Trevor wrote:

>
> No, Pauly's model is _not_ mechanistic. It is purely descriptive. There
> is no evidence, drawn across a wide variety of fish (as was Pauly's
> regression), that water temperature and/or growth patterns are factors
> that control the rate of natural mortality.

My answer:

Although Pauly (1980) established the equation as a general descriptive model, his publications
and earlier publication by Beverton (1959) on the same topic show some inherent relation among
curvature parameter, maximum length and natural mortality for fish species.  Pauly, dealing with
tropical fish, considered the habitat temperature as an important factor that has a control over
maximum size and natural mortality. Tropical fish grow to smaller maximum size than their
temperate counterparts and their natural mortality values are also higher. Therefore, as you have
mentioned in your earlier post that you were happy with using Pauly's model to predict M for a
species you were working with, this model has been used on a wide variety of fish and the general
explanation given in those publications have been largely accepted although specific scientific
evidences are lacking for causality for all the fish species found in the world.  So, in the
strict sense you may be correct to say that it is descriptive, but have you got any other (strict
sense) mechanistic model applicable to entire species groups?

Trevor wrote:

>
> > Your answer raises another question
> > whether one could conclude cause and effect based on good fit alone. This is one of the key
> > issues in model fitting.
>
> It is not an issue at all, let alone a "key" one. Quite simply,
> correlation can never be evidence of causality, no matter how good the fit.
>

My answer:

I do not agree with your statement that correlation can never be evidence of causality. It is a
sign  for the scientist to investigate further to find the causality. My understanding is that
with high causality you will get high correlation.

Trevor wrote:

>
> > What we have
> > discussed is for fish having continuous growth, not for invertebrates where length-based
> > model is most needed.
>
> Most (not all) fish have continuous growth but so do some invertebrates.
> Others, such as the clawed lobsters, have growth patterns that
> approximate to von Bertalanffy curves, allbeit overlain by fluctuations
> corresponding to moults. (Many teleosts have similar fluctuations driven
> by the seasonal cycle, though they are usually ignored when drawing
> growth curves.)
>
> However, I would dispute your supposition that length-based models are
> most needed for invertebrates. There are a great many, valuable finfish
> resources for which the collection of adequate age data is not
> practical. Conversely, there are important shellfish resources that can
> be easily and cheaply aged.
>

My answer:

Yes, most fish and some invertebrates have continuous growth. Indeed, I have fitted the special
von Bertalanffy growth equation to shrimp in the tropic. This is explainable, because growth (by
molting) processes are continuous in the protracted  warm environment.  Anyway success or failure
of fitting von Bertalanffy growth model is not going to answer the question whether natural
mortality sub model is age specific or length specific.

Yes, some shellfish can be aged, e.g., abalone; however, there are a number of commercially
important shrimp, crab, and lobster species that cannot be aged and live in a very contrasting
environment (in higher latitudes).  As you have mentioned there are some commercially important
fish species as well for which collection of age data as well as age reading are not practicable.
I agree with you they need length based models as well.

Best regards.

Shareef Siddeek
Alaska Department of Fish and Game
P.O. Box 25526
Juneau
Alaska, 99802

Note: By the way, someone did not like seeing my address card attached to each of my posts, hence
I have suppressed it. I hope he is happy now.

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