A Computer Program for Age–Length Keys Incorporating Age Assignment to Individual Fish

Abstract

We provide a computer program (AGEKEY) that quickly implements an age–length key approach for estimating fish population age structure and mean lengths at age. The program's approach to age assignment differs from previous approaches in that ages are explicitly assigned to individual unaged fish. Because individual age assignment is necessary to describe the variation around estimates of mean catch per effort (CPE) at age and to compare these estimates through standard statistical procedures, the number of fish of a specified age captured in each unit of effort must be known or estimated. Individual age assignment can be confounded by instances of fractionality, where ages cannot be evenly assigned to fish within individual length intervals. Program AGEKEY resolves the problem of fractionality by applying an integer-based approach to age assignment. When we tested the validity of the program with two sample data sets, AGEKEY yielded estimates of age frequency and mean lengths at age that were very similar to estimates calculated through spreadsheet approaches. Program AGEKEY can quickly derive estimates of age frequency and mean lengths at age using an age–length key approach that will provide users with the data necessary to describe the precision associated with estimates of CPE at age and allow for statistical comparisons of CPE at age across surveys.