Ashley John Williams

Marine resources are often shared among countries, with some fish stocks straddling multiple Exclusive Economic Zones, therefore understanding the structure of populations is important for the effective management of fish stocks. Otolith chemical analyses could discriminate among populations based on differences in the chemical composition of otoliths. We used otoliths from two deepwater snappers (flame snapper Etelis coruscans and ruby snapper Etelis boweni) to examine the evidence for population structure across six Pacific Island countries using solution‐based inductively coupled plasma mass spectrometry (ICP‐MS) for otolith core and whole otolith samples and laser ablation ICP‐MS (LA‐ICP‐MS) for core and edge areas of a cross‐sectioned otolith. The inter‐species comparison of these methods is important as the two species are often managed under the same regulations. For both species, the two methods demonstrated separation among the locations sampled with high classification accuracy. Smaller laser ablation spot size gave greater temporal resolution over the life‐history transect. Comparing the early life‐history section of the otoliths (i.e., the core), one interpretation is that young fish experienced more uniform environments in the open ocean as larvae than adults, as the elemental fingerprints had greater overlap among multiple locations. LA‐ICP‐MS methods had some advantages over solution‐based ICP‐MS and generally better discrimination for the trace elements investigated. There were substantial differences between species, but both methods suggested nonmixing populations at the regional scale. Otolith chemistry can be an effective tool in discriminating variation for deepwater marine species in multispecies fisheries, and edge measurements from LA‐ICP‐MS provided the greatest resolution. Although caution should be taken in interpreting the results from relatively small samples sizes, otolith chemical analyses could be useful at these spatial scales to investigate population structure. This information on separate or overlapping populations could be used in future regional fishery management plans.

Risks to deepwater chondrichthyans (sharks, rays, and chimaeras) from fishing are poorly understood, particularly in areas beyond national jurisdiction. We adapted productivity–susceptibility analysis (PSA) and sustainability assessment for fishing effects (SAFE) to assess the vulnerability of 173 deepwater chondrichthyans to various demersal fishing gears in the Southern Indian and South Pacific Oceans. Several species were categorized as being at high or extreme vulnerability, including some deepwater shark species in the Southern Indian Ocean that are reported to be commercially targeted. There was good concurrence between PSA and SAFE results for species categorized as being at high or extreme vulnerability by the SAFE, but as expected there was an overall greater number assessed to be as higher vulnerability using PSA due to its precautionary nature. Our results indicate probable misclassifications in the PSA relative vulnerability rankings, highlighting the value of applying more quantitative tools, such as SAFE, when adequate data are available. Our findings indicate that better catch, effort, and biological information are needed to inform the assessment and management of deepwater chondrichthyans. If targeted fishing of deepwater shark species continues in the Southern Indian Ocean, improved assessments and estimates of sustainable yields are urgently required to mitigate the risk of overexploitation.

Minimizing fishing-induced mortality on bycatch and endangered, threatened or protected species is a necessity for fisheries managers. Estimating individual vessel bycatch rates by dividing the amount of bycatch by effort (nominal rate) can be biased, as it does not consider effort heterogeneity within the fleet and ignores prior knowledge of fleet bycatch rates. We develop an empirical Bayesian approach for estimating individual vessel and fleet bycatch rates that: (i) considers effort heterogeneity among vessels and; (ii) pools data from similar vessels for more accurate estimation. The proposed standardized bycatch rate of a vessel is, therefore, the weighted average of the pool rate and nominal rate of the vessel; where the weights are functions of the vessel’s fishing effort and a constant estimated from the model. We apply this inference method to the estimation of seabird bycatch rates in the component of the Australian Eastern Tuna and Billfish Fishery targeting yellowfin tuna. We illustrate the capability of the method for providing fishery managers with insights on fleet-wide bycatch mitigation performance and the identification of outperforming and underperforming vessels. This method can also be used by fishery managers to develop fleet-wide performance measures or quantitative evaluation standards.