Review of ICES sole stock MSY and reference points

Meta-analyses, applying generic methods within a species group, have benefits in terms of providing more stable estimates of FMSY reference points and estimates for individual stocks which by themselves may otherwise have insufficient data. Current ICES advice for seven sole stocks for which there is sufficient data gives Fmsy estimates which vary from 0.16 to 0.38, but these are specified for differing age ranges so are not directly comparable. Bpa is estimated for six of these seven stocks but Blim is available for only two. Slope at the origin estimates for HS models for the seven stocks vary from 1.32, to 4.45 largely due to observed exploitation rates. Here an evaluation using a consistent framework to compare Fmsy targets and the sensitivity of long term yield to the choice of this value across these stocks. Recruitment is modelled though stochastic multiple model based simulation for 1000 constructed ¿populations¿ for each stock by randomly sampling with replacement selection at age in the fishery, weights at age in the catch and weights at age in the stock for the period 2001-2009. S-R models were fitted in a Bayesian framework under the assumption is that sole has generic exploitation form which can be scaled to the carrying capacity for each stock, but the resilience, or slope (R/SSB) to the origin, is consistent across stocks assuming each stock retains its own estimated or assumed growth and maturation. Three models S-R models were applied, Hockey-stick, Ricker and Beverton- Holt with each model formulated so that the A parameter defined the slope to the origin which was assumed to be common for all stocks, whereas the B parameter related to density dependence was assumed to be independent. The probability of each model type is selected for the set using the method described in Simmonds et al (2011). Results show that changing from only HS to multiple models has a minor impact on the estimate of Fmsy or the F that maximizes mean catch, though the range of estimates is reduced. There is a small reduction in range through standardization of F bar over the same ages (3-8). A slightly larger reduction using mean selection at age in the fishery. The largest contribution comes from the different growth of the sole in different areas. In most cases if exploitation is at Fmsy there is less than 5% probability of SSB being less than Blim, except for NS sole. For NS sole measurement error will have some impact on the results whereas for other stocks the influence is limited. The combined analysis (Table x1) shows that the range of F within which catch is within 5% of maximum catch is substantial. Only for Skagerrak-Kattegat sole is the target near the lower bound. For most stocks an F target (ages 3-8) of 0.25 is a good choice. The new Blim and Bpa values are an important change, they are coherent with the simulations contribute to the impression of safe exploitation. However they need to be considered carefully before acceptance, this further analysis would be most appropriately conducted by individuals familiar with the stocks. The higher exploitation rates for Irish Sea and Skagerrak are conditional on the growth and this need to be verified, particularly for Skagerrak-Kattegat.

SIMMONDS Edmund; 

2011-06-08

International Council for the Exploration of the Sea

JRC63782

http://www.ices.dk/workinggroups/ViewWorkingGroup.aspx?ID=494,    https://publications.jrc.ec.europa.eu/repository/handle/JRC63782,