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Subject: Re: Stock Assessment models
From: Campbell Davies <[log in to unmask]>
Reply-To:Scientific forum on fish and fisheries <[log in to unmask]>
Date:Tue, 18 Apr 2000 14:14:28 +1000

text/plain (153 lines)

Dear all,

Further comments on the discussion of Pauly's equation for estimation of M to follow on from Steve Oakley's:

Earlier commment:
Tropical fish grow to smaller maximum size than their temperate counterparts.

Steve wrote: 
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.

This pattern has also been taken to imply that tropical fish are faster growing and shorter lived than their temperate counter parts. A growing body of work on demographics of tropical reef fish using validated age-based methods is showing that the relationship between average maximum size, longevity and M is very loose and varies considerably within and among taxonomic groups of reef fish. Specifically, that many species of small reef fish have substantially greater longevity than larger commercial species. Whether this is a natural pattern or an artifact of a history of selective fishing is yet to be demonstrated. Nevertheless, comparison of estimates of M derived from Pauly's equation and estimated obtained from age based catch curves suggest that the former are quite positively biased. This is the result of extremely "flat-topped" growth curves and considerable longevity beyond the inflection point.

For examples see:

Newman et al 1995. Fish Bull 94:313-329
Hart and Russ 1996 MEPS 136: 25-35
Fowler  1990 MEPS 64:25-38
Doherty and Fowler 1994 Science 263:935-939
Choat and Axe 1996 MEPS 134:15-26
Ferreira and Russ 1991 AJFMR 43:1301-1312


Campbell Davies
Research Fellow
Effects of Line Fishing Project

CRC Reef Research Centre
James Cook University
Queensland  4811

Ph: [61] 07 47815114 Facsimile: [61] 07 47814099  Email: ntrol 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
>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,
Institute of Biodiversity & Environmental Conservation,  Universiti Malaysia
Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia  [log in to unmask]
Fax  082 671903  Tel 082 671000 x 254 or 257

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