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'.
In contrast, primary producers such as plants (including algae) are considered to be at a low trophic level, that is level 1. Those fishes that eat plants are at trophic level 2. Those eating the plant eaters are trophic level 3 and so on. The trophic levels are shown on the left hand scale of the figure above.

There are three ways that the trophic levels of marine systems have been changing.

Firstly those animals with higher trophic levels (typically fish eaters) are selectively fished (removed), resulting in a decline in the average trophic level of the marine ecosystem (see figures 2 and 3 below).

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.

Thirdly 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).

There is evidence that this change is occurring in most of the world's oceans and that it is caused by over-fishing.

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.

Unfortunately, there is more at risk for man than knowing whether marine systems are in a 'healthy' state, and whether our children will enjoy the same bountiful diversity in our oceans. The populations of the world need the protein that the marine systems of the world supply. Fishing down the food web may endanger our very food supply. Fish farming can not make up the shortfall and may further imperil marine systems if not done in a responsible manner. We can not divorce ourselves from our relationship with nature.

More Reading

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.
Chassot, E., Bonhommeau, S., Dulvy, N.K., Mélin, F., Watson, R., Gascuel, D., Le Pape, O. 2010. Global marine primary production constrains fisheries catches. Ecological Letters 13(4): 495-505. Pauly, D., Christensen, V., Dalsgaard, J., Froese, R., and Francisco T. Jr. 1998. Fishing down marine food webs. Science 279:860-863.
Pauly, D., Christensen, V., Froese, R., and Palomares, M.L. 2000. Fishing down aquatic food webs. American Scientist, January-February 46-52.
Pauly, D., Palomares, M.L., Froese, R., Sa-a, P., Vakily, M., Preikshot, D., and Wallace, S. 2001. Fishing down Canadian aquatic food webs. Canadian Journal of Fisheries and Aquatic Sciences, 58(1): 51-62.
Pauly, D. and Watson, R. 2005. Background and interpretation of the 'Marine Trophic Index' as a measure of biodiversity. Philosophical Transactions of The Royal Society: Biological Sciences 360: 415-423
Sethi, S.A., Branch, T.A. and Watson, R. 2010. Global fishery development plans are driven by profit but not trophic level. Proc. National Acad. Science. 107(27): 12163-12167

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