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Starving oceans


Jo Pitt <[log in to unmask]>


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


Wed, 8 Mar 2000 17:37:19 -0300





text/plain (1 lines)

I thought I'd add my two cents worth to this thread. I actually went to
Debbie's website, mostly for a laugh, and found it quite interesting. A
few of the replies seem to have misinterpreted her premise (or at least my
interpretation differs from theirs).

As I understand it, her theory is centred on the assumption that marine
food webs are highly dependent on recycling. The follow-through of the
theory then makes some sense if you assume that phytoplankton are dependent
on the decomposed carcasses of higher organisms as a nutrient source. She
starts with the statement that ocean ecosystems have a restricted capacity
to fix nitrogen, then moves on to the availability of fixed nitrogen as the
limiting factor for the system, restricting the ability of the plankton to
produce protein despite their carbon fixation efforts. The next assumption
is that tight cycling of organic material in the oceans provides
phytoplankton with the fixed nitrogen necessary to produce enough protein
to send around the food web again. Thus the tight cycling plus limited
additional nitrogen fixation allowed a slow increase in the total biomass
of the oceans over geological time. Consistent removal of consumer biomass
through fishing has supposedly broken the fixed nitrogen cycle! We could
conceivably remove the amount of consumer biomass that is equivalent to the
protein produced from the small amounts of newly fixed nitrogen, less the
amount required to compensate for nitrogen lost through inefficient
assimilation during the previous cycle. The cycling would then continue at
the same biomass levels ad infinitum. If we take more than this, then the
poor little phytoplankton have less and less fixed nitrogen with which to
make their protein, and the volume of material cycling through the system
is gradually diminished leading to the collapse of fish populations.
Therefore the 'obvious' solution is to replace the fish protein we have
removed from the system with our kitchen scraps, much like a compost heap
for one's veggie patch.

It all sounds nice and tidy. And is based on the oceans being an entirely
closed system that receives no additional nutrient inputs from terrestrial
ecosystems (such as suspended particulates in rivers or, more
appropriately, fertilizers in run-off from farm land or sewage - which,
without tertiary treatment, conveniently returns all that fish nitrogen we
ate minus the 10% we assimilated of course!). And the 'solution' is (as
she mentions) not suitable for oligotrophic waters. It is ironic that the
marine ecosystem with the tightest nutrient cycling (coral reefs) would be
devastated by this 'solution'. If a break in the nutrient cycle is the
cause of fisheries decline, the solution should be the same everywhere,
especially where nutrients are limited. Excretion of unassimilated
material (generally thought to be about 90% of the energy that is consumed)
is usually more important for nutrient cycling than the decomposition of
carcasses (the assimilated 10%). It happens over a much shorter time frame
(i.e. unassimilated material is returned to the system within hours of
consumption, rather than at the end of the individual's life, plus the time
it takes to cycle through the decomposers) and is already in a convenient
form (urea). There have been studies in which schools of juvenile grunts
appear to purposefully defecate near the coral heads they use as shelter
(unlike most terrestrial aminals which usually make the effort to defecate
as far as possible from their preferred hang-outs unless marking
territory). This has been interpreted as an important part of the process
by which the zooxanthellae (endosymbiotic algae living in the coral tissue,
for anyone who is not a coral reef person or a fan of big words) get enough
nutrients (can't remember the ref for this). Even if protein is
assimilated with greater efficiency than overall energy content (as is
presumed in the protein-poor marine environment), the percentage of
assimilated material removed from the system by catching a fish should not
disrupt the nitrogen component of the nutrient cycle enough to cause the
system to crash in the spectacular manner that has been observed in many
commercial fisheries. This, along with other flaws explained by previous
contributors and several responses in which the evidence cited in support
of the theory has been refuted or re-interpreted, should dictate that the
rationale behind the theory must fall.

Having said all that, significant loss of organic material from the system
may be one of the many factors contributing to the sad state of the world's
fishery resources. The very different phases of a fish's life cycle mean
that so many different things affect survival rates. And of course the
relative importance of these is different for every species. Putting a
stop to fishing to allow time for populations to recover and scientists to
better understand them so that we can manage them better after recovery is
the obvious answer to all except the fishermen, the politicians they vote
for and those who eat a lot of seafood. Is there anybody left? Meanwhile,
we do what we can. Being selective in your own purchases of seafood
(avoiding at-risk species and products from countries with bad management
reputations) is a way to contribute to the better management of fisheries
around the world. And of course, it is important to continue with our
research efforts aimed at understanding the system.

Another small point: I do think that it is important not to write off
approaches like this one, as Debbie has clearly made an effort with her
theory and submitted it (quite bravely) for examination by a scientific
forum in the appropriate field. Many people would be happy to simply
circulate this type of info through non-challenging, "preaching to the
converted" channels, stirring up all kinds of tree/dolphin huggers, without
trying to get feedback from those who would presumably be its most ardent
critics. I have to admit I was tempted to just delete Debbie's message
after the first paragraph. I didn't for two reasons: 1) rational feedback
in good faith (i.e. considered responses rather than rubbishing) often
produces a rational response, while lack of it might lead to greater
scaremongering (e.g. the scientists are writing us off because they don't
want to see the truth), and 2) even if the theory as a whole is a bit far
off the mark, there is usually a grain of truth (sometimes even a pebble or
a rock) in there somewhere.


Joanna Pitt, Ph.D.
Benthic Ecology Lab

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