The Tasmanian Commercial Dive Fishery (TCDF) is transitioning from a fishery with minimal controls on catch with no annual assessment, to a carefully managed fishery with annual robust fishery assessments. Two of the three key species Shortspined Sea Urchin (Heliocidaris erythrogramma) and Wavy Periwinkle (Lunella undulata), have similar life histories to abalone, while the third – Longspined Sea Urchin (Centrostephanus rodgersii), has an intermediate life history. All three species have a time-series of catch and effort data, and some level of biological data, and thus don’t fall into the class of data-poor fisheries. However, just as for haliotid fisheries, all the complexities identified by Orensanz et al (1995) for S-Fisheries (small-scale, spatial structured, sedentary target species) apply here, meaning that integrated assessment models assuming Dynamic Pool should not be used. Instead, Empirical Harvest Strategies based on standardised catch rates are the most appropriate approach to underpin annual assessments and determination of stock status. Given the similarities between these species and abalone (life history, habitat, fishing practices), it is desirable to utilise the considerable investment in research on data collection and assessment methods in that fishery. This project will therefore attempt to adopt and adapt ground-breaking research on Abalone, including utilization of high-resolution spatial data from GPS and depth data loggers, as a short-cut to worlds best practices in the TCDF.

Consideration of stock status for the TCDF species to date has been on the basis of graphical inspection of crude catch rates. As Empirical Harvest Strategies rely heavily on robust catch rate metrics, adopting a model for standardizing catch rates is a critical step for the TCDF fisheries. While there are a diversity of philosophies around catch rate standardization, choosing the optimal standardization model is only part of the challenge. The key challenge for the three key TCDF species are that catch is spatially and temporally fragmented. Both urchin species are highly seasonal, and Shortspined Sea Urchin and Wavy Periwinkle fisheries are spatially discrete with a few high catch areas, and a larger proportion of spatially and temporally disparate low productivity fishing grounds. Similarly, catch is largely landed by a small number of highly active participants, with a larger number of primarily part-time fisheries. This fragmented nature (time, space, people) of the dataset creates challenges for applying standardisation models to establish a robust time-series. Primarily this manifests as highly volatile time-series, that can flip above or below Reference Points over consecutive years. In some fisheries, this challenge is addressed by fitting a multi-year running mean through catch rates, essentially smoothing the trend. While this might have the desired effect of removing hyper-variability in catch rates, it will slow action required as the stocks decline, as well slow catch increases under rebuilding. Preferably, we will instead use environmental variables, quantify fisher experience and make use of fine-scale location data on fishing activity in our catch rate standardization methods.

An additional complication in the Tasmanian Commercial Dive fishery is mixed species fishing. This feature is also present in a sub-set of assessment areas within the Tasmanian Abalone fishery where H. rubra and H. laevigata co-occur, as well as in the Central and Western Zone abalone fisheries in South Australia. Developing a clear catch rate signal in these mixed species fisheries is challenging and an optimal solution has not yet been found, although spatial and depth information most likely will enable differentiation of fishing grounds across species. Through this project we will engage with South Australia specifically to collaborate on logger based approaches to improve assessment of mixed species fishing regions.

As with abalone fisheries, TCDA divers are able to modify their in-water fishing behaviour (swim faster and further) to achieve desired daily catches which has the effect of masking decline in abundance. However, this is not the only form of hyperstability expected in the TCDA fisheries. The highly fragmented stock structure and diver experience are likely to interact to create a second form of hyperstability more common in emerging fisheries, and that is serial depletion, or at least serial fishing of discrete patches. This latter form of hyperstability occurs when more experienced divers with greater knowledge are able to move when more accessible patches become over-exploited.

Local scale depletion is addressed by the establishment of size limits for Wavy Periwinkle and the Shortspined Sea Urchin. Robust size limits have recently been adjusted for Wavy Periwinkle (increase from 30 to 45 mm; FRDC 2011-024) and Shortspined Sea Urchin (increase from 60 to 75 mm FRDC 2017-033); No size limit is intended to be set for range-extending Longspined Sea Urchin, as the objective in this fishery continues to be to achieve localised depletion.

From the 2022/2023 season diver GPS and depth loggers will be compulsory to collect detailed spatial information about the fishery. Several key divers have been using dataloggers for the past eight years, providing a valuable time-series from the more experienced TCDF fishers. The availability of such data creates a new opportunity and leads to the key aim of this project, to develop methodologies and tools to incorporate the detailed spatial catch data into formal stock assessment and harvest strategies. In doing so, methods to facilitate the accurate standardisation of historical logbook data to facilitate transition into logger data to preserving the continuum of the time series is required. The development of reliable fishery dependent assessment metrics is essential in these smaller fisheries where there is no scope for cost-prohibitive fishery independent surveys.

In summary, this project is needed to develop strategies to produce robust standardized catch rate trends, that can be utilized in an Empirical Harvest Strategy. Spatial based Catch Per Unit Effort (CPUE) measures and spatial performance indicators and indices of hyperstability will be critical to facilitate the creation of appropriate stock assessment and harvest strategies for small scale dive fisheries, including smaller haliotid fisheries.