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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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