A popular idea in marine conservation and fisheries management is that of “fishing down the food web”, first coined in a paper by Pauly et al. (2000). The phrase describes the observation that the average catch from global fisheries in 2000 was of a lower average trophic level than over the last 50 years prior. So what does this mean?

Trophic levels are essentially the position a species takes in the food chain. For centuries, fishermen have targeted large apex predators like cod and tuna, which happily swim at the top of the food chain (and occupy the higher trophic levels). But through years of overexploiting a single species, there are now far fewer individuals. This means that fisheries are being forced to catch the smaller fish (on lower rungs of the food chain, such as sardines and mackerel), of which there are theoretically many more. This explanation is called the “collapse-replacement mechanism”, whereby fishing targets another species when the population of apex predators has significantly declined.

The scientific literature has been nearly unanimous in interpreting this trend, termed ‘‘fishing down the food web,’’ as being symptomatic of overfishing, unsustainable harvest, and unintended ecological changes induced by widespread removal of species from high trophic levels.” – Essington et al. (2006)

This diagram explains the idea of “trophic levels”, which are essentially groups of organisms which share a similar spot in the food chain. Top predators in this example include tuna and sharks, which are comparatively rarer (1 pound in weight compared to 1000 pounds in weight of the sea plants or primary producers).  The collapse-replacement mechanism suggests that when tuna are overfished, their population declines dramatically, so that fisheries will instead target smaller fish, such as the black grouper or Bar Jack in this example.

A study into this phenomena in Florida supported the theory of “fishing down the food web”. Researchers conducted an archaeological study into the harvesting of fish from the 16th Century, finding that changes in the diversity, biomass contribution and trophic level of fish caught may have been happening since the 18th Century (Reitz, 2004). However, they emphasised that overfishing was not the only reason for this trend – cultural and climate changes may also play a role in changing tastes and subsequently fishing practices.

It is difficult to isolate the causes for [fishing down the food web] because (1) people change their technology, fish in different places at different times, and target preferred resources which may lead to overfishing; and 2) there may be changes in the marine environment independent of human activity.” – Reitz (2004)

This led to other explanations being developed. Six years after the original Pauly (2000) paper was released, a contrasting report was published by Essington et al., (2006), who found that instead of fishing down the food web, fisheries were instead fishing through the food web. This took the focus from the decline in top predators being the root cause of the problem, and instead focused on the expansion of the number of fish species being caught from the lower trophic levels. In their analysis, Essington (2006) found that this “sequential adding” (where more species of fish from lower in the food chain are being prioritised by an increased number of fisheries) was far more pervasive a mechanism than the loss of apex predators; out of 30 marine ecosystems which experienced fishing down the food web, only 9 of these were linked to a statistically significant reduction in apex predators (Essington et al., 2006).

A diagram to explain the difference between fishing down and fishing through the food web. One focuses on exploiting another species due to a population crash of the bigger species, whilst the other focuses on increasing number of fisheries fishing different species.

Our results indicated that fishing down the food web is prevalent among marine ecosystems worldwide and that fishing down the food web was most commonly associated with the sequential addition of new fisheries.” – Essington et al. (2006)

But why is this theory relevant to the fisheries of today? Many would argue that overexploitation and overfishing is in fact the main reason why fish stocks are suffering today. However, not all fisheries are unsustainable, and many exemplify what can be done within the industry to improve the system. In their research on sustainable fisheries, Hilborn (2007) suggested that the primary reasons for unsustainable fishing are poor governance, pressure for employment and incentives which promote maximum harvests (as many fish should be caught as possible). To combat this, many ecologists support the idea of incentivising sustainability and stock rebuilding (allowing fish populations to recover) rather than encouraging fishing fleets to grow and more fish to be taken. But this, according to Hilborn (2007), should be paired with improved governance, improved ecosystem understanding, reduced demand for resources and eliminating poverty.

Sustainable fishing may look different in different places. In the Pacific Islands, the right to fish is strictly controlled by local people, and no outsiders are allowed to fish without permission. This limits the likelihood of unsustainable harvesting, and ensures that the responsibility of the ecosystem is with the people who know it best (Hilborn, 2007). In Canada, the Netherlands and the US, “individual tradable quotas” are one way through which fishing is regulated. When a total allowable catch (TAC) is set by the government, individual fishermen in a fishery are assigned a proportion of this catch, which can be bought/sold to others. This eliminate seasonal overfishing and boom-bust market systems which had previously plagued fisheries, and encourages fishermen to preserve the fish stocks for the future –  the quotas of a successful, sustainable fishery are worth a lot more than those of a collapsed one (Heal & Schlenker, 2008).

With open access to the world’s seas, a “Tragedy of the Commons” scenario plays out, where everyone seeks to get the most from the shared resource of the ocean (Heal & Schlenker, 2008). When one boat overharvests fish, not only will other competing fishermen suffer, but fish stocks take a massive hit. And when these fish stocks reduce, others are exploited in their place. Regulation seems to be key in managing our oceans, and as with everything, when a holistic approach to their management is taken, everyone stands to benefit.

References

Essington, T.E., Beaudreau, A.H. & Wiedenmann, J. (2006) Fishing through marine food webs. Proceedings of the National Academy of Sciences103(9), pp.3171-3175

Heal, G. & Schlenker, W. (2008) Sustainable fisheries. Nature455(7216), pp.1044-1045

Hilborn, R. (2007) Moving to sustainability by learning from successful fisheries. Ambio, pp.296-303

Pauly, D., Froese, R. & Palomares, M.L. (2000) Fishing down aquatic food webs: Industrial fishing over the past half-century has noticeably depleted the topmost links in aquatic food chains. American Scientist88(1), pp.46-51

Reitz, E.J. (2004) ” Fishing down the food web”: A case study from St. Augustine, Florida, USA. American Antiquity, pp.63-83

Published by avleveri

Hi! I'm Anna, an environmental science graduate from the UK. My main interests (if you can't already tell from my blog posts) are sustainability, consumption, conservation, nutrition, fitness and food! Lots of food.

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