Ventana software support forum

Posted by <Richard.Dudley@attglobal.net>

Some of you may be interested in a recent paper in Nature which explains
why fishing can cause even more disruption of fish populations than
previously thought by creating instability in population dynamics ....
causing boom and busts in a fished population independent of variations
in the amount of fishing.

From the abstract: ""....increased temporal variability in the
population does not arise from variable exploitation, nor does it
reflect direct environmental tracking. .......it arises from increased
instability in dynamics. This finding has implications for resource
management as an empirical example of how selective harvesting can alter
the basic dynamics of exploited populations, and lead to unstable booms
and busts that can precede systematic declines in stock levels.""

From conclusions: ""In summary, fishing for big individuals without
consideration of the impact on the age distribution can lead to unstable
nonlinear population dynamics, and this enhanced nonlinearity helps to
explain much of the volatility seen in fish stocks today.""

Source:

Anderson, C.N.K., Hsieh, C.-h., Sandin, S.A., Hewitt, R., Hollowed, A.,
Beddington, J., May, R.M., Sugihara, G., 2008. Why fishing magnifies
fluctuations in fish abundance. Nature 452, 835-839.

Posted by <Richard.Dudley@attglobal.net>
posting date Thu, 29 May 2008 09:28:44 +0700
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Posted by ""Steinar Moen"" <steinar.moen@powersim.no>

Hi,
Thanks for the information!

Disruption of fish population has happened too many times, and we should
be careful of doing over fishing. In collaboration with Prof. Johannes
Hamre in the Norwegian Institute of Marine Research we have during the
last 6 years made a model using System Dynamics that implements three
different species: Cod, Herring and Capelin. The model is based on
research over the last 15 years, and use historical data from 1982 until
now. We get different results compared to non System Dynamic models,
partly because the other models do not implement several species in the
same model.

What we have learned of the research and the model:

The influence of fishing on the ecological system is dependent on where
in the food chain we are hunting. If we over fish the species eating
plankton (herring and capelin), that will have negative influence, not
only for those species, but also for all the other species higher up in
the food chain that need these species (cod, coalfish/pollack, seal,
whale, fishing industry). We regulate the fishing to prevent over
fishing, and the cod regulates its own species by cannibalism. That is
highly demonstrated by the model. The mechanisms (dynamics) are the
same, but the controlling tools are different. We control by using
intelligence, the cod do it by using its biological survival mechanism
(survival of the fittest).

The lack of using System Dynamics for such a complex system may result
in disruption on one side, but also loss of millions of dollars through
wrong assumptions, and using mathematical correlations that may have
other causes.

Best regards,
Steinar Moen
Posted by ""Steinar Moen"" <steinar.moen@powersim.no>
posting date Sun, 15 Jun 2008 22:54:52 +0200
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