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Sanctuary Rocky
Intertidal Open
Ocean & Endangered
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Population
Change of Common Murres: Perspectives from the Bordering the northern boundary of the Monterey Bay National Marine Sanctuary, the largest Common Murre colony in central California occurs on Southeast Farallon Island (SEFI). The Farallon population may once have numbered close to a million birds but is now a fraction of its historic size due to commercial egg collecting, chronic and large-scale oil spills, pesticide contamination, over-exploitation of prey resources, and mortality in fisheries. Variability in forage fish and zooplankton availability linked to the El Niño phenomenon and longer-term climate change may also limit population growth. With the multitude of factors influencing this population, how can we separate natural and man-caused population fluctuations?
Research on the Farallones has revealed striking fluctuations in population size (Figure 1). The population increased from the early 1970s to early 1980s, declined precipitously between 1982 and 1988, demonstrated a short-term increase in the early 1990s, but has experienced limited growth (or even decline) in recent years. What explains these changes? In the 1970s there was a slight drop in population size in 1978, but in general numbers increased at about 8 percent per year from 1972 through 1982, reflecting high levels of prey availability during a period of moderate mortality. However, during the early 1980s, a near-shore set gillnet fishery caused substantial mortality through drowning of adult and sub-adult birds. Estimates indicate that upwards of 100,000 birds were drowned in this fishery. Major oil spills in the region in 1984 (Puerto Rican) and 1986 (Apex Houston) also caused significant mortality. There were also indications that the overall prey base was changing, thereby limiting opportunities for the birds to switch to alternative prey. For example, in years prior to 1989, murres alternated the chick diet between juvenile rockfish (mostly Sebastes jordani) and northern anchovy (Engraulis mordax). The availability of some species of juvenile rockfish has declined in this ecosystem, especially in the last decade, and this change is clearly evident in the food habits of murres on the Farallones. In the 1990s, while much of the set gillnet fishery was closed, a small fishery persisted in southern Monterey Bay and was responsible for an average of about 1,200 deaths each year. An emergency closure of fishing by this method within the foraging range of the murres in Monterey Bay, imposed by the California Department of Fish and Game in summer 2000, may help to resolve this mortality if it becomes a permanent law. Oil spills and chronic oil pollution have also continued to take a toll. For example, during the fall and winter of 1997-1998, hundreds of "tar balls" and oiled murres were found on beaches in the Gulf of the Farallones and Monterey Bay National Marine Sanctuaries. Entering the fourth decade of research on murres and other marine
birds on the Farallon Islands, the prognosis for conservation
is good if we can effectively manage human-related mortality factors.
Although the ecosystem has undergone change that might have limited
growth in the 1990s, indications are that the ocean is changing
again back to the cold-water regime that characterized the system
in the early 1970s. This purported "regime shift" may
result in a resurgence of certain prey populations that would
help support the large murre population in the region. William J. Sydeman
The Monterey Bay National Marine Sanctuary's beach monitoring program, Beach COMBERS (Coastal Ocean Mammal / Bird Education and Research Surveys), began its fourth year documenting monthly deposition of dead marine birds and mammals on sandy beaches within the Monterey Bay area. Since the program's inception in May 1997, Beach COMBERS volunteers have spent almost 1,500 hours counting and marking beachcast seabirds and marine mammals found on designated beach segments, covering fifty-one kilometers of sandy beaches within and around Monterey Bay. Counts of beachcast seabirds during January-October 2000 were more variable and over two times higher than during the same period in 1999, but lower than totals for 1997 and 1998 (Figure 1). The greatest deposition occurred during April and was dominated by Western Grebes (Aechmophorus occidentalis) and Clark's Grebes (Aechmophorus clarkii; Figure 2). The spring deposition of grebes was the third major deposition event detected by the monitoring program since its inception. Previous deposition peaks differed in the timing and composition of birds found beachcast. The first peak occurred during August-September 1997 and was dominated by Common Murres (Uria aalge). An increase in set gillnet fishing effort in southern Monterey Bay was suspected to be the primary cause of that deposition. The second peak, spanning nearly five months during the spring and summer of 1998, was composed of a wide assemblage of seabird species and was likely related to local El Niño conditions and a toxic diatom bloom within Monterey Bay.
The sharp spike in grebe deposition this year was not confined to the Monterey Bay area. U.S. Geological Survey biologist Brian Hatfield reported that deposition of Western / Clark's Grebes during February-April 2000 was significantly higher than the twenty-year average at beaches near Point Piedras Blancas (in the southern part of the Sanctuary). Volunteers monitoring beaches in the Gulf of the Farallones National Marine Sanctuary, to the north, also reported high numbers of beachcast grebes. The cause of the widespread event is un-known; however, all birds necropsied at the California Department of Fish and Game, Oil Spill Prevention and Response (CDFG-OSPR) facility in Santa Cruz were severely emaciated and many were immature. Biologists at the International Bird Rescue and Research Center reported very high reproductive success for Western and Clarke's Grebes during the summer prior to the spring beachcast event. Because mortality is high for first year grebes as they migrate from fresh water lakes to the ocean, it is likely that the increased deposition during the spring of 2000 was simply a reflection of the very successful 1999 breeding season.
Beach COMBERS volunteers will continue to collect fresh beachcast
seabirds for necropsy by personnel at CDFG-OSPR and graduate students
at Moss Landing Marine Laboratories. We anticipate that necropsied
seabirds will improve our ability to assess causes of death and
increase our understanding of the health of nearshore habitats.
We greatly appreciate the dedication of the many highly-skilled
volunteers who make this monitoring project possible. Scott R. Benson |
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For comments or question please refer to the Webmaster Last modified on: Jan 15, 2000 |
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