title: The Effects of Oceanographic Drivers on the Catchability of Spanner Crabs
Creators: name: Spencer, David
Publisher: name: Griffith University
Year: 2018
Subjects: subject: Physical oceanographic drivers
subject: Spanner crab
subject: Ranina ranina
subject: Spatiotemporal scales
subject: Hydrodynamic
subject: Biogeochemical

description: The world’s largest commercial spanner crab (Ranina ranina) fishery, situated on the east coast of Australia, is showing signs of a fishery-wide population decline. This trend has become ubiquitous in many global fishery stocks and is most commonly attributed to anthropogenic pressure and the rapidly changing climate. Thus, the importance of better understanding how commercially-important fishery species thrive in their immediate and surrounding environment is imperative. Catchability is an important parameter used to describe the relationship between catch rates and the abundance of the target species. Variations in catchability are often attributed to fluctuations in environmental and oceanographic conditions in the species’ preferred habitat. Earlier research had suggested several hydrodynamic and hydrographic parameters affect the catchability of spanner crabs. However, a review of the literature (thesis chapter 2) indicated the need for further investigations, particularly on the specific oceanographic processes responsible for these effects. This thesis examines the effects of several oceanographic parameters on spanner crab catch rates at a range of spatiotemporal scales. At large spatial scales (100:500 km) over several years, higher surface chlorophyll-a concentrations, relative to the range of chlorophyll-a observed in each management region, were correlated with lower catch rates in fishing grounds close to the coast and bays. Additionally, region-specific processes responsible for bringing oceanic waters into spanner crab fishing grounds were associated with an increase in catch rates. The link between oceanic water and increased catch rates was further supported by a more localised study (~100 km) that showed cooler ambient temperatures, occasionally attributed to wind-driven upwelling, increased catch rates of spanner crabs at both seasonal and shorter (days-months) temporal scales. At various fishing locations off the Gold Coast, Australia, the effects of current speed, direction and turbidity were examined to determine how specific conditions (day to day) and changes in conditions during soak times (hours) affected catch rates. The most significant finding from this work suggests that catch rates of spanner crabs benefit from a specific range of current speeds (~0.07-0.12 m.s-1), and current speeds exhibiting a large amount of variability may trigger a spike in catch rates over a period of hours. Results from this work show current speed can help explain short-term catch rate anomalies that are currently deemed “random” in stock assessment models. Incorporating environmental and oceanographic parameters into stock assessment models has been an ever-evolving challenge for fishery management. Depending on the region-specific oceanographic and coastal processes, various remotely-sensed oceanographic parameters are also useful in explaining catch rates. The most significant outcomes from this thesis indicate bottom temperature, alongshore wind stress, and bottom current speed are suitable in explaining variability in spanner crab catch rates. A coupled hydrodynamic and biogeochemical model capable of resolving and predicting fluctuations in these oceanographic parameters will help support studies in other areas of the Australian fishery and smaller fisheries in the Indo-Pacific, help improve the accuracy of stock assessment models, and greatly benefit the economic efficiency of commercial crabbing operations.