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Comments on Peter's remarks:
Your note suggests that age specific natural mortality and maturity exist in some
fish stocks. Is this correct? Initially I posed the question somewhat on a neutral
ground. However, I am inclined to think of natural mortality having two major
components: an age specific part (not necessarily a constant value), regulated by
genetic make up of the species; and a size specific part, governed by the
vulnerability of the species to predators. I think onset of maturity is also a
function of age. Of course, we do not have hard evidences to prove either of these
assumptions broadly across the species groups. So, you may be correct in saying
" 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."
Cheers
Shareef Siddeek
Peter Hagen wrote:
> 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!.
>
> regards,
>
> -Pete
>
> * 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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