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Subject: Re: depensatory exploitation
From: "Trevor J. Kenchington" <[log in to unmask]>
Reply-To:Scientific forum on fish and fisheries <[log in to unmask]>
Date:Wed, 2 Aug 2006 14:40:03 -0300

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Further to my message of a few minutes ago, Patrick Cordue has written:

> Hi Rom,
> Yes, I see the difference. The problem is that you keep using the term
> "function" (and this has consequences in interpretation of  
> results). In
> specific cases, exploitation rate will often be an  "empirical  
> function" of
> abundance. However, suppose that the abundance tracks down and then  
> recovers
> (and possibly the cycle repeats a number of times). Then for a given
> abundance there will be, in real fisheries, more than one  
> exploitation rate -
> and the "empirical function" is not, strictly speaking, a function.
> This is just a point of terminology. I can see that it would be  
> interesting
> to look at the shape of the relationships between exploitation rate  
> and
> abundance for a number of fisheries. However, without an  
> understanding of the
> underlying mechanisms (the real functional response - i.e., all the
> parameters, not just abundance) I am not sure what usefully  
> conclusions could
> be reached.
> But, thank you for an interesting discussion.
> Regards
> Patrick

In fisheries science, we have and need expertise drawn from many  
disciplines, which will often use the same terms with subtly  
different meanings. (And often radically-different ones, though those  
are less likely to cause confusion.) I was taught that one variable  
is a function of another when changing the second changes the first,  
whether or not additional variables can also influence the outcome.  
Hence, the strength of gravitational attraction between two bodies is  
a function (specifically an inverse linear function) of the square of  
the distance between them, even though it is also a function of the  
sum of the masses of the two bodies. That has nothing to do with  
purely-empirical relationships.

My previous message should be read with that understanding.

I do, however, fully agree with Patrick that an understanding of  
underlying mechanisms is needed before useful conclusions can be  
drawn from studies of relationships between exploitation rates and  
resource abundance.

Trevor Kenchington

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