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Aggregation and marine biological value

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This article continues the discussion on identifying the biological value in the marine environment. It considers the difficulties of defining areas where aggregation of species may occur, especially for widely dispersed highly mobile species. It is part of a summary of the discussions held during a workshop on marine biological valuation,held from 6 to 8 December 2006 at Ghent (Belgium). The workshop was a joint venture of the EU CA ENCORA (http://www.encora.org) and the EU NoE MARBEF (http://www.marbef.org). Both Theme 7 within ENCORA and Theme 3 within MARBEF deal with marine/coastal biological valuation and the workshop aimed to reach a consensus on this topic.


The ‘aggregation’ criterion is defined[1] as the degree to which an area is a site where most individuals of a species are aggregated for some part of the year or a site which most individuals use for some important function in their life history or a site where some structural property or ecological process occurs with exceptionally high density.[2]

The ‘aggregation’ and ‘fitness consequences’ criteria will mainly identify subzones that have high ecological importance for the wider environment. Evaluation of these criteria therefore lies at the heart of an ecosystem approach to management, assigns value to subzones that ‘drive’ ecological processes, and is one way to achieve preservation of the larger marine ecosystem. Ecosystem management forces us to adopt a holistic view of the components as parts of the system, rather than the reductionist view of single-species management, which ignores the fact that species exist only as part of the ecosystem. This is in agreement with the present concept of including as many components of biodiversity (both structural components and processes) in the criteria assessment as possible.

Application of the criterion

If data on the population size of a species are available at the scale of the study area, it is possible to determine whether a high percentage of a species’ population is located within a cluster of subzones of the study area.

If these data are lacking and qualitative information exists on certain areas where species aggregate (wintering, resting, feeding, spawning, breeding, nursery, rearing area or migration routes), this information should be used as an alternative or addition to broad-scale quantitative abundance data.

When the location of these areas is not documented, their existence and location may be predicted by examination of physical processes (incl. modelling) or remote sensing data, for example as indicated by Roff & Evans (2002)[3] in their survey of distinctive marine areas.

Alternatively, traditional ecological knowledge may assist in the definition of aggregation areas. It needs to be emphasized that any data, modelled or otherwise, needs to be assessed for its reliability and degree of confidence.

The inclusion of aggregation as a criterion for biological valuation introduces a certain degree of connectivity into the valuation concept, because this criterion is used to determine the aggregation value of subzones relative to the subzones adjacent to them, allowing the clustering of those subzones with equal value.

The aggregation criterion is especially important for highly mobile species like birds, mammals or fish. For the preservation of such wide ranging species, information on their full distribution is less useful than the localisation of areas which are critical for foraging, nursing, haul-out, breeding or spawning; it is these areas that should be included when a biological valuation is done. When the study area under consideration is relatively small, the foraging areas of such highly mobile species could cover the whole study area, but it is still important to include them in the biological valuation, as this can be an important signal to management as well.

Owing to the continuous nature of the marine environment, it is difficult to identify the boundaries of such aggregation areas, especially for widely dispersed, highly mobile species. This can be seen in the difficulties encountered by many countries to implement the EC Bird Directive (1979) and Ramsar Convention (1971), which both select important bird areas based on high densities of bird species.


These paragraphs are based on the paper of Derous et al. (2007). A concept for biological valuation in the marine environment. Oceanologia 49 (1). A concept for biological valuation in the marine environment. Oceanologia 49 (1). See FLANDERS MARINE INSTITUTE web site at [1] for the full citation and to download a copy of the paper.


  1. Derous S., Agardy T., Hillewaert H., Hostens K., Jamieson G., Lieberknecht L., Mees J., Moulaert I., Olenin S., Paelinckx D., Rabaut M., Rachor E., Roff J., Stienen E.W.M., van der Wal J.T., Van Lancker V., Verfaillie E., Vincx M., Weslawski J.M., Degraer S. (2007). A concept for biological valuation in the marine environment. Oceanologia 49 (1).
  2. DFO (2004). Identification of ecologically and biologically significant areas. DFO Can. Sci. Adv. Sec. Ecosystem Status Report 2004/006.
  3. Roff J.C., Evans S.(2002). Frameworks for marine conservation - non-hierarchical approaches and distinctive areas. Aquatic Conservation: Marine and Freshwater Ecosystems 12, 635-648.