period, ovotestes with relatively substantial amounts of both mature testicular and immature ovarian tissue, could function as males early in adult life and then change to females. Although R. sarba spawns at some time between late winter and late spring in Western Australia, spawning peaks later in the Swan River Estuary than in coastal, marine waters at the same latitude and Shark Bay, in which salinities are always close to or above that of full strength sea water, i.e. 35 0/00. While the males and females attain sexual maturity at very similar lengths in the Swan River Estuary and Shark Bay, i.e. L50s all between 170 and 177 mm, they typically reach maturity at an earlier age in the former environment, i.e. 2 vs 3 years old. Thus, length and consequently growth rate influence the timing of maturity rather than age. During the spawning period, only 9 % of the fish caught between 180 and 260 mm in nearshore, shallow marine waters had become mature, whereas 91 % of those in this length range over reefs were mature, indicating that R. sarba tends to move offshore only when it has become physiologically ready to mature. The L50s at first maturity indicate that the current minimum legal length in Western Australia (230 mm) is appropriate for managing this species. Oocyte diameter frequency distributions, stages in oocyte development, duration of oocyte hydration and time of formation of post-ovulatory follicles in mature ovaries of Rhabdosargus sarba in the lower Swan River Estuary (32 degrees 03'S, 115 degrees 44'E) were used, in conjunction with data on tidal cycles, to elucidate specific aspects of the reproductive biology of this sparid in an estuarine environment. The results demonstrated the following. (i) Rhabdosargus sarba has indeterminate fecundity sensu Hunter et al. (1985). (ii) Oocyte hydration commences at about dusk (18:30 h) and is completed by ca 01:30-04:30 h, at which time ovulation, as revealed by the presence of hydrated oocytes in the ovarian duct and appearance of newlyformed post-ovulatory follicles, commences. (iii) The prevalence of spawning was positively correlated with tidal strength and was greatest on days when the tide changed from flood to ebb at ca 06:00 h, i.e. approximately when spawning ceases. Spawning just prior to strong ebb tides would lead to the transport of eggs out of the estuary and thus into salinities that remain at ca 35 0/00. The likelihood of eggs being transported downstream is further enhanced by R. sarba spawning in deeper waters in the estuary, where the flow is greatest. (iv) Although mature ovaries were found in R. sarba in the estuary between early July and December, the prevalence of atretic oocytes was high until September, when salinities started rising markedly from their winter minima. Batch fecundities ranged from 2,416 for a 188 mm fish to 53,707 for a 266 mm fish. The average daily prevalence of spawning amongst mature females during the spawning period of R. sarba caught in the lower estuary, i.e. July to end of October, was 36.5 %. Thus, individual female R. sarba spawned, on average, at intervals of ca 2.7 days in each spawning season. Female R. sarba with total lengths of 200, 250 and 300 mm were estimated to have a batch fecundity of 7,400, 20,100 and 54,800 eggs, respectively and annual fecundities of 332,000, 903,000 and 2,461,000 eggs, respectively. Rhabdosargus sarba is shown to undergo size-related movements in each of the three very different environments in which it was studied. In temperate coastal waters, R. sarba settles in unvegetated nearshore areas and then moves progressively firstly to nearby seagrass beds and then to exposed unvegetated nearshore areas and finally to areas around reefs where spawning occurs. Although R. sarba spawns in the lower Swan River Estuary, relatively few of its early 0+ recruits remain in the estuary and substantial numbers of this species do not start reappearing in the estuary until they are ca 140 mm. In Shark Bay, R. sarba uses nearshore mangroves as a nursery area and later moves into areas around reefs. The maximum ages recorded for R. sarba in coastal marine waters (11 years) and Shark Bay (13 years) were far greater than in the lower Swan River Estuary (6 years). However, the maximum lengths recorded in these three environments were all ca 350 mm. Due to the production by size-related movements of differences amongst the lengths of R. sarba at given ages in different habitats in coastal marine waters, the composite suite of lengths at age was not fully representative of the population of this species as a whole in this environment. A von Bertalanffy growth curve, which was adjusted to take into account size related changes in habitat type, significantly improved the fit to the lengths at age of individuals in the composite samples for the population beyond that provided by the unadjusted von Bertalanffy growth curve. This resulted in the maximum difference between the estimates of length at age from the two growth curves, relative to the L derived from the unadjusted von Bertalanffy curve, reaching a value equivalent to 8 %. However, the maximum differences for the corresponding curves for populations in the lower Swan River Estuary and Shark Bay were far less, i.e. 1.7 and 3.2 %, respectively, and thus not considered biologically significant. Rhabdosargus sarba grew slightly faster in the lower Swan River Estuary than in either coastal marine waters or Shark Bay, possibly reflecting the greater productivity of estuarine environments. Acanthopagrus latus is a protandrous hermaphrodite. Detailed macroscopic and histological examination of the gonads of a wide size range of fish, together with a quantification of how the prevalences of the different categories of gonad change with size and age and during the year, were used to elucidate the sequence of changes that occur in the ovotestes of A. latus during life. The scheme proposed in the present study for the protandrous changes in A. latus differed from those proposed for this species elsewhere, but was similar to that of Pollock (1985) for the congeneric Acanthopagrus australis. The ovotestes of functional males develop from gonads which, as in older juveniles, contain substantial amounts of testicular and ovarian tissue. Such ovotestes, and particularly their testicular component, regress markedly after spawning and then, during the next spawning season, either again become ovotestes in which the testicular zone predominates and contains spermatids and spermatozoa (functional males), or become ovotestes in which the ovarian zone predominates and contains vitellogenic oocytes (functional females). Once a fish has become a functional female, it remains a female throughout the rest of its life. The trends exhibited during the year by reproductive variables demonstrate that A. latus in Shark Bay typically spawns on a very limited number of occasions during a short period in August and September and has determinate fecundity. The potential annual fecundities of 24 A. latus ranged from 764,000 in a 600 g fish to 7,910,000 in a 2,050 g fish and produced a mean [plus-minus]1SE of 1,935,000 [plus-minus] 281,000. The total length at which 50 % of A. latus become identifiable as males (245 mm) is very similar to the current minimum legal length (MLL) of 250 mm, which corresponds to an age of 2.5 years less than the age at which 50 % of males become females. Current spawning potential ratios calculated over a range of alternative values for natural mortality (M) for A. latus in Shark Bay suggests that the present fishing pressure is sustainable, but that the current MLL should be reviewed if recreational fishing pressure continues to increase. The age composition and von Bertalanffy growth parameters for Acanthopagrus latus have been determined. The relevant parameters were inserted into the empirical equations of Pauly (1980) and Ralston (1987) for estimating natural mortality (M). Total mortality (Z) was calculated using Hoenig's (1983) equations, relative abundance analysis and a simulation based on maximum age and sample size.The two point estimates for M for A. latus, which were both 0.70 year-1, greatly exceeded all estimates for Z (range 0.18 to 0.30 year-1), which is clearly an erroneous result. To resolve this problem of inconsistent estimates, a Bayesian approach was developed, which, through combining the likelihood distributions of the various mortality estimates, produced integrated estimates for M and Z that are more consistent and precise than those produced for these two variables using the above methods individually. This approach now yielded lower values for M than Z and a measure of fishing mortality that appears to be consistent with the current status of the fishery. This approach is equally applicable to other fish species

Gillnet selectivity for non-targeted shark species in temperate Australia

The use of size-selective fishing gear has been fundamental for the successful management of shark populations. For the main Australian shark fisheries, available data sets on gillnet selectivity experiments conducted since the 1990s were used to estimate gillnet selectivity parameters for nine commonly caught non-target species: broadnose, draughtboard, Port Jackson and school sharks, common and southern sawsharks, smooth hammerhead and spikey and whitespotted dogfish. Size at selection increased with mesh size, but it varied with species, with some species having broader selectivity curves due to their particular morphological features. In particular, smooth hammerhead had much broader selectivity curves and larger length at maximum selection than other species of comparable size such as the broadnose shark. The selectivity curves derived in this study will allow a more accurate interpretation of catch size composition and model-based assessments of stock status of these specie

Risk-based weight of evidence assessment of commercial sharks in western Australia

Sharks are generally taken in multispecies fisheries worldwide so in many cases it is logistically infeasible and cost prohibitive to assess the impact on all species caught. Risks of fishing impacts to the sustainability of shark species were assessed using a weight of evidence risk-based framework. Whiskery (Furgaleus macki), gummy (Mustelus antarcticus), dusky (Carcharhinus obscurus), and sandbar (C. plumbeus) sharks provide a good case study as they are the main (~80% of the catch) shark species taken in Western Australia (WA) and represent the range of life history strategies of other shark species typically caught. Based on the available lines of evidence for stock status (spatial and temporal time series of catch, effort, catch rates, size composition and population modelling), the 2015–16 risk level for the four species was scored as medium―i.e. it is unlikely (5–20% chance) that there is a high level of stock depletion. The status of each of the four species was scored as ‘acceptable’ with risk control measures in place, indicating that no additional restrictions are required

Exceptional longevity in a lightly exploited, semi-anadromous clupeid (Perth herring Nematalosa vlaminghi) within a degraded estuarine environment

Many anadromous (and semi-anadromous) fish species, which migrate from marine to freshwater ecosystems to spawn and to complete their life cycle, are currently threatened by habitat degradation in the upper parts of estuaries and rivers, where spawning and juvenile nursery areas occur. This situation pertains to Nematalosa vlaminghi, a semi-anadromous gizzard shad (Clupeidae: Dorosomatinae) endemic to south-western Australia. More information on the biology of N. vlaminghi is required for its effective management and conservation. This study estimated growth, longevity and natural mortality of N. vlaminghi. Ages were determined by counting validated annual growth increments in thin sections of sagittal otoliths. Fish were sampled in the Swan-Canning Estuary, which historically hosted the main commercial fishery for N. vlaminghi. Since the late 1990s, however, only very minor catches of this species have been taken from this estuary and none since 2007. Given the essentially unexploited state of the current population, the estimate of total mortality (Z, y-1 ) from the catch curve analysis in this study provides a direct estimate of natural mortality (M, y-1 ) for N. vlaminghi. Somatic growth during this study was substantially slower than that historically reported for N. vlaminghi. Various processes operating in this estuary since the 1970s may have contributed to slower growth, including increased hypoxia, higher primary productivity due to eutrophication and cessation of fishing for N. vlaminghi. The maximum observed age of 19.8 years for N. vlaminghi is the highest reported for any gizzard shad globally and one of the highest reported for any clupeid species. This exceptional longevity is likely part of a life-history strategy that allows N. vlaminghi, which exhibits substantial variation in annual recruitment success, to persist in the intermittently closed estuaries of south-western Australia where environmental factors, including low flow and hypoxia, can create unfavourable conditions for reproduction for extended periods

Stock enhancement in greenlip abalone part III: bioeconomic evaluation

This study presents a bioeconomic evaluation of the effect of stock enhancement on biomass, net present value, profitability, and gross value of product of the Australian greenlip abalone (Haliotis laevigata) fishery. Enhancement targets were defined as a function of natural recruitment (Nr) and compared with current harvest strategies. The model was conditioned on a Western Australian fishery, then applied to greenlip stocks throughout Australia. Two levels of releases (50% Nr and 100% Nr) at varying fishing mortality (F), size at harvest, and size at release were evaluated in detail. Model validation was also undertaken by comparing the model-derived spawning biomass (SSb) with an alternative estimate (SSbf) obtained using in-water surveys and a different growth model. Economic profitability and increased spawning biomass were achieved for most stock enhancement scenarios, and optimal profitability occurred with a 10–20% decrease in F from current levels, a 10% decrease in minimum legal length, and an annual enhancement of Nr juveniles to match natural recruitment. More radical scenarios, such as an annual release of 150% Nr combined with a 30% decrease in size at harvest resulted in greater profitability (+175%) but presented a higher risk of wild stocks being replaced with hatchery genotypes. Sensitivity analysis revealed that mortality, size at release, and harvest price were the critical parameters, while costs of production and fishing were less important. At the national scale, an enhancement scenario involving an annual release of 6.1 million 4-cm juveniles (∼age 2) resulted in a 60% increase in gross value of product ($25 to$40 million), a 120% increase in profitability ($12 to$26 million), and net present value ($190 to$420 million; 6% discount), and a 25% increase in SSb

Differential changes in production measures for an estuarine-resident sparid in deep and shallow waters following increases in hypoxia

This study determined how productivity measures for a fish species in different water depths of an estuary changed in response to the increase in hypoxia in deep waters, which had previously been shown to occur between 1993–95 and 2007–11. Annual data on length and age compositions, body mass, growth, abundance, biomass, production and production to biomass ratio (P/B) were thus determined for the estuarine-resident Acanthopagrus butcheri in nearshore shallow (\u3c2 m) and offshore deep waters (2–6 m) of the upper Swan River Estuary in those two periods. Length and age compositions imply that the increase in hypoxia was accompanied by the distribution of the majority of the older and larger A. butcheri changing from deep to shallow waters, where the small fish typically reside. Annual densities, biomass and production in shallow waters of \u3c0.02 fish m−2, 2–4 g m−2 and ∼2 g m−2 y−1 in the earlier period were far lower than the 0.1–0.2 fish m−2, 8–15 g m−2 and 5–10 g m−2 y−1 in the later period, whereas the reverse trend occurred in deep waters, with values of 6–9 fish net−1, 2000–3900 g net−1, 900–1700 g net−1 y−1 in the earlier period vs \u3c 1.5 fish net−1, ∼110 g net−1 and 27–45 g net−1 y−1 in the later period. Within the later period, and in contrast to the trends with annual abundance and biomass, the production in shallow waters was least during 2008/09, rather than greatest, reflecting the slow growth in that particularly cool year. The presence of substantial aggregations of both small and large fish in shallow waters accounts for the abundance, biomass and production in those waters increasing between those periods and thus, through a density-dependent effect, provide a basis for the overall reduction in growth. In marked contrast to the trends with the other three production measures, annual production to biomass ratios (P/B) in shallow waters in the two years in the earlier period, and in three of the four years of the later period, fell within the same range, i.e. 0.6–0.9 y−1, but was only 0.2 y−1 in 2008/09, reflecting the poor growth in that year. This emphasises the need to obtain data on P/B for a number of years when considering the implications of the typical P/B for a species in an estuary, in which environmental conditions and the growth of a species can fluctuate markedly between years