SAFS Research Roundup: Washington’s Sea Otters and Whale Twins
New model improves accuracy of Washington State’s sea otter estimates
Graduate student Jessie Hale (Laidre Lab) recently published a paper rethinking the status, trends, and equilibrium abundance estimates of Washington State’s sea otter population on January 27 in The Journal of Wildlife Management.
Reintroduced after being locally extirpated from Washington’s coastal waters, sea otters (Enhydra lutris kenyoni) have steadily increased in number over the past 50 years. The Washington Department of Fish and Wildlife (WDFW) has outlined two goals for recovery: a target population level and a target geographic distribution. Criteria for these thresholds are determined by estimates of population abundance, equilibrium abundance, and geographic distribution. In 2018, WDFW downlisted the sea otter from state endangered to state threatened as the best available information indicated the population had achieved a 3-year average of 1,752 individuals, exceeding 60% of the previously estimated equilibrium abundance threshold for downlisting (1,640 individuals).
The new model developed by Hale and her co-authors improves upon previous analyses of sea otter population dynamics in Washington by partitioning and quantifying sources of estimation error to estimate population dynamics, by providing robust estimates of equilibrium abundance, and by simulating long-term population growth and range expansion under a range of realistic parameters. The authors state that this population model can provide a template for studying the recovery of other small, fragmented populations of endangered or translocated species.
Included in the study results are estimates of equilibrium abundance considerably higher than what earlier models have shown, and under these new estimates, Washington’s sea otter population has not yet reached the 60% threshold required for downlisting.
The authors recommend the WDFW incorporate these new estimates in future decision-making processes for sea otters. Their forward projections of population dynamics also highlight the potential for increased competition between sea otters and valuable state and tribal shellfisheries, as sea otters are, in general, predicted to continue to increase in number and expand their geographic range in Washington.
“This work is the product of a large and long-term collaborative effort to study sea otters in Washington State and highlights the importance of long-term monitoring efforts in understanding sea otter population dynamics,” added Jessie.
Learn more about Jessie’s sea otter research
Prevalence of identical twins and twin survival rates in whales
A new UW study investigated the proportions of identical twins and twin survival rates in whales. Utilizing whaling data from 1908 to 2020, Ruth Drinkwater (BS 2021) and co-author Trevor Branch examined the instances of multiple pregnancies in 16 species of whales. They found that .87% (2,197 out of 252,651) of pregnancies resulted in multiple fetuses, including 12 instances of five or more fetuses. The results were published April 1 in Marine Mammal Science and were the culmination of her capstone project.
The authors then estimated the proportion of identical twins in six of the sixteen species, and found identical twins to be less common than fraternal twins in all six species, excluding humpback whales. Similar to human statistics, the proportion of identical twins is one-third of all twins. Furthermore, strong evidence was found that twin survival declined with fetal length in blue, sei, and fin whales, but not in sperm, humpback, and Antarctic minke whales.
The study shows that successful births of multiples in large whales is exceedingly rare, and thus in predicting future population growth, births can be assumed to be single.