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Indicator Assessment
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.
www.seaaroundus.org
Marine Trophic Index for selected European seas (B)
Note: How to read the graph: in the Mediterranean Sea, MTI has been stable since 1950.
www.seaaroundus.org.
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 .
NOTES
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.
FURTHER INFORMATION
Trends in mean trophic levels of fisheries landings per European sea.
%
trophic level units
It has been suggested that high trophic levels reflect a high level of evolved biodiversity.
Preferred fish catches consist of large, high value predatory fishes, such as tuna, cod, sea bass and swordfish. The intensification of fishing has led to the decline of these large fishes, 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 increases, and the mean trophic level (i.e. the mean position of the catch in the food chain) of fisheries landings, goes down.
Fisheries, since 1950, are increasingly relying on the smaller, short-lived fish and on the invertebrates from the lower parts of both marine and freshwater food webs. If decline in trophic levels continues at the current rate, the preferred fish for human consumption will become increasingly rare, forcing a shift for fisheries and human consumption to smaller fish and invertebrates.
The mean trophic level of a species is a calculated value which reflects the species abundance balance across a trophic range from large long lived and slow growing predators to fast growing microscopic primary producers and 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.
If decline in mean trophic levels of fisheries landings continues, the resulting smaller food chains leave marine ecosystems increasingly vulnerable to natural and human induced stresses, and reduce the overall supply of fish for human consumption. Thus the indicator is well suited to illustrate the focal area on ecosystem integrity and the provision of goods and services provided by biodiversity in support of human well being.
No targets have been specified
No related policy documents have been specified
See below extracts from Pauly and Watson (2005).
'The original demonstration of the effect now widely known as 'fishing down marine food webs' by Pauly et al. (1998a) relied upon the global database of fishes landing assembled and maintained by the Food and Agricultural Organization (FAO) of the United Nations. This database includes, based on voluntary submissions, the annual fisheries catches (since 1950) of member countries, by species or groups of species (genera or families or larger groupings such as 'miscellaneous fishes'). Importantly, these statistics are aggregated by the countries where the catches were landed, and not by the countries where they were taken (Watson et al. 2004). However, FAO also assigns the marine components of these catches to 18 large statistical areas (e.g. Northeast Atlantic; West Central Pacific), thus allowing at least some spatial disaggregation.
Using the FAO data and TL [trophic level] estimates for over 200 species (or groups thereof; see below), mean TLs were computed, for each year k from
TLk=((N-ary summation)i(TLi))*(Yik)/((N-ary summation)iYik)
Where Yi refers to the landings of species (group) i, as included in fisheries statistics.
(Note that, ideally, mean TL should be based on catches, i.e. all animals killed by fishing (i.e. landings and discards; Alverson et al. 1994), rather than only on the landings included in FAO statistics).'
N/A
No methodology references available.
No uncertainty has been specified
No uncertainty has been specified
MAIN DISADVANTAGES OF THE INDICATOR
Most fisheries and their effects have been studied in shallow/ continental shelf waters and deep water fisheries are not yet well covered and surveyed, so in fact deep oceanic waters may not be well represented by the present data sets and calculations of the indicator.
The indicator, when calculated by using the landings data, can sometimes show a different than expected picture, in certain sub regions (e.g. the North Sea), due to the history of fishing in the region. The interpretation could be improved by using data on the size of the landings or surveys.
The use of commercial landings is not optimal since this does not include illegal landings and species which are discarded. Furthermore, a change in the MTI based on commercial landings might reflect changes in gear technology and taste rather than in population changes. The use of survey data instead of commercial data has been suggested to overcome this bias. However trawl surveys are primarily aimed at determining the recruitment and thus targeting the young fish.
The methodology has been criticised: short-term fluctuations in lower trophic level species may temporarily exaggerate or skew the mean trophic level computation, such as the effect of periodic eutrophication in the Mediterranean, with increased biomass and production of small pelagic fishes. Thus, the original authors have proposed a modified version, termed 'cut MTI', which excludes lower (below a cut value) trophic levels.
Based on the caveats described above it is suggested to use both commercial landings and scientific trawl surveys. Furthermore the 'cut MTI' should be used as suggested above.
ANALYSIS OF OPTIONS
This indicator, adopted by CBD, is proposed for adoption for the European Seas with two formulations: (a) by using fishery/ landings data (as in CBD) and (b) by using survey based data. The usefulness of using landings data is related to the comparability with the global index, but is also important as sometimes these are the only available data for north Africa and the Black Sea.
For references, please go to https://www.eea.europa.eu/data-and-maps/indicators/marine-trophic-index-of-european-seas/marine-trophic-index-of-european or scan the QR code.
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