Marine trophic index of European seas

Assessment versions

Published (reviewed and quality assured)

• No published assessments

 

Rationale

Justification for indicator selection

MAIN ADVANTAGES OF THE INDICATOR

• Policy relevance: the Marine Trophic Index gives a simple, clear and consistent message to policy-makers and the public.
• The indicator can be applied to all European seas and can be aggregated to different scales/levels. The preferred option for European assessments is to have it calculated for each regional sea, or to the ecological regions used by ICES.
• Its temporal coverage for fisheries landings is quite good for European countries. The indicator could also be calculated on the basis of national survey data deriving from the implementation of EU fisheries regulations for many regional seas, which tend to be fairly high resolution and consistent over time. Calculations per regional sea for Europe are not currently available yet (because survey data are not available yet).

Scientific references

• No rationale references available

Indicator definition

Trends in mean trophic levels of fisheries landings per European sea.

Units

%
trophic level units

 

Policy context and targets

Context description

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.

Relation of the indicator to the focal area

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.

Targets

No targets have been specified

Related policy documents

No related policy documents have been specified

 

Methodology

Methodology for indicator calculation

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).'

Methodology for gap filling

N/A

Methodology references

No methodology references available.

 

Data specifications

EEA data references

• No datasets have been specified here.

External data references

• Marine Trophic Index pr High Seas Areas

Data sources in latest figures

 

Uncertainties

Methodology uncertainty

No uncertainty has been specified

Data sets uncertainty

No uncertainty has been specified

Rationale uncertainty

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.

Work description

SUGGESTIONS FOR IMPROVEMENT The interpretation of this indicator could be improved by using data on the size of the landings or of the survey samples. It will be also very useful to use the 'cut' MTI calculation and carry out a sensitivity testing. This would involve proposals of threshold values and precautionary reference points. Based on the bias in connection with both the commercial data and the survey data it is suggested to explore and test a third method: size of the large specimen in a fish population. By using the size distributions of some selected fish species (e.g. 10 common species) from the survey data a measure off the median or some higher percentile (e.g. 95 percentile) could be assessed over time. The median or high percentile is used in order to minimize the 'noise' of varying recruitment of juvenile fish.

Resource needs

No resource needs have been specified

Status

Not started

Deadline

2099/01/01 00:00:00 GMT+1

Work description

COSTS RELATED TO DEVELOPING, PRODUCING AND UPDATING THE INDICATOR (as available) The costs of production of this indicator are expected to be minimal as much of the development work has been done already.

Resource needs

No resource needs have been specified

Status

Not started

Deadline

2099/01/01 00:00:00 GMT+1