Sanctuary Integrated Monitoring Network
Monitoring Project

Long-term monitoring of Northern Elephant Seals: colony development and growth rates in the Monterey Bay National Marine Sanctuary

Principal Investigator(s)

  • Burney Le Boeuf
    University of California, Santa Cruz
  • Richard Condit
    Smithsonian Tropical Research Institute (STRI)


  • SIMoN
Start Date: December 31, 1969
End Date: March 15, 2008

Northern elephant seals (Mirounga angustirostris) offer important practical, economic and scientific advantages for long-term monitoring of a top trophic level predator in the Monterey Bay National Marine Sanctuary (MBNMS), relative to other pinnipeds, whales, sea otters and large fishes. They breed at rookeries along the length of the Sanctuary and are abundant in these waters throughout the year. The growth of the population, since near extinction in 1884 and subsequent recolonization of California from the mother colony in Baja California, Mexico, is a model for a recovering and expanding mammal population that is exceptionally well documented.

We initiated a study of the population in 1968 involving systematic censuses and mark/recapture studies on the major rookeries which continues to the present; this long-term study permits a detailed documentation of population growth and colonization of the Sanctuary via dispersion and emigration. We request funds to complete data entry of censuses and tagging records and to convert the data into a modern, relational database.

Summary to Date

In late 2003, we were awarded $20,000 by the Monterey Bay Marine Sanctuary
SIMoN program to convert 40 years of paper records on the northern elephant seal
population at Año Nuevo into a thorough and modern computer database. The funds were
used during 2004 to 2005 to hire assistants to code data. A total of 2500 hours of data
entry was accomplished. The funds were exhausted in December, 2005. Three different
sets of data were included in the work:
1) Counts of pinnipeds at Año Nuevo Island and mainland since 1968.
2) Tags placed on elephant seals at Año Nuevo, 1968 to 2003.
3) Resightings of the same tags subsequent to the original tagging, 1987 to 2003.

A regular program for marking elephant seals at Año Nuevo with permanent numbered
tags began in 1964 by Tom Poulter of Stanford Research Institute.  Le Boeuf took over
the program in 1967. Most of these tagging records were computerized during the mid­
1990s, and organized into a Panorama database. Starting in 2004, we reorganized this
into a MySQL database on a web­accessible server, and researchers began entering new
tag information directly (via another PHP interface). With the SIMoN funds, student
assistants entered 2102 old records that had been missed, nearly all prior to 1985. 
The database is now essentially complete for elephant seals tagged at Año Nuevo since
1969.  A total of 17,672 individual animals are in the database, including 14,031 tagged
as pups. The latter are those whose ages are known exactly at any later sighting. Modest
error­screening has been done, but there are still several problems that need attention.

The most important part of our SIMoN funded project involved creating a database of
tag observed on juvenile and adult elephant seals. Tagged animals identified years after
birth provide information on movements, lifetime pupping success, and longevity. They
form the basis of mark­recapture analyses of survival and dispersal, and sightings have
been carefully cataloged by UCSC researchers since 1969. As of 2004, these records
were poorly organized, with most not even typed into computer files. In 2004, we
designed a MySQL database to house the resightings and set up web­programs via PHP
to enter data. 

With the SIMoN grant, we began entering tag­sightings on females during the breeding
season and importing the records into the MySQL database. We have completed all 1987­
2003 records, including error­screening, including 61,000 sightings. Since 2004, tag
sightings have been entered directly by researchers who make the sightings. All together,
the database includes 20 years of breeding records for 3697 animals. Sightings prior to
1987 were organized into a SAS database in 1988, and we have begun the task of
merging this into the MySQL format, but not completed it.

Monitoring Trends

  • These tag sightings provide lifetime survival and breeding histories for substantial samples. One female tagged at the Año Nuevo mainland in 1984 (tag H257) was seen breeding near her birthplace in 13 out of 18 breeding seasons, and raised a pup at age 21 in 2005. 
  • An animal branded in 1987 (#315) was seen in 14 out of 17 seasons and is still alive in 2006.
  • The age distribution produces an estimate of 77% for annual survival rate of adult females (figure below). This method ignores tag loss, though; true survival  rate is thus probably 80 to 83% per year. 

Study Parameters

  • Growth
  • Range/Biogeography
  • Migration/movement patterns
  • Distribution
  • Tagging

Figures and Images

Figure 1. Age distribution of female northern elephant seals (Mirounga angustirostris) at Año Nuevo. The blue line indicates an adult survival curve, at about 77% per year.

Figure 2. Male northern elephant seal (Mirounga angustirostris) emerging from the surf to haul out on the beach.

Figure 3. Male northern elephant seals (Mirounga angustirostris) fight to establish territories on the beach and then attract females to their harem. They must also defend their harem from other males. These are likely young males just going through the motions.

Figure 4. Female (or young?) northern elephant seal (Mirounga angustirostris) at Año Nuevo Island.


  • Condit and Le Boeuf (2006)
    Long-term Monitoring of Northern Elephant Seals: Colony Development and Population Growth in the Monterey Bay National Marine Sanctuary
    508 KB PDF
  • Condit et al. (2007)
    Estimating population size in asynchronous aggregations: a Bayesian approach and test with Elephant Seal censuses
    240 KB PDF