Beached bachelors: An extensive study on the largest recorded sperm whale Physeter macrocephalus mortality event in the North Sea.

Between the 8th January and the 25th February 2016, the largest sperm whale Physeter macrocephalus mortality event ever recorded in the North Sea occurred with 30 sperm whales stranding in five countries within six weeks. All sperm whales were immature males. Groups were stratified by size, with the smaller animals stranding in the Netherlands, and the largest in England. The majority (n = 27) of the stranded animals were necropsied and/or sampled, allowing for an international and comprehensive investigation into this mortality event. The animals were in fair to good nutritional condition and, aside from the pathologies caused by stranding, did not exhibit significant evidence of disease or trauma. Infectious agents were found, including various parasite species, several bacterial and fungal pathogens and a novel alphaherpesvirus. In nine of the sperm whales a variety of marine litter was found. However, none of these findings were considered to have been the primary cause of the stranding event. Potential anthropogenic and environmental factors that may have caused the sperm whales to enter the North Sea were assessed. Once sperm whales enter the North Sea and head south, the water becomes progressively shallower (<40 m), making this region a global hotspot for sperm whale strandings. We conclude that the reasons for sperm whales to enter the southern North Sea are the result of complex interactions of extrinsic environmental factors. As such, these large mortality events seldom have a single ultimate cause and it is only through multidisciplinary, collaborative approaches that potentially multifactorial large-scale stranding events can be effectively investigated.

[Testing fish vaccines with an in vitro lymphocyte blastogenesis assay]

Lethal-challenge experiments are provided by German law till now for testing fish vaccines. Finding an in vitro-assay which could replace this experiments was subject of our work. Two questions must have been answered to develop the lymphocyte blastogenesis assay: Do lymphocytes of immunised fish react the same way as lymphocytes of naive fish, being incubated with antigen and is there a coherence between the lymphocyte's in-vitro-response and the immune status of challenged fish. Methods to take blood samples from anaesthetised fish and to obtain lymphocytes by centrifugation on gradients were worked out and the lymphocyte's demands on culture requirements were evaluated thoroughly because the ability to proliferate depends on many exogenous parameters. Proliferation was represented and measured by a tetrazolium based colorimetric assay (MTT).