My response to Steve, Ben-Yami and others have created a little confusion on the
form of natural mortality partitioning. Let me clarify this with some
modification of my previous wording:
Natural mortality can broadly be partitioned into two components: an age specific
( I am inclined to think this way) part + a size specific (predation) part
The age specific part is determined, mostly as a constant average value, using
various relations, such as mortality-growth & temperature, mortality-maximum age,
mortality-mass maturity age, and mark-recaptures.
One cannot correctly model the size specific part without incorporating the
predator abundance. This belongs to multispecies aspect of modeling, which is
I agree with the comments made by Peter and others that natural mortality (age
specific part in my presentation) is unlikely to be a constant. That is why
age-specific component has been proposed.
Shareef Siddeek wrote:
> > Shareef Siddeek wrote:
> > Are we correct in assuming these vital parameters length dependent?
> > Steve Gutreuter wrote:
> > This is an important question, I think. In my opinion, the answer is
> > 'sometimes (most of the time?), yes.' There is a substantial literature
> > on this issue.
> > In general, we might expect body size to mediate life history parameters
> > in organisms that have indeterminate growth, show a wide range in
> > body size, show size-selective predation (e.g., via gape limitation
> > of animals that swallow food whole), are affected by size-selective
> > predators, and etc. These features apply to many (most?) fishes.
> > I think the inclusion of size-based assessments is entirely appropriate and
> > natural.
> In general what Steve has said is true mostly for finfish, young stages of
> animals and small and edible (to predators) form of animals (not animals with
> spines and other abnormal morphology and behavior). That is another story. 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?