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Trevor,
You should read the last few chapters of "The Selfish Gene" by Richard Dawkins. He has an interesting theory about how ideas are passed through a population and compares/contrasts that with how DNA is passed. If you read the book strictly to understand evolution, you would have been dissapointed with the last couple of chapters. However, in this case, the question as to why people believe that an every individual should be given an opportunity to reproduce before we catch it, his theory would be applicable. I doubt that the belief being held by many people is an inherited genetic trait. More likely it spread in the manner that Richard suggests and stems from social ideals adopted while most people are very young.
Cheers,
Justin Johnston
-----Original Message-----
From: Scientific forum on fish and fisheries [mailto:[log in to unmask]] On Behalf Of Trevor J. Kenchington
Sent: Wednesday, April 23, 2008 7:50 AM
To: [log in to unmask]
Subject: Re: Fishing Throws Targeted Species Off Balance
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Dr. Pranaya Parida wrote:
> Another principle/ethics in fisheries management is " we should give a
> chance to individuals to breed at least once in their life time."
>
>
>
> For the stock assessment for fisheries management the calculation
> of the
> Length at maturity (Lm50), generally helps the fisheries scientist
> to decide
> the minimum size of the species to be caught.
>
>
>
> No fish less than Lm50 should be caught for the fisheries management
> purpose.
>>
I most strongly disagree. That is the notion which I dismissed as an
"absurd oversimplification" in my contribution to this thread two
days ago. I did not expect to see it supported on this mailing list.
The idea that individuals should be given a chance to breed at least
once before being caught is neither a necessary nor a sufficient
criterion for effective conservation of a resource.
It is not _sufficient_ because a single spawning by every female that
reaches reproductive maturity in the absence of fishing will not (in
most species) contribute enough eggs to optimize future recruitment.
Such a strategy would also raise risks with long-lived species that
are adapted to exploit variable environments through long
reproductive lives (spanning across the variations). There is also
increasing evidence that the quality of the eggs produced by first-
time spawners is markedly less than that from older females (in at
least some species), plus hints that individuals with prior spawning
experience may play essential roles in some spawning behaviours.
[Then there are the protogynous hermaphrodites, which have to go
through multiple years as spawning females before becoming males.]
For all of those reasons, effective conservation requires that the
fishing mortality rate applied to mature adults must be kept within
reasonable bounds.
But if fishing mortality is to be limited, there is no absolute
requirement to prevent any harvest of juveniles. It is fully
possible to have a controlled harvest of young animals, provided that
sufficient numbers are allowed to escape so that the spawning
population is maintained at an appropriate level. Hence, the notion
of allowing each individual to breed once is not a _necessary_
criterion for effective conservation.
Two extreme examples:
1: Pacific salmon die after spawning once. Any fishery for them must
necessarily harvest pre-spawners, yet some of those fisheries (at
some times and places) have been and still are examples of excellence
in resource conservation.
2: Atlantic Canadian sealers could kill adult seals and devastate the
harp seal population as the southern fur seals were once devastated.
Instead, they take only new-born pups in numbers that are now
(thankfully) tightly controlled. The adult population remains abundant.
For an alternative perspective, consider what happens in an
unexploited population of a typical marine teleost: The average
lifetime fecundity of a female reaching maturity will vary from
species to species but can run up to 100,000,000 eggs or thereabouts.
Of all those, on average, only 2 survive to reach maturity in their
turn. Hence, whatever we do with our fisheries, the overwhelming
majority of individuals will NOT get even one chance to breed.
Altering juvenile survival rates by imposing some fishing mortality
is not the critical concern. What we need to worry about is the total
amount of fishing mortality and its distribution across the resource
population.
Yet the notion persists that we should allow each individual to breed
before we try to catch it. Indeed, that idea is so persistent, in the
face of its obvious logical deficiencies, as to suggest that it is
deeply embedded in our own inherited behaviour patterns. It might be
interesting to figure out why a bipedal marine predator carries such
counter-productive baggage in its DNA -- if, in fact, we do.
Trevor Kenchington
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