Project number: 2005-023
Project Status:
Budget expenditure: $99,882.65
Principal Investigator: Simon Robertson
Organisation: Agriculture Victoria
Project start/end date: 30 Aug 2005 - 30 Jun 2009


Age composition data provides the key information necessary to effectively manage fisheries. The proposal provides a mechanism where age composition data can be gained using length frequency data and age composition data from different years and sampling events, which has previously been impossible. The benefits will be a reduced need for production ageing, more timely age composition data and the ability to construct age composition data from historical length frequency data where no samples were collected for ageing.

Currently the Age-Length Key (ALK) is the most widely used numerical method for assessing the age composition in a large sample of length-frequency data. However, the application of ageing data in this approach is restricted to the original sample of length distribution (ageing data from the same year the length-frequency sample is taken). Due to this severe limitation, the ageing information must be regenerated for each new data sample. Using the Fredholm First Kind equations, previous years ageing data can be used to generate the underlying age composition from the current length-frequency data. Furthermore, the ageing data may be added to include many years, improving the robustness of the statistic which can then be used to decompose the underlying age distribution from the given length frequency.

As noted by a number of referees, the major problem with the current methods is variable recruitment. We have demonstrated that the technique is tolerant to the most extreme changes in age frequency (see accompanying text). These extreme changes in age frequency are greater than any changes that could occur naturally through recruitment. The issue of variable growth may affect the efficacy of the approach, but to our knowledge, has only been observed in two species. These are black bream and blue grenadier. It is proposed that the technique be demonstrated on blue grenadier in the first year.

The cost of collecting ageing data is high, with approximately $150,000 spent each year on ageing samples from commercially important species within the South East Fishery. Due to the cost, the number of species aged is not optimal and species are prioritised on a scientific and social-political basis. The cost-benefit of applying this approach is intuitively a large reduction in cost of ageing to industry and more timely information on the age structure of the population. A formal cost benefit analyses will need to be conducted on a species by species basis. This is a function of different cost structures for ageing different species, different numbers of samples that need to aged for each species. These different numbers of estimates that need to be made for each species is primarily due to longevity and stock structuring.

The age-structured data obtained from this project will benefit the South East Trawl Fishery, the Great Australian Bight Trawl Fishery and the Gillnet, Hook and Trap Fishery which are supported by The Integrated Scientific Monitoring Program (ISMP) and various other stock assessment programs that rely on age-structured data.

Further, age composition data will be able to be reconstructed historically from species where samples were not aged but length-frequency data were collected. This will enable age-structured population analysis where the lack of ageing data prevented these stock assessment techniques from being previously used. The net effect of this approach is to greatly improve the knowledge base from which species are managed. One of key advantages of this approach is, if successful, will at the very least compliment current methods and provide temporal and spatial coverage of age composition information which is currently cost prohibitive and only collected for a few, high value species.

The implication of a technique that can provide age-composition data free from the restriction of those associated with the ALK is more cost-effective resource management.

The proposal has been developed in two parts, the first component is a 'Proof of Concept Study' where the use of the Fredholm First Kind Equations to provide age compositions from length frequency data will be further examined. If this is not assessed as successful in a workshop environment, the project will be terminated at the end of the first year. The second and third year will examine a broad range of species.


1. During first 12 months provide a Proof of Concept Study on two species. One of the species with a short longevity and stable age composition over time and the second, with variable recruitment and higher longevity. The proposed species for this objective will be school whiting and blue grenadier.
2. Using Central Ageing Facility data, determine which commercially important fish stocks and associated data provide sufficient information for estimation of the age composition from length frequencies
3. Apply limited length-at-age data with extensive length-frequency data to improve time series of age compositions for fish stock assessment.
4. Determine the appropriate sample size for collection of age data.
5. Examine robustness of the numerical methods to length-specific selectivity.
6. Compare the new techniques with existing numerical methods.
7. Develop user friendly software and data environment for numerical reconstruction of cohorts and age groups.
8. Disseminate method to a broad audience of end-users
9. Provide cost-benefit analysis for each species undertaken comparing the financial benefit of using the new technique to the cost associated using traditional techniques. The objective will be delivered as species are examined.

Final report

ISBN: 978‐1‐74264‐135‐5
Author: Simon Robertson

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