For the assessment of status, a mean value based on data from a recent five to 10 year period for each of the three parameters is evaluated against its threshold value (and when appropriate, tested with the chi-square or equivalent method). Generalized Additive Models are used to investigate average changes over time (R-code available for all analyses and graphs presented in this report is available from Peter Hellström, Swedish Museum of Natural History). To check for significant nonlinear trend components, a GAM-smoother is applied (e.g. Wood 2017). Statistical power analysis is used to estimate the minimum annual trend likely to be detected at a statistical power of 80% during a monitoring period of 10 years.

Methodology of data analyses

In the following three paragraphs, n1 denotes the number of nests containing 1 young (etcetera).

Productivity

The mean number of nestlings, of at least three weeks of age, out of all occupied nests:

(n1 + [n2x2] + [n3x3]) / (n0 + n1 + n2 + n3).

For nests with young that were observed only from the ground, the numbers of nestlings is underestimated since sometimes not all nestlings are visible. A correction is necessary before the total number of nestlings from such observations can be incorporated with the total number of nestlings from climbed nests, to make up the total number of nestlings for the productivity assessment. A correction is calculated based on the mean number of nestlings in climbed nests divided by the mean number of nestlings observed from the ground, or by applying the mean nestling brood size in climbed nests to all successful nests that were observed only from the ground.

Brood size

The mean number of nestlings, of at least three weeks of age, in nests containing young:

(n1 + [n2x2] + [n3x3]) / (n1 + n2 + n3).

For the calculations of mean brood size only data obtained from nests that have been climbed are included. Even big nestlings that are lying down in the nest are easily overlooked when observations are made from the ground. Data received from observations made from the ground in Germany underestimated the real number of nestlings by 11% (Hauff & Wölfel 2002), using an extended data set (updated until 2014) the difference was 14% (Herrmann, unpublished). For all data used in the State of the Baltic Sea (2017/2018)-evaluation pooled across regions, there was a 10%-difference between ground observations and climbed nests. For some regions the differences were quite large, e.g. up to 50% for instance in Sweden. There was a trend that suggests that the difference is larger in regions with a higher fraction of climbed nests and also with denser populations. In dense populations, less time and fewer visits can be invested to correctly assign brood size. Under such circumstances, brood sizes have in many cases been counted as "at least one nestling" for nests where no nestlings where observed visually during the time of visit, but rather inferred from begging calls, white-wash, droppings, pellets, prey remains etc. In the future it is highly recommended that such observations are identified as brood size "larger than zero, but unknown" rather than lump such observations with brood sizes of 1. This procedure has been suggested by Estonia, and should be preferably be used in the future.

Breeding success

The proportion of nests containing at least one nestling, of at least three weeks of age, out of all occupied nests:

(n1 + [n2] + [n3]) / (n0 + n1 + n2 + n3).

Assessment units

White-tailed sea eagles are presently breeding in coastal areas of the whole Baltic Sea. In this evaluation the HELCOM assessment unit scale 3 'Open sub-basin and coastal waters' has been applied with modifications (see Key message figure 1 + Appendix 1 for explanations). The assessment units are defined in the HELCOM Monitoring and Assessment Strategy Annex 4.

Where a sub-population within a coastal strip of a marine assessment unit is too small from a statistical point of view, data from coastal strips of adjacent units can be combined. However, since the assessment of status is based on data from a period of at least five years, it will yield a reasonable sample size even for small sub-populations. For example, the sub-population on coastal strip # 15 (Russian part of the Gulf of Finland) has been reported to be only 6 pairs. Over a five year period this would yield a potential sample size of 30, provided that data from all pairs can be collected in all years. This is not the case so far, and the currently available samples are useful only for calculations of mean nestling brood size (Key message figure 1).

Besides breeding in coastal areas, white-tailed sea eagles also breed inland within all Baltic Sea coastal countries. The boundaries for coastal areas where this indicator applies are set in accordance with Article 1 of the Helsinki Convention (Convention Area) to include landward internal waters (lagoons and estuaries) (see Assessment protocol figure 1). The inner landward boundary is set in accordance with the Guidelines for the identification of coastal ecosystems proposed by EC Nature (EC Nat 2-5, 1993) and approved by EC 4, stating under point 1.2: 'For practical reasons in cases where the extension of coastal ecosystems is difficult to define according to a) – c), a strip in a width of at most 10 kilometres inland from the coastal mean water line is taken for a working area of Art. 15.