Measuring uncertainty in age-structured fisheries stock assessment models using MCMC, bootstrap, and Hessian methods

Arni Magnusson
UW School of Aquatic & Fishery Sciences
April 27, 2005

Uncertainty is a fundamental part of fisheries stock assessment, that needs to be quantified to successfully manage the resource. Among the statistical methods that are used to measure uncertainty are Markov chain Monte Carlo (MCMC) simulations, bootstrap, and Hessian delta-method approximation. In this study, a large number of stochastic datasets is generated, where the true parameter values are known. Confidence bounds are then estimated using the different methods, and the claimed uncertainty is compared with how often they contain the true value. The findings from this simulation study are reviewed, as well as theoretical and practical differences between the methods.


Analysis of survey data using GLMMs: dealing with the “complete survey”

Tom Helser
NOAA Northwest Fisheries Science Center
April 13, 2005

A generalized linear mixed model (GLMM) that treats year and spatial cell as fixed effects while treating vessel as a random effect is used to estimate biomass and variances for 11 slope species in the NMFS bottom trawl surveys on the upper continental slope of U.S. West coast. A Bernoulli distribution is used to model the probability of a non-zero haul and we examine alternative error distributions from the exponential family using AIC to model the non-zero catch rates.

Progress towards rebuilding

André E. Punt
UW School of Aquatic & Fishery Sciences
February 9, 2005

The Pacific Fishery Management Council adopted rebuilding plans for eight groundfish species in 2004 in the form of Amendments 16-2 and 16-3 to the groundfish FMP. Each of these eight stocks will be re-assessed during 2005 and, as a consequence, there will be an opportunity to determine whether or not they have responded to recovery efforts and are on track to rebuild as previously projected. It is to be expected that the results of the 2005 groundfish assessments with not conform exactly with the results expected based on the previous assessments. The question that arises then is whether the fishing mortality rate used to set harvest guidelines specified as part of the rebuilding plan should be changed, and if so how. A further consideration is that data now available may show that the original basis for the rebuilding plan is no longer valid (e.g. because the values assumed for natural mortality or stock recruitment steepness have changed markedly). Although guidelines exist regarding how rebuilding analyses are to be conducted, there no guidelines to determine whether (and to what extent) rebuilding plans are to be updated given new information. The objectives of this presentation are to outline: a) a set of possible “rebuilding revision rules” which could be used to measure progress towards rebuilding (and make appropriate adjustments to rebuilding plans as needed), and b) a framework which uses simulation to provide a quantitative means to compare alternative rebuilding revision rules in terms of their effectiveness at correctly (and adequately) making adjustments to rebuilding plans.

Good and ugly multispecies fisheries: ITQs in British Columbia versus retention limits on the U.S. West Coast

Trevor Branch,
UW School of Aquatic & Fishery Sciences
November 3, 2004

Can we devise a set of regulations for multispecies fisheries so that productive species are not undercaught (resulting in economic loss) while preventing overfishing of unproductive species? The groundfish fisheries of British Columbia and the U.S. West Coast offer some insights. They were managed by individual trip limits (with all their associated problems) until 1996, but thereafter diverged. The B.C. fishery implemented a 100% on-board observer system, permitting individual accountability of catch and discard mortality, followed in 1997 by the introduction of Individual Transferable Quotas (ITQs). The West Coast fishery, forbidden to consider ITQs by the Sustainable Fisheries Act, continued on the path of increasingly restrictive retention limits, and ever-increasing regulatory-induced discarding, as additional species were declared overfished. By almost any measure, the B.C. fishery is now in a better state of health: ITQs have resulted in greater flexibility, increased profitability, reduced overcapitalization, compliance with TACs, and the reduction of marketable discards to near zero. The reduction in discards alone provides additional income sufficient to cover the costs of observer coverage. A model of fishermen’s location choice highlights the benefits of accounting for catches and discards under 100% observer coverage and the benefits of increased flexibility under ITQs.

Moving beyond MSY: making Alaska’s salmon fisheries socially and economically sustainable

Ray Hilborn
UW School of Aquatic & Fishery Sciences
November 3, 2004

Many of Alaska’s salmon fisheries are models of biological success, with management structures that have maintained biomass, stock diversity and biological yield. At the same time the fisheries face severe challenges due to low price for the product, and the fisheries have been declared formal “economic” disasters by state and federal agencies in recent years. From many perspectives these fisheries are in crisis. I explore how the governance system for Alaskan salmon has led to biological success and economic failure. I review a range of alternative governance structures that are in place or being considered that might provide for social and economic sustainability. I also demonstrate that the basic biological principal that has guided management, Maximum Sustainable Yield, is a serious impediment to social and economic sustainability.