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Subject: Are Oceans really starving??
From: Andreas Svensson <[log in to unmask]>
Reply-To:Scientific forum on fish and fisheries <[log in to unmask]>
Date:Thu, 9 Mar 2000 09:44:15 -0500

text/plain (68 lines)

Dear Debbie and others

Although not entirely applicable to the oligotrophic ocean environment,
experiences from the Baltic might shed some light on this issue.

The Baltic Sea has been grossly eutrophied for the last 50 years. Nitrogen
and phosphorous input has increased with 8 and 4 times since 1900,
respectively. In the 1980's the cod stock was severly depleted due to
aggressive fishing. Herring (which is the cod's main prey) landings
increased for a few years but are now much lower than before. A peculiar
increase in PCB's in herring was later on explained my decreasing fat
reserves, i.e the herring is starving, despite the low numbers. Much of the
macroalgae (Fucus) have dissapeared. Zooplankton catches are often very low
in highly eutrophied areas. Planktotrophic fish with freshwater origin (e.g.
roach), that normally benefit from eutrophication, are showing signs of
decrease. Pike and perch have virtually nonexistent recruitment in many
areas. At the moment, few species seem to be doing well.

Is this du to starvation? Not likely. First of all, what we take out of the
Baltic goes back in there. Little of the fish produce leaves the catchment
area. Second, a Whole Lot more nutrients are added via farming, urban
sewage, NO3 deposition etc.

Who is benefitting from this then? What is growing when zooplankton,
macroalgae, planktivorous and predatory fish all show a similar pattern of
Well, a whole lot of phytoplankton enjoy this new condition, that's for
sure. And a whole lot of filamentous algae like it too.

It is not the amount of food that decides what diversity and abundance we
have of our marine species. Our richest marine (and terrestrial)
environments are not the fat ones.

When analyzing Baltic Sea, I come to the conclusion that the Entire
Ecosystem is disturbed by man. Both bottom-up via nutrients, and top-down
via overfishing. The result is a more unstable condition, dominated by
opportunistic, fast growing small species. And the equilibrium shift from
year to year, sometimes even faster. Slower growing species, with behavioral
and physiological demands that are not met by this new shaky environment,
are all loosing out.

From our fresh water colleagues, we have learned that biotic factors are
just as likely controllers of the species equilibria as abiotic ones.
Removing predatory fish are not necessarily influencing their prey in a
positive way, atleast not in a longer perspective. Even in small lakes, food
webs are too complex to predict a certain trophic chain reaction. Removing
cod from the Baltic may have been the death sentence to the macroalgae,
since small cod eat the grazers. But when the macroalgae are gone, grazers
may perish too. And the few remaining juvenlie cod could go without food.

Unless we want to to live off diatoms and dinoflagellates, I think
sustainable fishing is the way to go.
Not (additional) fertilization.

Andreas Svensson
Kalmar University

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