Marine trophic index of European seas (SEBI 012) - Assessment published May 2010
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Biodiversity (Primary topic)
- SEBI 012
Key policy question: What is the impact of existing fisheries and maritime policies on the health of fish stocks in European seas?
In the majority of European seas, the Marine Trophic Index (MTI) has been declining since the mid - 1950s, which means that populations of predatory fishes decline to the benefit of smaller fish and invertebrates.
Marine Trophic Index percentage change between 1950 and 2004
Note: How to read the graph: The MTI for the Black Sea was about 13 % lower in 2004 than it was in 1950.
Marine Trophic Index for EEZs and LMEs, 2009. Sea Around Us Project, www.seaaroundus.org.
SEBI indicators, 2010 - SEBI indicator 12.
Marine Trophic Index for selected European seas (A)
Note: How to read the graph: in the Baltic Sea, MTI has been decreasing since 1950.
Marine Trophic Index for selected European seas (B)
Note: How to read the graph: in the Mediterranean Sea, MTI has been stable since 1950.
A multispecies fishery can be assumed to be unsustainable if the mean Trophic Level of the species it exploits keeps declining. The decline in MTI is happening at different rates in different seas and four seas have shown no overall changes in their MTI since 1950. A more thorough analysis of the individual fisheries is required to assess causes of declines and specific effects on the wider marine ecosystems. Figures 2 and 3 show the MTI in European seas in two groups. The seas have been grouped according to the evolution in their MTI since 1950. Figure 2 shows seas with a more or less continuous decline in MTI. Figure 3 shows those seas where the trend is more stable. It is noteworthy that the trend since 1950 is different for most seas from the trend considered over a shorter recent time period (since 2000 MTI declines seem less severe or MTI is even going up).
The levelling off since 2000, however, may still mean that biodiversity has been lost, because considerable declines had already taken place before 1950 (e.g. in the North Sea). Increases in the Barents and Norwegian Seas since 1980, and in the Greenland Sea and on the Iceland shelf since 2000, signify a potential positive sign for biodiversity. It is also worth noting that when a country halts the fishery of a species with a low trophic level, the calculated MTI for the sea will go up, which distorts the message, for example if a fishery is halted because the stock is at a very low level.
Pursuant to the Marine Strategy Framework Directive (Art. 8) the EU requires that by mid-2012 the Member States should make an integrated 'initial assessment' of the environmental situation of their marine waters .
Most preferred fish catches consist of large, high value predatory fish, such as tuna, cod, sea bass and swordfish. The intensification of fishing has led to the decline of these large fish, which are high up in the food chain. As predators are removed, the relative number of small fish and invertebrates lower in the food chain tends to increase and the mean trophic level (i.e. the mean position of the catch in the food chain) of fisheries landings, goes down. The mean trophic level of a species is a calculated value, which reflects the species abundance balance across a trophic range from large long living and slow growing predators to fast growing microscopic primary producers. It is therefore a reflection of the biodiversity status of the system. It is derived by assigning a numerical trophic level to selected taxa, established by size, diet or nitrogen isotope levels.
Thus, the MTI describes a major aspect of the complex interactions between fisheries and marine ecosystems and communicates a measure of species replacement induced by fisheries. What is most important in the MTI is the trend, rather than the specific value.
Some improvements of this indicator (calculating an MTI using commercial landings and existing lists of trophic level of adult fish by species) as well as supplementary indicators have been suggested. Some of these will be explored during 2009 - 2010.
- Marine Trophic Index at the Sea Around Us Project: www.seaaroundus.org/sponsor/cbd.aspx
Marine Trophic Index pr High Seas Areas
provided by Sea around us project
More information about this indicator
See this indicator specification for more details.