Browsing by Author "Hatch, Joshua M."

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    A comparison between traditional and measurement-error growth models for weakfish Cynoscion regalis
    Hatch, Joshua M.; Jiao, Yan (PeerJ, 2016-09-21)
    Inferring growth for aquatic species is dependent upon accurate descriptions of age-length relationships, which may be degraded by measurement error in observed ages. Ageing error arises from biased and/or imprecise age determinations as a consequence of misinterpretation by readers or inability of ageing structures to accurately reflect true age. A Bayesian errors-in-variables (EIV) approach (i.e., measurement-error modeling) can account for ageing uncertainty during nonlinear growth curve estimation by allowing observed ages to be parametrically modeled as random deviates. Information on the latent age composition then comes from the specified prior distribution, which represents the true age structure of the sampled fish population. In this study, weakfish growth was modeled by means of traditional and measurement-error von Bertalanffy growth curves using otolith- or scale-estimated ages. Age determinations were assumed to be log-normally distributed, thereby incorporating multiplicative error with respect to ageing uncertainty. The prior distribution for true age was assumed to be uniformly distributed between ±4 of the observed age (yr) for each individual. Measurement-error growth models described weakfish that reached larger sizes but at slower rates, with median length-at-age being overestimated by traditional growth curves for the observed age range. In addition, measurement-error models produced slightly narrower credible intervals for parameters of the von Bertalanffy growth function, which may be an artifact of the specified prior distributions. Subjectivity is always apparent in the ageing of fishes and it is recommended that measurement-error growth models be used in conjunction with otolith-estimated ages to accurately capture the age-length relationship that is subsequently used in fisheries stock assessment and management.
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    The Effects of Ageing Error on Stock Assessment for Weakfish Cynoscion regalis
    Hatch, Joshua M. (Virginia Tech, 2013-06-12)
    Inherent uncertainties in the stock assessment for weakfish have precluded accurate and consistent advice concerning the management of commercial and recreational fisheries. Error within ageing techniques, used to assess relative age frequencies within commercial and recreational harvest, has been cited as a potential source for uncertainty during assessments of the weakfish fishery. The implications for age-reading error on weakfish stock assessment were explored using measurement-error growth models (i.e. Chapter 1), ageing error matrices within a statistical catch-at-age framework (i.e. Chapter 2), and Monte Carlo simulations to gauge robustness of ignoring this type of uncertainty during fisheries stock assessment (i.e. Chapter 3). Measurement-error growth models typically resulted in weakfish that grew to reach larger sizes, but at slower rates, with median length-at-age being overestimated by traditional von Bertalanffy growth curves, at least for the observed age range. Measurement-error growth models allow for incorporation of ageing uncertainty during nonlinear growth curve estimation, as well as the ability to estimate the ageing error variance. Age-reading error was further considered during statistical catch-at-age analysis of the weakfish fishery, mainly through permutations of true catch-at-age via ageing error matrices constructed from estimates of the ageing error variance, thus reflecting changes in relative age compositions as a consequence of ageing uncertainty. As a result, absolute levels of key population parameters were influenced, but general trends in those parameters tended to be similar, with strong congruency across models as to weakfish stock dynamics in most recent years. Finally, Monte Carlo simulations showed that implications for age-reading error on weakfish stock assessment are varied, depending upon the direction and magnitude of the ageing uncertainty. However, relative trends of parameter estimates over time tended to be similar, resulting in proper allocation of weakfish stock status, regardless of the type of ageing error considered. Furthermore, assuming negligible ageing uncertainty within fishery-independent surveys appears reasonable, as simulations incorporating ageing error within indices of relative abundance showed similar patterns to situations that only considered observation noise.