We are NOT Fishing Down the Food Chain

#fishmyth

In 1998 Daniel Pauly and others published a paper entitled “Fishing down marine food webs,” in the journal Science that argued that the mean trophic level (where in the food chain a fish feeds) of the global catch was declining. This implied that we had fished out the large, valuable high trophic level fish of the world, and were moving down the food chain to fish out the smaller, less valuable fish at lower trophic levels. See Figure 1.

The paper has become one of the most referenced in fisheries history, cited over 3,600 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 NGOs 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 Myth is totally wrong -- globally the mean trophic level of the catch is not declining and the underlying theory that we begin fisheries by catching large valuable fish is incorrect.

Figure 1. Pauly’s original data on decline in mean trophic level of global catch.

Figure 1. Pauly’s original data on decline in mean trophic level of global catch.

Fisheries impact every part of marine food webs, from upper-trophic-level tunas and sharks to lower-trophic-level oysters and abalone. Though we eat seafood across a range of trophic levels, the fishing down food webs myth has become pervasive in the popular media. This myth has great appeal to common sense, we start fisheries by eating and depleting the high value, bigger, higher trophic level fish and when they are gone we are work our way down to smaller, less valuable lower trophic level fish.  This myth overlaps with the “All large fish of the ocean are gone” myth, and is part of a common narrative that (1) we have depleted the big fish already, (2) now we are working on the small fish and thus (3) all stocks will be collapsed by 2048.

The evidence for fishing down the food chain was declining mean trophic levels in global catch shown in this 1998 paper by Daniel Pauly. 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.

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

Not only is the trend towards higher trophic level of catch, but 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.

Additionally, if fishing down the food web was occurring, 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 (see figure 3).

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

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

Mean trophic level of catch has been reported in many ecosystems, but this decline is confounded by studies that only report a decline in mean trophic level after adjusting or excluding particular data (See Branch 2015 for details). When fully tested at a global scale, removing all species below the average (thus looking at only the higher trophic level fish) initially resulted in a decline in mean trophic level; however, this actually turned into an increase when the trophic level of Atlantic Cod data was updated in FishBase. This has subsequently turned into a stable trend following the most recent Atlantic Cod update.

Recently, researchers have been trying to blame fisheries expansion for evidence of fishing down food webs. The basic premise is that fisheries first fished down low-trophic-level, near-shore species and then expanded to offshore, higher trophic-level predators like tunas. Fisheries expansion has certainly occurred, but shifting from low- to high-trophic level species over time is evidence for fishing up marine food webs, not fishing down marine food webs. This is confirmed by trends in mean trophic level across different latitudes and distance from the coast, which, between 1985-2004 show increases for 74% of all latitudes, with more increases than decreases at every distance from the coast.