Last year’s unusual mortality event brought 215 dead gray whales to the west coast of North America and the trend continues with at least 114 stranded grey whales recorded in 2020. Still, the documented strandings are thought to only represent a small fraction of the die-off as most animals might have sunk to the ocean floor or stranded at inaccessible locations.
Though reasons for the high number of stranding events are still unclear, several hypotheses have emerged. Among the top are changes in the arctic habitats due to climate change and food depletion due to a population size near capacity as we have covered here. Now, necropsies on 50 whales stranded last year unveil further information.
The study published by the International Whaling Commission, with a total of 23 people from the US, Mexico and Canada participating in the enormous task, revealed that food depletion might not have been the main direct factor. Out of the 50 whales examined, 19 most likely died from killer whale attacks with another eight showing rake marks that suggest similar incidents. These animals were mostly found in Alaskan waters. Mainly around California, 10 gray whales had injuries consistent with ship strikes and another one showed evidence of past entanglement with fishing gear. For the rest, the exact cause of death could not be identified.
Additionally, eighty-nine of the stranded gray whales had been rated for body condition, with only one-quarter showing average to good condition. Since gray whales calve in Mexican waters before travelling back to their northern feeding grounds, they are not unlikely to be low on fat reserves during their northward migration. However, the conditions last year were worse. Fifty per cent of the examined animals were thin, and one-third classified as emaciated. Even at their starting point in Mexico, one half of the living gray whales studied showed signs of poor nutritional condition compared to only 10 per cent in the previous year.
The scientists assume that the main hypotheses all contribute to the mortality event. An unprecedented loss of sea ice and increasing ocean temperatures may have impacted the food availability in the northern feeding grounds negatively. By looking for food in new areas, the gray whales could “have also been at a greater risk from shipping, commercial fisheries, killer whale predation, even exposure to new pathogens or toxins.” In their weakened condition, they might also be “more susceptible to predation by killer whales, which themselves have become more common in Arctic waters as a result of climate change”. Lastly, if a population is already close to its capacity, a year of bad feeding conditions will affect a large part of the population.
While the unusual mortality event continues, the researchers recommend that the use of drones could help to assess the physical status of the animals during their southward migration. They also stress that environmental conditions such as prey availability and oceanographic and meteorological factors need to be considered when assessing the population status.
The full report can be found here: https://www.hakaimagazine.com/news/necropsies-unveil-more-about-mass-gray-whale-deaths/
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