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Sancutary Program Accomplishments Open Ocean & Deep Water Systems Endangered & Threatened Species Marine Mammals & Bird Surveys
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Preliminary
results: Seabirds: Among individual seabird species, Surf Scoters and Cassin's Auklets exhibited the greatest difference between years and Common Murre carcasses were found three months earlier than the previous year. The causes for these differences are not known; however, several necropsied Surf Scoters exhibited a high parasite load of banjo worm, and the Cassin's Auklet, a small planktivore, may have been more susceptible to El Niño-enhanced storms than larger fish-eating species. Marine Mammals: The increase in marine mammal deposition during May and early June 1998 may have been caused by the unusual presence of female California sea lions in Monterey Bay during late spring/early summer. Normally, these animals would be expected to be at breeding rookeries off Southern California; however, El Niño conditions may have forced them to move north in search of resources. Although the causes of the increase in marine mammal mortality are not known with certainty, it is suspected that a toxic diatom bloom during May (Pseudo-nitzschia australis) was partly responsible. The number of marine mammal carcasses encountered remained high during July and August, but these were likely the same carcasses as those counted during the June survey. Scott R. Benson1, Andrew
DeVogelaere2, and James T. Harvey1
A total of 402 marine mammal sightings encompassing seventeen species were made during the seven surveys. The California sea lion was the most frequently encountered marine mammal. Pacific white-sided dolphins and humpback whales were the most frequently-encountered toothed cetacean and baleen whale, respectively. Common dolphins were numerically the most abundant cetacean, because this species was seen in large groups. A total of 2,359 sightings of 9,994 seabirds were made while the ship was on transect. Species composition varied from month to month. The most abundant families of seabirds were the shearwaters and alcids (80 percent and 11 percent of all birds seen, respectively). Species composition was similar from May through September, but changed notably in October and November, with fewer Sooty Shearwaters and Common Murres, and an increased number of Buller's Shearwaters, Black-vented Shearwaters, and Rhinoceros Auklets. Preliminary results indicate that El Niño conditions likely affected the number and types of marine mammals encountered during the surveys. Dall's porpoises were seen less frequently than in previous years, whereas common dolphins were more numerous. In 1998 California sea lions were unusually abundant in early summer, when they normally would be expected to be at breeding rookeries off Southern California. The number of baleen whale sightings was greater in 1998 than 1997, and peak abundance was observed later in the season (August vs. May-June). The observed pattern of abundance was similar for zooplankton, the whales' primary prey. Seabird assemblages were roughly similar to those seen in 1997, but abundances of several species were notably lower (e.g., Common Murres, Cassin's Auklets, and Brandt's Cormorants). Furthermore, a large number of Storm Petrels was seen in September 1998, whereas this group of species was rare on all previous 1996-97 surveys. Scott R. Benson1, Donald
A. Croll2, and Baldo Marinovic2 |
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The
results outlined below refer only to
newly-deposited birds. Beachcast deposition of
marine birds from January to August 1998 was
extremely variable. A low beachcast deposition was
recorded in February and high deposition was
recorded during April. Beachcast deposition
remained high from early April through early July.
The amount of deposition was greater at all beaches
throughout the study area from May through July,
compared to the same period in 1997. The causes of
this elevated deposition during 1998 are not clear.
Many foraging guilds seem to have been affected and
overall species diversity was greater in 1998,
although Common Murres dominated the composition of
seabird carcasses encountered. In particular,
overwintering and resident seabirds were more
frequently found beachcast during the May - July
1998 surveys compared to the same period in 1997.
Although no comparable data exist for April 1997,
deposition rates of overwintering and resident
birds for April 1998 were also high. The extended
period of high beachcast indicates that resource
limitations (perhaps caused by El Niño) may
have been a factor contributing to elevated seabird
mortality from March through early July. Seabird
deposition recorded during August was much lower
than the four previous months and remained low
through the fall.