The effects of the abundance and availability of prey on otariid foraging duration and time ashore has been well documented in many pinniped species (e.g. Trillmich 1990, Horning and Trillmich 1999).  In otariids, the cyclic behavior of females alternating between foraging at sea and nursing their offspring ashore is referred to as female attendance patterns (Gentry and Kooyman 1986, Trillmich 1990).  Longer foraging trip durations have been attributed to low food availability in Antarctic fur seals (Arctocephalus gazelle; Lunn and Boyd 1993, Boyd 1999), California sea lions (Zalophus californianus; Ono et al. 1987), Galapagos fur seals and sea lions (Arctocephalus galapagoensis and Zalophus californianus wollebaecki; Trillmich 1986, 1990) and Steller sea lions (Eumetopias jubatus; Hood and Ono 1997).  El Niño events may cause changes in sea lion prey distribution and abundance resulting in changes in otariid attendance patterns (Heath et al. 1991, Majluf 1991, Trillmich and Dellinger 1991).  Recent hypoxic events off coastal Oregon waters have caused widespread fish and invertebrate mortality (Grantham et al. 2004, PISCO 2006) and regime changes have affected forage fish abundance (Emmett and Brodeur 2000, Peterson and Schwing 2003, Brodeur et al. 2005), but the effect on pinnipeds is largely unknown.

Over the past 30 years the eastern population (EP) and western population (WP) of Steller sea lions have exhibited opposing population trends (see Pendleton et al. 2006), with the WP declining by over 80% (Loughlin et al. 1992, Trites and Larkin 1996) and the EP increasing to near historic levels (Calkins et al. 1999).  While causes of the decline remain a mystery, one prominent hypothesis is that of reduced prey availability resulting in nutritional stress (Loughlin and Merrick 1989, Rosen and Trites 2000, Trites and Donnelly 2003).  One method of inferring the availability and quality of food resources is by examining patterns of maternal attendance and nursing durations (Trites and Porter 2002, Milette and Trites 2003, Brandon et al. 2005, Maniscalco et al. 2006, Trites et al. 2006).  Contrary to expectations, when maternal Steller sea lion foraging trips were compared between the declining WP and the stable EP in Alaska during the 1990s, foraging trips of adult females were significantly shorter in areas of the population decline (Milette and Trites 2003, Brandon et al. 2005).  Similar results were found at Chiswell Island (WP) from 2000-2004 (Maniscalco et al. 2006).  Trites et al. (2006) found significant differences in Steller sea lion attendance behavior among seasons (winter, spring, and summer), but not among haul-out sites in the declining Gulf of Alaska region and the increasing southeast Alaska region.  Although none of these study sites were observed year-round, it appeared that adult females in the WP had no difficulty finding sufficient prey resources during the period of these studies.

If insufficient prey resources contributed to the decline of the WP during the 1970s and 1980s, studies such as those mentioned above may have been conducted too late to detect this.  Revealing why a species declines without having information prior to and during the decline is often difficult, if not impossible.  Numerous studies have been conducted between the WP and EP of Steller sea lions in Alaska but there is a large gap in our knowledge of the Steller sea lion population off the Oregon coast, a second replicate region where the Steller sea lion population is increasing.  Establishing a long-term Steller sea lion behavioral study in Oregon is timely given the 1998 climate regime shift from a warm, low production regime to a cool, highly productive regime (Peterson and Schwing 2003, Brodeur 2005).  Between 1998 and 2002, species composition shifted from a community dominated by southern species (mackerels, Scomber japonicus, Trachyurus symetricus; and hake, Merluccius productus) to one dominated by northern species (squid; smelts, Osmeriidae; and salmon) (Peterson and Schwing 2003, Brodeur 2005).  However, these same investigators note that conditions have fluctuated dramatically in more recent years and it is unknown what regime the California Current System is currently in.  How these changes in prey resources may affect upper trophic level species such as Steller sea lions is yet to be determined.

We will use remotely operated video cameras to record attendance patterns of Steller sea lions at Sea Lion Caves haul-out.  Sea lion Caves is an ideal location to study Steller sea lion behavior, as it is the primary nursery area for Steller sea lions in Oregon (Scordino 2006).  Scordino (2006) reported that during the winter months from 2002-2005, 60-78% of Steller sea lion females counted on the Oregon and northern California coast were observed at Sea Lion Caves and in January of each year, over 90% of the pups counted were present at Sea Lion Caves.  The importance of Sea Lion Caves to Steller sea lions may be tied to its proximity to nearby Heceta Bank.  Heceta Bank is among the largest submarine banks on the west coast (Hickey 1998).  The enhanced vertical and horizontal mixing associated with Heceta Bank is linked to higher prey densities attracting seabirds (Ainley et al. 2005) and cetaceans (Tynan et al. 2005).

The high wintertime abundance of females and pups at Sea Lion Caves make it an ideal location to conduct the proposed study.  Using permanently marked individuals will allow us the ability to track the same animal over successive seasons and years eliminating the variability associated with sampling different individuals.  It is our hope that this could be the beginning of a long-term behavioral study that will shed light on inter-seasonal and inter-annual variability in Steller sea lion foraging and attendance patterns.  Moreover, it may help determine which behavioral factors could be influencing the decline of the WP Steller sea lions.


Lack of sufficient prey resources can have a direct effect on pinniped attendance patterns resulting in increased foraging durations, less time ashore, and ultimately, reduced pup survival.  One method of inferring the availability and quality of food resources is by examining maternal attendance patterns.  We will use video cameras to record Steller sea lion attendance patterns at Sea Lion Caves, the primary Steller sea lion nursery area in Oregon, and compare to similar studies in Alaska.  Furthermore, this study is timely, as the northern California Current System has recently exhibited dramatic annual fluctuations affecting prey resources for top predators. 

This project is funded by

The North Pacific Universities Marine Mammal Research Consortium through the North Pacific Marine Science Foundation.

Improved technology in digital imaging systems has been beneficial for studying the behavior of numerous species of wildlife without the inherent risk of disturbance to the animals.  Video can record during all hours of the day, often during inclement weather, and has the ability to view a large area at one time.  Moreover, because images are stored to disk, data can be viewed more than once and archived for future reference.

One method of inferring the availability and quality of food resources of Steller sea lions is by examining patterns of maternal attendance and nursing durations of the same individuals between seasons and among years. To study these behaviors at Sea Lion Caves requires many hours of observation, and often one observer to record behaviors of numerous (up to 400) sea lions in a large and often quite dark cave.  Using video capable of collecting images of the entire haul-out during all daylight hours and during low light conditions reduces the chances that the observer will miss collecting important behavioral data.  The imaging system we will use consists of IQeye megapixel network cameras which provide high speed resolution with exceptional low-light performance.  Video images will be automatically recorded and stored to hard disk and then taken to the lab for subsequent analysis.  Specialized software includes features such as pan, tilt, and zoom which will help us identify particular individuals and analyze their behaviors.

  • Determine Steller sea lion maternal attendance patterns including: duration of foraging trips, time ashore between foraging trips, and duration of nursing bouts
  • Compare attendance patterns between females with pups (< 12 mo) vs. those with juveniles (> 12 mo)
  • Compare inter-seasonal and inter-annual attendance patterns
  • Determine if offspring of lactating females make independent trips to sea and compare duration of trips to sea between adult females and dependent offspring
  • Compare differences in duration of foraging trips and nursing bouts for adult females with marked male versus female juveniles and pups
  • Validate video collected data by comparing with simultaneous observations by field personnel
  • Compare results to concurrent and past studies in Alaska
  • Establish a research and monitoring program at Sea Lion Caves to address long term ecological interactions of Steller sea lions and marine ecosystem changes.
  1. Foraging trip duration and time ashore will be similar between lactating Steller sea lion females in Oregon and Alaska.
  2. Foraging trip duration will be greater for females nursing juveniles (> 12 mo) than females nursing pups.
  3. Attendance patterns will not differ between autumn and winter seasons, but will between years depending on ocean conditions.
  4. Females nursing male pups or juveniles will have longer foraging trips than those nursing females.

February 7th, 2008 - Camera equipment purchased.  Camera system tested successfully in laboratory.  Owners of Sea Lion Caves has agreed upon locations of the cameras in the cave.

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