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Addendum: On Assessment of the Atlantic Menhaden Population

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posted on 29.04.2022, 06:59 by Doug ButterworthDoug Butterworth, Rebecca A Rademeyer

The possibility of a single change in the selectivity function for each of the four fisheries during recent years is introduced into the assessment model. This leads to fits of the data to the model which are considerably preferred in terms of AIC. For scenario II for which only the JAI abundance index is used in the fit, recent fishing mortalities no longer increase substantially, and the recent spawning biomasses become much higher. For scenario III for which the SAD and NAD abundance indices are used, high recent fishing mortalities are no longer evident when such changes in selectivity are admitted. The main selectivity change is in the north reduction fishery, where the peak changes from age 3 to age 2. Of note is that fishing mortality and spawning biomasses estimates become fairly similar over time for the two scenarios, given this selectivity change possibility, except for a recent decrease in spawning biomass and larger fishing mortality for recent years when using the JAI rather than the SAD and NAD indices. Viewed overall, allowing for these selectivity changes leads to considerably improved fits to the data, and to a more positive estimate of the status for the resource over recent years.

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Department of Mathematics and Applied Mathematics, University of Cape Town