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?

As a side observation, in the tropical fisheries, scientists have
estimated the average instantaneous natural mortality for a variety of
fish and invertebrate stocks using an established multiple regression
equation relating natural mortality to growth parameters and
environmental temperature. Although the multiple regression has been
established using strictly fish data, scientists have liberally used
this equation for invertebrate stocks as well. One reason for using this
equation more liberally is that natural mortality is a hard to determine
parameter for a heavily exploited stock.  By applying this equation to
any marine animal one implicitly assumes that natural mortality is
primarily governed by growth and environment not by any other genetic
functions. Is this assumption correct?

Cheers
Shareef Siddeek.