Harvest rates

John Shuey jshuey at tnc.org
Fri Dec 17 15:08:52 EST 1999

The key thing to remember here is that all mortality is not equal.  For any
stable population, the bottom line is that each female is able to replace
herself and a mate during her lifetime (and that pair have to successfully mate
themselves).  Hence a salmon which lays 4000 eggs produces two breeding adults
seven years later (if population are stable), just a a bear which produces maybe
one cub every two years - maybe 10 cubs total - needs to generate exactly a pair
by the time she dies in a stable population.

Hence mortality of immatures in these populations has very different
consequences.  99.99% of immature salmon are destined to die.  80% percent of
bear cubs might suffer the same fate.  But the point that everyone forgets, is
that adult mortality is very different than immature mortality.  Yes, salmon
have higher reproductive "potential" than do bears, but only in optimal years,
when they do better than replace themselves.  This very basic point is why
people justify over fishing commercial stock, and then seem surprised that by
eliminating all the reproducing adults (literally the 2 in 4,000 that won the
lottery and are still alive), they place population in a downward trend.  The
argument that 99.99% of all salmon die is true from egg to adult only - yet
applied to adults where breeding success in a stable population probably
approaches 75%.

In many ways, this is the invalid argument that we here over and over again with
regard to collecting adult insects.  To me the logic (and theoretical
underpinnings) of the argument seem invalid at best.

Hence in addition to "lies, damn lies and statistics", I would add  "lousy
population ecologists" to the litany of misuse.

Norbert.Kondla at gems3.gov.bc.ca wrote:

> Agreed. I am cautious about all statistics.  I seem to recall a famous quote
> about "lies, damn lies and statistics" :):) Maybe best to use the following
> example with general numbers to illustrate the point that perhaps I did not
> make very well.  A female salmon has been reported to produce between 3 and
> 4 thousand eggs in a season.  A female bear will produce at best one or two
> offspring every few seasons.  Given these fundamental differences in
> fecundity; it is reasonable to expect that the impacts of mortality on
> population levels and ability of populations to recover from crashes will be
> fundamentally different.  (and yes, there have been some classic management
> errors in fisheries management - in this country we use the phrase "cod
> scenario" to jokingly refer to harvest levels that are likely to be
> dangerous)
> -----Original Message-----
> From: Michael Gochfeld [mailto:gochfeld at EOHSI.RUTGERS.EDU]
> Sent: Thursday, December 16, 1999 5:43 PM
> To: leps-l at lists.yale.edu
> Subject: Harvest rates
> I would be cautious about accepting statistics on fish harvest rates,
> because they may not take into account sustainability.  I grew up with
> what I call the "weekly reader" concept of resources. That is fish were
> a renewable resources, and pulp and timber companies replanted forests
> and sustained their yields indefinitely.  My experience both here and
> abroad is that the reality is different----harvest all the fish as
> quickly as possible and sell your boats to some other country so they
> can do the same.  Fishery folk may be more enamored of models than of
> reality, and when fish populations decline or the size of the fish
> declines, they are loathe to abandon the models and instead blame
> sampling.
> Mike Gochfeld

John Shuey
Director of Conservation Science
Indiana Office of The Nature Conservancy

phone:  317-923-7547
fax:  317-923-7582
email:  Jshuey at tnc.org

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