Fishing Down the Food Chain?

Fishing Down Marine Food Webs

In 1998, a paper called, Fishing down marine food webs, was published in the journal Science. It argued that the mean trophic level (how high in the food chain a fish eats) of the global catch was declining. This implied that we had fished out the large, valuable, high trophic level fish of the world (like tuna & billfish), and were moving down the food chain to fish out the smaller, less valuable fish at lower trophic levels (like sardines).

fishing down

The paper has become one of the most referenced in fisheries history, cited over 4,500 times. It has also spawned a mini-industry of other papers looking at trophic level changes in the catch of many marine ecosystems, and is often used to suggest that we are eating our way through the oceans and all that will be left is jellyfish.

Further, it was then proposed and accepted by some conservation organizations to use mean trophic level of catch as a measure of the “health” of a marine ecosystem. Low trophic level of catch was thought to be an indication that the system was overfished and depleted.

As it turns out, this is totally wrong—globally, the mean trophic level of the world’s catch is not declining and the underlying theory that we begin fisheries by catching large valuable fish is incorrect. Fisheries impact every part of marine food webs, from upper-trophic-level tunas and sharks to lower-trophic-level oysters and abalone.

The fishing down the food chain myth has great appeal to common sense: we start fisheries by eating and depleting the bigger, valuable, higher trophic level fish; then when they are gone we are work our way down to smaller, less valuable lower trophic level fish.

The 1998 paper by Pauly et al., did show that mean trophic level in global fisheries was declining, however, in 2010, Trevor Branch and others updated the data set and found that mean trophic level of catch was increasing, not declining.

Figure 2.  Mean trophic level of global catch as updated by Branch et al. 2010.

Further, mean-trophic level decline can happen for a number of reasons, not necessarily unsustainable fishing practices. For instance, catch of high trophic level species could be stable but catch of lower trophic levels could be increasing, skewing the average.

Additionally, if fishing down the food web was pervasive, we would see a collapse in top predators; however data actually show that lower trophic-level species like abalone and oysters are more likely to collapse from fishing pressure. Finally, one of the reasons that fishing down food webs seems so likely to happen is the idea that high trophic level fish like tunas, swordfish, and cod are valuable, and low trophic level fish like anchovies are much less valuable. However, in 2010, Suresh Sethi and others actually looked at the relationship between price and trophic level for marine food products (fish and shellfish) and found absolutely no relationship.

Figure 3.  The relationship between price of fish and their trophic level.  From Sethi et al. 2010.

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