Terence I. Walker

Standardization of catch and effort data in a spatially-structured shark fishery

Abstract The methods used to develop catch rate based indices of relative abundance for the school shark Galeorhinus galeus resource off southern Australia are outlined. These methods are based on fitting generalized linear models to catch and effort data for several regions in this fishery. This is to take account of the multi-gear nature of the fishery and the spatial structure of the trends in catch rate. The data on whether or not the catch rate is zero and the catch rate given that it is non-zero are analysed separately and then combined to provide indices of abundance. The former analysis is based on assuming the data are Bernoulli random variables. Given the uncertainty about the appropriate error-model to assume when fitting generalized linear models to catch and effort data, four alternative error-models — log-normal, log-gamma, Poisson, and negative binomial — were explored when modelling the non-zero catch rates.

Stock assessment of school shark, Galeorhinus galeus, based on a spatially explicit population dynamics model

The school shark (Galeorhinus galeus) resource off southern Australia is assessed by use of an assessment approach that takes account of the spatial structure of the population. The population dynamics model underlying the assessment considers the spatial as well as the age-specific characteristics of school shark. It allows for a series of fisheries (each based on a different gear type), explicitly models the pupping/recruitment process, and allows for multiple stocks. The values for the parameters of this model are determined by fitting it to catch-rate data and information from tagging studies. The point estimates of the pup production at the start of 1997 range from 12% to 18% of the pre-exploitation equilibrium size, depending on the specifications of the assessment. Allowing for spatial structure and incorporating tag release–recapture data lead to reduced uncertainty compared with earlier assessments. The status of the resource, as reflected by the ratio of present to virgin pup production and total (1+) biomass, is sensitive to the assumed level of movement between the stocks in New Zealand and those in Australia, with lower values corresponding to higher levels of movement.

Spatial and temporal variation in the reproductive biology of gummy shark Mustelus antarcticus (Chondrichthyes : Triakidae) harvested off southern Australia

Mustelus antarcticus, endemic to southern Australia, exhibits matrotrophic aplacental viviparity. Differences in synchronous ovarian and parturition cycles, mostly annual west and biennial east of longitude 138°E, are explained by environmental differences. Ovulation and parturition peak during November–December and the gestation period is ~12 months. Largest ovarian follicle diameter ranges from 15 to 28 mm at ovulation, and mean wet mass gain is 10-fold from in utero egg (~10 g) to full-term embryo (~100 g) at ~330 mm total length. The sex ratio of embryos in utero is 1:1, and litter size (1 to 57 embryos) rises curvilinearly with maternal length. Length-at-maternity and length-at-maturity increased with rising fishing mortality and subsequently decreased with falling fishing mortality. These patterns are explained by the hypothesis on the ‘phenomenon of apparent change of size-at-maternity’ (and size-at-maturity) caused by gill-net length-selective fishing mortality, which masks any potential density-dependent responses. Male length-at-maturity estimates from seminal vesicle condition, testis development and spermatogenesis stages are similar, but are less than estimates from clasper calcification. Maximum body mass of females is double that of males and, at any length >700 mm, mean body mass of females exceeds that of males.

Determining reproductive parameters for population assessments of chondrichthyan species with asynchronous ovulation and parturition: piked spurdog (Squalus megalops) as a case study

Population assessments of chondrichthyan species require several key parameters of their reproductive biology, which were estimated for Squalus megalops (Macleay, 1881). Length-at-maturity differed depending on the criterion adopted for defining maturity. In the case of males, length-at-maturity was smallest when condition of seminal vesicles was adopted as a maturity criterion. For females, length-at-maturity was smallest when the largest follicle diameter >3 mm was adopted as the criterion for maturity; this was appropriate only as an indicator of the onset of maturity. Mature males are capable of mating throughout the year. Females have a continuous asynchronous reproductive cycle. The sex ratio of embryos is 1 : 1 and litter size and near-term embryo length increase with maternal length. Females have an ovarian cycle and gestation period of two years. This was reflected in the differences found between the maturity and maternity ogives. Although all females are mature at 600 mm, only 50% of them contribute to annual recruitment each year. Hence, for chondrichthyan species with reproductive cycles of two, three or more years, if maturity ogives are used in population assessments instead of maternity ogives, the models will overestimate recruitment rates.

Size-selectivity of lobster pots with escape-gaps: application of the SELECT method to the southern rock lobster (Jasus edwardsii) fishery in Victoria, Australia

We have utilised a relatively new modelling method, SELECT, to calculate size-selectivity curves for data from two escape-gap field experiments on southern rock lobster (Jasus edwardsii) in Victoria, Australia. Size-selectivity curves based on an asymmetric Richards model fitted the data better than the commonly used logistic model, possibly because of higher than expected retention of small lobsters. Theoretical size-selectivity curves, calculated from morphometric data, were remarkably close to size-selectivity curves obtained from one experiment. In a second experiment, we showed that retention probabilities for lobsters close to the legal minimum length were lower than that predicted by the theoretical size-selectivity curves. Size-selectivity curves confirm that the current escape-gap size of 60 mm is close to optimum for the legal minimum lengths used in the Victorian southern rock lobster fishery. Our analyses failed to support the common assertion that escape-gaps increase the fishing power of lobster pots.

The Physiological Response of Port Jackson Sharks and Australian Swellsharks to Sedation, Gill-Net Capture, and Repeated Sampling in Captivity

Abstract Studying postrelease effects of fisheries capture on chondrichthyans in the wild poses considerable logistical challenges. We report a laboratory-based technique to (1) simulate gill-net capture of sharks, which allows monitoring the condition of animals during recovery from a controlled capture event, and (2) assess effects of sedation, serial blood sampling, and repeated exposure to experimental treatment on stress-related blood variables. Exposing Port Jackson sharks Heterodontus portusjacksoni and Australian swellsharks Cephaloscyllium laticeps to 30 min of simulated gill-net capture elicited behavioral stress (struggling and elevated ventilation rate) and minor physiological stress (elevated plasma lactate) responses but did not cause any mortality. Sedation of Australian swellsharks affected some stress-related blood variables. Repeated handling of Port Jackson sharks and Australian swellsharks at short intervals may result in elevated stress levels, but repeated exposure to simulated captu...

Comparison of deterministic growth models fitted to length-at-age data of the piked spurdog (Squalus megalops) in south-eastern Australia

Age and growth estimates of Squalus megalops were derived from the first dorsal fin spine of 452 sharks, ranging from 274 to 622 mm total length. Age bias plots and indices of precision indicated that the ageing method was precise and unbiased. Edge analysis of the enameled surface of whole spines and similarities in the banding pattern deposited in the enameled surface of spines and in spine sections supported the hypothesis of annual band formation. Multiple versions of two growth models were fitted to length-at-age data, from which a two-phase von Bertalanffy model produced the best fit. For males, the change in growth rate corresponded with size-at-maturity, whereas for females, the change was slightly before size-at-maturity. Regardless of the growth model used, growth rate of females (0.034 to 0.098 years −1 ) was very low, making S. megalops highly susceptible to overexploitation by fisheries.

Immediate and delayed effects of gill-net capture on acid-base balance and intramuscular lactate concentration of gummy sharks, Mustelus antarcticus☆

Many sharks are captured as untargeted by-catch during commercial fishing operations and are subsequently discarded. A reliable assessment of the proportion of discarded sharks that die post-release as a result of excessive physiological stress is important for fisheries management and conservation purposes, but a reliable physiological predictor of post-release mortality has not been identified. To investigate effects of gill-net capture on the acid-base balance of sharks, we exposed gummy sharks, Mustelus antarcticus, to 60 min of gill-net capture in a controlled setting, and obtained multiple blood and muscle tissue samples during a 72-h recovery period following the capture event. Overall mortality of gummy sharks was low (9%). Blood pH was significantly depressed immediately after the capture event due to a combination of respiratory and metabolic acidosis. Maximum concentrations of plasma lactate (9.9 ± 1.5 mmol L(-1)) were measured 3h after the capture event. Maximum intramuscular lactate concentrations (37.0 ± 4.6 μmol g(-1)) were measured immediately after the capture event, and intramuscular lactate concentrations were substantially higher than plasma lactate concentrations at all times. Sharks in poor condition had low blood pH and high intramuscular lactate concentration, but blood pH does not appear to be a reliable predictor of survival. Suitability of intramuscular lactate concentration as predictor of delayed mortality deserves further investigation.

Microscopic organization of the sperm storage tubules in the oviducal gland of the female gummy shark (Mustelus antarcticus), with observations on sperm distribution and storage

Oviducal gland morphology, the microscopic organization of the terminal zone, and sperm storage were described in the female gummy shark (Mustelus antarcticus). Mustelus antarcticus is a nonplacental viviparous hound shark, which displays minimal histotrophy during embryonic development. The animals examined represented all stages of maturity and gestation. The oviducal gland was found to have the same fundamental zonation as in most chondrichthyans. Using recent terminology, the oviducal gland of chondrichthyans has an anterior club zone, followed by a papillary zone, both of which produce jelly that surrounds the egg, a baffle zone that elaborates the tertiary egg envelope and a terminal zone, where sperm storage occurs. Each zone is composed of simple tubular glands that connect to transverse grooves, which extend the full width of the gland. The exception is the terminal zone, which does not have transverse grooves but consists of individual tubules. The microscopic organization and histochemical nature of the zones display similar patterns to those of other chondrichthyan genera. Tubules of the terminal zone contain four types of cell: ciliated cells, alcian blue‐positive secretory cells, periodic acid‐Schiff and alcian blue‐negative secretory cells, and secretory columnar cells. These tubules end in recesses, the sperm storage tubules, which extend beyond the periphery of the baffle zone. Sperm were stored in the sperm storage tubules of all maturing and mature animals examined. Of note is the observation of stored sperm in an animal 1 year prior to first ovulation. Sperm were also observed throughout the uterine sphincter, body of the uterus, isthmus, and oviduct of maturing and mature animals, and in the uterine sphincter of an immature animal. These sperm represent immediately postcopulation aggregations of sperm and sperm in the process of migrating to the site of storage or to the site of fertilization. J. Morphol., 2008.

The state of research on chondrichthyan fishes

More than a decade has passed since the Convention for Trade in Endangered Species (CITES) first alerted the United Nations Food and Agriculture Organization (FAO) of the urgent need to establish programs for collecting biological and trade data and for managing the impacts of fishing on shark populations. By 2000, FAO had developed the International Plan of Action for the Conservation and Management of Sharks (IPOA-Sharks) to form part of the Code of Conduct for Responsible Fisheries. Under the IPOA-Sharks, each of more than 80 signatory nations is obliged to develop and implement a National Plan of Action for the Conservation and Management of Sharks, where the term ‘shark’ covers all chondrichthyan groups (sharks, batoids, and holocephalans) (Anon. 2000). The IPOA-Sharks emphasises that the harvest of chondrichthyan fishes should be biologically sustainable, economically rational, utilising all body parts of the sharks killed, and managed to ensure biodiversity conservation and maintenance of ecosystem structure and function. However, despite the best intentions for better management by many nations, fishing for these species continues to increase in response to the evergrowing demand for shark meat and other products from these animals. Shark fins, for example, are among the most highly priced fisheries products in eastern Asia and this is stimulating the targeting of sharks and retention of only their fins, the practice known as ‘finning’. Direct fishing mortality is not the only impact on chondrichthyan populations. There are fishing impacts on habitats through disturbance of biotic communities and substrates. Shipping and underwater exploration, construction, mining, and electrical installation also affect habitats, and increasing ambient sound, light, electromagnetic fields, and chemical contamination stimulate the sensory systems of chondrichthyan fishes. Understanding and measuring responses to these impacts and others such as from invasive species and perhaps aquaculture pose enormous research challenges. An overlay to complicate the research difficulties addressing these impacts is the uncertain magnitude of climate change, the effects of which have the potential to obscure the effects of fishing and other anthropogenic activities.