Yongshun Xiao wrote:
> "Sizebased models=agebased ones +a growth in size curve, or > Agebased models =sizebased ones a growth in size curve." > > Shareef is correct in saying that my comments did not answer the basic > biological question: whether natural mortality is age or length dependent. > The conclusion that I would wish you to draw from my previous message is > that, if natural mortality is age dependent, then it must be length > dependent, or vice versa.
Response:
Yongshun is mathematically correct to say "Agebased models =sizebased ones  a growth in size curve,"
> but in reality when we want to use age based model, we do not formulate > sizebased model first and then deduct the growth curve. One should not > blindly hold on to mathematical equality without considering biological > reality. It is mathematically correct to say, if you take 4 from 2, you end up > with 2, but when you catch 4 fish in a stock containing only 2 fish you will > end up with 2 fish taken from another stock, if it exists. This is a minor > comment, but make sense in explaining the next part.
If natural mortality is a function of age, generally it will behave as a step function, changing at each step of age. Therefore, for a range of length within an age group, it will have the same value. On the other hand, if natural mortality is to change as a function of length, then it will change continuously within an age group, unless the length function is kept as a step function of age. In that case, natural mortality is assumed to be a function of age. Therefore, I imagine a difference in the outcome between the two biological concepts, although one can mathematically transform one to another to get what he wants.
Regards.
Shareef Siddeek
