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Subject: Re: Stock Assessment models
From: Mathew Cieri <[log in to unmask]>
Reply-To:Scientific forum on fish and fisheries <[log in to unmask]>
Date:Thu, 20 Apr 2000 13:38:31 -0400

text/plain (65 lines)

 So the question is...Why use age at all to determine survival? Is being
older impart an ability to reduce either an individual predation or
starvation probability? Given that "Bigger" is supposedly "better", why
bother with age at all? Why not base larval (invertebrate and veterbrate)
mortality on size alone?

 Perhaps a better predictor of larval survival is a combination of size (as
it effect predation), condition (as if effects both starvation and
predation), and hydrology.

Just a random thought.


-----Original Message-----
From: Shareef Siddeek
To: [log in to unmask]
Sent: 4/20/00 12:49 PM
Subject: Re: Stock Assessment models

Campbell Davies wrote:

> 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.

My comment:
Yes, when fish stop growing at the maximum length and continues to live,
natural mortality to growth parameter relation becomes blurred. That is
why I queried about the validity of directly transferring continuous
growth ideas to modeling of non-ageable animals, which either grow
severely discontinuous way throughout their life span, stop growing at
some stage of their life, or both.


Shareef Siddeek
Alaska Department of Fish and Game
Juneau, Alaska

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