Interim report of the HELCOM CORESET project

areas where the index does not signal good environmental status, GES may be achieved by restoration of
recruitment habitats, improving water quality or regulating fi shing pressure.
Coastal Fish - Species Demographic Index (D1.3.1)
The index refl ects the size structure of a key fi sh species in Baltic Sea coastal areas, such as Perch (Perca fl uviatilis),
which was described in the section above. The index is based on the metric mean length of Perch,
but the metric abundance of large Perch could be used as additional information or complement.
The estimate mean length of Perch is expected to refl ect changes in recruitment success as well as in mortality.
Low levels may signal the appearance of a strong year class of recruits, decreased top down control
in the ecosystem, or high fi shing mortality, but potentially also density-dependent growth. High levels in
the indicator may signal a high trophic state, but potentially also decreased recruitment success. Because of
this, the indicator should be interpreted together with the Species Abundance Index.
The estimate abundance of large Perch is calculated as the catch per unit effort of Perch larger than 25
cm. The estimate is expected to provide a more direct measure of fi shing mortality on the actual species,
as well as of ecosystem health. The index will, however, not be indicative of changes in the younger yearclasses
of the population.
Policy relevance: The index will show if the size structure of Perch is at an appropriate level for supporting
coastal ecosystem function, including food provision with a focus on trophic state. In areas where the index
does not signal good environmental status, GES may be achieved mainly by regulating fi shing pressure.
Coastal Fish - Community Diversity Index (D1.6.1)
The index refl ects biological diversity at the community level and is based on the Shannon Index. High values
refl ect high species richness and low dominance of single species, whereas low values refl ect the opposite.
The index has both an upper and a lower boundary since very high levels of the index potentially also may
refl ect a decrease in the abundance of a naturally dominating species.
Policy relevance: The index will indicate whether the biodiversity structure of coastal fi sh communities is at
an appropriate level for supporting coastal ecosystem function, including ecosystem resilience, or not. The
index refl ects the general state of the fi sh community. In areas with sub-GES conditions, actions to achieve
good ecological status should target the species level.
Coastal Fish - Community Size Index (D1.6.1)
The index refl ects the general size structure at the community level and is based on estimates of the abundance
of large fi sh (measured as catch per unit effort). Depending on the maximum mesh size of the gear,
large fi sh are defi ned as individuals larger than 30 (Net series) or 40 (Coastal survey nets, Nordic coastal
multi-mesh gillnets) cm.
Generally, large fi sh are abundant in coastal communities indicative of good ecological status in the Baltic
Sea (Pauly et al. 1998). The index is expected to mainly refl ect changes in fi shing mortality at the community
level, where low values refl ect increased fi shing mortality. However, the value of the index may to some
extent also be infl uenced by environmental conditions such as temperature and nutrient status.
Policy relevance: The index will indicate if the size structure of coastal fi sh communities is at appropriate
level for supporting coastal ecosystem function, including food provision and ecosystem resilience. In areas
where the index does not signal good environmental status, GES may be achieved by mainly regulating
fi shing pressure.
49