One particular facet of salmon life history- the relationship between ocean survival and variations in environmental conditions- has been greatly ignored until recent years. To stimulate interest in this issue, the National Marine Fisheries Service and Oregon State University convened a workshop on Estuarine and Ocean Survival of Pacific Salmonids this past March in Newport, OR. The workshop brought together oceanographers, fisheries scientists, ecologists and managers to synthesize information relating physical, chemical and biological conditions and factors to salmon survival, and to identify questions, hypotheses and research strategies to improve understanding of how estuarine and ocean habitats affect survival.
PFEG oceanographer Frank Schwing was an invited speaker, and addressed the approximately 100 attendees on the patterns of decadal variability in the Northeast Pacific (NEP) and how pelagic stocks may be affected. The focus of the talk was to describe decadal-scale differences in the spatial texture of the NEP, to provide a base from which to evaluate the effect of climate change on the area's ecosystems, and particularly on its salmonid populations and fisheries. An extended abstract of this presentation will appear in a NOAA Technical Memorandum published later this year.
The California Current System (CCS) can be divided into three distinct geographical regions. The northern region (north of 40°N) features a rapid transition from strongly equatorward to poleward wind stress with distance north. The mean stress north of 44°N is poleward and has become increasingly poleward over time. SSTs north of 40°N vary temporally in unison, and exhibit little spatial difference in magnitude. Winds south of 40°N are equatorward and can be described in terms of central and southern regions. The central region (32-40°N) exhibits the greatest coastal wind stress magnitudes, and the greatest scales of interannual-to-decadal variation in stress and SST as well. Stress in the southern region (22-32°N) has become increasingly equatorward over time as well, but with relatively little interannual variation.
SSTs in the CCS off California appear to warm rapidly in response to the 1957 and 1983 ENSO events as well as the 1976 regime shift. SSTs off Washington and Oregon, on the other hand, take several months to years following the 1976 shift to warm by similar amounts, and are less sensitive to ENSOs. Although CCS SSTs exhibit a warming tendency south of 36°N, they show a cooling tendency north of 36°N. However shore-based SST trends along the entire U.S. west coast display a significant warming tendency over the past several decades. A lack of correspondence between the CCS and shore SST time series north of 36°N suggests there is considerable cross-shelf as well as latitudinal variability in the NEP. This is confirmed by analysis of the decadal differences before and after 1976 for the North Pacific, as well as examination of data in the NEP on 1° space scales.
SST shows decadal-scale periods of warming and cooling that extend along the entire coast. Wind stress anomalies are less coherent latitudinally and are uncorrelated with local SST, suggesting that decadal-scale SST variability in the coastal NEP is controlled by the basin- to global-scale meteorological fields, rather than local wind forcing. Preliminary analysis of decadal changes beginning in 1976 shows a strong correlation between anomalously high wind divergence, hence greater upwelling, and cool SST in winter over the central north Pacific. Anomalously weak divergence and warm SSTs are seen in a broad stretch along the North American west coast. Cool anomalies propagate eastward until they extend along the West Wind Drift to within a few degrees of the coast off Oregon and Washington. Thus it appears that wintertime anomalies in wind forcing over the central Pacific, rather than changes in coastal wind, lead to the observed regional SST pattern in the coastal NEP. A complex interaction of spatially as well as temporally varying Ekman transport, wind mixing and direct heating appears to be responsible for the long-term fluctuations in SST.
The correlation between poleward stress and SST is positive in much of the central region; increasing equatorward stress coincides with a cooling trend. Outside of the central region, however, the trends in stress and SST are negatively correlated. Increasing equatorward stress coincides with warming south of 34°N, while greater poleward stress accompanies a cooling trend north of 44°N. It is apparent that over the last several decades surface coastal waters in the northern portion of the California Current have either experienced a different set of forcing conditions from those further south, or have responded differently to large-scale atmospheric forcing. Not only are the tendencies of wind and SST different in these regions, but the different linear relationships between stress and SST implies that the primary mechanisms driving variability in SST, and probably the general ocean circulation, on decadal time scales is fundamentally different in these regions.
The results presented at this workshop clearly demonstrate the highly variable nature of the NEP environment in time and space, and argue against assuming climate change is constant, or that it can be represented by a record from a single location. The distinct latitudinal regionalization and cross-shelf variability of wind and SST fields have key implications for ecosystem studies as well as fisheries management, particularly for salmonids, which have latitudinally distinct stocks. These results stimulated considerable discussion during the oral presentation sessions and the subsequent working groups. The participants agreed that the ocean conditions and variability plays a much more critical role in salmon survival than has been traditionally accounted for in conceptual models. There was also a general consensus that our analysis of historical data demonstrate the need for a long-term interdisciplinary ocean monitoring program.
The analysis of ocean variables led many to consider that the mechanisms leading to such fluctuations may account for variability in other factors (e.g., precipitation) that could determine interannual changes in survival. It was agreed that the salmon's complex life history combined with the numerous physical forces affecting their distribution, migration, growth, and mortality make understanding salmon survival an extremely complicated issue. However it was brought to focus that freshwater survival is not the exclusive factor in determining the successful return of smolts, and that estuarine and ocean variability, related to many different processes and forces on a number of time and space scales, may well be the critical factor in controlling fluctuations in salmon populations, and specifically the long-term declines in the size of the Pacific Northwest fisheries. (F. Schwing, (831) 648-9034)