Utility of sighting and stranding data to document marine mammal occurrence: A case study in Puget Sound, Washington, USA
Huggins, J, J Calambokidis, D Lambourn, S Norman, J Evenson, D Anderson, B Hanson, S Jeffries, and S Berta. 2015. Utility of sighting and stranding data to document marine mammal occurrence: A case study in Puget Sound, Washington, USA. Abstract (Proceedings) 21st Biennial Conference on the Biology of Marine Mammals, San Francisco, California, December 14-18, 2015.
Sighting and stranding data are often used to identify patterns in marine mammal occurrence. Here we evaluate the use of public sighting reports, systematic surveys (where available), and stranding data from 1995-2015 to test how well these reflect trends in nine cetacean species in Puget Sound, Washington, USA. In general we found good agreement in documenting trends in occurrence between one of the sighting sources and strandings for 7 of 9 species, though there were often some species-specific considerations. Changes in harbor and Dall's porpoise populations in recent years were documented by all three sources, as was the long-term presence of a few Risso's dolphins. Annual variation in gray whales was well supported by strandings and sighting reports and showed a clear association between increases in both reporting sources in unusual areas during the 1999-2000 major mortality event. The presence of unusual species such as Bryde's whales, bottlenose dolphins, and common dolphins were captured by stranding records and sightings reported by the public but were not detected in surveys; largely because their occurrence were over short periods not covered by a survey. For two species there were insightful disconnects between the sightings and strandings. An increase in humpback whales in the region was detected by public sighting reports but is not reflected in the stranding records because none of these humpback whales have stranded this region (but have elsewhere in Washington State) showing how for a long-lived species, strandings may not be a very sensitive indicator of the presence of a small number of animals. Fin whales were documented in stranding records only; these animals had been transported dead into the study area on bows of ships, indicating that stranding location may sometimes not reflect the location where the animal lived or died. Overall our findings reflect good agreement among these sources but point out some important considerations with each: sightings from the public can be biased by consistent reports in the same area from a single individual; systematic surveys may not document the occasional presence of unusual species; and stranding records may not reflect the animals' actual original location.