Abundance and Distribution of Coastal Bottlenose Dolphins
D1 - Biological Diversity
D1.1 - Species distribution D1.2 - Population size D4.3 - Abundance/distribution of key trophic groups/species
Key Message
Coastal bottlenose dolphin populations declined through the 19th and 20th century and have remained low, but stable, in the 21st century. However, the population in the Sado Estuary (Portugal) has declined since monitoring began (1980s). Abundance and distribution of bottlenose dolphins (as top predators) is indicative of environmental health.
Area assessed
Printable Summary
Background
Cetaceans are an important component of marine biodiversity. In European waters there is a large wide-ranging population of offshore bottlenose dolphin (Figures 1-3), as well as several much smaller coastal populations.
Figure 1: Bottlenose dolphin (Tursiops truncatus) mother and calf (courtesy of PGH Evans)
This assessment considers changes in abundance and distribution of coastal populations of bottlenose dolphin; offshore bottlenose dolphins are considered in the assessment of
Coastal populations of bottlenose dolphins reside in relatively small areas, close to shore. They have the potential to be exposed to a greater level of human activity due to their proximity to humans and due to the small size of the area they inhabit.
Bottlenose dolphins are long-lived top predators and are highly susceptible to change in their environment. Changes in abundance and distribution provide important information on the state of the population. Several populations of coastal bottlenose dolphin have been monitored for several decades, whereas monitoring for most is relatively recent (last ten years) or consists only of anecdotal information.
Bottlenose dolphins are vulnerable to the accumulation of pollutants through the food chain and to local disturbance from shipping, tourism, industrial development, and incidental . Underwater noise can have long and short-term effects on cetaceans (such as hearing loss or displacement from an area), but it is unclear to what extent coastal bottlenose dolphins are affected. The conservation status of bottlenose dolphin is assessed under the European Union Habitats Directive (Council Directive 92/43/EEC).
Figure 2: Bottlenose Dolphin (Tursiops truncatus) eating an eel, Cardigan Bay, Wales (courtesy of PGH Evans)
Figure 3: Pod of Bottlenose dolphins (Tursiops truncatus) in Cardigan Bay, Wales (courtesy of PGH Evans)
Bottlenose dolphins (Tursiops truncatus) are long-lived top predators, and are among the most iconic and recognisable of the cetacean species. Their abundance and distribution are indicative of specific aspects of the status of the marine environment, such as food web integrity and pollutant load.
Bottlenose dolphins occur in coastal waters of Spain, Portugal, north-west France, west and south Ireland (including a genetically distinct population in the Shannon Estuary and a more wide-ranging coastal population that moves along the west coast), north-east Scotland (particularly the Moray Firth south to the Firth of Forth), west Scotland, north and west Wales (including all of Cardigan Bay), and parts of the English Channel. In past centuries, the species appears to have regularly occupied the southern North Sea and a number of estuaries within the study area. However populations in these locations are no longer common.
While high mobility of the species facilitates interaction and gene flow over great distances (Hoelzel, 1998; Quérouil et al., 2007), bottlenose dolphins can also display fine-scale genetic population structure resulting from localised adaptations over small spatial scales (Ansmann et al., 2012). Genetic differentiation between neighbouring populations regularly occurs and may be related to habitat borders (Natoli et al., 2005; Bilgmann et al., 2007; Wiszniewski et al., 2009), sex-biased linked dispersal (Möller and Beheregaray, 2004; Bilgmann et al., 2007; Wiszniewski et al., 2010), niche specialisation (Louis et al., 2014a), human activities (Chilvers and Corkeron, 2001), and through isolation by distance without apparent boundaries separating populations (Krützen et al., 2004; Rosel et al., 2009). Consequently, bottlenose dolphins are sub-divided into small discrete coastal populations residing relatively close to shore and a much larger wide-ranging offshore population. This is reflected in the relatively large number of AUs for this species (Figure a). The relationships both within and between those coastal and offshore populations remain unclear (Rosel et al., 2009; Toth et al., 2012; Richards et al., 2013; Louis et al., 2014b). This report covers coastal populations, while the offshore population is considered in the assessment of Abundance and distribution of cetaceans . The reason for addressing the same species in two assessments reflects the differing threats to which each population is exposed (at the coast and offshore), and the differing methodologies used to assess each population. The coastal populations are potentially exposed to a greater level of human activity due to their proximity to humans. Since the 19th century a number of coastal bottlenose dolphin populations have declined or disappeared altogether.
Human Pressures
ICES (2015c) reviewed and prioritised the key human pressures on coastal bottlenose dolphin in the OSPAR Maritime Area. Pressures from human activities may affect bottlenose dolphins directly (Table a) or indirectly by changing the food web structure and thus prey availability. In coastal areas human disturbance is mainly from recreational activities (including commercial dolphin watching), with both short- and long-term impacts noted around the world (Bejder and Samuels, 2003; Bejder et al., 2006) including west Wales (Feingold and Evans, 2014a; Norrman et al., 2015) and east Scotland (Pirotta et al., 2014, 2015). Incidental bycatch of bottlenose dolphins through entanglement in fishing gear also occurs (mainly through gillnets and pelagic trawls) (ICES, 2015a,b). Within the OSPAR Maritime Area, bottlenose dolphin incidental bycatch appears to be highest (and potentially unsustainable) off the coasts of northern Spain (Galicia, Asturias, Cantabria, Basque Country), west Portugal, and south-west Spain (Andalucia) (López et al., 2003, 2012; Goetz et al., 2014; Vázquez et al., 2014; Vélez, 2014; ICES, 2015a). Fishing activities may also indirectly affect populations through depletion of the prey resource (ICES, 2015c). Habitat disturbance through fishing activities causing damage to the seabed and its benthic fauna has been suggested as a human pressure in some areas, although this remains to be supported by evidence (Feingold and Evans, 2014a; Norrman et al., 2015). Habitat loss has also been shown to affect coastal populations (Camphuysen and Peet, 2006; Camphuysen and Smeenk, 2016). Research has shown that high pollutant loads, in most of the investigated coastal bottlenose dolphin populations, can lead to health issues and reproductive failure (Jepson et al., 2013, 2016). Exposure to high pollutant levels has also been suggested as a reason for past declines and the disappearance of some populations (Jepson et al., 2013, 2016). Climate change may also affect bottlenose dolphins (positively and negatively) by altering human activities and thus pressures or by influencing the stock sizes and distribution of their prey. Increased underwater noise may also have negative effects (Bailey et al., 2010).
The pressures listed in Table a represent those considered to have most relevance for marine mammals (OSPAR, 2012; ICES, 2015c). The risks associated with the identified pressures are classified as high, medium or low using the following criteria:
- High: evidence or strong likelihood of negative population effects mediated through effects on individual mortality, health and / or reproduction;
- Medium: evidence or strong likelihood of impact at individual level on survival, health or reproduction. Effect at population level is not clear;
- Low: potential negative impact on individuals but evidence is weak and / or occurrences are infrequent; and
- No Information: cases where there was little or no information on the impact on coastal bottlenose dolphins and cases where the threat was absent or irrelevant for this species.
| Pressure Category | Pressure | Risk |
|---|---|---|
| Pollution & other chemical changes | Contaminants | High |
| Nutrient enrichment | Low | |
| Physical loss | Habitat loss | No information |
| Physical damage | Habitat degradation | Low |
| Other physical pressures | Litter (including microplastics and discarded fishing gear) | Low |
| Barrier to species movement (offshore windfarm, wave or tidal device arrays) | Low | |
| Underwater noise changes | Military sonar | Medium |
| Seismic surveys | Medium | |
| Pile-driving | Medium | |
| Shipping | Medium | |
| Death or injury by collision | Death or injury by collision (with ships) | Medium |
| Death or injury by collision (with tidal devices) | No information | |
| Biological pressures | Introduction of microbial pathogens | Low |
| Removal of target and non-target species (prey depletion) | Low | |
| Removal of non-target species (bycatch) | Low | |
| Disturbance (e.g. wildlife watching) | Medium | |
| Deliberate killing and hunting | No information |
The Indicator
The indicator is based on the trend in abundance per Assessment Unit (AU). As bottlenose dolphins are long-lived animals, with an estimated generation time of 23 years (Taylor et al., 2007), assessments of a temporal trend in the population should be carried out over a relatively long period (e.g. Thompson et al., 2004; Englund et al., 2007). Assessments based on very short time series may give misleading results. In small resident populations, local movement may account for apparent changes in abundance over short time scales. It is recommended to base assessment of trends in the population on a time series covering at least the last ten years, with a minimum of four counts during that period (ICES, 2015c).
Monitoring in several populations 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 (either mark-recapture or in the case of small populations, by direct census), although in their absence, abundance estimates are derived from line-transect surveys. Where possible, annual data on the estimated number of animals by population are provided in this assessment. Consideration is also given to populations known to have disappeared from their former range.
Baselines
Although the baseline should derive from historical data, these data are not available for any cetacean species. Historical abundance and distribution are therefore unknown. Even if numbers are suspected to have declined, they could probably not realistically be restored because today’s marine environment is very different, in part due to climate change and human impact. Consequently, a recent baseline must be used, which should then be assessed as a normal situation, or one that is already known to be degraded. In this assessment, the start of the data time series for each AU has been used as the baseline, with trends identified as a deviation from that baseline value.
Abundance estimates typically have wide confidence values, therefore they may not have the statistical power to detect even relatively strong shifts. Bottlenose dolphins are a long-lived and slow reproducing species, which means there may be significant time lags before impacts on reproductive success can be detected. As such, abundance data should always be considered separately for each AU, along with any associated data on changes in distribution, causes of death in stranded animals, and possible links to human activities.
Quality Status Report 2010
No comparable quantitative assessment of the distribution and abundance of common bottlenose dolphin was included in the OSPAR Quality Status Report (QSR) 2010. The report highlighted human pressures on marine mammals more generally, such as through bycatch, increased shipping and bioaccumulation of persistent hazardous substances.
The conservation status of bottlenose dolphins is also assessed under the European Union Habitats Directive (Council Directive 92/43/EEC), Article 17 reporting in 2013 (EU, 2013).
Assessment Units
Assessment units (AUs) for coastal bottlenose dolphin (Tursiops truncatus) have been determined on the basis of a combination of spatial separation, lack of photo-ID matches and genetic differences (Evans and Teilmann, 2009; ICES, 2013) as outlined by ICES (2014) (Figure a).
Abundance
The abundance for coastal bottlenose dolphin populations has been calculated for each assessment unit where sufficient data exist. Abundance estimates were made largely using Photo-ID capture-recapture methods, and an indication is given about the trend in the population since the start of monitoring: stable, declining, increasing or unknown. At least four abundance estimates from different years should be available before the population trend was assessed. On occasion, pooled estimates have been calculated from a period of years. Some small discrete populations of coastal bottlenose dolphin were assessed by a full census of individuals.
Distribution
Records of sightings and strandings were used to identify where populations of coastal bottlenose dolphins existed historically.
Assessment Value
No assessment value has been applied in this assessment.
Definition of Trends
Declining is defined as a decreasing trend of ≥5% over ten years (significance level p<0.05)
Increasing is defined as an increasing trend of ≥5% over ten years (significance level p<0.05)
Stable is defined as population changes of <5% over ten years
Figure a: Assessment units for coastal bottlenose dolphins
This percentage (i.e. 5%) is derived from the International Union for the Conservation of Nature (IUCN) criterion to detect a 30% decline over three generations for a species, which equates to slightly less than 0.5% per year for Odontocetes.
Results
Coastal bottlenose dolphins are observed along the Atlantic coast of Europe from Scotland in the north to Spain in the south. 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 have been monitored on an annual basis. The most extensive assessment was undertaken on the Sado Estuary population in Portugal (since 1986), and indicates that the population is in decline. Annual mark-recapture estimates for populations in East Coast Scotland (United Kingdom: observed since 1990), indicate that the population is stable and may be showing signs of increase. Estimates from the wider Cardigan Bay (United Kingdom: observed since 2002), the Gulf of St Malo including the Channel Islands (France, United Kingdom: observed since 2010), Ile de Sein (France: observed since 1992) and the Shannon Estuary (Ireland: observed since 1997) indicate broadly stable populations. However, in several of these locations the trend was assessed on fewer than four data points. A summary of available data and population trends is shown in Table 1.
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.
High pollutant loads in individuals leading to health problems and reproductive failure may have contributed to the disappearance of some populations of coastal bottlenose dolphins, and to a decline in bottlenose dolphin numbers in general.
There is low confidence in the method for this assessment and low/moderate for data availability.
| Assessment Unit | Length of time series | Population trend | |
|---|---|---|---|
| ≥10 year | ≥4 abundance assessments | ||
| West Coast Scotland | No | No | No assessment |
| East Coast Scotland | Yes | Yes | Possible Increase/Stable |
| Coastal Wales | Yes | Yes | Stable |
| Coastal Ireland | Yes | Yes | Stable |
| Coastal Southwest England | No | No | No assessment |
| Coastal Normandy and Brittany | No | Yes | Increase/Stable (indicative) |
| Northern Spain | No | No | No assessment |
| Southern Galician Rias (Spain) | No | No | No assessment |
| Coastal Portual | No | No | No assessment |
| Coastal Portual (Sado Estuary) | Yes | Yes | Decline |
| Gulf of Cadiz | No | No | No assessment |
It should be noted that the Sado Estuary population is considered to be a separate AU to the Coastal Portugal AU. The length of time series indicates whether the monitoring requirements have been met. If the time series is less than ten years in length and has fewer than four years of abundance estimates, no assessment was undertaken. Population trend indicates the result of the assessment (if undertaken).
There is estimated to be a minimum of 2700 coastal bottlenose dolphins Tursiops truncatus in the area assessed, with the overall population most likely to be between 3000 and 4000. However, separating coastal bottlenose dolphins from offshore populations remains a challenge in some areas. Recent population estimates and trends for coastal populations are summarised by Assessment Unit (AU) as follows:
West Coast Scotland (United Kingdom)
A small resident bottlenose dolphin population of around 15 animals inhabits the vicinity of the Sound of Barra in the Outer Hebrides (Grellier and Wilson, 2003; Cheney et al., 2013) while an estimated 30 bottlenose dolphins range around the Inner Hebrides spending time around Islay, the Small Isles, Skye, and occasionally the Minch north of Skye (Cheney et al., 2013). There are too few data to determine population trends, although numbers appear to have been stable over the past two decades.
East Coast Scotland (United Kingdom)
Monitoring of bottlenose dolphins in the inner Moray Firth began in 1990, and was later extended to cover a wider part of the Firth. Even though bottlenose dolphins ranged all along the north and south coasts of the Moray Firth during the 1990s, it was not until the mid-1990s that the species began to extend its range around the Grampian coast (Evans et al., 2003; Wilson et al., 2004). It is now regularly seen particularly off Aberdeen, the coast of Fife and in St Andrews Bay (Weir and Stockin, 2001; Cheney et al., 2013). Bottlenose dolphins, some of which have been photo-identified as belonging to the Moray Firth population, are now seen annually along the coast of north-east England as far south as Yorkshire (Sea Watch Foundation, unpubl. data).
Analysis of mark-recapture studies using a Bayesian approach estimates the population on the east coast of Scotland at 87–208 animals, with the latest estimate (2014) being 170 (95% Highest Posterior Density interval: 139–200). Despite interannual variability, the population is considered to be stable and may be showing signs of increase (Figure b: Cheney et al., 2014).
Figure b: Yearly abundance estimates for the population of bottlenose dolphins along the east coast of Scotland
Dashed lines show the 95% upper and lower confidence interval.
Although bottlenose dolphins are occasionally recorded offshore in the North Sea and in coastal waters off south-east England, northern France, Belgium, the Netherlands and Germany, there is no evidence that these are anything other than transient animals, probably from the east coast of Scotland population or further afield (Evans et al., 2003; Camphuysen and Peet, 2006; Evans and Teilmann, 2009; ICES, 2013).
Coastal Wales (United Kingdom)
Annual monitoring of bottlenose dolphins in Cardigan Bay Special Area of Conservation (SAC), west Wales (United Kingdom), began in 2001. This was extended to incorporate the wider Cardigan Bay area from 2005. In addition, since 2007, there have been opportunistic photo-ID surveys in the coastal waters of north Wales, and occasionally around the Isle of Man and in Liverpool Bay (Pesante et al., 2008; Feingold and Evans, 2014a; Norrman et al., 2015). A proportion of the population inhabiting Cardigan Bay in summer ranges more widely between November and April, occurring in particular off the north coast of Anglesey, the mainland coast of north Wales and further north around the Isle of Man (Feingold and Evans, 2014b). Summer mark-recapture estimates for Cardigan Bay SAC vary from 116 to 260 animals. The latest estimate (2015) is 159 animals (95% confidence interval (CI): 130–228). For the wider Cardigan Bay, summer mark-recapture estimates vary from 152 to 342 animals, with the 2015 estimate being 222 animals (95% CI: 184–300). The coastal Wales population is considered to be stable (Figure c). It should be noted that between 2013 and 2015, the population estimates have been among the lowest recorded but due to variability in the estimates it is too early to determine whether this indicates a decline.
Figure c: Yearly abundance estimates for the population of bottlenose dolphins in Cardigan Bay Special Area of Conservation (SAC), Wales
Dashed lines show the 95% upper and lower confidence interval.
Figure d: Yearly abundance estimates for the population of bottlenose dolphins in the wider Cardigan Bay area, Wales
Dashed lines show the 95% upper and lower confidence interval.
Coastal Ireland
Bottlenose dolphins are regularly recorded in several bays along the west coast of Ireland, notably Kenmare River and Brandon Bay (County Kerry), Clew Bay and adjacent coastal areas of Connemara (County Galway), Broadhaven Bay (County Mayo), and Donegal Bay (County Donegal) (Ingram et al., 2001, 2003; Evans et al., 2003; Ó Cadhla et al., 2003). They have also been recorded all along the south coast of Ireland, with sightings mainly around Cork Harbour (County Cork) and Rosslare Harbour (County Wexford) (Evans et al., 2003; O’Brien et al., 2009). Photo-ID matches indicate that individual animals may range all around the coast of Ireland, and although there is a more or less continuous distribution from inshore to offshore, there is both photo-ID and genetic evidence for an offshore ecotype west of Ireland (O’Brien et al., 2009; Mirimin et al., 2011; Oudejans et al., 2015). There are a number of mark-recapture population estimates for animals using the west coast of Ireland, but at different spatial scales. One estimate for north-west Connemara is 171 individuals (95% CI: 100–294) in 2009 (Ingram et al., 2009) and a second estimate for a much larger area, including Connemara, Mayo and Donegal, of 151 (95% CI: 140–190) individuals in 2014 (Nykanen et al., 2015). This mobile population appears to range widely, with seasonal and patchy habitat use. There is not enough information to indicate population trends. Additional data not used for this assessment were used by Ireland for the 2013 European Union Habitats Directive (Council Directive 92/43/EEC) Article 17 reporting.
Bottlenose dolphins inhabit the Shannon Estuary all year round, and genetic studies indicate that they form a discrete population separate from those occurring elsewhere along the west coast of Ireland (Mirimin et al., 2011). Six mark-recapture population estimates produced between 1997 and 2015 range from 107 to 140 individuals (Ingram, 2000; Ingram et al., 2008; Berrow et al., 2012). The latest population estimate (2015) is 114 (95% CI: 90–143) (Rogan et al., 2015) indicating that the population is probably stable (Figure e).
Figure e: Yearly abundance estimates for the population of bottlenose dolphins in the Shannon Estuary, Ireland
Dashed lines show the upper and lower confidence interval.
Coastal South-West England (United Kingdom)
Bottlenose dolphins have regularly inhabited the south and south-west coasts of England since the 1990s, being most common around Cornwall but rare east of Dorset (Williams et al., 1997 Evans et al., 2003; Brereton et al., 2017). No systematic photo-ID surveys have been undertaken, but Brereton et al. (2017) have reported maximum abundance estimates for south-west England coastal waters, using two mark-recapture methods, ranging between 102 and 113 individuals (95% CI: 87–142) over the period 2008–2013. There are insufficient data to assess trends.
Coastal Normandy and Brittany (France / United Kingdom)
A resident population of bottlenose dolphins inhabits the Gulf of St Malo, ranging between the French coast of Normandy and the Channel Islands (Couet et al., 2015a,b; Louis et al., 2015). Mark-recapture estimates of this population in 2010 showed it numbering between 372 (95% CI: 347–405) and 319 (95% CI: 310–327) individuals, with a 2014 estimate of 340 (95% CI: 290–380) (Couet et al., 2015a,b; Louis et al., 2015), thus indicating no significant difference (Figure f).
Figure f: Yearly abundance estimates for the population of bottlenose dolphins in the Gulf of St Malo, France
Dashed lines show the upper and lower confidence interval.
Two small populations, which appear to be distinct, exist in the Iroise Sea, which is an OSPAR Marine Protected Area (MPA). One population is located around Ile de Sein and the other is located around the Molène Archeplago. Photo-ID surveys have been undertaken in the vicinity of Ile de Sein since 2001, ranging from 20 (2001) to 29 (2014) individuals. The earliest estimate for this population was 14 (1992), thus indicating a steady increase (Figure g) (Liret, 2001; Liret et al., 2006).
Figure g: Yearly abundance estimates for the population of bottlenose dolphins around Ile de Sein, France (counts only).
Around the Molène Archipelago, a mark-recapture estimate of 29 individuals (95% CI: 28–42) was produced from photographs taken between 1999 and 2001 (Le Berre and Liret, 2004; Liret et al., 2006; Louis and Ridoux, 2015). A new photo-ID analysis is currently being undertaken (V. Ridoux, CNRS, France, pers. comm.). It is currently not possible to assess trends in this Molène Archipelago population.
Northern Spain
In northern Spanish waters, only model-based abundance estimates exist, derived from line-transect surveys conducted between 2003 and 2011. These encompass both coastal and offshore animals (López et al., 2013). The annual uncorrected abundance estimate in the study area is 10,687 individuals (95% CI: 4,094–18,132). Abundance estimates for the different areas are: Euskadi (1,931 individuals), Cantabria (744), Asturias (1,214), Galicia (703), Galician Bank (108) and Aviles (234).
Although the distribution is homogeneous throughout the northern peninsula, there is a clear gradient in density, this being higher in eastern areas of the Bay of Biscay where the largest groups have been recorded (López et al., 2013). There are insufficient data to assess trends.
Southern Galician Rias (north-west Spain)
Photo-ID surveys were conducted along the Galician coast between 2006 and 2009, resulting in the identification of 255 individuals (García et al., 2011). A third of these (76 individuals) were considered to form the resident population inhabiting the Southern Galician Rias, as revealed by recapture histories, genetic studies and stable isotope analysis (Fernández et al., 2011a,b; García et al., 2011). Movements of individuals were recorded between Galicia and Euskadi in the Bay of Biscay (García et al., 2011). It is not currently possible to assess trends in this population.
Coastal Portugal
Bottlenose dolphins occur widely along the coast of Portugal as well as further offshore. Photo-ID surveys undertaken over two periods have been used to derive mark-recapture population estimates of bottlenose dolphins in coastal Setúbal Bay (Martinho, 2012; Martinho et al., 2015). Bottlenose dolphins identified between 1998 and 2001 were considered a closed and more cohesive group than those identified between 2007 and 2011, with stable associations and an abundance of 106 individuals (95% CI: 69–192). The more recent animals sampled seemed to comprise an open group of 108 individuals (95% CI: 83–177), with a migration rate of 19% per year and low association values.
A wider-scale analysis of animals photographed in central west coastal Portugal from Nazaré and Setúbal Bay between 2008 and 2014 resulted in an estimate of 352 individuals (95% CI: 294–437) (Martinho, 2012; Martinho et al., 2015).
There have been a number of line-transect surveys by both ship and plane undertaken off west Portugal between 2010 and 2014, covering the region between the coast and 50 nautical miles offshore. The estimated abundance for 2010–2014 from aerial surveys was 2,306 animals (34.7% coefficient of variation, CV), and 3,798 animals (87.6% CV) for 2011 using vessel surveys (Araújo et al., 2014). It is not possible to distinguish between the coastal population of the coast of Portugal AU and the offshore population.
The longest sequence of counts for a coastal bottlenose dolphin population in Europe is associated with the resident population in the Sado Estuary, where an annual census has been undertaken since 1986 (Gaspar, 2003; Lacey, 2015). Over this period, the population has shown a long-term decline from 39 individuals in 1986 to 28 individuals in 2014 (Figure h) (Lacey, 2015), with pollution of the estuary proposed as a possible cause (Van Bressem et al., 2003).
Figure h: Yearly abundance estimates for the population of bottlenose dolphins in the Sado Estuary, Portugal
Gulf of Cadiz (Spain)
Mark-recapture estimates for bottlenose dolphins in the coastal Gulf of Cadiz have been determined for two periods: 2005–2006 and 2009–2010 (MAGRAMA, 2012). These gave estimates of 347 individuals (95% CI: 264–503) for 2005–2006 and 397 individuals (95% CI: 300–562) for 2009–2010, suggesting no significant difference. A much larger population appears to occupy the offshore Gulf of Cadiz, estimated at 4391 individuals (95% CI: 2373–8356) for 2009–2010 (MAGRAMA, 2012). It is not possible to assess trends.
A bottlenose dolphin population also inhabits the area around the Strait of Gibraltar, on the edge of the Bay of Biscay and Iberian Coast. Photo-ID surveys in 2010 resulted in a mark-recapture population estimate of 297 individuals (95% CI: 276–332) (Portillo et al., 2011). This estimate can be used as a baseline for an assessment of trends.
Historic population losses of bottlenose dolphin
Since the 19th century several coastal bottlenose dolphin populations have declined or disappeared altogether, such as the decline that occurred until the end of the 1960s in the Southern North Sea, along the shores of northern France to the north of the Netherlands. This population appears to have used some coastal areas, such as the Marsdiep area, for only limited periods of time. The species was recorded regularly in the Marsdiep area and the area east of Texel (the Netherlands), and in relatively large numbers (up to 30 to 40 individuals at a time), between 1933 and 1939 by Verwey (1975), mainly between February and May, coinciding with the migration and spawning period of the Zuiderzee herring. After the closure of Zuiderzee Bay in 1932, the Zuiderzee herring gradually disappeared from the area, and in the late 1930s the regular occurrence of relatively large numbers of bottlenose dolphins ceased. Observations outside the Marsdiep area between the 1930s and 1970 are anecdotal, but the species was regarded as common in all Dutch waters and estuaries, second only to harbour porpoise (Phocoena phocoena) (Camphuysen and Peet, 2006; Camphuysen and Smeenk, 2016). After 1970, the species became scarce in Dutch waters, with strandings also declining rapidly (Figure i) (Kompanje, 2001; Camphuysen and Peet, 2006; Camphuysen and Smeenk, 2016). This occurred at a similar time to the reduction in strandings recorded in south-east and south-west England (Evans, 1980; Tregenza, 1992).
Figure i: Bottlenose dolphins stranded in the Netherlands since 1900
Earlier status changes are difficult to ascertain but historical accounts indicate that bottlenose dolphins occurred in the Severn, Thames and Humber estuaries of England, and in the Firth of Forth in Scotland until the late 19th century (Evans and Scanlan, 1990; Nichols et al., 2007). Along the coast of Germany, bottlenose dolphins occurred in the Elbe (Goethe, 1983) and Weser estuaries (Mohr, 1935; Goethe, 1983; Kölmel and Wurche, 1998) until the late 19th century. Further south, in Portuguese waters, a resident group of bottlenose dolphins was reported in the Tagus Estuary until 1960 (Teixeira, 1979).
Bottlenose dolphins appear to have formed ephemeral populations, using some coastal areas for limited periods. For example, a group of dolphins inhabited the Noirmourtier area (France) in the 1950s and 1960s, with similar reports made for the Quiberon-Houat-Hoedic area (France). It is unclear whether these were truly resident coastal populations or offshore visitors that remained in the areas for a limited period. A coastal group persisted at Arcachon (France) between the late 1980s until it disappeared in the early 2000s, and a group of six individuals occurred at Pertuis Charentais (France), between Ile de Ré and Ile d’Oléron and the French mainland, for a period in the late 1990s (V. Ridoux, CNRS, France; O. van Canneyt, and W. Dabin, Université de La Rochelle-CNRS, France pers comm.).
Confidence Assessment
The assessment is undertaken using limited data with poor spatial coverage within the area assessed. Some of the methods used in the assessment are well-established. However, there are uncertainties over the delineation of assessment units. Therefore, there is low confidence in the method for this assessment and low / moderate confidence in data availability.
Conclusion
Most populations of coastal bottlenose dolphins in the areas assessed are relatively small. In many coastal areas of the North-East Atlantic Ocean, populations declined or disappeared completely during the 19th and 20th centuries. Where trends could be assessed, the remaining populations show little long-term change with the exception of the declining population in the Sado Estuary in Portugal. The reasons for the decline in the Sado Estuary are unknown but could be related to estuarine pollution.
Bottlenose dolphins are vulnerable to the effects of persistent organic pollutants, with high levels occurring through bioaccumulation potentially inhibiting reproduction. Disturbance by recreational activities, such as whale watching, underwater noise , collision with ships and commercial fisheries are also identified as pressures for bottlenose dolphins.
The current bottlenose dolphin (Tursiops truncatus) population occupying coastal waters of western Europe is estimated at 3,000 to 4,000 individuals. The majority of populations for which there are sufficient data to estimate trends show little change, with the exception of the Sado Estuary population in Portugal, 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.
The consequences of human activities, particularly fisheries bycatch, prey depletion, habitat deterioration (including pollutants), and disturbance from recreational activities (including commercial dolphin watching) on these populations are unclear. Coastal bottlenose dolphin populations can have an ephemeral nature, potentially related to prey availability. The consequences of habitat alterations due to climate change on coastal bottlenose dolphin populations are unknown.
Knowledge Gaps
Historical data on abundance and distribution of coastal bottlenose dolphins are either scarce or lacking. As a result, assessment was only possible for five populations of coastal bottlenose dolphin, with an indicative assessment provided for one other population. The time series of monitoring data was too short to undertake an assessment for the remaining populations. The connectivity between coastal bottlenose dolphins and wider-ranging offshore populations remains unclear. The impacts of human activities on these populations remain to be studied. Some coastal populations might be ephemeral.
Information on historical distribution and, in particular, abundance is scarce or lacking. There are some published accounts, but much of the information is based on anecdotal accounts. Although historical reviews are currently being undertaken in some areas, it is unlikely that such information will become available in sufficient detail to qualify as a baseline.
Assessments can only be made for five coastal populations of bottlenose dolphin (Tursiops truncatus) as the monitoring in many assessment units (AUs) has not been undertaken for a sufficiently long period of time (i.e. at least ten years, with a minimum of four assessments during that period) (Table 1). An indicative assessment was made for another population where there had been four abundance estimates, but over a period of less than ten years.
Defining AUs at an appropriate scale for bottlenose dolphin is challenging. While the bottlenose dolphin was subdivided into two groups for the Intermediate Assessment (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.
Human activities have potential to affect coastal bottlenose dolphins. However, the relationship between human activities (e.g. disturbance, pollutant loads, overfishing, habitat alteration) and their impact on bottlenose dolphin populations requires further study.
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| Sheet reference | BDC17/D102 |
|---|---|
| Assessment type | Intermediate Assessment |
| Context (1) | Biological Diversity and Ecosystems - Targeted actions for the protection and conservation of species, habitats and ecosystem processes |
| Context (3) | D1 - Biological Diversity |
| Context (4) | D1.1 - Species Distribution, D1.2 - Population Size |
| Point of contact | OSPAR Secretariat |
secretariat@ospar.org | |
| Metadata date | 2016-03-31 |
| Title | M-4 A: Abundance and Distribution of Coastal Bottlenose Dolphins |
| Resource abstract | Common indicator assessment of M-4 Abundance and distribution of cetaceans: coastal bottlenose dolphins , Applicable to the Greater North Sea, Celtic Seas and the Bay of Biscay and Iberian coast (OSPAR Regions II, III and IV). |
| Linkage | http://www.iucnredlist.org/details/15421/0 |
| Topic category | Environment |
| Indirect spatial reference | L3.2.2.1;L3.3.1.1;L3.3.2.10;L3.3.2.13;L3.3.3.1;L3.3.3.2;L3.3.3.3;L3.3.3.4;L3.3.3.5;L3.3.3.6;L3.3.3.7;L3.3.3.8;L3.3.3.9;L3.3.3.11;L3.3.3.12;L3.3.3.13;L3.3.3.14;L3.3.3.15;L3.3.3.16; |
| N Lat | 58.8257468522355 |
| E Lon | 2.54486264569459 |
| S Lat | 36.0002573554926 |
| W Lon | -10.9863393542777 |
| Countries | BE, DE, ES, FR, IE, NL, PT, UK |
| Start date | 1985-01-01 |
| End date | 2016-12-31 |
| Conditions applying to access and use | TandCs for assessment and data: |
| Data Snapshot | https://odims.ospar.org/documents/197/download |
| Data Results | https://odims.ospar.org/documents/198/download |