Evaluation of Age–Length Key Sample Sizes Required to Estimate Fish Total Mortality and Growth.

Fisheries assessments often rely on age composition data to infer information about growth, mortality, and the outcome of various management policies. To reduce the costs of estimating the age of all fish collected (i.e., via direct aging), an age–length key (ALK) is often developed by directly aging only a subsample of fish and estimating the ages of the remaining fish based on observed lengths. Although this approach is common, there is little guidance in the literature for determining the appropriate total sample size and numbers of fish needed to directly age for each length bin. We developed a stochastic simulation to evaluate the sample-size requirement for using ALKs to estimate von Bertalanffy growth parameters and the instantaneous rate of total mortality (Z). The simulations accounted for variation in life history characteristics of stocks and exploitation rates of fisheries. Our simulations suggested that for overfished populations, aging more fish per bin provides negligible benefits whereas collecting larger total sample sizes could improve bias and precision ofZestimates. These results also indicated that bias and precision was relatively uninfluenced by fish life history type, which may allow for standardization of field collection methods across a wide range of fish species. In most cases, total sample sizes of 500–1,000 and 10 fish aged per bin provided near optimal performance in accuracy and precision, suggesting that this could be a guideline for many studies. Received August 21, 2012; accepted January 14, 2013 [ABSTRACT FROM PUBLISHER]