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Cassin’s auklet breeding success

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Cassin’s auklet breeding success

Over a 45 year period, the breeding success of this seabird on the Southeast Farallon Island near San Francisco has become more variable. Breeding success, or the number of chicks per breeding pair, reflects the availability of their prey, which is affected by ocean conditions. 

The Cassin’s auklet (Ptychoramphus aleuticus) is a small, dark seabird with an extraordinary ability for diving to catch prey underwater. Each breeding pair produces a single egg; after successfully fledging the first chick, the auklet can lay a second egg – an unusual trait among seabirds. Southeast Farallon Island, located 30 miles west of San Francisco, is home to a large colony of these seabirds. This region is rich in biodiversity, fueled by a wind-driven process known as upwelling, which brings deep, nutrient-rich waters to the surface. Seasonal upwelling promotes the growth of the algae that support a range of marine organisms, from krill to seabirds and marine mammals. For more information, download the Cassin’s Auklet Breeding Success chapter. 

The Cassin’s auklet’s breeding range extends from Alaska’s Aleutian Islands to islands off the middle Baja California peninsula. Scientists have been monitoring Cassin’s auklets (left, top and bottom) since the 1970s at a study site on Southeast Farallon Island (shown on map and pictured in inset, right).

Two photos of Cassin's auklet are on the left and a Map of the gulf of Farallonnes with an inset of the Farallon islands

Credit: L. Lauber, US Fish and Wildlife Service (birds); US Geological Survey (Map)

What does the indicator show?  

  • The average number of offspring produced per breeding pair of Cassin’s auklets on Southeast Farallon Island each year became increasingly variable between 1972 and 2016.  

Why is this indicator important?  

  • Seabirds serve as reliable visual indicators of ocean conditions. Tracking reproductive success among auklets provides clues into the impacts of climate change on the marine ecosystem.  

What factors influence this indicator? 

  • The abundance and quality of krill affect seabird breeding. Cassin’s auklet breeding success was high in years when the krill they feed on were abundant. In 2005 and 2006, when no live offspring were produced, scientists noted that krill were absent from the diet fed to chicks. 
  • Ocean warming may reduce upwelling off the California coast. With weaker upwelling, less nutrients are available to support the growth of algae, leading to a reduction in krill and other zooplankton eaten by fish, seabirds, and whales and other marine mammals. 

Additional resources

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