Spatiotemporal mortality and demographic trends in a small cetacean: Strandings to inform conservation management

With global increases in anthropogenic pressures on wildlife populations comes a responsibility to manage them effectively. The assessment of marine ecosystem health is challenging and often relies on monitoring indicator species, such as cetaceans. Most cetaceans are however highly mobile and spend the majority of their time hidden from direct view, resulting in uncertainty on even the most basic population metrics. Here, we discuss the value of long-term and internationally combined stranding records as a valuable source of information on the demographic and mortality trends of the harbour porpoise (Phocoena phocoena) in the North Sea. We analysed stranding records (n = 16,181) from 1990 to 2017 and demonstrate a strong heterogeneous seasonal pattern of strandings throughout the North Sea, indicative of season-specific distribution or habitat use, and season-specific mortality. The annual incidence of strandings has increased since 1990, with a notable steeper rise particularly in the southern North Sea since 2005. A high density of neonatal strandings occurred specifically in the eastern North Sea, indicative of areas important for calving, and large numbers of juvenile males stranded in the southern parts, indicative of a population sink or reflecting higher male dispersion. These findings highlight the power of stranding records to detect potentially vulnerable population groups in time and space. This knowledge is vital for managers and can guide, for example, conservation measures such as the establishment of time-area-specific limits to potentially harmful human activities, aiming to reduce the number and intensity of human-wildlife conflicts

Nutritional status and prey energy density govern reproductive success in a small cetacean

A variety of mammals suppress reproduction when they experience poor physical condition or environmental harshness. In many marine mammal species, reproductive impairment has been correlated to polychlorinated biphenyls (PCBs), the most frequently measured chemical pollutants, while the relative importance of other factors remains understudied. We investigate whether reproductively active females abandon investment in their foetus when conditions are poor, exemplified using an extensively studied cetacean species; the harbour porpoise (Phocoena phocoena). Data on disease, fat and muscle mass and diet obtained from necropsies in The Netherlands were used as proxies of health and nutritional status and related to pregnancy and foetal growth. This was combined with published life history parameters for 16 other areas to correlate to parameters reflecting environmental condition: mean energy density of prey constituting diets (MEDD), cumulative human impact and PCB contamination. Maternal nutritional status had significant effects on foetal size and females in poor health had lower probabilities of being pregnant and generally did not sustain pregnancy throughout gestation. Pregnancy rates across the Northern Hemisphere were best explained by MEDD. We demonstrate the importance of having undisturbed access to prey with high energy densities in determining reproductive success and ultimately population size for small cetaceans

The stranding anomaly as population indicator: the case of Harbour Porpoise <i>Phocoena phocoena</i> in North-Western Europe

Ecological indicators for monitoring strategies are expected to combine three major characteristics: ecological significance, statistical credibility, and cost-effectiveness. Strategies based on stranding networks rank highly in cost-effectiveness, but their ecological significance and statistical credibility are disputed. Our present goal is to improve the value of stranding data as population indicator as part of monitoring strategies by constructing the spatial and temporal null hypothesis for strandings. The null hypothesis is defined as: small cetacean distribution and mortality are uniform in space and constant in time. We used a drift model to map stranding probabilities and predict stranding patterns of cetacean carcasses under H-0 across the North Sea, the Channel and the Bay of Biscay, for the period 1990-2009. As the most common cetacean occurring in this area, we chose the harbour porpoise <i>Phocoena phocoena</i> for our modelling. The difference between these strandings expected under H-0 and observed strandings is defined as the stranding anomaly. It constituted the stranding data series corrected for drift conditions. Seasonal decomposition of stranding anomaly suggested that drift conditions did not explain observed seasonal variations of porpoise strandings. Long-term stranding anomalies increased first in the southern North Sea, the Channel and Bay of Biscay coasts, and finally the eastern North Sea. The hypothesis of changes in porpoise distribution was consistent with local visual surveys, mostly SCANS surveys (1994 and 2005). This new indicator could be applied to cetacean populations across the world and more widely to marine megafauna

Fuelling conditions at staging sites can mitigate Arctic warming effects in a migratory bird

Under climate warming, migratory birds should align reproduction dates with advancing plant and arthropod phenology. To arrive on the breeding grounds earlier, migrants may speed up spring migration by curtailing the time spent en route, possibly at the cost of decreased survival rates. Based on a decades-long series of observations along an entire flyway, we show that when refuelling time is limited, variation in food abundance in the spring staging area affects fitness. Bar-tailed godwits migrating from West Africa to the Siberian Arctic reduce refuelling time at their European staging site and thus maintain a close match between breeding and tundra phenology. Annual survival probability decreases with shorter refuelling times, but correlates positively with refuelling rate, which in turn is correlated with food abundance in the staging area. This chain of effects implies that conditions in the temperate zone determine the ability of godwits to cope with climate-related changes in the Arctic

walvisstrandingen_nl

The database of stranded cetaceans in The Netherlands contains of records of whales, dolphins and porpoises, order Cetacea. All records concern strandings of single individuals (there are four exceptions: one of 3-6 sperm whales and three of 11-61 long-finned pilot whales). The electronic database was compiled from published records on strandings (see Literature) gathered since 1914 and is supplemented with data on a regular basis. It contains all records that have been presented to the respective managers of the database, and was started by A.B. van Deinse. Van Deinse and successors have added strandings from before 1914 as well and the oldest record thus far is a sperm whale stranded in 1255. Anno 2020, the database contains over 12,500 unique records of 26 species (among which five subfossil records of the nowadays extinct Atlantic population of grey whale Eschrichtius robustus and a few dozens of unidentified specimens). The database is considered to be fairly complete since 1914 regarding species other than harbour porpoise Phocoena phocoena. Of the latter, records are up to date only since 1971. Harbour porpoise has always been resident species in Dutch waters and it was considered too common by Van Deinse – and records were too numerous – to collect data of stranded individuals until the late 1950s, when numbers were dwindling. Hence, records prior to 1971 are very incomplete, although records from the past are are added ad hoc. Nowadays, the database is updated at least once a week, and records are validated before they are entered. Individual records contain the Dutch and scientific name as far as animals can be identified (in several cases, stranded individuals have been identified only down to 'group' (e.g. 'small whale'). Identifications may be adjusted if new information becomes available. Included with every single record, apart from species name, are date and location of stranding, sex, length, state of carcass, storage of remains, autopsied or not, references, photographs, and name of finder/reporter, if appropriate. As of 1 January 2006 the database is freely accessible at www.walvisstrandingen.nl and managed by Naturalis Biodiversity Center in Leiden, The Netherlands. With the publication of the www.walvisstrandingen.nl-dataset with GBIF, all data concerning Dutch strandings are better searchable and the full set, or a selection, can be downloaded. However, it is recommended to contact Naturalis if the data are used for research.</p

Cetaceans stranded in the Netherlands in 2015-2019

This report presents a validated list of stranded cetaceans in the Netherlands, as well as some cetaceans accidentally brought in on ship bulbs, between 2015-2019. During this period 2701 cetaceans representing eleven species were reported. The list also includes a few bones and skulls, among others of killer whale (Orcinus orca) and bottlenose dolphin (Tursiops truncatus), species not reported otherwise during this period. The most common species was harbour porpoise (Phocoena phocoena), with 2651 individuals. The average number of porpoises per year was 530, lower than the preceding seven-year average, but numbers fluctuated largely between years and there has been no particular trend after the strong increase in the early 2000s. The monthly pattern of strandings as recorded during the previous periods, with peaks in March and July-September, remained the same. The number of stranded porpoise is equally spread along the entire coastline. The density, expressed as the number of stranded porpoise per kilometre per year, is 0.6 for the entire coastline, or 1.2 if the extensive and less well surveyed areas of Western Scheldt, Eastern Scheldt and Wadden Sea proper are omitted. Sex ratio remained stable over the years and is in line with results from before 2015, with a preponderance of males. On the basis of length, over half were immature and 8.7% neonate. Among neonates and immatures there was a preponderance of males as well, but not in adults. In the Wadden Sea area more neonates and adults were found than in the other two subareas. The major cause of death was infectious disease, followed by predation by grey seal (Halichoerus grypus). Accidental by-catch was identified as the cause of death of 11% of the stranded animals. Of particular interest was the stranding of six sperm whales (Physeter macrocephalus) in January 2016, the largest stranding event of this species in the Netherlands. It was part of a stranding event spread out over the Central and southern North Sea, involving thirty individuals. During 2015-2019 four dead fin whales (Balaenoptera physalus) and eight minke whales (B. acutorostrata) were reported, several of which were hit by ships. There seems to be a slight increase in the strandings of fin whale and Sowerby’s beaked whale (Mesoplodon bidens) since 2000, while white-beaked dolphin (Lagenorhynchus albirostris) shows a decrease. There is no temporal trend for any of the other species since 2000

Cetaceans stranded in the Netherlands in 2008-2014

We present a validated list of cetaceans stranded from 2008 up to and including 2014. A total of 4406 cetaceans was found on the Dutch coast during this period, comprising 4346 harbour porpoises (Phocoena phocoena) (98.6% of all strandings) and 59 individuals of twelve other species. The next most numerous species was white-beaked dolphin (Lagenorhynchus albirostris) (14 individuals). All individual cases of stranded cetaceans are included, excepting those of harbour porpoise. During the present period, the years with the highest numbers ever of harbour porpoise have been recorded, although numbers in the early twentieth century or before may have been equally high. Largest numbers of harbour porpoise, both absolute and expressed as average per stretching kilometre, are found in the Wadden Sea area, with a gradually decreasing density further south

Predicting Harbor Porpoise Strandings Based on Near-Shore Sightings Indicates Elevated Temporal Mortality Rates

The increase in anthropogenic activities and their potential impact on wildlife requires the establishment of monitoring programs and identification of indicator species. Within marine habitats, marine mammals are often used as ecosystem sentinels, which has led to investigations into their abundance, distribution, and mortality patterns. However, trends in sightings and strandings are rarely analyzed in combination. This is necessary to distinguish elevated stranding rates caused by changes in local abundance from increased mortality as a consequence of other natural, environmental or anthropogenic factors. Therefore, the objective of this study was to assess whether harbor porpoise (Phocoena phocoena) stranding frequency in the southern North Sea can be explained by local population density derived from more than 400 thousand hours of systematic observations along the Dutch coast between 1990 and 2018. Since the late 1990s, both the number of stranded porpoises and the sighting rate increased rapidly up to around the mid-2000s, after which they remained high, but with large inter-annual fluctuations. On an annual basis there was a strong correlation between porpoise strandings and sightings, but with a seasonal mismatch. Highest stranding rates occur in late summer, while highest sighting rates occur in early spring. Despite low sighting rates in late summer, August appears to be the best predictor for the monthly variation in the number of stranded porpoises, which could be explained by post-reproductive dispersal and mortality. Excessive high porpoise stranding numbers after accounting for variations in local density could signpost unusual mortality events (UMEs). The corrected stranding rates show that in the early 1990s, when porpoise sightings were rare, and after 2010, the number of stranded porpoises exceeds the expected number. Especially in the summer of 2011, the number of dead porpoises found ashore was excessively high and this might reflect an UME. These results demonstrate that a comparative interpretation of marine mammal strandings and coastal sightings can be a valuable management and conservation tool that could provide an early warning signal for population change

Spatiotemporal analysis in white-beaked dolphin strandings along the North Sea coast from 1991-2017

The white-beaked dolphin (Lagenorhynchus albirostris) is an endemic species in the North Sea with an estimated population of around 36,000 individuals. Recently, concerns have been raised among conservationists regarding increasing water temperatures as a result of climate change, which could result in a decline in population numbers in certain areas of the white-beaked dolphin’s range. Here we use stranding frequencies of white-beaked dolphins as an indicator of distribution and investigate whether there have been spatiotemporal patterns and changes in stranding frequencies in the south western North Sea in the last 27 years (1991-2017). A total of 407 strandings was recorded and the distribution of stranded animals throughout this period revealed a higher density of animals in the southern countries in earlier years, with slightly increased densities in the north western area more recently. This could be a first indication of a change in habitat use and population distribution from southern to northern regions. A potential explanation for the observed shift is climate change and its effect on prey distribution and availability. This study highlights the potential of using stranding records as a way to collect high resolution spatiotemporal data, making this a valuable addition to surveys of live animals assessing species distribution and abundance. Additional research into metrics such as causes of mortality, life history and diet parameters (all of which are currently largely unknown for this species) would provide a welcome contribution to assess more detailed measures of the status of the population