Limits of aquaculture

Common Grackle (Quiscalus quiscula), near Mud Lake, Ottawa

Seen from a simplistic and very selfish human perspective, ecosystems are devices for converting sunlight into human food. Sometimes, this happens fairly directly: sun hits soybean leaves, soybeans grow, and people eat them. In the case of the fish we eat, it is generally much less direct: sun hits phytoplankton, zooplankton eats that, they get eaten by fish that can eaten by successively larger fish, finally the largest fish get caught and eaten by us. In at least one important sense, this pyramid of energy use is quite different from the terrestrial one. In terrestrial agriculture, we manage the initial sun collection and can increase its amount in various ways. We are not, and perhaps never can be, farmers of plankton at the scale necessary to sustain the global marine food web. The effort involved in boosting the global plankton supply significantly would presumably be very large, given the immense biomass involved. Also, since energy is lost in each conversion, the amount of additional high-level species that would result from any increase would be smaller than the amount of additional plankton generated.

We are seriously overfishing the stocks that depend on the energy from existing phytoplankton stocks. If we start growing tuna and salmon in farms, feeding them fish from progressively lower in the marine ecosystem, we will eventually hit the bottom (if we keep having enough fuel for all those fishing boats). It is a fallacy to think that fish farms are like livestock farming on land. In the latter case, we are responsible for providing the inputs. In the former, we are still gathering from natural ecosystems, and doing so at an unsustainable rate.

Two partial solutions seem to exist. Firstly, we can get more fish per person by eating more plentiful species with lower trophic levels (closer to being creatures that eat plankton). That means anchovies for dinner, rather than tuna. Secondly, we could conceivably feed fish in farms using food from the land. That allows us to increase the basic solar energy being collected, and sustain a larger amount of tasty fish as a result. Of course, extending land-based agriculture entails other financial and environmental costs. Not least among these are the marine dead areas produced by pollution and fertilizer runoff.

The sensible way to run global fisheries is to avoid activities that cause disproportionate harm (dynamite fishing, catching juvenile fish) and then eat the sustainable portion of the output from different trophic levels. This means basically accepting a total level of sustainable human fish consumption for different species, then resisting political and financial pressures to exceed that limit. Of course, the record of human societies on doing this is dismal. We basically only fish sustainably when we are physically incapable of fishing more. Partly as a result of that, the general outlook for the world’s marine fisheries is dire.

Author: Milan

In the spring of 2005, I graduated from the University of British Columbia with a degree in International Relations and a general focus in the area of environmental politics. In the fall of 2005, I began reading for an M.Phil in IR at Wadham College, Oxford. Outside school, I am very interested in photography, writing, and the outdoors. I am writing this blog to keep in touch with friends and family around the world, provide a more personal view of graduate student life in Oxford, and pass on some lessons I've learned here.

7 thoughts on “Limits of aquaculture”

  1. We are not, and perhaps never can be, farmers of plankton at the scale necessary to sustain the global marine food web.
    What about the idea of adding iron to the oceans to grow plankton and sequester CO2?

    Might it also boost the primary productivity of fisheries?

  2. In four decades, stocks of Japanese eels have fallen by nine-tenths. Japanese wholesalers turned first to European eels. But they are now on the watchlist of the Convention on International Trade in Endangered Species (CITES). Among other things, the Atlantic eel is highly susceptible to a parasitic nematode introduced when stocks of the Japanese eel were imported to Europe for aquaculture. Japanese suppliers then shifted to farm-bred eels from China and South Korea. Eel farms, however, still depend on elvers netted from the wild, putting further pressure on populations.

  3. A common complaint by green types is that fish farming does not relieve as much pressure on the oceans as it appears to, because a lot of the feed it uses is made of fish meal. That simply transfers fishing pressure from species eaten by people directly to those that get turned into such meal. But fish meal is expensive, so researchers are trying to reduce the amount being used by substituting plant matter, such as soya. In this they have been successful. According to a paper published last year by researchers at NOFIMA, 90% of salmon feed used in Norway in 1990 was fish meal. In 2013 the comparable figure was 30%. Indeed, a report published in 2014 by the European Parliament found that fish-meal consumption in aquaculture peaked in 2005.

  4. Aquaculture boosters might answer that fleets are simply no longer needed. But farmed fish, particularly salmon and their ilk, are fed on smaller fish that themselves are caught at sea. Insects or algae might provide alternative fodder, but the companies involved are slow to embrace such novelties, according to Ari Jadwin of AquaSelect, which provides advice to Chinese fish farms. One issue, he says, is that Chinese consumers are not moved by sustainability arguments. But he thinks that concerns over food safety will lead to better practices in the long run.

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