Data on whale distribution and abundance in the polar oceans is rather sparse, as implementing the standard surveying method, line-transect surveys, is challenging and costly. To overcome this problem, we initiated a program to electronically log all opportunistic cetacean sightings during all Polarstern expeditions through the nautical officer on watch. Opportunistic (visual) sightings by naked eye were logged during Polarstern Cruise PS123 (Bremerhaven – Port Stanley) by the nautical officer on duty using a customized Software package (WALOG, WhAleLOGger) installed on a touch screen laptop located on the ship's bridge. Species were identified by naked eye or handheld binoculars (7x50) to the lowest possible taxonomical level and assigned a "certainty" level of identification. The number of animals were counted if possible or estimated for larger groups. Whenever identification to species level was not possible, the next identifiable taxonomical category was assigned. Information on sighting position, date and time are automatically transferred from the ship's DAVIS-Ship System (https://dship.awi.de/) to the WALOG software at the time of logging. Photographs were taken if possible for retrospective analysis. All data of acquired sightings were retrospectively validated by a marine biologist and converted to a standard format. To this end, plausibility of sighting time, location, standardization of species names, eventual comments added at the time of sighting, as well as additional information such as photographs (if available) were checked either to verify or improve species identification. Datasets are used in species distribution modelling and to inform interested parties about occurrences.
Date/Time is given in UTC. Certainty of identification: definite if observer clearly identified the species. The number of individuals is binned according to the options given in the data acquisition software: 1, 2, 3, less equal 5, less equal 10, greater than 10, greater than 20. More precise values may exist, due to observer comments.Data on visual cetacean sightings are collected systematically during all Polarstern expeditions by the nautical officer on duty, rendering WALOG to be "on effort" continuously. To log sightings, nautical officers are provided with the customized Software package (WALOG, WhAleLOGger) installed on a touch screen laptop located on the ship's bridge. However, during Polarstern expedition PS123 (Bremerhaven – Port Stanley), a malfunctioning of WALOG prevented its use altogether. Hence WALOG has to be considered "off effort" for this expedition. From a statistical point of view, this situation differs from expeditions where WALOG was "on effort", but no sightings were logged, i.e. a null sample.
