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dear all
two corrections to the thread of this arguement
Tropical fish grow to smaller maximum size than their
>temperate counterparts
while the pauly model has validity for many commercial fish - the three
largest fish species are all tropical, humphead wrasse, bumphead parrots and
giant grouper all occur in the tropics and are larger than their temperate
counterparts. so models such as this which lump all fish together need to be
approached with due caution and respect and not used without further
consideration of the species biology.
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).
lobsters and probably crabs and other crustaceans can be aged - the
technique is difficult but see papers by mat sheehy - can j of fish & aq sci
56 10 1999 or Mar ecol prog ser 143 99-111 1996
contact mat at [log in to unmask]
regards
steve oakley
At 10:52 AM 4/17/00 -0800, you wrote:
>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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Dr. Steve Oakley, Shell Prof. of Environmental Science,
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