Principal Investigator(s)
Jaime Jahncke (PRBO Conservation Science)
Meredith Elliott (PRBO Conservation Science)
Ben Saenz (Stanford University)
Start Date 2004-05-01
Funding
Resources Legacy Fund Foundation
National Fish and Wildlife Foundation
California Sea Grant
Overview
Krill (also known as euphausiids) are crustacean zooplankton found in marine environments, and they are unique from other zooplankton in that they are relatively large and abundant. Euphausiids are important in marine food webs; they consume phytoplankton and small zooplankton and then become prey to fish, seabirds, and whales.
There are two common species of krill found in the Gulf of the Farallones: Euphausia pacifica and Thysanoessa spinifera. These species vary in abundance and distribution depending on upwelling conditions. Cassin’s auklets (Ptychoramphus aleuticus) are zooplanktivorous seabirds that breed on the Farallones archipelago from March to July. Diet data from Cassin's auklets breeding on Southeast Farallon Island (SEFI) have shown that E. pacifica is eaten more in the early part of the breeding season, while T. spinifera is more prominent in the latter part of the breeding season. We wondered if this switch from one species to another was due to changes in availability of these prey or changes in foraging locations of the auklets.
The important objectives of this project are to:
•Investigate the krill populations in the Gulf of the Farallones;
•Investigate foraging locations of Cassin’s auklets in the Gulf of the Farallones;
•Investigate the diet of Cassin’s auklets on SEFI.
Summary to Date
Since the inception of this project, we have observed very different ocean conditions in each year. Upwelling conditions in 2004 were near average; 2005 and 2006 were warm-water years, characterized by weak, delayed upwelling; and 2007 and 2008 were cold-water years where upwelling was strong and started early. Our results on the krill populations (while not complete) reflect these varying ocean conditions. Analysis of July 2004 through October 2006 euphausiid data indicates that E. pacifica and T. spinifera are present in the upper 50 m of the water column year-round, with E. pacifica being more abundant than T. spinifera for most cruises, with the exception of some late-summer and autumn cruises (Figure 2). When looking at adult euphausiids (the main prey item of Cassin’s auklets), E. pacifica adults were present in spring and summer months, and T. spinifera adults only appeared in June 2005 (Figure 3). Age class composition of these two species varied. Younger age classes (zoea and juveniles) dominated in both species during most cruises in 2005 and 2006; then adults became more prevalent in 2007 and 2008 (Figures 4 and 5).
Hydroacoustic data on krill abundance showed dense layers of krill in 2004, which led to large and densely aggregated flocks of auklets that congregated over a smaller area at the edge of the shelf near SEFI. However, other years were characterized with more frequent but loosely packed swarms of krill, which resulted in small and dispersed flocks of auklets that occurred scattered along the shelf break from SEFI to Cordell Bank (Figure 6).
Diet results from Cassin’s auklets breeding on SEFI have shown that adult krill comprise ~80% of their diet, except during warm-water years. The diminished availability of adult krill in 2005 and 2006 was clearly reflected in their diet, as the few samples collected during these years consisted mostly of mysids. With the return of adult krill in 2007 and 2008, krill appeared in the diet again (Figure 7).
Reduced adult krill abundance during the auklet breeding season in 2005 and 2006 led to a delay in breeding, abandonment of eggs, and complete breeding failure of Cassin’s auklets – the first ever recorded collapse in auklet reproduction in the 30 years of research on the Farallones. Since adult euphausiids have increased in their relative abundance in 2007, the auklets have rebounded (Figure 8).
Springtime ocean conditions (temperatures, ocean productivity) are critical for auklets and other seabirds because that is when the birds are accumulating energy for and/or recovering from egg production. Egg abandonment occurred in 2005 because upwelling winds were particularly weak for most of the preceding 5 months, surface temperatures were elevated, and krill were very scarce near the nesting colonies. By June 2005, upwelling-favorable winds increased, sea surface temperature was near average, chlorophyll was above average and prey abundance in the upper 50 m increased. A few birds responded with a new breeding attempt at this time, but were unsuccessful as conditions quickly worsened again in July. Similar conditions occurred in 2006, with weak, delayed upwelling conditions that led to decreased krill abundance and the eventual abandonment of nests by Cassin’s auklets. Ocean conditions changed in 2007 and 2008, with stronger upwelling ensuing earlier in the spring, resulting in a relatively greater abundance of adult euphausiid abundance in the region. Auklet productivity has rebounded and returned to the long-term average.
Monitoring Trends
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Discussion
These results show how anomalous “bottom-up” forces can shape the ecology of upper trophic-level predators on relatively short temporal scales. Upwelling favorable winds regulate the overall amount of nutrients available for primary production; this determines the abundance of prey available for upper trophic-level predators. Seasonal wind anomalies can create mismatches between prey availability and predator demand for prey, leading, at times, to complete loss of an annual cohort, as observed for auklets in 2005. Mobile zooplankton (krill) and upper trophic-level predators (marine birds) have the ability to act in response to short-term changes in climate and food resources. As evidenced by the response of marine birds in this study, it is apparent that the timing of the climate anomalies, and not just their magnitude, play a large role in the structure and dynamics of the ecosystem. Timing of anomalous weather may have a similar effect on other upper trophic-level predators, and likely on krill as well, depending on the seasonality of their reproductive cycle and the pattern of climate variation.
Literature Cited
Jahncke, J., B.L. Saenz, C.L. Abraham, C. Rintoul, R.W. Bradley, W.J. Sydeman. 2008. Ecosystem responses to short-term climate variability in the Gulf of the Farallones, California. Progress in Oceanography 77:182-193.
Schwing, F. B., Bond, N. A. Bograd, S. J. Mitchell, T. Alexander, M.
& Mantua N. 2006. Delayed coastal upwelling along the U.S. West Coast in 2005: A historical perspective, Geophysical Research Letters, 33, L22S01, doi:10.1029/2006GL026911.
Sydeman, W. J., Bradley, R. W., Warzybok, P., Abraham, C. L.,
Jahncke, J., Hyrenbach, K. D., Kousky, V., Hipfner, J. M. & Ohman, M. D. 2006. Planktivorous auklet(Ptychoramphus aleuticus) responses to the anomaly of 2005 in the California Current, Geophysical Research Letters, 33, L22S09, doi:10.1029/2006GL026736.
Collaborators
Lisa Etherington (NOAA, Cordell Bank National Marine Sanctuary)
Jan Roletto (NOAA, Gulf of the Farallones National Marine Sanctuary)
Rachel Fontana (UC Davis, Bodega Marine Laboratory)
Nina Karnovsky (Pomona College)
Moira Galbraith (Fisheries and Oceans Canada, Institute of Ocean Sciences)
Tracy Shaw (NOAA, Hatfield Marine Science Center)
John Largier (UC Davis, Bodega Marine Laboratory)
Steve Morgan (UC Davis, Bodega Marine Laboratory)
William Sydeman (Farallon Institute)
Study Parameters Click here to view the parameters measured in this study.
Study Methods Click here to view the methods and materials used in this study.
Images and Documents Figure 1. Map of study area indicating survey lines, CTD (conductivity-temperature-depth recorder) stations and location of the National Data Buoy Center buoys used in this study. Figure 2. Average densities of Euphausia pacifica and Thysanoessa spinifera in the Gulf of the Farallones, July 2004 – October 2006. Figure 3. Average densities of adult Euphausia pacifica and Thysanoessa spinifera in the Gulf of the Farallones, July 2004 – October 2006. Figure 4. Age class composition of Euphausia pacifica in the upper 200 m, October 2004 – September 2008. Figure 5. Age class composition of in the upper 200 m, October 2004 – September 2008. Figure 6a. Cassin’s auklet foraging distributions during the month of May, 2004. Figure 6b. Cassin’s auklet foraging distributions during the month of May, 2005. Figure 6c. Cassin’s auklet foraging distributions during the month of May, 2006. Figure 6d. Cassin’s auklet foraging distributions during the month of May, 2007. Figure 6e. Cassin’s auklet foraging distributions during the month of May, 2008. Figure 7. Diet composition of Cassin’s auklets on Southeast Farallon Island, 1977-2007. Figure 8. Breeding success (chicks fledged per breeding pair) of Cassin’s auklets on Southeast Farallon Island, 1972-2007.
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