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Deroba J.J.,National Oceanic and Atmospheric Administration | Butterworth D.S.,University of South Africa | Methot R.D.,National Oceanic and Atmospheric Administration | DeOliveira J.A.A.,CEFAS - Center for Environment, Fisheries and Aquaculture Science | And 32 more authors.
ICES Journal of Marine Science

The World Conference on Stock Assessment Methods (July 2013) included a workshop on testing assessment methods through simulations. The exercise was made up of two steps applied to datasets from 14 representative fish stocks from around the world. Step 1 involved applying stock assessments to datasets with varying degrees of effort dedicated to optimizing fit. Step 2 was applied to a subset of the stocks and involved characteristics of given model fits being used to generate pseudo-data with error. These pseudo-data were then provided to assessment modellers and fits to the pseudo-data provided consistency checks within (self-tests) and among (cross-tests) assessment models. Although trends in biomass were often similar across models, the scaling of absolute biomass was not consistent across models. Similar types of models tended to perform similarly (e.g. age based or production models). Self-testing and cross-testing of models are a useful diagnostic approach, and suggested that estimates in the most recent years of time-series were the least robust. Results from the simulation exercise provide a basis for guidance on future large-scale simulation experiments and demonstrate the need for strategic investments in the evaluation and development of stock assessment methods. © 2014 Published by Oxford University Press 2014. This work is written by US Government employees and is in the public domain in the US. Source

Uhlmann S.S.,Wageningen Institute of Marine Resources and Ecosystem Studies IMARES | Bierman S.M.,Wageningen Institute of Marine Resources and Ecosystem Studies IMARES | Van Helmond A.T.M.,Wageningen Institute of Marine Resources and Ecosystem Studies IMARES
ICES Journal of Marine Science

In 2009, a self-sampling programme was organized in the Netherlands, fishers sampling ca. 80 kg of discards from randomly selected bottom trawls in the North Sea. A statistical procedure is proposed to highlight samples, trips (with multiple samples), or vessels (which may have multiple trips within a year) where extreme mean lengths of discarded fish were observed. Randomization methods were used to test for evidence of non-randomness in patterns of highlighted discard samples, e.g. repeated observations of extreme mean lengths for consecutive discard samples across trips from the same vessel. European plaice (Pleuronectes platessa), common dab (Limanda limanda), grey gurnard (Eutrigla gurnardus), and whiting (Merlangius merlangus) were considered because these were the most abundant species in most of the discard samples. A linear mixed model was used to estimate random-sample effects on the estimated mean lengths by species. These random effects were incorporated into uni- and bivariate procedures to identify extreme samples that were summed for each vessel, and the probability of observing such numbers was estimated. Excluding these samples from the dataset had marginal effects on estimated size distributions of fish. © 2011 International Council for the Exploration of the Sea. Source

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