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Re: The Ocean is.. fwd from E.Allison


Madeleine Hall-Arber <[log in to unmask]>


Scientific forum on fish and fisheries <[log in to unmask]>


Thu, 9 Mar 2000 12:41:04 -0500





text/plain (1 lines)

> -----Original Message-----
> From: Fisheries Social Science Network
> [mailto:[log in to unmask]]On Behalf Of Debbie MacKenzie
> Sent: 08 March 2000 13:00
> To: [log in to unmask]
> Subject: The Ocean is Starving!

> Dear Marine Scientist,
> Taking a fish out of the sea does NOT leave a type of "vacuum" that nature
> is somehow compelled to fill in with another fish!
> I ask you to consider a new explanation for the collapsing fisheries and
> other recent changes in the marine ecosystem. My theory is that the TOTAL
> result of fishing


> fairly well understood.)
> Carbon fixation and nitrogen fixation naturally occur in the ocean and are
> the mechanisms by which new "life" is constantly produced. Nitrogen
> fixation is more important in determining the absolute quantity
> that can be "made" each year since very few organisms (only some
blue-green algae) are
> able to accomplish it. Fixed nitrogen is the limiting nutrient for
> phytoplankton (at least in the Northern hemisphere) and the rate of
> nitrogen-fixation therefore determines how much new protein will
> ultimately
> be added to the marine ecosystem each year.


Unless I have misunderstood, your main argument seems to be that removing
fish from the sea removes nitrogen faster than it is replenished by nitrogen
fixation, and thus, biomass production at all trophic levels is decreasing,
leading to starving fish and collapsing fisheries, and turning formerly
productive seas (e.g. Bering) into biological deserts.

If you want to test your argument, how about doing some crude calculations?

Lets say that roughly 100 million tonnes of fish are removed from the oceans
each year. If you dried it all, it would be around 20 million tonnes.
Nitrogen content of a whole fish is around 7% of dry weight. So our fish
catch represents 1.4 x 10^12 (1.4 million million) grammes of Nitrogen.
Molar mass of nitrogen is 14, so we can calculate that 10 x 10^10 mol N per
year are extracted from the ocean in the form of fish. If these rough 'back
of envelope' calulations are right, then this is indeed a fairly big
nutrient flow! However, if you look up a nitrogen cycle diagram in any
introductory environmental sciences book, you will get figures of around 1.4
x 10^16 mol N of organic nitrogen 'pool' in the world ocean, so we are only
removing 0.0007% of it in the fish catch. Looking at N fixing rates (2.9 x
10^12 mol N/yr) we can calculate that the fish harvest removes only 0.48%
per year of the nitrogen fixed.

That begins to sound significant, but there is a further caveat: I can't see
mentioned anywhere in your argument the other major sources of nitrogen
inputs into the oceans. My environmental science text book puts figures of
2.1 x 12^12 mol N per year input from rivers, 2.9 x 10^12 from N fixation,
and 4.3 x 10^12 from precipitation - the biggest source. Total molar N in
the world ocean is of the order 5.7 X 10^16, around a quarter of which is
organic. Internal cycling between organic and inorganic fractions is of the
order 1.4 x 10^14 mol N/yr. If we add up all the inputs, you get 1.493 x
10^14. Fish catches remove less than 0.07% of this. Annual losses from
denitrification (a natural process) are around 6-7% of the total organic
nitrogen input.

As Gary Sharpe has pointed out, the system fluctuates, so these flows will
vary. I suspect that the removal of nitrogen in the form of fish landed is
within the natural range of variability in processes such as fixing,
denitrification and exchange between organic and inorganic fractions (e.g.
through variations in upwelling driven by climate variability). I don't know
of any calculations to confirm or refute this. My environmental science
text admits that the calculations of these flows are sometimes 'intelligent
guesses'. You should also be aware that industrial nitrogen fixation is
now of a similar order of magnitude to 'natural' fixation (2.9 x 10^12 mol N
/ year). Much of this excess organic N will end up in the seas. This will
have more than compensated for any nitrogen lost from them in the form of
fish catches.

If you want to play with these calculations some more, here are the sources
I used for the above doodling:

Jackson, A.R.W. & J.M. Jackson (1996). Environmental Science: The natural
environment and human impact. Longman, Harlow, U.K.

Lightfoot, C., P.A. Roger, A.G. Caguauan & C.R. Dela Cruz (1993).
Preliminary Steady-State Nitrogen Models of a Wetland Ricefield Ecosystem
With and Without Fish. In: Christensen, V. & D. Pauly, Trophic Models of
Aquatic Ecosystems. ICLARM, Manila, Philippines.

Even if you still think bio-available forms of nitrogen in fish removed from
the ocean by fisheries amounted to a significant fraction of the nitrogen
budget of the world ocean you still have to assume that this nitrogen is
'lost' from the system. In reality, an unknown, but probably substantial
proportion of that nitrogen will make its way back into the sea, some of it
rather quickly. A crude illustration would be someone eating a halibut
steak in a seaside restaurant, then going to the bathroom. Its also
important to bear in mind that different nitrogen compounds have differing
'bioavailability' and if you want to understand impacts on different
sources, you have to disagreggate the above analysis somewhat.


Feed them with what? If the transfer of nitrogen from the ocean to the land
(in the form of fish)doesn't make sense to you, then is there a better
justification for doing it the other way round, by throwing nitrogen (feed)
from the land into the sea? Or have I missed your point?
The best way to ensure a ready supply of nitrogen into the seas is to keep
pumping sewage into them, and to keep up those fertiliser application rates
on the land. A recent article in the 'Financial Times' attributes the
higher productivity in the North Sea in the 1970s to all the untreated
sewage and nitrogen-rich waste poured into the major rivers such as the
Rhine. Better pollution control in Germany is blamed for north sea
fisheries decline...

Of course those are simplistic generalisations, but the principles of
'fertilizing the ocean' are not new. What happended to the scheme to
fertilise Norwegian Fjiords to increase fish production? Or to pump deep,
nutrient rich waters to the surface, off the coast of Hawaii?

In freshwaters, playing around with food chains and nutrient budgets is well
established - the art or science of biomanipulation. A few kilometres from
where I write, there is a programme to remove fish from some small shallow
lakes called the Norfolk Broads. Removing these fish, which eat
zooplankton, leads to increased zooplankon populations. Zooplankton are
efficient grazers of microalgae, released from the pressure of fish
predation, they prevent algal blooms, so the water doesn't get all green and
yucky. It got green and yucky in the first place because of increased
nutrient (N and P) loadings. The fish removal is only necessary because
nutrients no longer constrain algal blooms. Water clarity is the main
objective for management, and fish removal the way to achieve it.

In many of the world's inland and coatal waters, too much nitrogen is seen
as a problem, not too little, as your arguments suggest. Eutrophication
control is one of the major activities of water boards and environmental
agencies in the industrialised countries. In other areas, nutrients are
percieved to limit productivity (e.g. norwegian fjiords example above). As
with all these issues, the big picture looks different if you disaggregate

> Regardless of what you think about my final conclusion, I am really
> interested in your opinion on the "starving ocean" theory, and would love
> to hear your comments. If you see a flaw in the logic, please
> tell me;

I'll concede you have constructed an argument with some degree of internal
consistency, but it is not well enough supported by evidence or argued
against alternative explanations (such as those provided by Gary Sharp) for
us to validate or refute in any detail. I hope this encourages you to work
on those arguments - these comments are meant constructively, and you
certainly stimulated me to take more interest in the Nitrogen cycle than
I've ever shown before!



Dr Edward H. Allison, Lecturer in Natural Resources
School of Development Studies, University of East Anglia,
Norwich NR4 7TJ, U.K. Web:
Tel +44 1603 593724 (direct) 456161 (switchboard)
Fax +44 1603 451999 Email: [log in to unmask]

> Sincerely,
> Debbie MacKenzie
> [log in to unmask]

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