We found that the rockfish proportion in SEFI murres' diet was strongly correlated to the frequent shifts (every 3-7 years) due to El Niño (warm-water) and La Niña (cold-water) events. The relationship is less obvious, however, with respect to multi-decadal shifts between warm and cold phases in the northern Pacific, as tracked by the Pacific Decadal Oscillation (PDO). The PDO switched from cold to warm in 1976-77 and is considered by many to have switched back to a cold phase in the late 1990s.
Understanding these correlations called for multiple data sets, spanning the same time period but on different temporal scales: one year by year, or inter-annual; and one seasonal within single years, or intra-annual.
|Figure 1. Components of Common Murre diet on Southeast Farallon Island, 1975-2002. Arrows at top indicate warm-water El Niño (gray) and cold-water La Niña (black) events.Figure 1.|
Figure 1 shows the proportion of various prey in the SEFI murres' diet over the 28 years of our study to date--year by year. Following the onset of a PDO warm phase in 1976-77 (which amplified El Niño events for the next 10-20 years!), one might expect the murres' use of rockfish to decline. Instead, rockfish use only declined beginning in 1989, but then rebounded shortly after the presumed return to a cool phase of the PDO in about 1999.
Data from the same 28 years on a seasonal scale (within years) help reveal the actual nature of this biological response. During the seabird breeding season, PRBO biologists on SEFI documented the prey that Common Murres brought to their colonies daily during the 30-40 days of chick rearing, as shown in Figure 2 (page 8).
At the beginning of the study, the PDO was still in a cool phase; the proportion of rockfish in the diet was often high (Figure 1) and unchanging within seasons (Figure 2). In the 1980s, after the 1976-77 shift to a warm phase of the PDO, the proportion of rockfish often declined during each season (Figure 2). Seasonal declines continued to be observed until the mid- to late 1990s, when the overall proportion of rockfish was extremely low, remaining low within and among seasons.
In 2001 and 2002, a few years after the presumed return to a cool regime, rockfish returned as a dominant prey item (Figure 1), with a seasonal pattern in 2002 similar to those of the 1970s during the last cool regime (Figure 2,).
Aspects of rockfish life history affect the availability of juvenile fish late in the season, when SEFI murres raising chicks may prey on them. The short-bellied rockfish, most common among Sebastes species used, generally live at least 30 years, reaching maturity at 2-5 years. This long life span may have led to a delayed effect on population size following the 1976-77 PDO shift. A prolonged period of sub-optimal reproductive conditions following the regime change may have ultimately led to a significant reduction in population size.
|Murre use of rockfish during chick-rearing on Southeast Farallon Island.Figure 1.|
Our evidence of seasonal changes in Common Murre diet (Figure 2) suggests that rockfish populations were gradually affected over the 20 years following the shift from a cool to a warm regime. The close parallels between (1) rockfish take by the birds, (2) El Niño and La Niña events, and (3) changes in inter-decadal climate patterns, including the PDO, make climate a compelling explanation for the intra-annual and inter-annual variability we observed.
Monitoring fish population dynamics through this kind of marine bird diet study provides a level of detail that is unlikely to be found in any direct monitoring of these fish populations. Without the use of daily observations, within 30- to 40-day seasons over 28 years, the effects of the regime changes on rockfish populations would have been obscured. When data were summarized on only an annual time scale, no changes were observed at the time of the 1976-77 regime shift--or even in the decade following. When we looked at seasonal as well as annual diet patterns, however, the influence of inter-decadal climate regimes on rockfish was revealed.
This study points to the value of using marine birds as indicators of the status of ecosystems and food webs. Using Common Murres as a sampling tool, we documented seasonal change in prey use and thus gained insight into the effects of inter-decadal climate change on rockfish abundance.This article is drawn from a paper by Aileen K. Miller and William J. Sydeman entitled "Rockfish response to low-frequency ocean climate change as revealed by the diet of a marine bird over multiple time scales," in press in Marine Ecology Progress Series. Claire Peaslee and Nadav Nur helped adapt it.