"Dr. Steve Oakley" wrote:

> 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]
>

My answer:

Yes, I agree that there are exceptions to the general observation made on the size
of tropical fish and one should be cautious in applying these models, which lump
all fish together. Anyway my first query was whether we could take these ideas
from fish to invertebrates.

To reply to your second comment, I just gave one example of an invertebrate
(abalone), which I know of, that could be aged. Of course, there are a number of
shelled  animals that can easily be aged using the marks on the shell provided age
can be validated. As you have mentioned lobsters and probably crabs and other
crustaceans can be aged, but the techniques currently available are difficult, not
used on a wider scale, and if used on some stocks, may have generated limited data
series. Until such time these techniques are widely used and generated a long time
series of age data, one has to look for non age-based models, i.e., length-based
models, for stock assessment.

Regards
Shareef Siddeek
Alaska Department of Fish and Game
Juneau, Alaska

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