Title: A Diagnostic tool for displaying stock history and simulated equilibrium exploita-tion, and an example using North Sea herring
Authors: SIMMONDS EDMUND
Publisher: International Council for the Exploration of the Sea
Publication Year: 2010
JRC N°: JRC58235
URI: http://www.ices.dk/indexfla.asp
http://publications.jrc.ec.europa.eu/repository/handle/JRC58235
Type: Articles in books
Abstract: The objective To provide a single page display to compare historic exploitation, and stochastic equilibrium exploitation with estimates of risk of SSB<Blim and probability of F=Fmsy. Uncertainty is optionally included in the modelling. The basis Simulated recruitment based on a fitted S/R relationship from a year based data pe-riod set by user. Number of iterations (stocks) selected by user ¿ (100 ). Number of years to project to equilibrium (100). This longer period is required near F crash where equilibrium can be more difficult to find ¿ 30 years is sufficient elsewhere. Se-lected F steps to scan over (0 to 1 in steps of 0.05). Example analysis based on North Sea herring Some exploration to show effect of S/R function on NS herring ¿ comparing Beverton Holt and Ricker for two periods 1975-2008 (post crash) and 2002 to 2008 (recent re-duced recruitment). Stock Recruit model fits are given in Figures 1,3,5,7 Stochastic equilibrium, SSB, catch and recruitment compared with historic values are shown in Figures, 2,4,6,8 along with estimates of risk to SSB<Blim and <Bpa,the estimated pdf of Fmsy and the for comparison the exploitation F rule for the existing management plan. The results are sensitive to recruitment assumptions but the management plan seems to appropriate for recent recruitment. Figures 9-12 show the changes in estimates of the pdf of Fmsy for the Ricker S/R rela-tionship for differing varying components in the simulation. Figure 9 gives the mean yield per recruit, a deterministic result showing a single value. Figure 10 shows the impact of the inclusion of just stochastic recruitment, which gives Fmsy as a spread of values symmetrically around the mean. Figure 11 shows the impact of addition of uncertainty in the functional form of the S/R relationship, through bootstrap using var/cov of the fit, this gives increased uncertainty at high F. Figure 12 includes vari-ability from the last 10 years in weights at age, maturity and selection in the fishery.
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