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Modeling Otolith Weight using Fish Age and Length: Applications to Age Determination

Hanson, S. Dale, Stafford, Craig P.
Transactions of the American Fisheries Society 2017 v.146 no.4 pp. 778-790
Salvelinus namaycush, age determination, cost effectiveness, fish, hatcheries, models, otoliths, quality control, seasonal variation, statistical analysis, stocking rate, temperature
Relationships among otolith weight and age have been explored widely as cost-effective means to predict age. However, otolith weight is influenced by both fish age and somatic growth, making it necessary to partition these confounding effects to best use otolith weight to predict age. We used several hatchery strains of Lake Trout Salvelinus namaycush that varied in capture season, year, location, and mean size at stocking to develop a maximum likelihood model of otolith weight as complementary but independent functions of age (mg/year) and fish length (mg/mm). Once these relationships were established, we determined age probabilities for each fish based on their measurements. Our best model included age and somatic growth components, with cumulative mean temperature as a variable to scale for seasonal differences in otolith accretion. For fish between 3 and 19 years of age, otolith weight increased by 1.16 mg/year, with an additional somatic growth component of 0.62 × exp(fish length × 0.0038) mg/mm. We correctly assigned age to 60% of the age-3 fish, but this decreased to about 10% among the oldest age-classes; however, predicted age residuals approximated normal distributions within each age-class. The model was robust to seasonal and annual variation, but somatic growth differences among some strain and stocking-size groupings resulted in modal aging biases of ±1 year. Given the stability in otolith weight components arising from fish age and somatic growth, we suggest that modeling otolith weight using reader age estimates offers a new quality assurance/quality control tool to assess whether reader ages support the widely documented properties of otolith growth. When otolith weight and fish length are more informative of age, our model may also be useful in estimating age composition data similar to an age–length key.