Statistical Catch-Free Age-Structured Assessment to Estimate Year-Class Strength for Inland Recreational Fisheries: A Largemouth Bass Example

Abstract

Statistical catch-at-age analysis (SCAA) is commonly used to estimate recruitment and mortality for large-scale, economically important fish stocks, but fishery-dependent catch-at-age data are often unavailable for smaller-scale inland fisheries, thus limiting the use of SCAA. In these systems, a catch-free approach to age-structure modeling may be useful for estimating recruitment and mortality rates solely from a time series of survey age composition and catch rate data by estimating recruitment as a relative index of year-class strength (R_t). Here, we demonstrate a statistical catch-free age-structured assessment (SCFASA) model for small-scale inland fish stocks as an alternative to traditional catch curve approaches by using Largemouth Bass Micropterus salmoides as an example. We fitted the SCFASA to fishery-independent survey age composition and catch-per-effort data from Largemouth Bass stocks in three Alabama reservoirs to estimate R_t, annual fishing mortality rates, and age-specific electrofishing survey vulnerability; we compared R_t to catch curve residual estimates and performed a sensitivity analysis under a variety of model assumptions. Model estimates of R_t and temporal trends in fishing mortality were robust to changes in assumptions about the effective sample size of multinomial age samples, natural mortality rate, and temporal variability in fishing mortality. However, the magnitude of instantaneous fishing mortality rates at two of the three reservoirs was somewhat higher than empirical estimates for similar stocks and was sensitive to model assumptions. Our application of the SCFASA to survey data sets routinely collected by management agencies for Largemouth Bass potentially provides a comprehensive approach to estimating R_t, mortality, and sampling vulnerability for these stocks without increasing sampling effort. Future investigations of SCFASA’s utility for small-scale inland stocks should address the potential for bias in the magnitude of fishing mortality estimates due to incorrectly specified vulnerability schedules.

Received October 5, 2015; accepted September 19, 2016 Published online December 21, 2016

ACKNOWLEDGMENTS

We thank D. DeVries and R. Wright for providing helpful input throughout the study and for their comments on previous drafts of the manuscript. We are grateful to Keith Floyd and Phil Ekema (ADCNR district biologists) for help with logistics and for providing data sets; and to Kurt Lakin (TVA biologist) for supplying data. This research was funded by a grant from the ADCNR through Federal Aid in Sportfish Restoration Project F-40-R to M.J.C.