Rias of Galicia IMMA

Size in Square Kilometres

1,346 km2

Qualifying Species and Criteria

Common bottlenose dolphin – Tursiops truncatus

Criterion B (1)



The Galician Rias in northwest Spain are of exceptional ecological significance for common bottlenose dolphins (Tursiops truncatus). The Galician Rias host one resident coastal bottlenose dolphin population, with strong group associations, unusually large aggregations, and seasonal patterns reflecting the population’s dependency on the unique habitats provided by these inlets and coastal embayments. The Galician Rias serve as essential areas for reproduction, confirmed by observed newborn dolphins and lactating females. At the same time, the Rias offer highly productive feeding grounds year-round, where dolphins display adaptable foraging strategies. The bottlenose dolphins in these areas display unique behaviors and low genetic diversity, while also being exposed to significant threats, including bycatch, overfishing, marine pollution, and aquaculture impacts, necessitating urgent conservation efforts.

Description of Qualifying Criteria

Criterion B: Distribution and Abundance

Sub-criterion B1: Small and Resident Populations

The Galician Rias host one resident population of coastal bottlenose dolphins, as extensively documented by various studies (López et al., 2002; Díaz López et al., 2008; Pierce et al. 2010; Díaz López & Methion, 2017; Díaz López & Methion, 2018; López et al., 2019; Methion & Díaz López, 2018; Methion & Díaz López, 2019a; Methion & Díaz López, 2019b; Methion & Díaz López, 2021; Methion et al., 2023). Through long-term photo-identification and mark-recapture methods spanning over two decades, these studies have provided a wealth of insights into the characteristics of this resident population. Individual characteristics, including age and sex, as well as ecological and social preferences, may explain the movement of individuals across the Galician rias (Methion & Díaz López, 2018, 2019b). Notably, the presence of photo-identified bottlenose dolphins in the Galician Rias for periods exceeding 20 years underscores the enduring nature of their residency. This extended timeframe of observation enhances our understanding of their long-term interactions, behaviors, and adaptations within the unique environment of the Galician rias.

Bottlenose dolphins are present in the Galician Rias throughout the entire year, and there are no substantial seasonal fluctuations in their presence. Research has consistently shown that the largest Galician Rias, including Arousa, Vigo, Pontevedra, Ares-Betanzos, and Muros-Noia serve as primary aggregation sites for most of the individuals of the resident population (López et al., 2004; Díaz López et al., 2008; Methion & Díaz López, 2018, 2020; López et al., 2019). Studies reveal that the movement of individuals between Rias within the IMMA may vary seasonally, with lower movement during the winter months (Methion & Díaz López, 2018). The data collected from various surveys and transects in the largest and most productive Galician ria (Ria of Arousa) reveal an average of 1.4 sightings per hour. In contrast, outside the rias, the presence of bottlenose dolphins is less frequent. The surveys and transects conducted outside the rias report an average of 0.1 sightings per hour. These few observations are characterized by the movement of groups between Rias or the presence of transient bottlenose dolphins with a more pelagic behavior. In a comprehensive study conducted by the Bottlenose Dolphin Research Institute between 2014 and 2022, it was observed that 94% of the bottlenose dolphin groups were located within the Rias, reaffirming their strong association with these inlets (Díaz López et al., 2023). Moreover, photo-identification studies carried out to date have further confirmed the regular movement of individuals between the Rias and their preferences for these habitats (Díaz López et al., 2008; López et al., 2019; Díaz López et al., 2023). A recent ongoing study conducted by researchers at the BDRI Institute for the Spanish government between 2023 and 2025, in its first year of observations, indicated that the estimates of abundance, based on mark-recapture techniques (photo-identification) using the robust Pollock method, were 391 ± 25 bottlenose dolphins for the whole Galician Rias.

Ecological and genetic distinctions are evident between bottlenose dolphins stranded within the Galician Rias (suggested as a coastal ecotype) and those stranded outside the Rias. This differentiation is supported by studies (Fernández et al., 2011a, b). The existence of these two ecotypes is further substantiated by the high variability in isotopic values (δ13C) observed in bottlenose dolphins along the Galician coast (Méndez-Fernández et al., 2012, 2013). The low levels of genetic diversity observed in bottlenose dolphins stranded in the Galician Rias suggest potential restrictions on gene flow with adjacent sites, despite the absence of physical barriers (Fernández, 2011b).

The interplay between social and oceanographic conditions, both on a small and large scale, directly shapes the spatio-temporal aggregation patterns of common bottlenose dolphins within the Galician Rias (Methion et al., 2023). Abundance estimates of bottlenose dolphins in Galician waters are subject to fluctuations influenced by oceanographic variables. Water oxygenation, chlorophyll-a concentration, and the North Atlantic Oscillation (NAO) index play pivotal roles in determining the spatio-temporal aggregation patterns of these dolphins (Methion et al., 2023). The observed aggregations in the Galician Rias, with an average group size of 10.7 ± SE 0.3, are larger than the average group size in several other coastal bottlenose dolphin populations in different regions (e.g., California, Shannon Estuary, Kvarneric Bay, Sarasota, Sardinia Island). Notably, the group size in the Galician Rias is comparable to or even exceeds that of some other populations with larger average group sizes (e.g., Moray Firth, Golfo San José, Doubtful Sound).

Bottlenose dolphins utilize Galician rias as a nursery area, where evidence of newborn dolphins has been observed from June to September (Díaz López et al., 2018; Methion, 2021; Methion et al., 2023). This observation indicates that the Galician rias are a critical location for calving, with the majority of calving events occurring during the summer (Methion, 2021). Evidence of infanticides during the summer months (July-September), supported by post-mortem examinations, further substantiates the significance of these estuaries as calving and nursery areas (Díaz López et al., 2018). Evidence from stranded pregnant and/or lactating females, as well as stranded neonates and juveniles, confirms that bottlenose dolphins give birth and provide care for their young in Galician rias. This nurturing process continues until the calves are weaned (Fernández et al., 2011b; Read, 2014; Díaz López et al., 2017). Dependent calves, including both immature and newborn dolphins, are present in 55% of the observed groups (Methion, 2019; Methion et al., 2023a).

The productive waters of the Galician Rias serve as vital foraging grounds for common bottlenose dolphins, offering an abundance of high-quality food resources. The Galician Rias are recognized for their rich food resources, both in terms of quality and quantity. Studies have documented the importance of these productive waters in supporting the dietary needs of bottlenose dolphins, indicating that the region provides a reliable source of high-quality prey species (Díaz López et al., 2008; Díaz López & Methion, 2017; Methion & Díaz López, 2019; Methion et al., 2023a). The Galician Rias not only offer a bountiful and diverse array of food resources for common bottlenose dolphins but also highlight their adaptability in navigating human-altered environments. The interactions with shellfish aquaculture areas and the application of specific foraging strategies underscore the resourcefulness of these dolphins within their feeding grounds.

Studies in Galician rias have revealed that bottlenose dolphins employ various foraging techniques, including cooperative feeding. These techniques involve a high degree of social organization and behavioral adaptation, emphasizing the adaptability of these dolphins in obtaining their food resources (Methion and Díaz López, 2019, 2020). Shellfish aquaculture within the Rias has introduced spatial habitat complexity. This complexity has led to variations in resource distribution and abundance, effectively fragmenting the habitat of bottlenose dolphins (Díaz López & Methion, 2017). Some individual bottlenose dolphins have adapted to these changes, frequently utilizing shellfish farm areas as foraging grounds. They employ specific foraging strategies to catch prey within the floating rafts, benefiting from a reliable and easily located food source. This results in lower energetic and time expenditure during foraging and potentially increases their energy intake (Methion, 2019; Methion & Díaz López, 2019). Ongoing interactions with shellfish aquaculture may have endowed specific individuals with an intimate understanding of the characteristics of these human-altered areas. This knowledge enables them to efficiently obtain the food resources concentrated within these structures.

Supporting Information

Díaz López B, Shirai JAB, Bilbao Prieto A, Méndez Fernández P (2008). Diving activity of a solitary wild free ranging bottlenose dolphin (Tursiops truncatus). Journal Marine Biological Association of U.K 88, Special Issue 06: 1153-1157.

Diaz Lopez B. and Methion S., 2017. The impact of shellfish farming on common bottlenose dolphins’ use of habitat. Marine Biology 164: 83.

Díaz López, B & Methion, S., 2018. Does interspecific competition drive patterns of habitat use and relative density in harbour porpoises? Marine Biology (2018) 165:92.

Díaz López, B., López, A., Methion, S., & Covelo, P. (2017). Infanticide attacks and associated epimeletic behaviour in free-ranging common bottlenose dolphins (Tursiops truncatus). Journal of the Marine Biological Association of the United Kingdom, 98(5), 1159-1167.

Díaz López, B., Methion, S., Giralt Paradell, O., 2019. Living on the edge: Overlap between a marine predator’s habitat use and fisheries in the Northeast Atlantic waters (NW Spain). Progress in Oceanography. 175, 115–123.

Díaz López, B., Methion, S., Mosca, O. & Dunel-Roig, N. 2023. Determinación de áreas de especial sensibilidad para cetáceos y aves marinas en aguas del sur de Galicia. Informe del Bottlenose Dolphin Research Institute para Portos de Galicia, O Grove. 71pp.

Evans, G., & Prego, R. (2003). Rias, estuaries and incised valleys: is a ria an estuary? Marine Geology, 196(3), 171–175. https://doi.org/https://doi.org/10.1016/S00253227(03)000483

Fernández, R., Santos, M.B., Pierce, G.J., Llavona, A., López, A., Silva, M.A., Ferreira, M., Carrillo, M., Cermeño, P., Lens, S. and Piertney, S.B. (2011a). Fine-scale genetic structure of bottlenose dolphins (Tursiops truncatus) in Atlantic waters of the Iberian Peninsula. Hydrobiologia 670, 111-125.

Fernández, R., García-Tiscar, S., Santos, M.B., López, A., Martínez-Cedeira, J.A., Newton, J. and Pierce, G.J. (2011b). Stable isotope analysis in two sympatric populations of bottlenose dolphins Tursiops truncatus: evidence of resource partitioning? Marine Biology 158, 1043–1055.

Fernández, R., MacLeod, C.D., Pierce, G.J., Covelo, P., López, A., Torres-Palenzuela, J., Valavanis, V. and Santos, M.B. (2013). Inter-specific and seasonal comparison of the niches occupied by small cetaceans off north-west Iberia. Continental Shelf Research, 64: 88–98.

Giralt Paradell, O., Díaz López, B., Methion, S., 2019. Modelling common dolphin (Delphinus delphis) coastal distribution and habitat use: Insights for conservation. Ocean and Coastal Managament. 179, 104836.

Giralt Paradell, O., Díaz López, B., Methion, S., 2020. Food-web interactions in a coastal ecosystem influenced by upwelling and terrestrial runoff off North-West Spain. Marine Environmental Research 157:104933.

Giralt Paradell O, Methion S, Rogan E, Díaz López B (2021). Modelling ecosystem dynamics to assess the effect of coastal fisheries on cetacean species. Journal of Environmental Management 265 (112175).

Goetz, S., Read, F. L., Begoña, S. M., Pita, C., & Pierce, G. J. (2014). Cetacean–fishery interactions in Galicia (NW Spain): results and management implications of a facetoface interview survey of local fishers. ICES J Mar Sci, 71(3), 604–617. https://doi.org/10.1093/icesjms/fst149

López, A., Santos, M.B., Pierce, G.J., González, AF, Valeiras, X. and Guerra, A. (2002). Trends in strandings and by-catch of marine mammals in northwest Spain during the 1990s. Journal of the Marine Biological Association of the United Kingdom, 82: 513–521.

López, A., Pierce, G.J., Santos, M.B., Gracia, J. & Guerra, A. 2003. Fishery by-catches of marine mammals in Galician waters: Results from on-board observations and an interview survey of fishermen. Biological Conservation 111: 25-40.

López, A., Pierce, G. J., Valeiras, X., Santos, M. B., & Guerra, A. (2004). Distribution patterns of small cetaceans in Galician waters. Journal of the Marine Biological Association of the United Kingdom, 84(1), 283-294.

López A., Martínez Cedeira J., Mariscal P. (2019). Catálogo TurGaSur de fotoidentificación dos arroaces (Tursiops truncatus) de Galicia Sur e manual de boas prácticas de navegación. CEMMA. Fundación Biodiversidad-Ministerio para la Transición Ecológica.

Louis M, Viricel A, Lucas T, Peltier H, Alfonsi A, Berrow S, Brownlow A, Covelo P, Dabin W, Deaville R, de Stephanis R, Gally F, Gauffier P, Penrose R, Silva M, Guinet C, Simon-Bohuet B. 2014. Habitat-driven population structure of bottlenose dolphins, Tursiops truncatus, in the North-East Atlantic. Molecular Ecology 23(4):857-874.

Méndez Fernandez, P., Pierce, G. J., Bustamante, P., Chouvelon, T., Ferreira, M., González, A. F., López, A., Read, F. L., Begoña, S. M., Spitz, J., Vingada, José V, & Caurant, F. (2013). Ecological niche segregation among five toothed whale species off the NW Iberian Peninsula using ecological tracers as multiapproach. Marine Biology, 160(11), 2825–2840. https://doi.org/10.1007/s0022701322749

Méndez Fernandez, P., Webster, L., Chouvelon, T., Bustamante, P., Ferreira, M., González, A. F., López, A., Moffat, C. F., Pierce, G. J., Read, F. L., Russell, M., Santos, M. B., Spitz, J., Vingada, José V, & Caurant, F. (2014a). An assessment of contaminant concentrations in toothed whale species of the NW Iberian Peninsula: Part I. Persistent organic pollutants. Science of the Total Environment, 484, 196–205. https://doi.org/https://doi.org/10.1016/j.scitotenv.2014.02.045

Méndez Fernandez, P., Webster, L., Chouvelon, T., Bustamante, P., Ferreira, M., González, A. F., López, A., Moffat, C. F., Pierce, G. J., Read, F. L., Russell, M., Santos, M. B., Spitz, J., Vingada, José V, & Caurant, F. (2014b). An assessment of contaminant concentrations in toothed whale species of the NW Iberian Peninsula: Part II. Trace element concentrations. Science of the Total Environment, 484, 206–217. https://doi.org/https://doi.org/10.1016/j.scitotenv.2014.03.001

Methion, S. (2019). Ecology and behaviour of bottlenose dolphins (Tursiops truncatus) in a coastal area subject to shellfish farming (378 pp.) [PhD Thesis University of Bordeaux: Écologie évolutive, fonctionnelle et des communautés].

Methion, S., Díaz López, B., 2018. Abundance and demographic parameters of bottlenose dolphins in a highly affected coastal ecosystem. Marine and Freshwater Research 69, 1355.

Methion, S., Díaz López, B., 2019. Natural and anthropogenic drivers of foraging behaviour in bottlenose dolphins: Influence of shellfish aquaculture. Aquat. Conserv. Mar. Freshw. Ecosyst. 29, 927–937.

Methion, S., Díaz López, B., 2020. Individual foraging variation drives social organization in bottlenose dolphins. Behavioral Ecology, 31(1), 97-106.

Methion, S., & Díaz López, B. 2021. Spatial segregation and interspecific killing of common dolphins (Delphinus delphis) by bottlenose dolphins (Tursiops truncatus). Acta ethologica, 24(2), 95-106.

Methion S, Giralt Paradell O, Padín XA, Corrège T, Díaz López B (2023a). Group size varies with climate and oceanographic conditions in bottlenose dolphins. Marine Biology 170:7. Doi.org/10.1007/s00227-022-04154-4

Methion S, Mosca O, Díaz López B (2023b). Epimeletic behavior in a free-ranging female Risso’s dolphin (Grampus griseus). acta ethologica Doi.org/10.1007/s10211-023-00417-5

Pierce, G.J., Santos, M.B., Murphy, S., Learmonth, J.A., Zuur, A.F., Rogan, E., Bustamante, P., Caurant, F., Lahaye, V., Ridoux, V. and Zegers, B.N., 2008. Bioaccumulation of persistent organic pollutants in female common dolphins (Delphinus delphis) and harbour porpoises (Phocoena phocoena) from western European seas: Geographical trends, causal factors and effects on reproduction and mortality. Environmental Pollution, 153(2), pp.401-415.

Pierce GJ, Caldas M, Cedeira J, Santos MB, Llavona A, Covelo P, Martinez G, Torres J, Sacau M, López A (2010) Trends in cetacean sightings along the Galician coast, north–west Spain, 2003–2007, and inferences about cetacean habitat preferences. J Mar Biol Assoc UK 90 (Special issue 08):1547–1560.

Santos, M.B., Fernández, R., López, A., Martínez, J.A., Pierce, G.J.. 2007. Variability in the diet of Bottlenose dolphin, Tursiops truncatus, in Galician waters, North Western Spain, 1990– 2005. Journal of the Marine Biological Association of the United Kingdom 87 (01):231-241.

Spyrakos, E., Santos-Diniz, T.C., Martinez-Iglesias, G., Torres-Palenzuela, J.M. and Pierce, G.J. (2011). Spatiotemporal patterns of marine mammal distribution in coastal waters of Galicia, NW Spain. Hydrobiologia, 670: 87–109.

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