Sanctuary Integrated Monitoring Network
Monitoring Project

Long-term Studies of Seabirds on Año Nuevo Island and Mainland

Principal Investigator(s)

  • William Sydeman
    PRBO Conservation Science
  • Julie Thayer
    PRBO Conservation Science


  • SIMoN
  • Exxon/Mobil Corporation
  • Bella Vista Foundation
  • Oracle Corporation
Start Date: January 01, 1992

Several new seabird colonies have been established in the last decade in central coastal California. Año Nuevo Island (ANI) is the largest and most diverse seabird breeding colony in the Monterey Bay National Marine Sanctuary (MBNMS), hosting breeding populations of Rhinoceros Auklets (Cerorhinca monocerata), Cassin's Auklets (Ptychoramphus aleuticus), Brandt’s Cormorants (Phalacrocorax penicillatus), Pelagic Cormorants (P. pelagicus), Western Gulls (Larus occidentalis), Pigeon Guillemots (Cepphus columba) and Black Oystercatchers (Haematopus bachmani). Additionally, ANI is an important roosting site for Heerman's Gulls (Larus heermani) and the endangered Brown Pelican (Pelicanus occidentalis). Other species that have occurred on ANI include state-listed Ashy Storm-Petrels (Oceanodromo homochroa) and Fork-tailed Storm-Petrels (O. furcata).

There are several threats to marine bird populations of the MBNMS. Environmental variability (e.g., climate change) and a variety of human factors (e.g., bycatch and indirect effects from commercial fisheries, oil and pesticide pollution) affect seabird populations in this region. We have initiated what we hope will be long-term (i.e., multi-decadal) studies of breeding seabird population dynamics, demography and prey use in the MBNMS, with an emphasis on ANI. Such information will provide valuable insights into changes of these upper-trophic level predators in central California and help marine wildlife managers develop appropriate conservation strategies.

Summary to Date

The year 2003 represents the 11th year of monitoring seabirds on Año Nuevo Island (ANI). Rhinoceros Auklet, Brandt’s Cormorant, and Western Gull populations increased from 2002 to 2003, to the highest levels yet recorded for ANI. Reproductive success, however, has been variable. Rhinoceros Auklets banded as chicks in nest boxes earlier in our project are now recruiting into the ANI breeding population, providing novel information on age-at-first-breeding, immigration and recruitment rates for this species worldwide. Auklet diet reveals a shift from an anchovy-based diet in the 1990s to rockfish based diet in recent years, corresponding to a shift in ocean climate in 1998-1999.

We continue to provide scientific advice to the California Department of Fish and Game (CDFG) on how to manage fisheries from an ecosystem perspective. Most recently, we reviewed the draft Market Squid Fishery Management Plan and provided recommendations to protect squid as one of the primary prey resources for marine birds and mammals in the MBNMS. Future monitoring should include analyses of food habits of other seabirds of the MBNMS.

For more information download the full PDF report below.

Monitoring Trends

  • Rhinoceros Auklets totaled approximately 270 breeding birds in 2003. Brandt’s Cormorant and Western Gull populations on the island numbered ~ 1740 and ~2370 birds, respectively.
  • Productivity (chicks fledged per pair) in 2003 was lower than in 2002 for all species except Pelagic Cormorants breeding on the mainland.
  • We collected and archived diet samples Rhinoceros Auklets, Pelagic Cormorants, and Brandt’s Cormorants in 2003, adding to our long-term databases of marine bird food habits. See Figure 2 below.


In the past 5 years, marine bird populations have increased on ANI yet reproductive success has been variable. Through these studies, we have gained valuable new information on survival, recruitment and immigration, demographic processes that are important to better understand seabird population dynamics. Prey is also key to population dynamics, but highly variable due to climate change and at risk from certain fisheries. Sustained time-series of marine bird population trends, productivity and prey use are valuable for tracking long-term changes in community and ecosystem health.

Future work should involve analysis of cormorant diet. Analyses of ANI seabird population dynamics, reproductive success and prey use in relation to other colonies would also be beneficial to understand how populations of these species in the MBNMS compare to the dynamics of these populations elsewhere.

Study Parameters

  • Age & Growth
  • Abundance
  • Dispersal & Recruitment
  • Stock assessment
  • Trophic association

Figures and Images

Aerial view of Ano Nuevo Island and mainland. Photo: PRBO Conservation Science

Figure 1. Population dynamics of Pelagic Cormorants, Pigeon Guillemots and Cassin’s Auklets on Ano Nuevo Island, 1976-2003.

Figure 2. Percent number of prey species in Rhinoceros Auklet chick diet on Año Nuevo Island over 11 years, 1993-2003.