Fishing down the food web

Fishing down the food web is the process whereby fisheries in a given ecosystem, 'having depleted the large predatory fish on top of the food web, turn to increasingly smaller species, finally ending up with previously spurned small fish and invertebrates'. Fishing down the food web is the process whereby fisheries in a given ecosystem, 'having depleted the large predatory fish on top of the food web, turn to increasingly smaller species, finally ending up with previously spurned small fish and invertebrates'. The process was first demonstrated by the fisheries scientist Daniel Pauly and others in an article published in the journal Science in 1998. Large predator fish with higher trophic levels have been depleted in wild fisheries. As a result, the fishing industry has been systematically 'fishing down the food web', targeting fish species at progressively decreasing trophic levels. The trophic level of a fish is the position it occupies on the food chain. The article establishes the importance of the mean trophic level of fisheries as a tool for measuring the health of ocean ecosystems. In 2000, the Convention on Biological Diversity selected the mean trophic level of fisheries catch, renamed the 'Marine Trophic Index' (MTI), as one of eight indicators of ecosystem health. However, many of the world's most lucrative fisheries are crustacean and mollusk fisheries, which are at low trophic levels and thus result in lower MTI values. Over the last 50 years, the abundance of large predator fish, such as cod, swordfish and tuna, has dropped 90 percent. Fishing vessels now increasingly pursue the smaller forage fish, such as herrings, sardines, menhaden and anchovies, that are lower on the food chain. 'We are eating bait and moving on to jellyfish and plankton' says Pauly. Beyond this, the overall global volume of fish captured has been declining since the late 1980s. The mean trophic level is calculated by assigning each fish or invertebrate species a number based on its trophic level. The trophic level is a measure of the position of an organism in a food web, starting at level 1 with primary producers, such as phytoplankton and seaweed, then moving through the primary consumers at level 2 that eat the primary producers to the secondary consumers at level 3 that eat the primary consumers, and so on. In marine environments, trophic levels range from two to five for the apex predators. The mean trophic level can then be calculated for fishery catches by averaging trophic levels for the overall catch using the datasets for commercial fish landings. Pauly's team used the catch data from the FAO which it fed into an Ecopath model. Ecopath is a computerised ecosystem modelling system. The functioning of an ecosystem can be described using path analysis to track the direction and influence of the many factors controlling the ecosystem. The original Ecopath model was applied to a coral reef food web. Scientists tracked tiger sharks at the top of the food web and collected data on their feeding behaviour, what they ate and how much. Likewise, they collected feeding data on the other organisms in the food chains down to the primary producers, such as algae. This data was fed into an Ecopath model, which then described the energy flow, in terms of food, as it moved from the primary producers up the food web to the apex predator. Such models allow scientists to compute the complex effects that occur, both direct and indirect, from the interactions of the many ecosystem components. The model showed that over the last 50 years the mean trophic level of fish catches has declined by somewhere between 0.5 and 1.0 trophic levels. This decline applied both globally, on a worldwide scale, and more locally on a scale specific to oceans, that is, for the separate FAO subareas: the Atlantic, Indian and Pacific Oceans, and the Mediterranean-Black Seas. Pauly's team argued in their 1998 paper that the larger, more valuable predatory fish, such as tuna, cod and grouper, had been systematically overfished, with the result that fishing effort was shifting to less desirable species further down the food chain. This 'fishing down the food web', said Pauly, would in time reduce people to a diet of 'jellyfish and plankton soup'. The colourful language and innovative statistical modelling by Pauly's team triggered critical reactions. Later in the same year, Caddy and his team from the FAO argued a counter position in a paper also published in Science. They argued that Pauly's team had oversimplified the situation and may have 'misinterpreted the FAO statistics'. The response of Pauly's team was published in the same paper, claiming that the corrections suggested by the FAO, such as accounting for aquaculture, actually made the trend worse. The concerns raised by the FAO were further countered by Pauly and others in 2005. Other researchers have established that 'fishing down' also applies to smaller, regional areas, such as the Mediterranean, North Sea, Celtic Sea, and in Canadian, Cuban and Icelandic waters.