Work package 5.
Density dependent growth, maturity and cannibalism.
The scientific knowledge on DD growth and maturity by stock will be reviewed.
New meta-analysis will be performed based on WEST (matrix of weight at age by year) and MATURITY (matrix of maturity at age by year) files from ICES working group reports. These exists for all the stocks dealt with in the current project.
We will define criteria where a simple functional regression analysis (SSB also carry substantial uncertainty) between weight at age (and maturity at age) and stock size is likely be appropriate. These could be where there is no suspicion that the large stock sizes observed historically is caused by good growth of individuals, as seems to be case for most cod stocks in the North Atlantic as investigated by Brander about a decade ago. Now however, we have cod stocks exposed to substantially reduced F and might be able to see more clearly the expected negative relationship.
As we a priori would expect the relationship to be negative we will use such a model and select that one which fits data best, a la David Andersons 2007 approach. We will apply a similar approach to ICES for stock-recruitment relationships as outlined in the ICES guidelines to the assessment working group in SGPRP (2003, ICES CM 2003/ACFM:15). The approach is to outline a set of typical relationships between e.g. weight-at-age and SSB and for each type prescribe how to obtain the DD relationship formulae.
DD natural mortality will be treated in the same way for those stocks which are cannibals.
We will perform standard Fmsy calculations including the DD growth and maturity (and cannibalism). The PROST Software is ideal for this.
We will also estimate F0.1 and Fmax for each stock. This is likely to be a quite small extra work because PROST probably has options to that. These are the well-known parameters based on calculations per recruit and can therefore not take into account density dependence of any kind. In principle there will be one set of these two points for each stock size bin for a given stock, as weight-at-age etc. varies with stocks size, but we will focus on some relevant ones, maybe the present stock size and the stock size corresponding to the new Fmsy.
Key persons: Ray, Bjarte, Henrik.