Steller and California sea lion count and diet data in northwest Washington, 2010-2013

Published: 07-04-2021| Version 1 | DOI: 10.17632/npdzxcsfh9.1
Jonathan Scordino,
Adrianne Akmajian,
Susan Riemer


In this submission, we share data collected on Steller (Eumetopias jubatus) and California sea lion (Zalophus californianus) in northwest Washington year round from 2010-2013. Our primary objectives were to characterize the diets of Steller and California sea lions and to determine if the sympatric species compete for the same prey resources. We also wanted to determine how much prey each of the sea lions were eating. To determine prey consumption we collected data on the abundance of California and Steller sea lions during surveys. Generally only male California sea lions are present in northwest Washington. For Steller sea lions, all age groups are present and so we also counted by demographic group. We found that Steller and California sea lions have significant dietary niche overlap as the species utilize very similar assemblages of prey. We developed a procedure for estimating prey consumption by the sea lions using published data and our counts and have made our code publically available at We found that Steller sea lions ate roughly 11,000 metric tons of prey per year from 2010-2013 and that California sea lions ate roughly 9,500 metric tons of prey. A manuscript of our results has been submitted to Fishery Bulletin in April of 2021.


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All research was conducted in northwest Washington. Sea lion haulouts in northwest Washington include Waadah Island, Tatoosh Island (including subsites of Tatoosh East and Tatoosh Cut), Duncan Rock, Guano Rock, East Bodelteh Island, West Bodelteh Island, Umatilla Reef, Carroll Island, and Sea Lion Rock. Sea lion haulouts were surveyed throughout the year with the goal of surveying all sites at least once a month. Surveys were conducted by boat and occasionally by landing an observer onto the haulout to conduct a land based count. Counts were aided by the use of 7x50 and 8x40 binoculars. Counts were always conducted of total California and Steller sea lions present first. After the species count was completed, counts by demographic group were conducted for Steller sea lions to determine the numbers of adult males, adult females, juveniles, and pups. Our goal was to collect 30 Steller sea lion scat a month and 50 California sea lion scat a season. Scat samples were only conducted from sites in which 95% or more of the sea lions present were composed of the target species. Scat samples were collected following the techniques described by Lance et al. (2001). Scat samples were cleaned by either placing in paint strainer bags and washing in a residential style washing machine or by washing through nested sieves. Cleaned prey hard parts were stored in isopropyl alcohol and later dried. All prey hard part structures (i.e. vertebrae, otoliths, gill rakers, and beaks) were used for identifying the prey to the lowest taxonomic level possible. Prey were identified by examining under a dissecting microscope and comparing to a reference collection of known prey specimens from the Eastern North Pacific and adjacent estuaries. Some species have very distinct bones for each species within a family whereas others, like rockfish and salmon, have very similar bone structures for all species within a family. As a result, some species are identified to the species level and others to higher taxonomic levels. We have presented our prey data with a 1 for presence in each scat sample for all prey levels identified and for prey families (bold font). We reconstructed the sea lion diet for each species using split sample frequency of occurrence. The data we collected can also be used to report frequency of occurrence. We combined our split sample frequency of occurrence results with an estimate of how much prey the sea lions were eating to determine an estimate of how much of each prey species was consumed per year in northwest Washington during our 2010-2013 study. Our code for estimating prey consumption, which utilizes the counts described above, is available at A manuscript presenting our results was submitted to Fishery Bulletin in April 2021.