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Re: Stock Assessment models


Peter Hagen <[log in to unmask]>


Scientific forum on fish and fisheries <[log in to unmask]>


Fri, 7 Apr 2000 13:29:49 -0800





text/plain (1 lines)

Shareef Siddeek wrote:
> Some thoughts!
> I am puzzled by the way a number of stock assessment models are being
> developed and applied in fisheries management.
> Sophisticated length based models have been developed for hard to age
> fish and invertebrate stocks. Because, the animals cannot be aged the
> underlying principle in incorporating various sub-models has been that
> of assuming length dependent vital parameters (e.g., natural mortality,
> maturity, etc.). Are we correct in assuming these vital parameters
> length dependent?

It is an interesting question. From a fish point of view, it would seem to
make some intuitive sense that natural mortality would largely be size
dependent. And others who responded made the case.

But I'm not sure if that is necessarily the case with regards to other
parameters such as sexual maturity which you included in your question.

Pink salmon are the classic example of age dependence with their strict
schedule of a two year maturation/life cycle regardless of size. Then there
are examples from fresh water lakes where, with a population of "stunted"
individuals, size at sexual maturity can be quite small, while in other more
typical lakes, size at sexual maturity is much larger. Competition limited
growth (and not density dependent mortality) are likely to be critical factors
that gives rise to those contrasting conditions.

However despite the above, until recently I've always assumed that marine fish
would respond more directly to size dependency. However a recent paper
focusing on decade long changes in Pacific halibut (Clark et al.1999 *) have
shown dramatic a decrease in size at age of halibut over the last twenty years
in the Gulf of Alaska, yet there has been little or no change in the age of
sexual maturity. Smaller halibut are now more likely to be sexually mature
than they were in the past.

On that basis, I suspect the ticking of the biological clock and not size is
more relevant to the 'programed' life schedule changes in fish perhaps the
same may be true for invertebrates as well.

With regards to the follow up question:

Shareef Siddeek wrote:
> My specific question is that if we are to partition, for example, natural
> mortality into two components: a constant part + a size specific part; then
> the size specific part is a function of prey size and predator abundance in
> the vicinity, etc. But how does the constant part behave? Is it size or age
> specific. The constant part includes all other causes (diseases,
> environmental stress, old age, etc.). My thinking is that this constant part
> results from a cumulative effect over time; therefore should be age specific
> not size specific. Is this correct?

I would say that to the extent that individuals have their own programed
longevity (assuming they survive that long), then certainly some part of
natural mortality is age dependent more so than size. But even that part of
the constant part it is not likely to be constant!.



* Clark, W.G., S. R. Hare, A.M. Parma, P.J. Sullivan, and R. J. Trumble. 1999
Decal changes in growth and recuitment of Pacific halibut (Hippoglossus
stenolepis). Can. J. Fish. Aquat. Sci. 56: 242-252

Peter Hagen, ADF&G email: [log in to unmask]
P.0. Box 25526, phone: 907-465-3499
Juneau AK USA, 99801-5526 Fax: 907-465-2765

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