The Marine Trophic Index (MTI) is used to measure the degree to which countries are “fishing down the food chain,” i.e., catching smaller and smaller fish within their exclusive economic zones (Pauly 1999). It is considered to be a measure of overall ecosystem health and stability, but also serves as a proxy measure for overfishing. Human fishing practices have tended to start at the top of food webs – culling large, predatory fish before moving down to lower trophic levels. When the average trophic value of a marine ecosystem is low it indicates that many of the large predators have been removed through excessive fishing pressure.
The consequences of moving to a lower average Marine Trophic Index include lower ecosystem complexity, which can make the system more susceptible to disease and more sensitive to the pressures of fishing or climate change. A lack of species and genetic diversity means there are fewer variants with potential resistance to new environmental challenges. Overall, low MTIs put fisheries at much greater risk of collapse (Pauly 2006). The Convention on Biological Diversity has also identified the Marine Trophic Index as a key measure for setting biodiversity targets.
To calculate the Marine Trophic Index, each fish or invertebrate species is assigned a number based on its location in the food chain. Carnivores are assigned high numbers, and herbivores lower ones. The Index is calculated from datasets of commercial fish landings by averaging trophic levels for the overall catch.
For the purposes of the Marine Trophic Index indicator used in the 2008 EPI, we are interested in monitoring the direction of change in average trophic index over the last several decades. The Sea Around Us website has data from 1970-2005. We measured the slope of the trend line and set the target score as zero, i.e. no further decline in trophic level.
