Australia’s crustacean fisheries generate over a billion dollars annually. Age information is necessary for calculating growth, mortality, recruitment and productivity, and is a critical component for any sustainable fisheries management plan. Until recently, direct age determination was considered impossible for crustaceans and only indirect methods existed (e.g. size frequency distributions). However, recent publications (Leland et al., 2011; Kilada et al., 2012) have rejected this paradigm and demonstrated that direct age determination of crustaceans is possible. Clear growth marks have been identified in ossicles from several key Australian species – e.g. western rock lobster (de Lestang, unpublished data), redclaw crayfish and mud crab (Leland et al., 2011; Leland et al., in prep). This project is needed to produce validated ages for mostly long-lived slow-growing species, across their full size range and in climatic regions other than the cool temperate conditions assessed previously. Validated age information will facilitate the sustainable management of Australia’s crustacean fisheries.
This project aligns with three FRDC Strategic Priority Areas: Theme 4 (Ecologically sustainable development), Theme 13 (Innovation skills) and Theme 14 (Extension and Adoption). All FRDC stakeholders will benefit by: i) ensuring the sustainable management of Australia’s relevant crustacean fisheries (e.g. through improved growth modelling and age-based stock assessments), ii) improving knowledge of key biological attributes (i.e. age and growth), iii) developing a national network of Australian fisheries and academic scientists capable of applying innovative crustacean age validation methods, iv) achieving change in crustacean fisheries management, while v) improving social license and vi) reducing uncertainty.
This research project was undertaken by a national collaboration of government and academic scientists representing key Australian crustacean fisheries. The collaborating institutions were the: Marine Ecology Research Centre – Southern Cross University, Department of Fisheries Western Australia, Institute for Marine and Antarctic Studies – University of Tasmania, New South Wales Department of Primary Industries – Fisheries, Northern Territory Department of Primary Industry and Fisheries, South Australian Research and Development Institute and James Cook University. The project was initiated in response to the need for validated age information for crustacean fisheries management. We applied a novel direct age-determination method to seven commercially important Australian crustaceans sourced from tropical to temperate habitats, shallow to deep water and including both short- and long-lived species. Similar to fish ageing, the direct ageing method applied here involves cross-sectioning gastric ossicles (i.e. semi-calcified structures within the stomach) to enable the extraction of a chronological record (i.e. by counting growth marks) for subsequent growth modelling. For the first time, we have demonstrated the widespread applicability of direct ageing to Australian crustaceans and validated that ossicular growth marks in Western, Eastern and Ornate Rock Lobster and Crystal Crab ossicles are deposited annually. Validation of the direct ageing method, allowed for the construction of the world’s first directly determined growth models for any Rock Lobster, with most comparisons to existing indirect estimates corroborating annual periodicity.
The ability to procure accurate age information is important for any sustainable fisheries management plan. Age information underpins growth and productivity estimates and also informs the selection of input control regulations (e.g. minimum legal size). For many fin fish and invertebrate species, age determination is relatively straightforward and involves counting growth increments in calcified structures. Because crustaceans grow via consecutive moult events, it was always presumed that their hard parts could not retain a chronological growth record and fisheries scientist have relied solely on less-accurate indirect methods (e.g. tag-and-recapture) that infer age. However, recent studies have demonstrated that crustacean ossicles contain growth marks that can be used for direct age determination, but species-specific periodicity validation (i.e. proof of accuracy) is needed before widespread use of the method occurs. The need for a validated direct ageing method for crustaceans was recognised throughout Australia and resulted in this project being strongly supported by relevant industry bodies, state government fisheries departments and academic institutions. Although indirect techniques provide useful information, a validated direct ageing method is highly desirable and could substantially increase the resolution of age-related data for crustacean fisheries management in Australia.
The objectives of this research project were to:
- assess the relationship between estimated age and size, compared with existing growth models for Western and Eastern Rock Lobster,
- evaluate growth mark periodicity for Western and Eastern Rock Lobster and Crystal Crab by vital staining and long-term grow-out,
- investigate the applicability of direct ageing methods to other commercially important crustaceans (Western, Eastern, Southern and Ornate Rock Lobsters and Giant, Crystal and Mud Crabs) – validated with laser ablation induction-coupled plasma mass spectroscopy and known-age individuals and
- establish a network of Australian government and academic fisheries researchers that can consistently apply direct ageing methods to decapod crustaceans.
For Western and Eastern Rock Lobster, sectioned ossicles contain regular primary growth marks that are positively correlated with body size. Ossicular growth mark counts were converted to age estimates and used to generate von Bertalanffy growth models that were not significantly different to those from comparable tag-and-recapture studies. For Western Rock Lobster, the directly determined ages closely agreed with indirect longevity estimates and the age at fishery-specific milestones (i.e. minimum legal size and size-at-sexual maturity), with the relationship between direct and indirect age (i.e. derived from both wild-caught and known-age individuals) being approximately 1:1 and providing strong corroborative support for annual periodicity. For Eastern Rock Lobster, the directly determined putative ages broadly agreed with indirect maximum longevity estimates, but yielded consistently older ages at fishery-specific milestones (i.e. minimum legal size, size-at-sexual maturity and maximum legal size), with the relationship between direct and indirect age estimates for some locations being approximately 1:1 (i.e. providing support for annual periodicity), but for others it was markedly different (i.e. for Jervis Bay and some Coffs Harbour individuals).
For Crystal Crab, there was ossicular extension during the 18 month grow-out, with primary growth mark formation occurring during the inter-moult. Irrespective of the sampling period, most Crystal Crab deposited one new-formed primary growth mark (n = 12) during the grow-out. For Western Rock Lobster (n = 1), the periodicity evaluation indicated that a single primary growth mark was deposited during the 18 month grow-out. For Eastern Rock Lobster (n = 1), the periodicity evaluation indicated that a single primary growth mark was deposited during the 12 month grow-out. For both rock lobster species, there were other ossicles that had material deposited beyond the calcein stain, but were without an identifiable growth mark. For all species, the common outcome of the periodicity evaluation was that a single new growth mark was deposited during the grow-out, indicating that the primary marks are deposited annually.
The direct ageing method was readily applied to Ornate Rock Lobster, Southern Rock Lobster, Mud Crab and Crystal Crab ossicles. Giant Crab (n = 3 individuals) ossicles contained some primary growth marks, but complete counts were not possible. For Ornate Rock Lobster (n = 5) and Southern Rock Lobster (n = 5), the direct ageing method allowed for the rapid estimation of preliminary von Bertalanffy growth parameters that were not significantly different to those derived from tag-and-recapture studies at the same location. Some LA-ICPMS results (e.g. for Mud Crab and Western and Eastern Rock Lobster) could be interpreted as supporting annual periodicity, but emerging uncertainties around ossicular decalcification and potential re-deposition of mineral features precluded a positive validation outcome. The direct ageing method was also validated by the use of known-age Ornate Rock Lobster (n = 13) and Western Rock Lobster (n = 3).
The immediate impact from this project will be jurisdiction- and species-specific, because each state fisheries department has different needs, priorities and validation expectations. However, the ability to directly determine (i.e. and validate) crustacean age provides another tool for fisheries scientists to enhance the resolution of current growth models, while decreasing research costs. Validation of the ageing method for Western, Eastern and Ornate Rock Lobster and Crystal Crab also opens the way for preliminary trials using the technique in stock assessments. Further, the validated technique will allow for rapid location-specific growth assessments and more accurate longevity estimates. This will be particularly important for long-lived species that present difficulties for tagging studies (e.g. Crystal Crab and Tasmanian Southern Rock Lobster) and would be useful for securing fishery sustainability certifications (e.g. Marine Stewardship Council). For shorter-lived species (e.g. Ornate Rock Lobster and Mud Crab), direct ageing could improve the assessment of population dynamics. The financial gains are difficult to quantify, but even a 1% improvement in decision making, and/or decrease in research costs (i.e. across multiple valuable fisheries), would equate to a substantial return of investment from this project. Such gains will translate into improved sustainability among Australia’s crustacean fisheries, with flow-on benefits to the relevant fishing industry and across other sectors.
The broad-ranging nature (i.e. in terms of species and fisheries jurisdictions) of this project made definitive recommendations difficult. However, further species-specific research should: i) validate periodicity across the entire age range, ii) determine the age at first growth mark formation and iii) assess ageing accuracy. Concurrent studies trialling the direct ageing method during ongoing stock monitoring programs would be beneficial. This would allow for direct methodological comparison and growth model construction for the exact same location(s) and temporal period. For some species (e.g. Eastern Rock Lobster), the direct ageing method should be used to assess the potential for location-specific differences in growth. Application of the direct method to Crystal Crab is needed to provide the first solid (i.e. non-preliminary) assessment of growth and longevity for this species. Such research should encompass the relevant priorities for further development, particularly the requirement for concurrent species-specific precision assessments. The provision of this report to the relevant state fisheries departments is expected to initiate further jurisdiction- and stock-specific recommendations that will form the basis for further research and development applications.