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Jim Mackie wrote:
>On average size another thought. Two things control the catching of wild
>salmon. Time of year and net sizes. Net sizes are designed to let
>small fish escape therefore we selectively kill the large and presumably
>faster growing fish. Fish lay eggs in numbers in proportion to their
size.
>Have we selectively culled large fish and therefore the fish we protect are
>actually laying few eggs and breeding a smaller fish which in turn will
>aggravate the situation?
A couple of months ago I read that the mean size of chinook salmon has
declined by nearly 50% in the last 60 years. Their mean age at maturity has
supposedly also declined by 2 years. I no longer have a copy of the article,
nor do I remember the journal or author, so I'm not sure of the validity of
those data. Perhaps someone on the list who works with chinook salmon can
verify/refute that information?
Here in the Turks & Caicos, fisheries managers are worried that spearfishing
for grouper (which is highly size-selective) will remove the larger, more
fecund individuals, eventually leading to a depleted stock consisting of
smaller fish which mature earlier and have lower reproductive outputs.
Unfortunately, fishing is not like animal husbandry, in which animals are
specifically bred to ensure that desired traits continue in future
generations. Fishing removes the desirable individuals from the population
so that their genes are not passed on. It's possible that, many years ago,
this type of selection was mitigated somewhat by gene flow from inaccessible
populations (those too deep or too far away to be harvested). Today we have
the
ability to fish pretty much every square meter of the ocean floor.
I'm sure the Turks & Caicos are far from being the only country wrestling
with size limits of large, slow-growing species like groupers (I don't think
there are any limits at this time - certainly none are being observed).
Should a large size limit be set, to ensure that fish get a chance to spawn
at least once or twice? Or would be a slot be better, in order to protect
existing large individuals and - theoretically - perhaps remove some of the
smaller, "genetically disadvantaged" individuals (if such a thing exists)?
Mark Tupper
Dr. Mark H. Tupper, Resident Faculty
Center for Marine Resource Studies
PO Box 007, South Caicos
Turks & Caicos Islands, British West Indies
ph. (649) 946-3362, FAX (649) 946-3246
email: [log in to unmask]
check out www.sfs-tci.org and www.marine-ecology.com
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