Trevor Kenchington wrote:

Pauly's equations (assuming those are the ones you mean) have come
in for their share of criticism but they do seem to work -- for teleosts.

Yes, I was referring to his equation when I mentioned about its application to
a number of tropical fish and invertebrates. I have also used this equation
for a number of fish species.  You are correct that it is not appropriate to
use  the same relationship between length, growth coefficient and mortality
rate to non teleosts. This has been mentioned in the FAO tropical fish stock
assessment manual as well.

My question is not directly related to the use and misuse of this equation.
But the underlying principle behind this and other length based models which
makes one to think that vital parameters (here it is natural mortality) are
length dependent.  When one faced with modeling hard to age animals (e.g.,
shrimp, lobster, crab, etc.), one of his concerns will be how to incorporate
natural mortality and other vital parameter sub-models into the main
length-based model. Should he use a direct length dependent sub model or an
age dependent (and then he may convert into a length dependent model using a
growth function) sub model?

Cheers.
Shareef Siddeek