Shark depredation: future directions in research and management

Shark depredation is a complex social-ecological issue that affects a range of fisheries worldwide. Increasing concern about the impacts of shark depredation, and how it intersects with the broader context of fisheries management, has driven recent research in this area, especially in Australia and the United States. This review synthesises these recent advances and provides strategic guidance for researchers aiming to characterise the occurrence of depredation, identify the shark species responsible, and test deterrent and management approaches to reduce its impacts. Specifically, the review covers the application of social science approaches, as well as advances in video camera and genetic methods for identifying depredating species. The practicalities and considerations for testing magnetic, electrical, and acoustic deterrent devices are discussed in light of recent research. Key concepts for the management of shark depredation are reviewed, with recommendations made to guide future research and policy development. Specific management responses to address shark depredation are lacking, and this review emphasizes that a "silver bullet" approach for mitigating depredation does not yet exist. Rather, future efforts to manage shark depredation must rely on a diverse range of integrated approaches involving those in the fishery (fishers, scientists and fishery managers), social scientists, educators, and other stakeholders.

A rare capture of two Japanese rubyfish (Erythrocles schlegelii) (Richardson, 1846) off southern Western Australia, including their otolith characteristics and age

Due to advances in fish-finding equipment and vessel capabilities, commercial and recreational fishers are now regularly accessing deep waters off the edge of the continental shelf and catching fish species that are rarely encountered in shallower waters or that have not previously been caught. This study details the capture of two Japanese rubyfish (Erythrocles schlegelii) in deep, offshore waters on the south coast of Western Australia, and in waters further south than previously encountered. This study also provides the first age estimates from counts of growth (opaque) zones in sectioned sagittal otoliths. The estimated ages of 8 years for both individuals, one female and the other male, are much lower than what would have been expected based on age estimates for this species derived from scales undertaken in an earlier study. Understanding the biological characteristics of these deepwater fish species will assist in future management of these resources.

Biological characteristics of three co-occurring species of armorhead from different genera vary markedly from previous results for the Pentacerotidae

Biological characteristics of Pentaceropsis recurvirostris, Paristiopterus gallipavo and Parazanclistius hutchinsi were determined from commercial gillnet samples from temperate south-western Australian coastal waters. Growth zones in otoliths, with more than a few such zones, were readily detectable only after the otoliths had been sectioned. Visual analyses and modelling of the trends in marginal increments on sectioned otoliths demonstrate that these opaque zones are formed annually. Maximum ages of 55, 36 and 49 years, derived for P. recurvirostris, P. gallipavo and P. hutchinsi, respectively, reflect relatively low mortalities. These longevities greatly exceed those estimated, using otoliths, for Pentaceros wheeleri and Pentaceros richardsoni, which belong to the other pentacerotid subfamily. These differences may be due to the counts of 'daily' growth zones in sectioned otoliths of P. wheeleri not representing the complete age range of that species and the zones detected in whole otoliths of P. richardsoni not constituting the complete range of annually-formed zones. Pentaceropsis recurvirostris, P. gallipavo and P. hutchinsi recruited into the fishery in the sampling area as 2-3 year-old fishes. Pentaceropsis recurvirostris and P. hutchinsi exhibited little or no subsequent growth throughout the remainder of their protracted life, whereas, P. gallipavo continued to grow for c. 5 years and then underwent little further growth. Spawning of P. recurvirostris and P. hutchinsi peaked in the austral winter and autumn, respectively, but in the austral spring and summer with P. gallipavo, which is more typical of temperate species. Although the females of P. gallipavo and P. hutchinsi were mature, this did not apply to a few P. recurvirostris, some of which were >20 years old, implying that any given female of this species does not always spawn every year. Ovarian mass greatly exceeded testis mass, indicative of pair spawning, which is consistent with field observations. In contrast to P. recurvirostris and P. hutchinsi, the sex ratio was heavily biased towards males and the spawning period longer in P. gallipavo, suggesting that selection pressures for spawning success were greater for this latter species.

Dietary variations within a family of ambush predators (Platycephalidae) occupying different habitats and environments in the same geographical region

This study has determined the extents and basis for variations in the composition of the prey ingested by the abundant species of a family highly adapted for ambush predation, i.e. Platycephalidae, in a region (south-western Australia) where that family is found in different habitats and environments. Dietary data were thus collected for Leviprora inops and Platycephalus laevigatus from seagrass in marine embayments and for Platycephalus westraliae from over sand in an estuary. These were then collated with those recorded previously for Platycephalus speculator from over sand and in seagrass in an estuary and for Platycephalus longispinis from over sand in coastal marine waters. While crustaceans and teleosts together dominated the diet of all five species, their percentage volumetric dietary contributions varied greatly, with those of crustaceans ranging from 7% for L. inops to 65% for P. speculator and those of teleosts ranging from 29% for P. longispinis to 91% for L. inops. For analyses, the data were separated into two sets. The first comprised the 17 dietary categories of invertebrates and all identified and unidentified teleosts collectively, while the second consisted of the 23 identified teleost families, both of which were subjected to permutational analysis of variance (PERMANOVA), analysis of similarities (ANOSIM) and a new (two-way) version of the RELATE procedure. The diets of three species changed seasonally, when using invertebrate dietary categories and teleosts collectively, but with only one species, when employing identified teleost families, probably reflecting a greater tendency for invertebrate than teleost prey abundance to change during the year. On the basis of dietary data for invertebrate taxa + teleosts collectively, the diets of three of the five species changed serially with body size, with a fourth species feeding, throughout life, predominantly on the carid Palaemonetes australis. Based on identified teleost families, the diets of the three species that fed predominantly on teleosts underwent serial size-related changes. Although L. inops and the co-occurring P. laevigatus both consume large volumes of teleosts, the former ingests larger, less demersal and more mobile prey, e.g. the labrids Haletta semifasciata and Neoodax balteatus, than the latter, e.g. the scorpaenid Gymnapistes marmoratus, reflecting the possession by L. inops of a far longer head and larger buccal cavity. Circumstantial evidence suggests that the large differences in the volumes of crustaceans and teleosts consumed by each platycephalid species are related to differences in the relative availability of these prey in the different habitats or environments of each species.

Biology of the harlequin fish Othos dentex (Serranidae), with particular emphasis on sexual pattern and other reproductive characteristics

Key biological characteristics of the harlequin fish Othos dentex, a representative of a monospecific genus of the Anthiinae (Serranidae), were determined from samples collected around reefs on the south coast of Western Australia. The females of this relatively long-lived species (maximum recorded age in this study = 37 years) attained only a slightly greater maximum total length and age than males and neither the length nor the age-frequency distributions showed a conspicuous sex-based bimodality. Furthermore, gonads from a wide size and age range of O. dentex were shown by histology, at several locations along their length, to always comprise exclusively either ovarian or testicular tissues. Thus, O. dentex is a gonochorist, a sexual pattern only previously recorded definitively for one other anthiine serranid, i.e. Epinephelides armatus, which also occurs in south-western Australia. Similar to E. armatus, O. dentex possesses 'solid' testes with a central sperm duct, thereby differing in structure from those typically found in serranids, in which there is a central membrane-bound 'ovarian' lumen and peripherally located sperm sinuses. The gonadal characteristics and sexual pattern of these two gonochoristic anthiines are not consistent with a recent proposal for the trends exhibited by the evolution of gonochorism and protogyny within the Serranidae. Othos dentex has indeterminate fecundity and a protracted spawning period (7 months) and, on the basis of underwater observations and a low gonado-somatic index (I(G)) for males, is a pair spawner, which is unusual for a gonochorist of a serranid or member of a related family. While the large spots on the lower half of the body of O. dentex are shown quantitatively to be similarly yellow in juveniles and adult females, they then become blue in males at maturity and this intensifies during the spawning period, when they presumably play an important role in agonistic interactions among males and courtship with females. The attainment of maturity and rapid growth by O. dentex early in life may reflect selection pressures to reduce predation mortality during that period. Total mortality in the population is moderately low during later life, implying that the current fishing pressure on O. dentex is relatively light.