Statistical estimation of mean values of fish stock indicators from trawl surveys

Free access article

Issue		Aquat. Living Resour. Volume 22, Number 2, April-June 2009 Fish Stock Assessments Using Surveys and Indicators


Page(s)		127 - 133
DOI		10.1051/alr/2009015
Published online		17 June 2009

Aquat. Living Resour. 22 (2009) 127-133
DOI: 10.1051/alr/2009015

Statistical estimation of mean values of fish stock indicators from trawl surveys

John Cotter

Cefas, Lowestoft, NR33 0HT, UK

Received 26 June 2008; Accepted 2 April 2009; Published online 17 June 2009.

Abstract - The mean or average value of a measure or indicator variable used to monitor a fish population can be defined in various ways, each with a correspondingly different statistical estimator for use in the context of a trawl survey. When, as is typical for many species, fish are heterogeneously clustered over space, then scooped in clusters from restricted localities using a trawl, these different estimators can produce different sample mean values with contrasting variations over time, possibly leading to different inferences about the fish population. Two mean parameters and their intuitive estimators, the mean “over fish”, and the mean “over stations”, are discussed and found to present contrasting statistical properties. A third estimator based on fitting a mixed model is proposed which has intermediate properties based on the within-haul correlation. The three estimators are applied illustratively to length data for cod caught in the North Sea by the English groundfish survey from 1992 to 2007. The time series of the mean over stations was smoothest, that for the mean over fish much more variable, and the mixed mean fell between in all years. Variance estimators derived from the fitted mixed model are also put forward. Estimates made from the example suggested that the mixed mean is most efficient. The type of estimator used for the mean should always be considered carefully and mentioned when reporting indicator studies.

Key words: Statistics / Mean over fish / Mean over stations / Trawl survey / Indicator / Mixed model

Corresponding author: john.cotter@fishworldscience.com

© EDP Sciences, IFREMER, IRD 2009

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