Decline of the Common Murre Colony at Redding Rock, California, and Restoration Options
The decline of the Common Murre (Uria aalge) colony on Redding (or ―Reading‖) Rock, Humboldt County, California, was investigated and restoration options examined for preventing colony loss and increasing numbers of breeding birds to self sustaining levels. Restoring the Redding Rock murre colony was targeted in the Stuyvesant Oil Spill Restoration Plan, as well as the Kure and Luckenbach oil spill restoration plans, to partly repair impacts to the northern California murre population caused by mortalities of thousands of murres in these oil spills and their lost progeny. This assessment includes: 1) population trends of Common Murres, California sea lions (Zalophus californianus), Steller sea lions (Eumatopius jubatus), and other breeding seabirds on Redding Rock in 1979-2008 from nearly annual aerial photographic surveys; 2) status of the murre colony and sea lion haul-out in 2009 from more intensive boat and aerial photographic surveys; 3) potential long-term impacts to the murre colony from sea lions and maintenance of the Redding Rock Aid to Navigation (ATON); and 4) possible methods for restoring the Redding Rock murre colony. In 1979-1990, Redding Rock supported about 1,600 breeding murres (800 pairs) per year. While annual variation occurred, colony size appeared to be fairly stable. After a two-year hiatus in surveys in 1991-1992, the colony was found to be nearly empty (29 birds) in 1993 during intense El Niño conditions when large numbers of California sea lions were first noted. Several distinct breeding areas were empty, including the largest breeding cluster on the top plateau of the rock. From 1994-2002, the murre colony continued to decline and by 2002 most historic nesting areas were abandoned; counts in breeding areas averaged only 204 birds (range = 83-375), a 79% decline from 1980-1990 average counts. The timing of this decline was associated with: 1) an increase in the numbers of California sea lions hauling out high up on the rock; 2) continued human disturbances during ATON maintenance; and 3) significant mortalities of murres during the 1997 Kure and 1999 Stuyvesant oil spills. Unlike threatened Steller sea lions which also haul-out on Redding Rock, California sea lions now regularly climb to the top plateau; this apparently displaces murres from traditional breeding areas. Between 1995 and 1999, murres initiated breeding at two new areas (Areas H and I) on cliff ledges inaccessible to sea lions and humans on foot maintaining the ATON. These have been the only known breeding areas since at least 2003. However, limited habitat on these ledges allows for only small numbers of breeding murres. In 2003-2008, annual counts in breeding areas averaged only 102 birds (range = 76-114 birds), 50% lower than in 1994-2002 and 89% lower than in 1980-1990. Between 2003 and 2008, numbers of sea lions (especially California) on Redding Rock remained high, and California sea lions hauled out in historic murre breeding areas nearly every year. Concurrent with the appearance of large numbers of sea lions was a decline in number and frequency of breeding by Brandt’s Cormorants (Phalacrocorax penicillatus) and recent breeding of small numbers of Western Gulls (Larus occidentalis). 12 More intensive data collection in 2009 and the early part of 2010 confirmed and expanded upon information from previous annual surveys. Murres bred only on the cliff ledges of Areas H and I, where average peak season counts were101 birds (range = 77-125), similar to 2003-2008 counts. We estimated a breeding population of about 50 pairs of murres (100 breeding birds), or only 6% of the 1980-1990 average. Large numbers of sea lions consistently hauled out on the rock, including in historic murre breeding areas. Monitoring of sample nest sites confirmed successful breeding. Egg laying began about 1 May, peaked in mid-May, and continued until about mid-June. Chicks began departing from the rock in late June and the last chicks and adults departed between mid-July and early August. Breeding phenology appeared to be somewhat later at Redding Rock than other nearby reference colonies at Castle Rock and the Trinidad area. Brandt’s Cormorants and Western Gulls also nested but had low breeding success. Use of the rock as a sea lion haul out changed dramatically during the study period. In 1979-1990, the rock was frequently used by small numbers of Steller sea lions (average = 6) and rarely by small numbers of California sea lions (maximum = 7). After 1990, use increased for both species, especially California sea lions. Since 1993, both species have been consistently present but California sea lions became the dominant species. Although numbers of both species increased between all study periods, the most increase occurred between the 1979-1990 and 1993-2002 periods. Up to 2008, counts ranged as high as about 90 and 673 Steller and California sea lions, respectively. Surveys in 2009-2010 showed constant high use of the rock by sea lions; counts during the seabird breeding season were similar to other high-use years. While Redding Rock is not a pupping site, female Steller sea lions nursed pups (maximum = 19) in the fall and winter months. Coast-wide sea lion surveys in 2009 from Crescent City to Trinidad showed that Redding Rock was one of the most consistently used haul outs for Steller sea lions and the largest haul out for California sea lions in this region. To prevent colony loss and increase murre numbers to self-sustaining levels on Redding Rock, restoration efforts are required. No action may result in loss of the currently very small colony in the near future. Key restoration issues of concern are: a) the degree of benefits to murres; b) the degree of impacts to sea lions; c) continued disturbance issues; and d) permanent addition of visible artificial structures including consideration of the cultural value of the rock to the local Yurok Tribe. We have identified two main restoration options for murres which would address the need to redevelop a self-sustaining murre colony: 1) Sea Lion Barrier Option: three approaches were provided to increase the current murre colony by 200 to 1,000 pairs by installing barriers (such as a concrete wall designed to visually blend into the rock) to block climbing access to California sea lions to portions of the top of the rock. Under these options, loss of California sea lion habitat varied but would be negligible and no loss of Steller sea lion habitat would occur. One option would limit re-population of murres to areas away from human foot access to the ATON. 13 2) Artificial Ledge Option: the murre colony would be increased by about 200 pairs by installing artificial ledges on the western cliffs adjacent to current breeding areas. Under this option, no loss of sea lion habitat would occur, artificial ledges would visually blend into the rock and be designed to exclude pelicans, and murres would not be accessible to humans on foot maintaining the ATON. Under either option, we recommend that if breeding or significant attendance with breeding-related behaviors in restoration habitats have not occurred within two years, social attraction techniques should be considered to stimulate murre breeding. Also, it is necessary to address human disturbance issues to murres by preventing ATON maintenance during the breeding season and prohibiting boats from close approach to the rock.