Factors driving patterns and trends in strandings of small cetaceans

The incidence of cetacean strandings is expected to depend on a combination of factors, including the dis- tribution and abundance of the cetaceans, their prey, and causes of mortality (e.g. natural, fishery bycatch), as well as currents and winds which affect whether carcasses reach the shore. We investigated spatiotemporal patterns and trends in the numbers of strandings of three species of small cetacean in Galicia (NW Spain) and their relationships with meteoro- logical, oceanographic, prey abundance and fishing-related variables, aiming to disentangle the relationship that may exist between these factors, cetacean abundance and mor- tality off the coast. Strandings of 1166 common dolphins (Delphinus delphis), 118 bottlenose dolphins (Tursiops truncatus) and 90 harbour porpoises (Phocoena phocoena) during 2000–2013 were analysed. Generalised additive and generalised additive-mixed model results showed that the variables which best explained the pattern of strandings of the three cetacean species were those related with local ocean meteorology (strength and direction of the North– South component of the winds and the number of days with South-West winds) and the winter North Atlantic Oscil- lation Index. There were no significant relationships with indices of fishing effort or landings. Only bottlenose dolphin showed possible fluctuations in local abundance over the study period. There was no evidence of long-term trends in number of strandings in any of the species and their abun- dances were, therefore, considered to have been relatively stable during the study period.Versión del editor2,01

Stable isotope analysis in two sympatric populations of bottlenose dolphins Tursiops truncatus: evidence of resource partitioning? Marine Biology 158

Abstract Skin and muscle from 43 bottlenose dolphins (38 juveniles/adults, 5 calves) stranded in NW Spain were analysed to determine whether stable isotope ratios (d 13 C and d 15 N) could be used to assess dietary variation, habitat segregation and population substructure. Results were compared with published stomach contents data. Stable isotope ratios from 17 known prey species were also determined. Isotope ratios of the main prey (blue whiting, hake) varied significantly in relation to fish body size. Dolphin calves showed significant heavy isotope enrichments compared to adult females. Excluding calves, d 15 N decreased with increasing dolphin body size, probably related to an ontogenetic shift in diet towards species at lower trophic levels, e.g. on blue whiting as suggested by stomach content results. Bottlenose dolphins were divided into two putative populations (North, South) based on previous genetic studies, and values of d 13 C and d 15 N differed significantly between these two groups, confirming the existence of population structuring

An Endangered Population of Harbour Porpoise Phocoena phocoena Hidden in Plain Sight: Biology, Ecology and Conservation of the Iberian Porpoise

International audienceThe Iberian harbour porpoise (Phocoena phocoena) reaches a larger body size than most other harbour porpoise populations and is genetically distinct, albeit closely related to the population in Northwest Africa. Currently comprising an estimated 3000–4000 individuals, genetic evidence and strandings data suggest that the population has declined in recent times, and it is considered to be at risk of extinction. It is distributed all around the Atlantic coast of the Iberian Peninsula, with the highest densities off Galicia in Northwest Spain and Northern and Central Portugal, a highly productive upwelling area characterised by cold-water upwelling. There are occasional reports from the Mediterranean and Macaronesia and some evidence of emigration into the Celtic Sea. It feeds mostly on fish, with pelagic fish being more important than in the diet of porpoises from northern Europe, perhaps due to excursions beyond the narrow continental shelf.The population faces a number of anthropogenic threats. Historically, porpoises were used for human consumption while current threats include polychlorinated biphenyls (PCBs), with some individuals having concentrations in their blubber above the threshold for impairment of reproduction, and nematode infections, probably also prey depletion, underwater noise and fatal attacks by bottlenose dolphins. The most serious current threat is fishery bycatch mortality. Stranding data suggest that the bycatch mortality increased in the last decade. Although based on information from a small number of documented mortalities (reflecting limited observer coverage especially for small-scale fishing as well as a low number of reported strandings), annual bycatch mortality estimates are in the order of a few hundred animals, which is clearly unsustainable. There is, however, an apparent incompatibility between the high bycatch estimates and the rather similar abundance estimates obtained from large-scale abundance surveys in 2005, 2016 and 2022.Consistent with population status assessments by Spain and Portugal, OMMEG 1 (Convention for the Protection of the Marine Environment of the North-East Atlantic) concluded that bycatch mortality in Iberian porpoise “is critically exceeding the agreed threshold” of zero. There are several national initiatives in Spain and Portugal including the development of species conservation plans. Continuous reduction of bycatch mortality, preferably until such mortality is eliminated, is a priority to ensure that this population does not disappear in the near future