I was away from my PC for a number of days, which delayed my response to
interesting replies to my post.

Yongshun Xiao wrote:

> "I would think that the size-based models are the same as age-based ones."

For a species that cannot be aged, there is no question about using size-based
models in place of age based models. However, in your size based model
specification, you started with an  age based Z(t(s)), then integrated in terms
of length.  What should be the mathematical function of the non predatory part
of the natural mortality component (assuming M not a constant) in the numerator
of  Z(t(s))/f(L(s))? One can assume it to be either M (L) or M(t).  Of course,
in either case, the entire integrant can be transformed into a function in
length. But the basic biological question remains unanswered, whether natural
mortality is age or length dependent.