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Estimates of natural mortality for flatfish in the Northwest Atlantic: a comparison of model predicted estimates
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2014
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Description:The main motivation of this paper is to compare empirical estimates of M and compare it with the current estimate used for natural mortality (M of 0.2 yr-1) for the Georges Bank Yellowtail Flounder stock assessment. We examined a database of direct estimates of M, maximum age (tmax), von Bertalanffy growth parameters K and L∞, as described by Then et al. (2014), for flatfishes (Order Pleuronectiformes). A total of six flatfish species (Family Pleuronectidae) were available from Northwest Atlantic and Northeast Pacific ocean basins, with literature M estimates ranging from 0.18 to 0.39 yr-1. We applied the original and updated equations of four empirical estimators based on tmax and on the von Bertalanffy growth parameters presented in Then et al. (2014) to the flatfish dataset to obtain empirical estimates of M and bootstrap-derived standard errors. With the exception of one species, the range of the empirical M estimates encompassed the literature M estimates. The tmax-based M estimates more closely matched the literature values than the growth-based M estimates. However, all the empirical estimates derived using bootstrap resampling suggested that M for the Georges Bank yellowtail flounder is greater than 0.2 yr-1. Empirical M estimates derived using both historical and recent growth estimates for the Georges Bank stock also provided evidence for M > 0.2. Based on a non-exhaustive literature survey, sexual dimorphism in growth and lifespan is prevalent in flatfish; for the stocks that exhibit such sex-specific differences, the females are typically the larger and longer-lived. Sex-specific empirical M estimates suggest that males experience higher natural mortality than females. The databases examined for the Georges Bank yellowtail flounder stock suggest that females survive in greater numbers to older ages than males. Considering all of the data, and patterns for other flatfish, it is not unreasonable to expect that M is greater for males than for females.
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