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Age at length keys (ALKs), which give the probability of age given length, are a fundamental component of many age-based fish stock assessment methods. Usually, ALKs are compiled from readings of otoliths or scales taken from length-stratified sub-samples of fishery landings or research vessel trawls catches. The assessment process is data intensive when there are numerous fleets assessment sub-regions to be sampled over an annual cycle, making the collection and analysis of material costly and time consuming. Hence, the data are almost always sparse, often with voids that require in-filling before use in assessments. Though robust statistical procedures for automatically in-filling data voids have been developed, they have not been widely adopted, and procedures often remain manual. Here we use Generalized Linear Models to derive the probability of age given length from sparse data gathered during the International Bottom Trawl Survey and Scottish commercial sampling program. The keys are used to test statistically for differences between ALKs from different sampling regions, differing gear geometries, different sampling programs and fish at different life stages. The results of the comparisons suggest that ALKs from the commercial sampling program are not, in general, comparable to those generated by the IBTS program. We also found that ALKs from Nephrops trawls are significantly different from those generated by other trawls over the same sampling region. The tests also suggest that age at length distributions differ not only between but also within IBTS roundfish areas, in part due to differences between ALKs for mature and immature fish. These differences are an important factor when considering a reduction in the resolution of sampling areas and when combining data from countries with differing fleet compositions. They also raise important questions about present protocol for collection of otolith data from IBTS survey trawls. |
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