Focal Area: Ecosystem integrity and ecosystem goods and services

Headline Indictor: Marine Trophic Index

Key Indicator Partner:

Data Available: Global, regional and national time series, 1950  onwards 

Development Status: Ready for global use

Fish currently supply the greatest percentage of the world’s protein consumed by humans. However, most of the world’s fisheries are being fished at levels above their maximum sustainable yield and many regions are severely overfished. In the late 1990s it was demonstrated that the mean trophic level of fisheries is declining, i.e. that global fisheries catches increasingly consist of smaller fish and invertebrates lower in the food web. This process, now known as ‘fishing down marine food webs’ has become a major concern as it means that fish stocks especially of large bodied fish are being overexploited and fisheries are not being sustainably managed. This severely threatens marine biodiversity.

The Marine Trophic Index (MTI) has been developed by the Sea Around Us project at the UBC Fisheries Centre, and was established to investigate the impacts of fisheries on the world’s marine ecosystems. The MTI can be used to describe the complex interactions between fisheries and marine ecosystems and communicate a measure of species replacement indices by fisheries. The concept and underlying methods to estimate the MTI have been well –tested and have undergone substantial peer-review using existing information. The MTI is calculated from catch composition data collected by the Food and Agricultural Organization of the United Nations (FAO). The concept and approach is now widely accepted.

The MTI has been calculated for the Exclusive Economic Zone (EEZ) of each country and for all Large Marine Ecosystems (LME), from 1950 to present. Data for all LMEs are combined to generate the global index. The data for all EEZs and LMEs, as well as information on how the MTI is calculated can be viewed via the Sea Around Us website.

The indicator can be applied at different scales from global to subnational (e.g. portion of the EEZ of a country). For countries such as Malaysia and Indonesia with EEZs in different basins, the MTI can be calculated for those sub-national areas. The data is readily available at these different scales via the Sea Around Us website.

The data, although catch based, can be disaggregated into different taxonomic classifications from very broad groupings (e.g. Fish, crustaceans and molluscs) to habitat based fish (demersal, bathydemersal, etc.) to species and genus. It is possible to report on a combination of thematic and spatial themes.

Combined (Global) Marine Trophic Index for all Large Marine Ecosystems

The combined index excludes data for Peruvian anchoveta and large pelagic fisheries. The very localized fishery for Peruvian anchoveta, a low trophic level species, is the largest single-species

fishery in the world, and it exhibits extreme fluctuations in landings which would mask the comparatively more subtle patterns in trophic level changes by the rest of the world’s fisheries. Data for large tunas and billfishes have been excluded as much of their catch is taken in pelagic waters outside of currently defined LMEs. Inclusion of these species would artificially inflate trophic level patterns, especially in recent decades, as tuna fisheries have expanded tremendously.

Source: Pauly et al. 2008.

A decrease in Marine Trophic Index represents a decline in the abundance and diversity of fish species high in the food chain, such as cod. This process also termed ‘fishing down marine food webs’ means that fish stocks are being overexploited and fisheries are not being sustainably managed. As a result the biodiversity of these resources will be threatened. The loss of top predators and the reduction of the trophic structure in oceans will have consequences for ecosystem stability and function, threatening biodiversity more broadly.

An increase in MTI does not necessarily mean that abundances of higher tropic level species have increased. Instead an increase may be the result of geographical expansion of fisheries outside of the defined LMEs.

‘The trend in mean trophic level for all LMEs combined indicates a decline in the MTI from a peak in the 1950s to a low in the mid 1980s. The decline represents a global decrease in the abundance of high tropic level species, resulting in the phenomenon of ‘fishing down marine food webs’, in which fisheries catches increasingly consist of smaller fish and invertebrates lower in the food web.

From the mid 1980s there is a trend reversal and the global MTI increases. This increase does not necessarily represent improvements in the global sustainability of fisheries and in turn an increase in the abundance of higher trophic level species. In fact, data from the complementary Fishing-in-Balance (FiB) index (see www.seaaroundus.org), a measure of the ‘balance’ between catches and tropic level indicates that this increase is the result of offshore expansion of the fisheries into pelagic waters outside of the currently defined LMEs.’

The MTI is available for the EEZs of every coastal country in the world and for all currently defined LMEs. Therefore the MTI indicator can be applied at different scales from global to national. For countries such as Malaysia and Indonesia with EEZs in different LMEs, the MTI can be calculated for those sub-national areas.

All data, including country level MTIs are available from the Sea Around Us website. For advice about the interpretation of MTIs contactseaaroundus@fisheries.ubc.ca, or use the ‘feedback’ link.

There is no future development of the indicator planned at this time.

The underlying landing data are continually assessed for quality and the method for allocating the landings spatially has been peer-reviewed, and any improvements to the method is also peer reviewed. New landings data from FAO will be continuously incorporated into the database, enabling the MTI to be calculated up to the present date.