Download How Science Works – Trophic Levels – Fishing Down the Food Chain

How Science Works – Trophic Levels – Fishing Down the Food Chain
Early ecological studies (by famous ecologists such as Charles Elton) tended to treat the idea of trophic levels as predefined
categories into which a species could be fitted. So, many fish were put into trophic level three because they consume
zooplankton (trophic level two) which in turn consume phytoplankton (trophic level one).
There are obvious problems with the Eltonian concept of trophic levels because species may consume organisms from
several trophic levels and may operate in different trophic levels at different stages in their life cycles.
These problems led other ecologists such as William Odum to think of trophic levels as an empirical characteristic of a
species (in the same way as we talk about characteristics such as average size or metabolic rate) that can be calculated
through a detailed breakdown of the diet of the species. The way it is done is to work out the average trophic level of the
things that an organism feed on and add a one onto that level. Marine biologists typically assign Tuna a trophic level of
between 4.2 and 4.5 because it will be consuming organisms from different trophic levels that have an average trophic level
of between 3.2 and 3.5. The variation (from 4.2 - 4.5) arises because the diet of Tuna varies regionally depending on what
prey species are available.
Fishing down the Food Chain & Mean Trophic Levels
Marine biologists have used the concept of mean trophic level as a way of assessing the sustainability of commercial seafisheries.
References:
http://www.americanscientist.org/issues/id.764,y.0,no.,content.true,page.2,css.print/issue.aspx
http://en.wikipedia.org/wiki/Fishing_down_the_food_web
http://www.seafriends.org.nz/issues/fishing/pauly1.htm
… and more recently there have been a number of studies which question the reliability of this as a measure of the status
of a fishery:
http://www.nature.com/news/2010/101117/full/news.2010.619.html
The basic principles are outlined overleaf.
How Science Works – Trophic Levels – Fishing Down the Food Chain
Fishing down the Food Chain
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V
Trophic Level of Catch
Catch
Total
Time
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•
Total catch has been increasing over time.
Initially the species were large, long-lived, high trophic level species such as Cod (trophic level 5) – in time catches of
these species declined.
As stocks of Cod declined species lower down the food chain became more important, this diversification led to an
increase in the total catch.
In time the stocks of these species also decline and species further still down the food chain become the main target
species.
“This reflects a gradual transition in landings from long-lived, high trophic level, piscivorous bottom
fish toward short-lived, low trophic level invertebrates and planktivorous pelagic fish.”
“Fishing down food webs (that is, at lower trophic levels) leads at first to increasing catches, then to a
phase transition associated with stagnating or declining catches. These results indicate that present
exploitation patterns are unsustainable”
Fishing Down Marine Food Webs by Daniel Pauly*, Villy Christensen, Johanne Dalsgaard, Rainer
Froese and Francisco Torres Jr. Science 6 February 1998 Vol. 279 no. 5352 pp. 860-863
What does it say about the structure of the Marine
Food Web and Biodiversity?
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4
II
3
III
2
1
IV
V
Time
As the importance (in the catch) of species from lower trophic levels increases
then the mean trophic level for the catch declines
Trophic Level of Catch
What does a declining Mean Trophic Level say about
the sustainability of the fishery?
Total
Mean Trophic Level
The Mean Trophic Level is now used as a measure of
sustainability by the Convention on Biological
Diversity (2000) and there are well documented
declines in the mean trophic level in a number of
fisheries.