Aim To describe distribution patterns and species richness of cetaceans along a wide geographical range using occurrence data coupled with survey effort, from poorly studied oceanic areas. Specific objectives were to compare species richness and relative abundances among sub‐regions and to describe the distribution of each species. Location Eastern North Atlantic. Time period 2012–2017. Major taxa studied Cetacea. Methods Cetacean monitoring was performed by dedicated observers from cargo ships, used as platforms of opportunity, along routes between Iberian Peninsula, Macaronesia and north‐western Africa. We mapped relative abundance (encounter rates), survey effort and species richness. We examined the dependence of the number of sightings and species richness on survey effort. The area was divided into sub‐regions (according to the Exclusive Economic Zones and international waters), and relative abundances of the eight most frequently sighted species, as well as species richness, were compared among them. In addition, we describe the distribution of each species in relation to sea depth, distance to coast, latitude and longitude. Results A total of 1,989 sightings were logged, and 26 cetacean species were identified. Species richness and relative abundances of the eight most common species differed substantially between sub‐regions. Common and bottlenose dolphins distributed in shallow coastal waters contrasting with the oceanic distribution of Stenella dolphins. Cuvier's beaked whale and minke whale had similar distributions. Pilot and sperm whales were distributed in southern waters. Main conclusions A considerable amount of survey effort was needed to attain reliable estimates of species richness. In less surveyed areas, species richness and abundance are likely to be underestimated. The offshore waters presented high species richness and several hotspots of cetacean abundance. This work provides new knowledge on cetacean distribution at a large scale in the eastern North Atlantic, relevant to future conservation management.

The eastern North Atlantic (ENA) has many highly productive areas where several species of cetaceans have been recorded, with the common dolphin (Delphinus delphis) being one of the most frequently sighted species. However, its spatial and temporal distribution in high seas is poorly known. The study presents the results from 5 years of cetacean monitoring in the ENA (2012–2016) aboard cargo ships that follow the routes from Continental Portugal to the Macaronesian archipelagos and north-west Africa. Common dolphin was the most frequently sighted cetacean with 192 occurrences registered on effort and an overall encounter rate of 0.36 sightings/100 nmi. The species was distributed in coastal and offshore waters, but absent from the Canaries and Cape Verde islands. Statistical ‘habitat’ models were developed to describe and explain the occurrence of sightings of the species: variables affecting detection of dolphins had a small impact and there were clear spatiotemporal distribution patterns, influenced to some degree by environmental variables. Predicted probability of occurrence was highest in coastal waters of continental Portugal and around the Azores. The models, combined with maps of distribution, were useful to identify important areas for the species, which could be the focus of future conservation efforts. Common dolphin presence was related to depth, distance to coast and seamounts, seabed slope, chlorophyll concentration, sea-surface temperature and sea level anomalies; the possible ecological significance of these relationships is explored.

The CETUS project is a cetacean monitoring program that takes advantage of cargo ships to undertake survey routes between Continental Portugal, Macaronesian archipelagos and West Africa. From 2012 to 2017, over 50 volunteers participated in the program, actively surveying more than 124.000 km, mostly beyond national jurisdictions in the high seas, for which little or no previous data existed. In total, the collection comprises 3058 georeferenced transect lines and 8913 positions, which are associated with 2833 cetacean sightings, 362 occurrences of other pelagic megafauna, 5260 estimates of marine traffic and 8887 weather observations. This dataset may provide new insights into the distribution of marine mammals in the Eastern North Atlantic and was published following the OBIS-ENV-DATA format (with the most recent biodiversity data standards at the time of writing). Consequently, it may serve as a model for similar visual line transect data collections yet to be published.

Data on species occurrence at the scale of their distributional range and the determination of their habitat use requirements are essential to support conservation and define management plans that account for their habitat requirements. For wide-ranging species, such as cetaceans, especially considering that their marine habitats include offshore areas, collection of such data is challenging. In the absence of dedicated surveys, alternative methodologies are needed, such as the use of data collected from platforms of opportunity and modelling techniques to predict distribution in unsurveyed areas. Using 6 years of cetacean occurrence data collected along cargo ship routes between the Iberian Peninsula, northwestern African coasts and the Macaronesian islands, we developed ecological niche models to assess habitat preferences and predict suitable habitats of the eight most frequently sighted cetacean taxa in the area. Explanatory variables used for model fitting included topographic, oceanographic, detectability, geographic and seasonal features. To provide a robust habitat characterisation, along with predictions of habitat suitability, making best use of occurrence datasets, we applied two modelling techniques, GAM and Maxent, which offer complementary strengths. Coastal areas provide important habitats for common and bottlenose dophins, while other dolphin species (spotted and striped dolphins) have a more oceanic distribution. The predicted niches of Cuvier’s beaked whale and minke whales are mainly in the high seas at northern latitudes. Suitable habitats for sperm whales and pilot whales are mostly in southern areas in continental slope regions. For all the species, models indicated that areas around seamount features offer suitable habitats, likely of high relevance in oligotrophic offshore waters. As such, dedicated survey effort in such areas would facilitate development and implementation of appropriate management plans, which are currently lacking. Our models offer an important contribution to baseline knowledge of cetacean distribution at basin-scale in the region and could support the definition of priority areas, monitoring plans, and conservation measures, essential to comply with the requirements of the EU Marine Strategy Framework Directive.

Macaronesia, an oceanic area located in the Northeast Atlantic, exhibits a high topographic and oceanographic complexity that enables a rich biodiversity. So far, seven species of baleen whales have been recorded in the area. These are migratory animals that travel long distances between feeding grounds at high latitudes in summer and tropical areas for mating and breeding in winter. As baleen whales are such mobile animals, the conservation and management of their habitat is highly challenging. Hence, knowing and understanding the patterns of their distribution is fundamental. Despite the many records of baleen whales in Macaronesia, heterogeneity in research effort has resulted in scattered occurrence data that leads to an incomplete picture of their distribution in the area. We aim to increase knowledge of distribution patterns of baleen whales, and identify research effort gaps in Macaronesia. From a total of 14 peer‐reviewed publications, four public reports, two poster presentations, and four data bases, we gathered 1798 records in Macaronesia since 1990 for four species of baleen whale: minke whale Balaenoptera acutorostrata, fin whale Balaenoptera physalus, blue whale Balaenoptera musculus, and humpback whale Megaptera novaeangliae. Spatio‐temporal descriptive analyses and ecological modelling were carried out in order to understand occurrence patterns. The results show a big lack of research effort and occurrence data in offshore areas and in some coastal waters, namely off the West African coast. Moreover, blue, fin, and humpback whales are sighted mainly in spring in the north of Macaronesia, while minke whale distribution is clearly different: they have a stronger presence in summer. This study highlights areas in urgent need of research effort in order to inform decision‐makers and support effective measures for the protection and conservation of baleen whales in Macaronesia.

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Background: Harbour porpoise (Phocoena phocoena) is listed under the Annex II of the European Habitats Directive (Directive 92/43/CEE) and Member States are required to designate special areas of conservation to protect their populations. Although harbour porpoise distribution in the Iberian Peninsula has been described, the mouth of the Douro River was never reported as part of its range of distribution. Methods: Observations from a pier in the mouth of the Douro were carried out to monitor the presence of the harbour porpoises. A monthly measure of sightings per unit of effort was calculated. Information on the sightings, group sizes and records of the white individual is presented. Results: Twenty six surveys were performed resulting in 22 sightings with group size varying from one to three animals. The white individual was sighted 12 times and recorded both in photographs and video. July was the month with the lowest number of sightings per unit of effort (SPUE), while October had the highest SPUE. Conclusions: Evidences, such as the presence of an easily recognizable white individual and the reports from local sea-users suggest a degree of site fidelity from this group of harbour porpoises in the mouth of the Douro River. However, further studies are needed to support the effective management and conservation of this group.

Long-term monitoring datasets are fundamental to understand physical and ecological responses to environmental changes, supporting management and conservation. The data should be reliable, with the sources of bias identified and quantified. CETUS Project is a cetacean monitoring programme in the Eastern North Atlantic, based on visual methods of data collection. This study aims to assess data quality and bias in the CETUS dataset, by 1) applying validation methods, through photographic confirmation of species identification; 2) creating data quality criteria to evaluate the observer’s experience; and 3) assessing bias to the number of sightings collected and to the success in species identification. Through photographic validation, the species identification of 10 sightings was corrected and a new species was added to the CETUS dataset. The number of sightings collected was biased by external factors, mostly by sampling effort but also by weather conditions. Ultimately, results highlight the importance of identifying and quantifying data bias, while also yielding guidelines for data collection and processing, relevant for species monitoring programmes based on visual methods.