Dear Fish-Ecology members,

I am studying the ecology of Pangasiid catfish in the Mekong River Basin.
Pangasius is a family of migratory Asian catfish found throughout the
lower Mekong River.  There is a large wild capture fisheries of many
species of this family, and some of these same species are also common
in aquaculture.  The aquaculture of this species is in many instances
supported by a supply of fry and fingerlings caught from the wild - that
is to say, not raised in hatcheries.  Estimates put annual harvest of P.
hypophthalmus alone at about 250 million individuals, with potential to
meet any increase in demand by increasing catch.  In Vietnam, harvest of
another species, P. bocourti, is estimated at about 15 million
individuals.

My question is regarding the ecological consequences of such a large scale
harvest of fry and juveniles from the wild.  I am inclined to believe that
such a harvest is not as detrimental as one might first think, because of
the large number of fry one individual can produce and also because of the
incredible number of small fish which apparently exist.  Even juveniles of
species like P. sanitwongsei (adults are now very rare in the wild) are
caught in large numbers throughout the basin.  Moreover, in terms of
actual production for consumption, it seems as though catching and raising
fry might yield more fish (for humans) than if these same fry were allowed
to mature in the wild (because few would survive?).  Can anyone provide me
with information about the average rates of survival to adulthood of fry
of similar species?  What factors tends to limit survivorship?  Predation
seems one possible limit to survivorship; food probably less so because of
the extensive areas of flooded forest habitat available to fry and
fingerlings during the rainy season (spawning takes place at the beginning
of the rainy season).

I am beginning to question the Dept. of Fisheries practice of releasing
large numbers of fry into the wild to restore fisheries.  What is the
normal rational for such practices?  It seems as though there is an
abundance of fry and juveniles, but few adults.  Does this imply that the
population is about to crash, once the remaining adults are captured?
Then, doesn't it follow that it would be better to restrict fishing of
adults and allow collection of fry for aquaculture?  Few small fish appear
to live to adulthood (except if cultured), judging by the ease by which
small fish are caught when compared with the great effort expended to
catch few adults.  I should emphasize again that I do not believe that the
exploitation of small fish (for aquaculture) is seriously depleting
stocks.  In Thailand, at least, small fish are only occasionally targeted
in the mainstem Mekong.  Exploitation of small fish for consumption
appears, however, to be a much more serious problem.

Is there some model or general formula which might predict the optimal
harvest of small fish, given that these fish are long lived, yet seem to
produce many thousands of eggs each year?  Mature adults seem much more
valuable to the over-all health of populations than the vast numbers of
fry exploited for aquaculture.  Without any hard data on the age structure
of populations, what generalizations can be made?

Thank you for your feedback.

Zeb S. Hogan

c/o Environmental Risk Assessment
Faculty of Science
Chiang Mai University
Chiang Mai 50200
Thailand