Fishing Down Marine Food Webs
|Fig 1.||Watson and Pauly In: Atlas of the Ocean|
|designed by Daniel Pauly, original by Rachel Atanaceo, redrawn by NationalGeographic|
Strong evidence exists that overfishing has caused drastic changes in many marine ecosystems
Commercial fishing, by its nature,
usually targets the more valuable and larger fishes. Many of these
fishes eat other fish and are considered 'higher up the food chain'
(4 or 5 on the figure 1 above). These animals are described as being
at a high 'trophic level'.
Fig 2. Decrease in trophic level in the North Atlantic and globally in coastal waters since 1950.
Fig 3. Trophic level change from 1950-2000 (hot colours= greater decline)
The second way that trophic levels have been reduced is that marine animals have changed their diet to include more foods at lower trophic levels. For example, fishes that normally eat fish-eating fishes may find that they are not as available as they once were (as a result of overfishing) and must therefore now eat more plant-eating fishes then they used to (see figure 4 below). This change of diet lowers their trophic level, again reducing the average trophic level of the marine ecosystem.
Fig 4. Trophic web reduction - note the shorter food path to the right as intermediate trophic levels are removed.
we also now harvest smaller individuals of many species which typically
have smaller mouths and feed lower in the food web (see figure 5 below).
Fig 5. Decrease in average maximum length of commercial fishes (cm) with resulting change in trophic level.
What is the danger of reducing the trophic levels of marine systems?
Reducing the diversity and complexity of marine
systems is an unraveling process that puts individual species at greater
risk of local extinction. It causes changes that spread throughout
the food web of the marine system. Energy enters marine ecosystem
through the photosynthesis of plants. Energy from the sun is passed
throughout food webs, from trophic level to trophic level. This energy
transfer is not completely efficient but there is now evidence reductions
in the average trophic level can actually reduce the efficiency of
this energy flow. These changes may actually favour some groups of
animals. Some people have linked the great increase in the abundance
of jellyfishes in recent years with this loss of efficiency, caused
by fishing down the food web. Ironically, man has adapted quite well.
Whereas we used to focus our attentions primarily on larger fishes
(ignoring now precious shellfishes and crustaceans), we now harvest
smaller fish species. Not only are individuals within a species harvested
at smaller sizes but we actually actively seek smaller species (the
larger ones are too hard to find now). When larger fishes were readily
available lobsters were used as fertilizer in many places, now they
are highly valued. Many fisheries of the world are simply the bulk
harvest of lower trophic level fishes and crustaceans for the production
of animal foods and fertilzer. The most valuable fisheries in the
United States now are those harvesting either invertebrates (lobsters,
crabs and clams) or the 'reduction' Menhaden
(mossbunker or pogy) fishery. The latter are "not considered fit
for human consumption" because of their oiliness. Perhaps the fisheries
of the future will target jellyfish? By then, perhaps, these will
be very valuable and highly regarded in restaurants.
Branch, T, Watson, R., Fulton, E.A., Jennings, S., McGilliard, C.R.,
Pablico, G., Ricard, D., Tracey, S.R. 2010. The trophic fingerprint
of marine fisheries. Nature 468: 431-435.
|updated Nov 2011 - thanks to Cassandra Willet inspiring and being inspired by her 'kids' - Creating a brighter future!|