How should the sardine length frequencies be weighted? Exploring alternative methods
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2009
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University of Cape Town
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University of Cape Town
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Abstract
The effect of alternative methods of weighting trawls when estimating the proportions-at-length in November surveys is examined. The current method used, in which the length frequencies from the individual trawls are weighted by the acoustic weighting associated by that trawl as a proportion of the total acoustic weighting of all trawls, is compared to methods in which the length frequencies of individual trawls are weighted equally or those with low sample number are down weighted. In addition, the effect of using only night time trawls in this weighting is investigated. We recommend that a method of weighting the length frequencies of trawls equally, except that those with sample sizes less than 40, which should be down-weighted proportional to their size, replace the current method used to calculate the survey length frequencies.
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Reference:
de Moor, C. L., & Butterworth, D. S. (2009). How should the sardine length frequencies be weighted? Exploring alternative methods. Marine and Coastal Management document: MCM.