Abundance and Distribution of Cetaceans

Cetaceans are an important component of marine biodiversity in the North-East Atlantic Ocean, as is clearly shown by their distribution and density (Figure a). Data from the systematic large-scale surveys (e.g., Small Cetaceans in the European Atlantic and North Sea SCANS in 1994; SCANS-II in 2005, SCANS-III in 2016; Cetacean Offshore Distribution and Abundance in the European Atlantic CODA in 2007; etc.) have been analysed using models that relate the density of species to features of their habitat to produce maps of predicted density distribution in the North-East Atlantic Ocean. Cetaceans are distributed widely across this area and the relatively small amount of overlap in predicted high-use areas highlights how the different species utilise the marine area in different ways, at least in summer. These and other similar data have been analysed to provide abundance estimates, and trends when possible.

Results of these analyses show that whales, dolphins, and porpoises are abundant throughout the North-East Atlantic Ocean. Although 36 species of cetacean have been recorded within the Greater North Sea, Celtic Seas, and the Bay of Biscay and Iberian Coast (Waring et al., 2009; Evans 2020; https://www.ascobans.org/), many are widely dispersed oceanic species that are rarely seen in these waters and, thus, very difficult to monitor systematically, regularly, and frequently. For instance, the current European Red List (as of spring 2022) has a majority of cetacean species classified as data deficient (https://ec.europa.eu/environment/nature/conservation/species/redlist/mammals/status.htm). Accordingly, this Indicator Assessment is mainly dedicated to assessing those species for which consistently robust information on abundance and distribution is available.

The status of the following species is assessed: harbour porpoise (Phocoena phocoena). For this species an Indicator Assessment is possible because it hasdefined Assessment Units (AU) in the OSPAR Maritime Area and enough data to calculate at least three absolute abundance estimates. Coastal bottlenose dolphins (Tursiops truncatus) were assessed in five AUs, for which more than four estimates of abundance over a period of thirty years are available. No assessment could be made for the remaining AUs, because fewer than four abundance estimates during the last thirty years are available.

The status of the following species, for which AUs are defined but fewer than three estimates of abundance are available, is discussed: minke whale (Balaenoptera acutorostrata), fin whale (Balaenoptera physalus), offshore population of bottlenose dolphin , white-beaked dolphin (Lagenorhynchus albirostris), white-sided dolphin (Lagenorhynchus acutus), and short-beaked common dolphin (Delphinus delphis).
No assessment could be carried out on striped dolphin (Stenella coeruleoalba), Risso’s dolphin (Grampus griseus), long-finned pilot whale (Globicephala melas), sperm whale (Physeter macrocephalus), and beaked whales (as a combined species group, Ziphiidae) because there are no AUs defined for these species and fewer than three estimates of abundance are available. Their status with respect to distribution and abundance in the OSPAR Maritime Area is, however, discussed.

Killer whale (Orcinus orca) is not assessed, owing to a lack of additional data since the Pilot Assessment on Abundance and Distribution of killer whales in IA 2017. A review of killer whale populations in the North Atlantic identified three genetically distinguishable populations in the North-East Atlantic, each associated to a major fish stock (Jourdain et al., 2019). However, further photo-identification effort and genetic studies are required to delineate and define AUs.
Densities of killer whales are lower in OSPAR Regions II, III and IV where the indicator is common: few sightings of this species are registered as part of the large-scale surveys undertaken in these OSPAR Regions. In contrast, in Region I, the North Atlantic Sighting Surveys (NASS) estimated approximatively 15 000 killer whales (CV = 0,42) in 2001 (Jourdain et al., 2019). No abundance estimates for Regions II, III or IV are currently available, but small, localized populations have been identified (e.g. Gibraltar Strait, off the West Coast of the British Isles, Iberian Peninsula). Photographic capture-recapture studies are needed to obtain population size for these small, localized populations: large scale surveys have low coverage probability and are not designed to capture small-scale features in distribution and abundance, especially in areas of low abundance and / or patchy distribution (Hammond et al., 2014).

Human pressures

Cetaceans are subject to a range of human pressures, some of which have been shown to have negative impacts at the individual level, ranging from increased stress and higher energetic costs, through sub-lethal effects on reproduction and immune function, to mortality. Effects may manifest at population level (Hammond et al., 2008; MAGRAMA, 2012; JNCC, 2013; Pirotta et al., 2018; ICES 2019). An overview of drivers, pressures and activities impacting marine mammals is given in The Marine Mammals Thematic Assessment .

Historical data on abundance and distribution

There is very little information about historical distribution (pre-1980) for any species of cetacean in the area assessed and no information about historical abundance (save for catch statistics of whales; e.g., Brown 1976; Smith et al. 2012; https://iwc.int/whaling). Methods to estimate cetacean abundance were first developed in the 1980s (Hammond, 1986; Hiby and Hammond, 1989). Development in analytical and survey methods continues (e.g., Bravington et al., 2021). The most recent methods are developed during the SCANS surveys (Hammond et al., 2002, 2013, 2021).

Data on strandings have been recorded for more than 100 years, but these can provide only very limited information on the presence at sea of offshore species, and it is not possible to use these data to infer population trends without additional information on causes of death, search effort and drifting of dead animals (Authier et al., 2014; Ten Doeschate et al., 2018; ICES 2021). In some cases, strandings data can provide information about distribution and occurrence, such as with harbour porpoise, for which stranding numbers inclined steeply in the Southern North Sea due to a shift in distribution, with a relatively high level of strandings to date (IJsseldijk et al., 2020).

Quality Status Report 2010 and Intermediate Assessment 2017

No comparable quantitative assessment of the distribution and abundance of wide-ranging cetaceans was included in the Quality Status Report 2010. The report highlighted human pressures on marine mammals more generally, such as by-catch, increased shipping and bioaccumulation of persistent hazardous substances. The assessment is based on updated abundance estimates from the Intermediate Assessment 2017, and a modelling approach integrating the IA 2017 data with regional survey data for wide ranging species up to 2020.  For coastal bottlenose dolphin populations, the IA 2017 data were updated to 2020. After IA 2017 a threshold value for M4 abundance was approved and applied in the current assessment.  
Wide-ranging cetaceans are also assessed under the European Union Habitats Directive Article 17 reporting, latest in 2018 (https://www.eea.europa.eu/publications/state-of-nature-in-the-eu-2020/at_download/file; EEA, 2020).

Methods

The data used to infer distribution and to estimate abundance of wide-ranging species were mostly derived from dedicated aerial and shipboard surveys that used line-transect methodology to generate robust estimates of abundance: SCANS (Small Cetacean Abundance in the North Sea; Hammond et al., 2002), SCANS-II (Small Cetacean Abundance in the European Atlantic and North Sea; Hammond et al., 2013), CODA (Cetacean Offshore Distribution and Abundance in the European Atlantic; CODA, 2009) and SCANS-III (Hammond et al., 2021). Smaller-scale (mostly national) surveys have been conducted using the same or a similar methodology, such as in Belgium, Denmark, France, Germany, Ireland, the Netherlands, and Sweden (Scheidat et al., 2008; Viquerat et al., 2014; Gilles et al., 2016; Laran et al., 2017; Rogan et al., 2018). The large-scale surveys provide information on distribution and abundance over a large area but are infrequent, and only undertaken in summer. The smaller-scale surveys are undertaken more often, and sometimes reveal seasonal patterns at a local scale.

The international (SCANS, SCANS-II, SCANS-III and CODA) and national surveys (e.g., ObSERVE) were collated following a data call from OSPAR in 2021 (Figure a and Figure b). Data were collected on dedicated line-transect surveys (either aerial or ship-based) using a distance sampling protocol, that took place within the OSPAR Maritime Area between 2005 and 2020. The first SCANS survey in 1994 focused on the Greater North Sea subregion but also covering part of the Celtic Seas subregion (Figure b, leftmost panel). The second SCANS survey in 2005 extended into the Bay of Biscay and Iberian Coast subregion. In 2007, the CODA survey covered offshore waters of the Celtic Seas and Bay of Biscay and Iberian Coast subregion (Figure b, middle panel). SCANS-III in 2016 covered both inshore and offshore waters. The ObSERVE summer aerial survey covered waters around Ireland that same year (Figure b, rightmost panel). While the SCANS-II / CODA and SCANS-III / ObSERVE survey had a similar geographic coverage, the first SCANS 1994 survey was more limited in scope. The Greater North Sea subregion has been consistently surveyed by the three SCANS surveys.

The data used to assess abundance and distribution of coastal bottlenose dolphins are mostly derived from capture–recapture methods applied to photo-identification data available in each of the AUs for this species. In some AUs, data from ship-based surveys are available. Data are patchy, however, and in some AUs robust data are only available for a limited number of animals that use small (protected) areas within the AU. As a consequence, data derived from small-scale surveys that relate to a discrete group of animals may not always be a robust measure of abundance for the entire AU. In order to examine trends, only sites with at least four estimates calculated over a ten-year period were considered.

Species Distribution

Distribution maps were generated from a density surface model approach (Miller et al., 2013) fitted to the collated data by OSPAR. Environmental covariates used in modelling included physiographic predictors (e.g., bathymetry) and oceanographic variables (e.g., sea surface temperature; Table a). The model was used to predict the summer distribution between 2005 and 2020 for harbour porpoise, fin whale, minke whale and offshore bottlenose dolphin. Predicted distribution maps were averaged over the periods 2005 to 2009, and 2010 to 2020.

For other species, information on species distribution has been obtained from modelled density surfaces fitted to data collected during large-scale surveys (SCANS-II/CODA and SCANS-III): Lacey et al., (2021) fitted (i) a density surface model to the SCANS-II/CODA data and (ii) another model to the SCANS-III data to investigate the distribution and habitat of cetacean species (and change therein) in the North-East Atlantic. Sightings of these species during the SCANS-III surveys (summer 2016) are shown to illustrate recent distribution.

For coastal bottlenose dolphin, records of sightings and strandings were used to identify where populations existed historically.

Abundance

Abundance was estimated using line-transect distance sampling methods (conventional design-based (CDS) estimates; Hammond et al., 2013). Ship-based survey methods mostly used two observation teams on the same vessel so that animals missed on the transect line and any responsive movement could be accounted for in the analysis. However, for some species in some years, insufficient data were available for such extended analytical methods to be used. Several aerial surveys used tandem aircraft or the circle-back procedure for harbour porpoises to correct for animals missed on the transect line (Hiby and Lovell, 1998; Hiby, 1999); this was extended to dolphin species and minke whales in SCANS-III in 2016. For other species, conventional aerial survey methods were used, corrected for availability and observer bias where possible (Hammond et al., 2013, 2021).

The abundance of coastal bottlenose dolphin populations was estimated, where possible, using Photo-ID capture-recapture methods (Amstrup et al., 2010; e.g. Cheney et al. 2018) and ship-based surveys.

Metrics

Species Distribution

Density surface models have been used to predict the distribution of those species for which sufficient data are available from large-scale purpose-designed surveys.

A suite of physiographic and oceanographic covariates (Table a) was used to model, with data collated by OSPAR (Figure b), the distribution of cetaceans in a Density Surface Modelling framework (after Miller et al., 2013).

Table a: Assessment values for cetacean species. Source A: EMODnet DTM (https://www.emodnet-bathymetry.eu/). Slope, aspect and topography complexity index were derived from the bathymetry data. Source B: Copernicus database (https://resources.marine.copernicus.eu).

Source A: EMODnet DTM (https://www.emodnet-bathymetry.eu/). Slope, aspect and topography complexity index were derived from the bathymetry data.
Source B: Copernicus database (https://resources.marine.copernicus.eu).

The modelling framework uses generalised additive models to predict cetacean distribution from environmental covariates. This framework allowed to model non-linear relationships between cetacean habitat and the marine environment (Embling et al., 2010; Gilles et al., 2016; Formula a):
 

NB denotes the Negative Binomial distribution, ω is the overdispersion parameter, Effort is the prospected surface (including the effective strip width estimated by distance sampling) and µ is the linear predictor (Formula b):

where f(.) are cubic Bezier splines (Marx and Eilers 2010).

An extrapolation analysis in environmental space was undertaken to assess the robustness of predictions from this model-based approach (Bouchet et al., 2019). Environmental variables (Table a) define a new topology, or environmental space, which can be studied with geometric tools such as distances and polygons. In particular, an extrapolation analysis can be carried out in this environmental space to assess how the data collected thus far covers the whole environmental space available in the North-East Atlantic. This analysis informs on how pervasive extrapolation may be when predicting cetacean distribution and habitat from a model. Technically, extrapolation analysis compares convex polygons in environmental space: a first polygon is obtained with the effort data by taking all segments of effort and their associated environmental covariates (the calibration data). A second polygon is obtained from environmental data collected in the whole area for which a distribution map is desired. It is important to note that this area is larger than the geographic area covered by surveys (e.g., Figure a and Figure b). These environmental data are to be used to predict cetacean distribution using both the environmental data and the model calibrated with the survey effort data. Each prediction is thus defined by a set of coordinates (values) that locates it in environmental space: it is possible to assess for each prediction whether it falls within the convex hull defined by the calibration data (King & Zeng 2007, Authier et al., 2017, Bouchet et al., 2019). Extrapolation analysis thus allows to assess the robustness of model predictions by quantifying how much the latter are informed by these data.

Abundance

Abundance of animals per species AU has mostly been estimated using data collected from large-scale purpose-designed aerial or shipboard surveys using line-transect distance sampling methods (Buckland et al., 2001); these are known as conventional design-based estimates CDS (e.g., Hammond et al., 2013; 2021). When abundance estimates come from models fitted to these data to generate a density surface (Miller et al., 2013) from which abundance is derived; these are known as model-based estimates (e.g., Gilles et al., 2016). All the reported estimates of population size for the Indicator Assessment are design-based estimates, which are less model-dependent, but do not allow to map densities at a fine spatial scale.

Abundance of coastal bottlenose dolphins per population in an AU was estimated using capture–recapture methods applied to photo-identification data, and ship-based surveys.

Baselines

Although the baseline should derive from historical data, these data are not available for the vast majority of cetacean species (Bearzi and Reeves 2021). To assess change, a recent (modern) baseline must be used (Kopf et al., 2015), which should then be assessed as a “reference” situation, and possibly one that is already known to be degraded. The most useful baselines for wide-ranging cetacean species derive from the results of large-scale surveys such as SCANS for coastal species and SCANS-II/CODA for wider ranging species (e.g., CODA, 2009; Hammond et al., 2002, 2013).

For most species, only two abundance estimates are currently available so a robust assessment of a trend involving change from a baseline is not possible. For harbour porpoise and white-beaked dolphin in the Greater North Sea there are three estimates available.

Spatial Scope

Assessment Units (AUs) for assessing abundance and distribution were defined for a number of species (ICES, 2014b) in separate regions, or in the OSPAR Maritime Area. For harbour porpoise, several AUs have been reviewed and defined (Figure c; NAMMCO/IRM 2019). OSPAR’s Marine Mammal Expert Group (OMMEG) further reviewed these AUs in 2021 and merged the Celtic Sea and Irish Sea AUs into a single Irish and Celtic Seas AU. For bottlenose dolphin, eleven AUs have been defined for the relatively small coastal populations on the basis of a combination of spatial separation, lack of photo-ID matches and genetic differences (Evans and Teilmann, 2009; ICES, 2013; ICES 2014; IAMMWG 2015). (Figure d) and a single offshore AU for the relatively large and wide-ranging population(s) of bottlenose dolphin living offshore (Figure e).

Currently, a single AU covering all European Atlantic waters has been defined for minke whale (Figure f), short-beaked common dolphin (Figure f), and white-beaked dolphin and white-sided dolphin (Figure g). For fin whales, three AUs defined by the International Whaling Commission overlap with the OSPAR Maritime Area (Figure h). No AUs have been defined for other species included in this assessment.

Threshold Value

The assessment thresholds for populations of cetacean species are based on the generation criterion from the International Union for the Conservation of Nature (https://www.iucn.org/). In each AU, the (localized) population size of each species will be maintained at or above baseline levels (the first abundance estimate available or the closest to the date of the Habitats Directive (Council Directive 92 / 43 / EEC)), with (i) no absolute decrease of more than 30% and (ii) a rate of decrease no greater than 30% over three generations. The latter can be converted to threshold values that are an annual rate of decline in abundance that must not be exceeded without having to wait for three generations. The annual rates of decline, if sustained over three generations, will lead to 30% decline in abundance. Since generation time varies between species (Taylor et al., 2007), thresholds, which corresponds to average annual rate of decline in abundance that would result in an overall 30% decline if sustained over three generations, are species-specific and reported in Table b.

Table b: Threshold values for cetacean species

N/A: not available

Trend estimation

A trend is quantified as the percentage change (in abundance) over a specified time period (Link and Sauer 1997). To assess trends, at least three abundance estimates are required over a relevant time scale. Robust assessments will require additional abundance estimates. To assess trends, it is necessary to consider the statistical power to detect changes (Authier et al., 2020).
For coastal bottlenose dolphins, monitoring in several populations within AUs has been ongoing for long enough to enable an assessment of trends, while in others there are currently too few data available over a suitable time period. Current estimates of numbers of animals in a population are usually obtained by photo-ID, although in their absence, abundance estimates are derived from line-transect surveys. Where possible, annual data on the estimated number of animals per population are provided in this assessment. Consideration is also given to coastal bottlenose dolphin populations known to have disappeared from their former range.

Power Analysis

Prospective statistical power analyses (Hoenig and Heisey 2001) were performed for detecting a trend in short time series of abundance estimates for cetacean species using a statistical significance set to 0,2 (Authier et al., 2020). Power was increased (except type-I error rate; Authier et al., 2020) but remained low to detect a small decline.  The power to detect trends could be improved by increasing the frequency of the large-scale surveys (Authier et al., 2020).

Data Call

Approximately 698 500 km of survey effort were collated as part of the OSPAR data call for the Indicator Assessment. Both ship-based and aerial survey data were collated with approximatively 195 000 km of effort with ships and approximately 500 000 km with planes. These data span the period between 2005 and 2020, and cover a wide geographical range (Table c) but not evenly (Figure i): a lot of effort is concentrated in the Greater North Sea, and comparatively less effort is deployed in offshore areas of the Celtic Seas and Bay of Biscay and Iberian Coast. Geographic coverage is thus uneven across the OSPAR Maritime Area.

* includes several Contracting Parties (e.g., United Kingdom, Portugal, etc.)

Species Distribution

Extrapolation

The extrapolation analysis reveals that environmental coverage was uneven, with shelf areas being relatively well covered, but the Norwegian Trench, offshore areas off Portugal and in the central Bay of Biscay corresponded to extrapolations in environmental space (Figure j). In practice, model-based predictions that are extrapolations (both in environmental and geographical space) are intrinsically more fragile because less informed by data and more by assumptions embedded in modelling choices.

Harbour porpoise (small toothed-cetacean species group)

Harbour porpoises are distributed throughout the shelf waters of the Greater North Sea, Celtic Seas and the Bay of Biscay and Iberian Coast. The Iberian Peninsula constitutes a separate population / subspecies (Fontaine et al., 2007, NAMMCO/IRM 2019), while all other areas except the Kattegat / Belt Sea are sub-areas of the North-East Atlantic / Greater North Sea population. The area around the northern border of the Kattegat / Belt Sea probably contains a mix of two different populations (Sveegaard et al., 2015).

The presence of harbour porpoises is influenced by oceanographic and human pressure variables. Water depth, surface salinity, temperature differences within the water column, eddy activity, current speed, coarseness of surface sediments, seabed slope and ship density have been identified as relevant influencing factors, although the interactions depend on the area and habitat type (Pierpoint, 2008; Marubini et al., 2009; Embling et al., 2010; Isojunno et al., 2012; Booth et al., 2013; Heinänen and Skov, 2015). Oceanographic variables might indirectly influence the presence of porpoises through determining the presence and abundance of their prey (Gilles et al., 2016).

A comparison of harbour porpoise distribution between the SCANS and SCANS-II data sets showed a marked difference in summer distribution within the Greater North Sea, with a southern shift in the main concentrations of animals between 1994 and 2005. This southward shift in distribution is supported by data from smaller scale surveys which show increasing numbers of porpoises occurring in French, Belgian, Dutch and German waters (e.g., Gilles et al., 2009, 2011; Haelters et al., 2011; Scheidat et al., 2012; Peschko et al., 2016, Nachtsheim et al. 2021) and decreasing numbers in the northern North Sea (Øien, 1999, 2005, 2010, Hammond et al., 2013, 2021). The shift in harbour porpoise distribution is believed to be related to changes in prey distribution or abundance (Hammond et al., 2013). Data from the SCANS-III survey in 2016 confirms that the shift has been maintained (Figure k). More sightings were made throughout the English Channel in 2016 than in previous surveys.

Short-beaked common dolphin (small toothed-cetacean species group)

Common dolphins in the North-East Atlantic are distributed widely in the Celtic Seas, the Bay of Biscay and Iberian Coast, and the Wider Atlantic (Reid et al., 2003; Murphy et al., 2013, Saavedra et al., 2018; Astarloa et al., 2021; Murphy et al., 2021). Sightings in the Greater North Sea are thought to be associated with the North Atlantic Oscillation and the incursion of warmer water into the Northern North Sea (Murphy et al., 2013). The SCANS-II and SCANS-III surveys revealed no sightings of common dolphin in the Greater North Sea but there were many sightings throughout the southern part of the Celtic Seas and throughout the Bay of Biscay and Iberian Coast (Figure l). Modelling the distribution of common dolphin was met with several problems (overdispersion etc.; Lacey et al., 2021) that resulted in predictions with large standard errors.

White-beaked dolphin (small toothed-cetacean species group)

White-beaked dolphins have been shown to prefer water temperatures of 13°C or less (MacLeod et al., 2008) and depths of 50–100 m (Reid et al., 2003). They are mainly found in the northern part of the Greater North Sea and Celtic Seas, and have only been recorded as vagrants in the Bay of Biscay and Iberian Coast (Figure m). From the pattern of sightings from the SCANS, SCANS-II and SCANS-III surveys, the distribution of white-beaked dolphin does not appear to have changed since 1994 (Hammond et al., 2002, 2013, 2021).

White-sided dolphin (small toothed-cetacean species group)

White-sided dolphins are found in temperate and sub-arctic waters, where they mainly occur along the slope of the continental shelf at depths of 100-500 m (Reid et al., 2003). They are distributed in the western part of the Celtic Sea and the northern part of the Greater North Sea (Figure n).

Coastal bottlenose dolphin (small toothed-cetacean species group)

Coastal bottlenose dolphins are observed along the Atlantic coast of Europe from Scotland in the north to Spain in the south. Populations of unknown size disappeared from the Severn Estuary (United Kingdom) and estuaries in the east of England (United Kingdom), as well as from the Elbe and Weser estuaries of Germany towards the end of the 19th century. More recently, a population in the southern North Sea, which ranged from the northern French coast to the north of the Dutch coast disappeared towards the end of the 1960s. Other discrete populations, such as those from Arcachon (France) and the Tagus Estuary (Portugal) have also disappeared in recent decades.

Offshore bottlenose dolphin (small toothed-cetacean species group)

Bottlenose dolphins are widely distributed in offshore waters of the North-East Atlantic (Reid et al., 2003; Hammond et al., 2002, 2013; Laran et al., 2017; Rogan et al., 2018; Figure o). In contrast to the situation for coastal populations of this species, less is known about the animals in offshore waters: the ObSERVE surveys in the summers of 2015 and 2016 revealed a large number of sightings south-west of Ireland.

Striped dolphin (small toothed-cetacean species group)

Striped dolphins have been found to prefer specific water temperatures (such as 21–24°C within the Mediterranean Sea; Panigada et al., 2008). Within the OSPAR Maritime Area, they are generally found only within the southern part of the Celtic Seas and in the Bay of Biscay and Iberian Coast, primarily in waters off the continental shelf (CODA, 2009; Laran et al. 2017; Hammond et al. 2021; Figure p).

Minke whale (baleen whale species group)

Minke whales occur both on and off the shelf of the North-East Atlantic. The southern/central North Sea and Celtic Seas represent the southernmost limit of their summer range in the North-East Atlantic (Reid et al., 2003; Evans et al., 2003). There is some evidence that minke whale distribution shifted south between 1994 and 2005 (Hammond et al., 2013); the pattern of sightings from SCANS-III shows that this has been maintained (Hammond et al., 2021). Minke whales have rarely been recorded in the Bay of Biscay and Iberian Coast (Figure v).

Fin whale (baleen whale species group)

Fin whales primarily inhabit deeper waters (200–400 m) beyond the continental shelf edge (Reid et al., 2003). They are rarely seen in the Greater North Sea but are commonly sighted in the Celtic Seas and Bay of Biscay and Iberian Coast (Figure w). García-Barón et al., (2019) evidenced higher densities of fin whale in deep waters of the Bay of Biscay, associated with moderate sea surface temperature, a positive sea level anomaly and high chlorophyll a concentration. Data spanning the past 30 years do not indicate any changes in distribution (Hammond et al., 2011; Lacey et al., 2021).

Risso’s dolphin (deep-diver species group)

Risso’s dolphin is a deep-diving species mainly occurring along the edge of continental shelf waters and adjacent oceanic waters in the Celtic Seas and the Bay of Biscay, but also sighted in continental shelf waters (Laran et al., 2017, Rogan et al. 2018, Figure x). They are thought to prefer water temperatures of 12°C or more (Hartman 2018).

Long-finned pilot whale (deep-diver species group)

Long-finned pilot whales are deep divers and typically found only in waters around the continental slope and beyond the shelf. They occur throughout the Celtic Seas and the Bay of Biscay and Iberian Coast (Figure y). No pilot whales were seen in the SCANS, SCANS-II or SCANS-III surveys in the Greater North Sea (Hammond et al. 2002, 2013, 2021). Slope and distance to shelf edge are important predictors for the presence of this species (Rogan et al., 2017).

Beaked whales (all species) (deep-diver species group)

Beaked whales are deep-diving species found almost exclusively in deep waters. Sighting of beaked whales are relatively uncommon in most areas throughout the Celtic Seas and the Bay of Biscay and Iberian Coast (Rogan et al., 2017) and rare in the Greater North Sea. Deep divers are the most frequently sighted on continental slopes (Virgili et al. 2019). Sightings of northern bottlenose whale Hyperoodon ampullatus and Sowerby’s beaked whale Mesoplodon bidens have been made primarily in the northern part of the Celtic Seas, whilst Cuvier’s beaked whale has mostly been seen in the southern part of the Celtic Seas and the western part of the Bay of Biscay and Iberian Coast (Figure z).

Sperm whale (deep-diver species group)

The sperm whale is a deep-diving species found throughout deep waters of the Celtic Seas and the Bay of Biscay and Iberian Coast, and rarely in the Greater North Sea, except in the Faroes-Shetland channel between the Greater North Sea and Celtic Seas (Figure aa). Virgili et al., (2019) evidenced high densities of sperm whales in deep waters (>2 000 m) associated with high gradients of sea surface temperature and high primary productivity.

Abundance

The most recent SCANS III and ObSERVE survey in summer 2016 provide the best available population abundance estimates for harbour porpoise, common dolphin, striped dolphin, white-beaked dolphin, white-sided dolphin offshore bottlenose dolphin, fin whale, minke whale, Risso’s dolphin, long-finned pilot whale, beaked whales and sperm whale (Table d).

Estimate figures rounded to the nearest hundreds.
* post-stratified to correspond exactly to the defined AU.

(Note: The 'Estimate' and 'CV %' for Offshore bottlenose dolphin (SCANS-II / CODA) and White-beaked dolphin (SCANS-III / ObSERVE); the 'CV %' for White-sided dolphin; and the 'Estimate' for Minke whale (SCANS-III / CODA) were amended following the publication of the SCANS-IV report, September 2023).

Harbour porpoise (small toothed-cetacean species group)

Indicator Assessment for this species of small toothed-cetacean was only possible for the North Sea AU (Figure c) which has been surveyed three times. The design-based abundance estimates show a slight increase between 1994 and 2016 (Table d), but the trend was not significant using the robust method for estimating trend on short time-series (Authier et al., 2020). The estimated yearly change (in %) was 0,00 (p-value = 0,93) which is larger than the threshold of -1,6%: the threshold was achieved and no decline in abundance was evidenced for harbour porpoise in the North Sea AU.

Only two abundance estimates were available for the West Scotland and Ireland AU: abundance slightly decreased between 2005 and 2016 (Table d) although no trend was computed and statistical significance was not assessed. The decrease seen between 2005 and 2016 is small, especially compared to the large coefficient of variation of the 2005 estimate (Table d). A qualitative assessment based on the robust evidence provided by the design-based estimates is thus one of no decline in abundance of harbour porpoise in the West Scotland and Ireland AU.

Only two abundance estimates were available for the Celtic and Irish Seas AU: abundance decreased between 2005 and 2016 (Table d) although no trend was computed and statistical significance was not assessed. The decrease seen between 2005 and 2016 is large, approximately 40% (Table d). A qualitative assessment based on the robust evidence provided by the design-based estimates is thus one of a decline in abundance of harbour porpoise in the Celtic and Irish Seas AU. A quantitative assessment against the threshold was not carried out as no trend was estimated on only two estimates. A possible explanation for the decrease may be excessive by-catch (Marine Mammal By-catch (Harbour Porpoise; Common Dolphin; Grey Seal)).

Only two abundance estimates were available for the Iberian Peninsula AU: abundance showed no change between 2005 and 2016 (Table d). The coefficient of variation of the 2005 estimate is, however, quite large and could mask an increase or decrease. A qualitative assessment based on the evidence provided by the design-based estimates is thus one of a decline in abundance of harbour porpoise in the Iberian Peninsula AU.

Short-beaked common dolphin (small toothed-cetacean species group)

One single AU has been defined for this species in the OSPAR Maritime Area (Figure f). The abundance estimate in 2016 is similar to the revised estimate in 2005 / 2007 (Table d). A qualitative assessment based on the robust evidence provided by the design-based estimates is thus one of no decline in abundance of common dolphin in the North-East Atlantic AU. A quantitative assessment against the threshold was not carried out as no trend was estimated on only two estimates.

White-beaked dolphin (small toothed-cetacean species group)

One single AU has been defined for this species in the OSPAR Maritime Area (Figure g). The abundance estimate in 2016 is similar to the estimate in 2005 / 2007 (Table d). An estimate in 1994 is available but only cover a fraction of the AU. A qualitative assessment based on the robust evidence provided by the design-based estimates is thus one of no decline in abundance of white-beaked dolphin in the AU. A quantitative assessment against the threshold was not carried out as no trend was estimated on only two estimates.

White-sided dolphin (small toothed-cetacean species group)

One single AU has been defined for this species in the OSPAR Maritime Area (Figure g). The abundance estimate in 2016 is the first estimate available at the scale of the AU (Table d). No assessment, qualitative or quantitative, is possible with a single estimate.

Coastal bottlenose dolphin (small toothed-cetacean species group)

Eleven AUs have been defined for coastal bottlenose dolphin in the OSPAR Maritime Area (Figure d). The overall population size of coastal bottlenose dolphins in the Greater North Sea, Celtic Seas and Bay of Biscay and Iberian Coast regions is between 3 000 and 4 000 animals. Few locations within the species AUs have been monitored annually. The most extensive assessment was undertaken on the Sado Estuary population in Coastal Portugal AU (Figure q since 1986). This assessment shows that the threshold with a decline of 30% relative to the baseline defined as the level at the start of the time-series, is exceeded. The population, however, has remained stable over the last ten years. Annual mark-recapture estimates for populations in East Coast Scotland since 1990 (Figure r), indicate that the population is stable and may be showing signs of increase and range expansion (Cheney et al. 2018). Estimates from the wider Cardigan Bay in the Irish Sea and Coastal Wales AU (Figure s since 2002), the Shannon Estuary in Coastal Ireland AU (Figure t since 1997), and the Gulf of St Malo including the Channel Islands (Figure u since 2010) and two smaller populations Ile de Sein (since 1992) and the Molene Archipelago (since 1999) in the Coastal Normandy and Brittany AU indicate broadly stable populations. However, a quantitative assessment against the threshold was not carried out as no trend was estimated in locations in other AUs, due to a lack of abundance estimates. A summary of available data and population trends is shown in Table e.

Offshore bottlenose dolphin (small toothed-cetacean species group)

One single AU has been defined for this species in the OSPAR Maritime Area (Figure e). The North-East Atlantic AU has only been partially covered, the abundance estimate for this area in 2016 is higher than the estimate in 2005 / 2007 (Table d). This increase exceeds natural growth rates for a closed population, and is probably due to immigration from Atlantic waters outside the surveyed area. A quantitative assessment against the threshold was not carried out as no trend was estimated on only two estimates.

Striped dolphin (small toothed-cetacean species group)

There is no defined AU for this species. The abundance estimate in 2016 is larger than the revised estimate in 2005 / 2007 (Table d). The coefficient of variation associated with the latter is large. A qualitative assessment based on the evidence provided by the design-based estimates is thus one of no decline in abundance of striped dolphin in the North-East Atlantic AU. A quantitative assessment against the threshold was not carried out as there is no defined AU and no trend was estimated on only two estimates.

Minke whale (baleen whale species group)

One single AU has been defined for this species in the OSPAR Maritime Area (Figure f). The abundance estimate in 2016 is similar to the estimate in 2005 / 2007 (Table d). A qualitative assessment based on the robust evidence provided by the design-based estimates is thus one of no decline in abundance of minke whale in the North-East Atlantic AU. A quantitative assessment against the threshold was not carried out as no trend was estimated on only two estimates.

Fin whale (baleen whale species group)

Three AU has been defined by the International Whaling Commission for this species in the OSPAR Maritime Area (Figure h). The abundance estimate in 2016 for each AU is reported in Table d. The AU covering the Greater North Sea subregion is the one that has been covered by all SCANS surveys, but no sighting of fin whales was made. The vast majority sightings of fin whales were made in the southern most AU (AU ’Sp’, Figure h). In 2007, many sightings of fin whales were also made in this AU, along with sighting of unidentified large whales (Hammond et al., 2021). At the scale of the North-East Atlantic, and taking into account unidentified large whales (under the assumption that they correspond to fin whales), no change in abundance between 2007 and 2016 is evidenced (Hammond et al., 2021).

Risso’s dolphin (deep-diver species group)

There is no defined AU for this species. The abundance estimate in 2016 is reported in Table d. Without a defined AU and with only a single estimate, no assessment, qualitative or quantitative, is possible.

Long-finned pilot whale (deep-diver species group)

There is no defined AU for this species. The abundance estimates in 2005 / 2007 and 2016 are reported in Table d: the latter is considerably smaller than the former, and could be due to population movement across the wider North Atlantic. Without a defined AU, however, no assessment, qualitative or quantitative, is possible.

Beaked whales (all species; deep-diver species group)

There is no defined AU for this species. The abundance estimates in 2005 / 2007 and 2016 are similar (Table d) with the latter somewhat smaller than the former. The associated coefficient of variation of the two estimates are comparable and result in a large overlap once uncertainty is taken into account. Without a defined AU, no assessment, qualitative or quantitative, is possible.

Sperm whale (deep-diver species group)

There is no defined AU for this species. The abundance estimate in 2016 is reported in Table d. Without a defined AU and with only a single estimate, no assessment, qualitative or quantitative, is possible.

In the Greater North Sea subregion, only harbour porpoise, white-beaked dolphin and minke whale have more than two estimates of abundance, but there is no evidence that numbers have declined (or increased) since 1994. However, only for the harbour porpoise does this subregion maps to a species’ AU. For all other AU of harbour porpoise and all other species of cetaceans (excluding coastal bottlenose dolphins), no quantitative assessment could be carried out, either due to a shortage of abundance estimates or due to no defined AU. Given the limited number of estimates in the time series and the precision of the estimates, the power to detect trends is low.

Qualitative assessments were nevertheless provided when possible and with caveats. A possible decline in abundance of harbour porpoises in the Celtic and Irish Seas AU is suggested by the available evidence: if true, this decline would amount to an annual rate of approximatively -7%, exceeding the threshold of -1,6% for this species (Table b). This decline may be linked to excessive incidental by-catch (Marine Mammal By-catch (Harbour Porpoise, Common Dolphin, Grey Seal)). No change in abundance was evidenced for harbour porpoises in the West Scotland and Ireland AU or for the subspecies occurring in the Iberian Peninsula AU. For many species with defined AUs (short-beaked common dolphin, white-beaked dolphin, minke whales, fin whales) no evidence of change in abundance was found in the available data. In the case of offshore bottlenose dolphins, a large increase in abundance between 2005 / 2007 and 2016 is suggested by the available evidence, although the causes behind such a large increase are unknown but require immigration as the increase exceeds plausible natural rates of increase for a closed population. For species without defined AU, the abundance of long-finned pilot whales decreased in the surveyed OSPAR Maritime Area: this decrease could be due to emigration outside this area, possibly due to spatial variation in prey availability (Hammond et al., 2021).

Currently, bottlenose dolphins occupying coastal waters of western Europe are estimated to number at 3 000 to 4 000 individuals. In assessment units for which there are sufficient data, estimated trends show little change, with the exception of the Sado Estuary population within the Coastal Portugal AU, which continues to decline. Several estuaries and enclosed bays that were historically inhabited by bottlenose dolphin populations are no longer inhabited by this species. Some of these losses occurred over a century ago, while others are more recent. Coastal bottlenose dolphins disappeared from the Southern North Sea around the late 1960s.
Some local populations comprise 200 to 400 individuals, although some have less than 50. The Sado Estuary population is particularly vulnerable to local extinction given both its small population size and steady decline.

Confidence Statements

The assessment is undertaken using data on marine mammal abundance with a spatial coverage and temporal extent that is mostly sufficient for the area assessed. There remain substantial gaps in coverage resulting in a low data availability for offshore areas and in the winter season in particular. For these reasons, data availability is low to moderate.
The data availability for coastal bottlenose dolphins is low to moderate. Data on abundance are available for a limited number of coastal populations, and for a limited time period.
There is moderate / low confidence in the methodology used in this assessment. There is consensus within the scientific community regarding this methodology. However, as the method has been developed specifically for this assessment and not been more widely used it is considered moderate / low.

Time series of information to assess status

Many cetacean populations range over very large areas and, even though some species are very abundant, these large ranges mean that densities are typically very low. In addition, all cetaceans spend the large majority of their lives underwater and so are difficult to observe. Surveys to obtain robust information over the large scale are both logistically challenging and expensive to undertake and, as a result, there have been few in the region. The large-scale surveys covering a large proportion of North-East Atlantic waters have been organised only four times: SCANS in 1994, 2005 and 2016; CODA in 2007; A fourth SCANS survey is currently considered for the summer 2022. Although there have been some additional valuable and more frequent systematic aerial surveys generating robust information for the southern and central North Sea (Gilles et al., 2016, Nachtsheim et al. 2021), in general the time series of coordinated purpose-designed surveys for cetaceans is still too short to allow status to be assessed in relation to both indicators of abundance and distribution. The preferred option would be to establish a European Atlantic-wide framework to determine abundance and trends in cetacean populations to provide a mechanism for using appropriate available and newly commissioned data.

For species such as killer whales with small-scale features in distribution and abundance, additional studies are needed to estimate abundance using e.g. photographic capture-recapture (Jourdain et al., 2019).

For species with an offshore distribution that occur outside the Greater North Sea, Celtic Seas, and Bay of Biscay and Iberian Coast, coordination with other survey programmes such as the six-yearly NASS surveys in central North Atlantic waters and the Norwegian ‘mosaic’ NILS surveys that cover the North- East North Atlantic is desirable.

Another option is to further pursue a coordinated approach to collate and standardise effort-related cetacean sightings data across the OSPAR Maritime Area. This is currently being undertaken by the Joint Cetacean Data Protocol (JCDP; https://jncc.gov.uk/our-work/joint-cetacean-data-programme/) whose aims are to develop (i) an international platform to host cetacean survey datasets from the North-East Atlantic and (ii) a data standard. The JCDP aims to streamline the process of accessing and utilising cetacean survey data for a wide range of applications and its launching in 2022 will facilitate future data calls.

Lack of seasonal information at an appropriately large scale

For logistical reasons, large-scale surveys such as SCANS have been conducted during summer. Information is therefore lacking on large-scale seasonal changes in distribution. However, seasonal variation in distribution and abundance at a smaller spatial scale is available from some national survey programmes (e.g., Belgium, Denmark, France, Ireland, the Netherlands, Germany). In some cases, these surveys are coordinated. To obtain better information about seasonal variation in distribution and abundance, consideration needs to be given to the extension to wider areas and to the better coordination of smaller-scale surveys in time and space. The Joint Cetacean Protocol initiative could also be valuable in this respect. The data collated by OMMEG clearly illustrate this imbalance in sampling with respect to season (Figure ab).

Information on historical distribution and abundance of coastal bottlenose dolphins are scarce or lacking. There are some published accounts, but much of the information is anecdotal.  Assessments can only be made for five AUs of coastal bottlenose dolphin as monitoring has not been undertaken for a sufficiently long period of time (i.e., at least ten years, with a minimum of four abundance estimates during that period; Table d).

Assessment Units

Several species have no defined AU which prevented Indicator Assessment. AU delineation is required to enable consistent monitoring and interpretation of variations in abundance over time. This is clearly challenging for wide-ranging and highly mobile species: for example, the decrease in long-finned pilot whales and the increase in offshore bottlenose dolphins between 2005 / 2007 and 2016 are too large in magnitude for closed populations. Migration in and out of the area are likely to contribute to the observed changes.

Defining AUs at an appropriate scale for bottlenose dolphin is challenging. While the bottlenose dolphin was subdivided into two groups (coastal and offshore) in future assessments the species could be divided into three groups related to their patterns of mobility and habitat use: truly resident in a small area; coastal - ranging over a larger area; and oceanic or offshore. The connectivity between the different groups is poorly understood, although they are considered to be distinct populations. As a result, within the AUs identified for coastal bottlenose dolphins, the smaller resident populations have often been included. In addition, where coastal and offshore populations mix, it is often difficult to identify which population has been surveyed.

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