In terms of their value to humanity, marine capture fisheries are probably the single most important way in which wild species are directly used. Global capture fisheries production in 2006 was about 92 million tonnes, with an estimated first-sale value of US$91.2 billion, comprising about 82 million tonnes from marine waters and a record 10 million tonnes from inland waters (FAO 2008). These fisheries provide the livelihoods for millions of people and are vital in food security for coastal communities in many parts of the world. These fisheries are also the most important way that human activities have a direct impact on marine ecosystems. There is increasing concern that many current fishing practices are unsustainable. This has major implications both for the health of marine ecosystems and for the ability of those ecosystems to continue delivering the goods and services on which so many people depend.
The Marine Trophic Index allows a country to look at whether their marine resources within their Exclusive Economic Zone (EEZ) are being exploited in a sustainable way by showing how the average trophic level of a country's catch changes over time. The trophic level of a marine species describes where it is in a ‘food web. Primary producers such as algae and plants are at the base of the food web and are assigned a trophic level of 1. Herbivorous species higher up the food web can be assigned a trophic level of 2 to top predators like sharks and tuna can have a trophic level of 3 or more. As fishing within an area increases the most valuable species are targeted first and are usually large predators that occupy high trophic levels, such as adult cod, tuna and swordfish. If these species are depleted an increasing proportion of the catch consists of invertebrates and smaller fish that are lower in the food chain and so have a lower trophic level. This is known as ‘fishing down the food web’ and leads to a decrease in the mean trophic level of the catch as a whole. This is reflected in a decrease in the Marine Trophic Index. If this trend continues, it indicates that there has been an underlying change in the marine ecosystem and that the fisheries within the area are being exploited unsustainably.