Use of otolith weight in length-mediated estimation of proportions at age
R. I. C. Chris Francis A E , Shelton J. Harley B , Steven E. Campana C and Peer Doering-Arjes DA National Institute of Water and Atmospheric Research, Private Bag 14901, Wellington, New Zealand.
B Inter-American Tropical Tuna Commission, 8604 La Jolla Shores Drive, La Jolla, CA 92037-1508, USA.
C Marine Fish Division, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia, Canada B2Y 4A2.
D Institute of Freshwater Research, Stångholmsvägen 2, SE-178 93 Drottningholm, Sweden.
E Corresponding author. Email: c.francis@niwa.co.nz
Marine and Freshwater Research 56(5) 735-743 https://doi.org/10.1071/MF04127
Submitted: 12 July 2004 Accepted: 28 January 2005 Published: 21 July 2005
Abstract
Each year almost a million fish are aged from otoliths, primarily to estimate proportions at age for use in stock assessments. The preparation and reading of otoliths is time-consuming and thus expensive. Two techniques have been proposed to reduce costs. The first is length-mediated estimation, in which the length distribution from a large sample of fish is converted to an age distribution, using information (usually in the form of an age–length key) from a smaller sample containing length and age data. The second is to infer age from otolith weight (and/or other otolith measurements). These two cost-saving ideas are combined in a new method, length-mediated mixture analysis. It requires three samples – one with lengths only, one with lengths and otolith measurements, and one with lengths, otolith measurements and ages – and estimation is by maximum likelihood. The use of this method, which can be thought of as a generalisation of three established methods of age inference, is illustrated in two simulation experiments in a cost-benefit framework.
Acknowledgments
We are grateful to Rick Fletcher, Ian Wright and an anonymous referee for reviewing an earlier version of the present paper.
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