Ocean Conditions Symposium - Role of the Columbia River estuary and plume in salmon productivity

Role of the Columbia River estuary and plume in salmon productivity

Ed Casillas
National Marine Fisheries Service
Northwest Fisheries Science Center
Seattle, Washington

Introduction

There has been a common assumption that the size of the oceans confers stability in oceanic processes. Although seasonal differences were recognized, the biological resources of oceans were considered stable and limitless. However, recent efforts documenting the existence of regime shifts (Beamish et al. 1999) and common and recurring weather phenomena such as El Ni? and La Ni?, and their respective impacts on fisheries, have forced a reevaluation of this basic premise. There is now improved understanding of the link between marine fisheries and climate (Francis and Hare 1994; Mantua 1997). Much effort is trained on identifying important climate indices that drive changing weather patterns, including the Pacific Decadal Oscillation (PDO), ENSO (El Ni? Southern Oscillation) events, the Aleutian Low Pressure (ALP) index, and the Pacific Northwest Index (PNI), as examples. Despite improved knowledge, the efforts to understand the impacts of ocean conditions on salmon survival have been minimal. When salmon production has dropped, it has generally been attributed to the degradation of freshwater habitat. However, an evaluation of survival records for the major salmon species of the Northeast Pacific now shows mortality rates in the freshwater and ocean environments (egg to smolt vs. smolt to adult) are essentially equivalent (Bradford 1995). Thus change in salmon survival can be attributable to either or both habitats. Because climate changes affects physical oceanic characteristics (fronts and eddies, upwelling intensity, temperature of the water, etc.), features important to fishes, it is easy to understand how the climate and ocean links could directly affect salmon production. At the very least, assessment of restoration efforts in freshwater to rehabilitate salmon runs will need to consider ocean conditions to properly value the success of any incremental improvement of freshwater habitats.

Local Ocean Conditions Are Important

What is the spatial scale we need to consider in understanding the role of the ocean in salmon productivity? Certainly large ocean features are important when considering large aggregate populations, such as Pacific coast salmon stocks. However, the local marine environments may be equally important. Several lines of evidence support this contention. Peterman et al. (1998) and Pyper (1999) assessed variation in survival rates, length-at-age 4, and age-at-maturity for nine stocks of Bristol Bay sockeye salmon from northern Alaska, and 16 stocks of Fraser River sockeye salmon in southern British Columbia. They argued that yearly variation in any of these parameters, relative to the stock averages, were a reflection of enhanced or sub-optimal environmental conditions. Further, yearly co-variation, when stock indices were compared, indicated they were experiencing similar environmental conditions. Predictably, significant co-variation within Bristol Bay and Fraser River stocks for each of the parameters was identified. More importantly, significant co-variation in length-at-age and age-at-maturity between Bristol Bay and Fraser River stocks was identified. This finding is consistent with the mixing of these two stocks in the Gulf of Alaska through much of their marine life. However, although significant co-variation in survival rates existed within stocks, no significant co-variation was identified between Fraser River and Bristol Bay sockeye; that is, Fraser River and Bristol Bay stocks were not experiencing similar environmental factors that affected survival. They concluded that much of the difference in survival rates is attributable to conditions in the first summer in the marine habitat. Local marine environmental conditions where salmon stocks originate greatly affected survival. A second line of evidence is derived from the positive relationship between abundance of precocious (jacks) males and adult survival rates (Pearcy 1992). Precociousness is a function of environmental conditions; higher growth rates translate to increased proportion of jacks (Friedland 1996). Because of the speed of returning jacks (coho jacks, for instance, return to spawn after only 3-4 months in the ocean), they cannot migrate far from their rivers of origin as some of their larger adult counterparts. This finding suggests the local marine environmental conditions greatly affect survival and year-class success for outmigrating stocks of juvenile salmon. Understanding local marine conditions and their influence on survival and health of outmigrating juvenile salmon should help in identifying important features that benefit or suppress growth, recovery, and resilience of specific salmon stocks.

Within the local marine environments, do we consider the entire coastal domain, or do salmon appear to seek specific marine habitats. If so, what are the features? Identifying important attributes that salmon seek could assist in developing appropriate monitoring plans. One of the best lines of evidence of habitat selection by salmon comes from temperature preferences identified for salmon in the Gulf of Alaska (Welch et al. 1995; 1998). Salmon were found in specific oceanic habitats that were delineated by an upper and lower boundary temperature limit. Further, these limits varied with the season, increasing through the summer and were the lowest during the winter. It is clear that not all the ocean presents itself as usable and acceptable habitat for salmon. Environmental conditions can clearly be envisioned that increase or decrease the quality and amount of habitat that may be preferred by salmon. Similarly, not all salmon are found together, suggesting species-specific habitat requirements that need to be identified.

Columbia River Plume

Salinity preferences may be an attribute to further identify and exploit, as suggested by Favorite (1969). Salmon are found in the less saline surface layers of the Northeast Pacific Ocean of the Subarctic Domain which is bounded to the south by the vertical structure (34 ? isohaline) of saline waters of the Transition Zone. Locally, the Columbia River plume may represent a habitat of less saline marine waters that is critical to salmon survival. The freshwater/saltwater interface is also considered a critical habitat. When coupled with our current inability to partition the contribution of the estuary factors alone from the marine environment, evaluating the contribution of the Columbia River estuary and the plume to salmon survival, recovery, and resilience may prove useful in assessing their overall contribution to Columbia River stocks. This concern is supported by recent studies assessing the importance and impact of the Fraser River plume to salmon. Beamish et al. (1994) found that the plume of the Fraser River affected survival of coho, and chinook salmon, with low flow years typically supporting higher productivity and salmon survival than high flow years. The mechanisms by which the Columbia River estuary and plume affect juvenile salmon survival have not been quantified, but likely include provision of food, refuge during transport away from coastal predation, and improvement of estuarine conditions for subyearling fish. Since the Columbia River estuary and plume have been significantly altered from historical conditions and hatchery stocks may be affected differently than natural stocks, the system's altered state likely contributes to the overall reduction of salmon. The impact of hydrosystem effects on reducing spring river flow and suspended particulate matter transport on salmon production in the estuarine and coastal plume environment may be large, as flows in most years may now be sub-optimal for salmon production.

Sources and Extent of Variability in the Columbia River Plume

The extent, propagation, and impact of the Columbia River plume on salmon productivity are affected by two dominant factors, one marine driven and one freshwater driven. The Coastal Upwelling Domain, which the plume enters, is part of the California Current (CC) system (Bakun 1996). The CC is a broad, slow, meandering, equatorward-moving flow that extends from the northern tip of Vancouver Island (50? N) to the southern tip of Baja California (25? N), from the shore to several hundred miles from land. In offshore waters, flows are southward all year round; however, over the continental shelf, southward flows occur only in spring, summer, and fall. During winter months, flow over the shelf reverses, and water moves northward as the Davidson Current. The transitions between northward and southward flows on the shelf bear the terms "spring transition" and "fall transition," because they occur in March/April and October/November, respectively. A deep, poleward-flowing undercurrent is found at depths of 100-300 m over the outer shelf and slope in spring, summer, and fall. This current seems to be continuous at least from Southern California (33? N) to the British Columbia coast (50? N).

Coastal upwelling is the dominant physical force affecting advection and production in the Coastal Upwelling Domain. Upwelling off Washington and Oregon occurs primarily over the continental shelf during the months of April-September, but can occur year round off Northern California. Upwelling also occurs in offshore waters through the action of Ekman pumping and through surface divergence in the centers of cyclonic eddies. The result of these several upwelling processes is a high biomass of both phytoplankton and zooplankton. Production is seasonal with periods of high and low productivity bounded by the spring and fall transition.

It is important to note that coastal upwelling is not a continuous process. Rather, it is a cyclic phenomenon, with favorable northerly winds blowing for periods of 1-2 weeks, interspersed by periods of calm or wind reversals. Interannual variations in the length and number of upwelling events lead to variations in the level of primary and secondary production, thus the overall level of production during any given year is highly variable. Any process that leads to reduction in the frequency and duration of northerly winds will result in decreased productivity. The most extreme of these processes is El Ni?, which disrupts coastal ecosystems every 5-10 years. Understanding how this variation (on a daily, monthly, seasonal, yearly, and longer time scale) translates to a change in Columbia River plume environments and its impact on survival potential of salmon species is a critical variable that needs to be considered.

Variability in productivity of the California Current and its interaction with the Columbia River plume occurs at varying climatic time scales, each of which must be taken into account when considering recruitment variability and fish growth. For example, the North Pacific experiences dramatic shifts in climate on a 30-40 year frequency, caused by eastward-westward jumps in the location of the Aleutian Low in winter. Shifts occurred in the 1920s, 1940s, and most recently in the winter of 1976/1977. One dramatic effect of these shifts (called regime shifts) is that large changes in biological productivity are seen in the Subarctic Pacific/Gulf of Alaska and the California Current, and they are opposite in trend. Under the present regime (known as a "warm regime"), zooplankton biomass in the southern sector of the California Current has declined by an order of magnitude whereas zooplankton biomass in the Subarctic Pacific has increased at least two-fold (Brodeur and Ware 1992). Salmonid abundance has never been higher in the Subarctic Pacific and never lower in the California Current. In contrast, during the past (cool) regime which extended from the 1940's through the mid-1970s, salmonid stocks were low in the Subarctic and high in the California Current. Groundfish species are also affected. Declining body weight of Pacific hake began near the regime shift and directly translates into reduced fishery yield. Declining recruitment for Dover sole and bocaccio rockfish in some areas also roughly coincides with the regime shift (Bakun 1996).

Since the early 1980s, the California Current has been experiencing an increased frequency of El Ni? events, with large El Ni? events occurring every 5-6 years: 1976-77, 1982-83, 1986-87, and 1991-92. Another large event occurred during 1997-98. Prior to 1982, El Ni? events seldom reached as far north as Oregon. Since 1992, the Oregon and Washington coasts have been experiencing almost continuous El Ni?-like conditions during summer (i.e., reduced upwelling and warmer ocean conditions in general). Whether these conditions will have a long-term effect on the fisheries of the Pacific Northwest has not yet been investigated. Changes in recruitment seem likely, however, with the decline in coho salmon survival since the onset of warm ocean conditions in 1992 a noteworthy example.

The shape and extent of the Columbia River plume is also controlled by the amount of freshwater flowing out of the Columbia River. Not only the flow or amount of water may be important but also the amount of sediment affecting turbidity, and the amount of nutrients fueling estuarine and oceanic productivity may be important to salmon growth and survival. Historical changes in flows of the Columbia River have been observed. Flow regulation, water withdrawal and climate change have reduced the average flow and altered the seasonality of Columbia river flows and sediment discharge, and have changed the estuarine ecosystem (NRC 1996; Sherwood et al. 1990; Simenstad et al. 1990; 1992; Weitkamp et al. 1995). Annual spring freshet flows through the Columbia River estuary are ~50% of the traditional levels that flushed the estuary and carried smolts to sea, and total sediment discharge is ~1/3 of 19^th Century levels. Decreased spring flows and sediment discharges have also reduced the extent, speed of movement, thickness, and turbidity of the plume that extended far out and south into the Pacific Ocean during the spring and summer (Barnes 1972; Cudaback and Jay 1996; Hickey et al. 1997). Pearcy (1992) suggests that low river inflow is unfavorable for juvenile salmonid survival despite some availability of nutrients from upwelling, because of: a) reduced turbidity in the plume (increasing foraging efficiency of birds and fish predators), b) increased residence time of the fish in the estuary and near the coast where predation is high, c) decreased incidence of fronts with concentrated food resources for juvenile salmonids, and d) reduced overall total secondary productivity based on upwelled and fluvial nutrients. Reduced secondary productivity affects not only salmonid food sources but focuses predation by other fishes and birds on the juvenile salmonids.

Characterizing the importance of the Columbia River plume to salmon will depend on identifying the attributes that are critical to salmon survival. Whether the important attributes are purely physical (e.g., a turbid environment) or biological (e.g., enhanced prey availability) in nature remains to be determined. It is likely the benefit of the plume will be derived from both physical and biological attributes that are powered by variation in the marine environment that the plume enters into and the quality and amount of freshwater flowing out of the Columbia River. Further, it is likely that the benefits will not be expressed in a linear manner, but more a dynamic interaction with no one combination of attributes working to benefit or suppress salmon production on a predictable frequency scale.

Columbia River Estuary

Estuaries appear to be uniquely important to salmon survival. Two separate studies in the early 80s (Emmett and Schiewe 1997) have shown that estuaries confer enhanced survival to salmon. In both cases, juvenile smolts (in one instance coho, and in the other, chinook smolts) were collected and released in the river, in the estuary, in the transition zone outside the estuary, and in the ocean. Both studies were repeated for multiple years. In both cases, smolts released in the estuaries consistently provided more to the fisheries or returned at higher rates than smolts released outside the estuaries. Interestingly, one of the studies, conducted in the Columbia River, showed that releasing smolts in the Columbia River plume was just as beneficial to survival. In another example, examination of adult returns from transportation studies conducted at Lower Granite Dam in 1990 showed that PIT-tagged adult salmonid returns varied dramatically through the seasons. Smolts transported and released below Bonneville Dam (at the head of the Columbia River estuary) during the early part of the migration season apparently had much lower survival than those transported during the later part for both hatchery and wild fish (Matthews et al. 1992; Hinrichsen et al. 1997). What is remarkable is the transition from a lower to a much higher survival rate occurred during a one-week transition period. The short time frame for the transition suggests that the events affecting survival are local, very likely within the estuarine domain. What have not been determined in any of these studies are the specific attributes of the estuary that confer enhanced survival to salmon.

What are the benefits of the Columbia River estuary to salmon productivity? Estuaries provide critically important habitats for numerous marine and anadromous fish and shellfish. Biologically, the estuaries of the PNW are perhaps most recognized as a transition habitat for salmon in their migrations to and from seawater. Although coho and stream-type chinook salmon, steelhead, and cutthroat trout spend relatively little time in estuaries (<6 weeks), ocean-type chinook salmon can reside in estuaries for up to 2 months or more. The influence of estuaries extends well beyond the immediate land boundaries of the coast, and the important linkages between estuaries and the nearshore ocean environment are greater than commonly recognized. Estuaries, for example, represent a means by which energy resulting from the action of climate (weather) on land masses is transmitted to the ocean. In addition, estuaries support important trophic interactions affecting a variety of marine species. For example, the planktonic larval stages of many estuarine invertebrates provide a major food source for many pelagic marine fish species. The nearshore ocean environment is continuously influenced by rivers (large or small) and their estuaries, and this influence can extend far beyond the coast for large river systems such as the Columbia and Sacramento/San Joaquin Rivers or for large bays such as Grays Harbor, Willapa Bay, Puget Sound, and San Francisco Bay. Humans have impacted estuaries in literally hundreds of different ways that have led to habitat destruction, habitat simplification, and loss of ecological function. Further, although urban lands make up only 2 to 3% of the land base of the West Coast, greater than 70% of the human population lives near estuaries or river and stream corridors that flow into estuaries. Most of these urban areas are located on historical wetlands, where drainage requirements have eliminated more than 90% of these productive aquatic habitats.

Sources and Effects of Variability in the Columbia River Estuary

The role of the Columbia River estuary in supporting or enhancing salmon survival may be diverse. The estuary may simply serve as a conduit to the ocean, transporting fish from the river to the ocean allowing them to complete their life cycle. Most important attributes affecting the outcome would be actual flow rates, timing of flow, and turbidity, as mentioned previously. This represents a practical role for the estuary. However, the Columbia River estuary may contribute in other ways. The estuary may represent an extension of the freshwater habitat of salmon, expanding the available habitat for rearing (Wissmar and Simenstad 1998). Obviously the number of salmon that could potentially be supported within the Columbia River system is increased if parts of the estuary served this role. Apart from contributing space or transporting fish, processes within the estuary can also affect survival of salmon. The impact of Caspian tern predation in the Columbia River estuary on salmon is clearly a recent example of concern. The outcome is that fewer salmon leave the estuary compared to the number that enter. Finally, estuarine processes may not directly affect salmon, but incur effects that are delayed. To this end, estuarine processes can directly benefit or negatively impact salmon. The outcome, with respect to survival, is manifested later, depending on conditions they encounter outside the estuary. Evidence of negative impacts has certainly been documented (Emmett and Schiewe 1997) and one can envision how improved availability of food, supporting enhanced growth, can make ocean survival more likely. With fish, as with all animals, increased size confers an enhanced survival potential. The actual role played by the Columbia River estuary is currently unknown, but likely incorporates components of all those listed above. Understanding the contribution will require a more comprehensive evaluation of the use of the estuary than we currently have.

The specific attributes in the Columbia River estuary that may be critical to salmon are varied and influenced by both external and internal forces. If these critical features could be identified empirically, then modifications could properly be targeted to restore or preserve these features. What are the characteristics of the estuarine features that possibly enhance salmon survival? Physical properties of estuaries that likely affect salmon include amount of flow and flow patterns, the degree of turbidity, as indicated previously, and the hypsometric curve. It is easy to see how climate, in the form of rainfall, could affect these features on a number of temporal scales (e.g., seasonal, yearly, decadal). There are, however, other physical variables of the estuary that may be critical. These include habitat types, influenced by landscape structure, the diversity of habitat types within the estuary itself, and the availability and amount of low velocity habitats. These could provide refuge and feeding habitats, particularly low velocity habitats, for salmon during critical migratory or residency periods. Food web structure within the estuary may also be important (Wissmar and Simenstad 1998). These are clearly influenced by the amount and type of nutrients and the type of organic matter sources that feed into the estuary. The impact of nutrient quality and quantity on prey availability and the timing and abundance of secondary productivity could certainly influence survival of juveniles. The Columbia River Data Development Program (CRDDP) studies, conducted in the 80s showed that a majority of the resident and outmigrating salmon had food in their stomachs. Factors that alter the food availability dynamics at the various scales of concern are likely to affect salmon survival. In addition, all of these elements contribute to the overriding ecological interactions that affect survival. Foremost of these are intra-specific and inter-specific competition, which are influenced by the abundance of salmon entering the estuary as well as the proportion of wild (naturally-produced) and hatchery-reared salmon. Finally, the biodiversity of the estuarine community confers benefits that are difficult to quantify, but certainly reflect on the quality of the estuarine habitat in general. The relative contribution and importance of all these elements, as with the role of the estuary, needs to be clearly identified before the impact of changes in the Columbia River estuary can be properly characterized. This has yet to be done empirically.

Do climate and ocean conditions affect estuarine condition directly or through the action of freshwater? Clearly a case can be made that the lower portion of the estuary is affected by ocean conditions. However, the upper portion of the estuary is more likely affected by freshwater inputs. As described previously, the evidence for change historically in terms of flow and climate in the Columbia River basin support the contention that the current day estuary is not the same (Sherwood et al. 1990). To the extent that understanding climate factors and their impact on estuarine conditions need to be addressed, clearly the modification of the Columbia River estuary by human-induced changes to the dynamics of the Columbia River flows is probably most dramatic. The human impact has likely affected the ability of the system to provide a diverse and acceptable habitat more so for salmon than any other factor in current historical terms. Nevertheless, both anthropogenic and natural factors need to be considered as driving factors affecting the beneficial use of the estuary by salmon. This is in contrast to the interaction of the Columbia River plume with the California Current, which is primarily driven by natural factors.

Management Issues

Should management of the Columbia River system incorporate the entire ecosystem important to salmon rather than concentrate on urban impacts on freshwater habitats? The answer is an unqualified yes, but the question is how. If the estuary and plume represent important habitat for critical periods in the salmon life cycle, we need to address the impact of the hydropower system, habitat, hatcheries, and harvest beyond the freshwater phase. For example, modification of the habitat by river flow amounts, timing of flow, as well as sediment input may be detrimental to the diversity of salmon life histories that occur in these areas. Clearly this would be affected by climate. But we need to examine the human-induced impact placed upon the Columbia River estuary and plume that are dynamically affected by natural forces. The integration of such an evaluation may provide insights into potential magnification of unforeseen problems simply because we artificially limit where and how we view the extent of the problem. Clearly if we ignore the dynamics and importance of the Columbia River estuary and plume environment to salmon stocks, our management success will be limited. One avenue that should be pursued is to improve our monitoring of the estuary and plume environment. The characteristics of the monitoring program at this stage should incorporate evaluation of the physical structure of the plume in relation to the ocean environment (e.g., strength of upwelling events, ocean temperatures, the timing of the spring transition) and characteristics of the biological environment (prey availability, condition of juvenile salmon during the summer period). Modification of the monitoring effort should take place as the important attributes affecting survival of salmon become better identified and articulated.

Conclusion

Both the freshwater and marine phases of salmon life history contribute significantly to survival. Further, the near coastal habitat, where salmon first enter the ocean environment appears to be a key area to recruitment success. This implies that the Columbia River estuary and plume may represent critical habitat for Columbia River basin salmon stocks. It is clear that natural oceanic and climatic forces affect the estuary and plume environment. In addition, consideration of human-induced changes to the estuarine and plume environment should be incorporated into any management plan that hopes to sustain or recover depressed salmon stocks of the Columbia River basin.

References

Bakun, A. 1996. Patterns in the Ocean; Ocean Processes and Marine Population Dynamics. California Sea Grant College System. 323 pp.

Barnes, C. A, C. Duxbury, and Morse, B.-A. 1972. Circulation and selected properties of the Columbia River plume at sea, p. 41-80. In The Columbia River Estuary and Adjacent Ocean Waters. Edited by A. T. Pruter, and D. L. Alverson. University of Washington Press, Seattle.

Beamish, R.J., C-E.M. Neville, and Thompson, B.L. 1994. A relationship between Fraser River discharge and interannual production of Pacific salmon (Oncorhynchus spp.) and Pacific herring (Clupea pallasi) in the Strait of Georgia. Can. J. Fish. Aquat. Sci. 51: 2843-2855.

Beamish, R.J., D.J. Noakes, G.A. McFarlane, L. Klyashtorin, V.V. Ivanov, and Kurashov, V. 1999. The regime concept and natural trends in the production of Pacific salmon. Can. J. Fish. Aquat. Sci. 56: 516-526.

Bradford, M.J. 1995. Comparative review of Pacific salmon survival rates. Can. J. Fish. Aquat. Sci. 52: 1327-1338.

Brodeur, R.D., and Hare, D.M. 1992. Long-term variability in zooplankton biomass in the subarctic Pacific Ocean. Fish. Oceanogr. 1: 32-38.

Clark, J., and Benson, N. 1981. Summary and recommendations of symposium, p. 523-528. In Proceedings of National Symposium on Freshwater Inflow to Estuaries, Vol. II. Edited by Cross, R. D. and D. L. Williams. US Fish and Wildlife Service Rep. FWS/OBS-81/04.

Cudaback, C. N., and Jay, D. A. 1996. Buoyant plume formation at the mouth of the Columbia River -- an example of internal hydraulic control? Buoyancy Effects on Coastal and Estuarine Dynamics, AGU Coastal and Estuarine Studies 53: 139-154.

Emmett, R.L., and Schiewe, M.H. 1997. Estuarine and ocean survival of Northeastern Pacific Salmon: Proceedings of a workshop. NOAA Technical Memorandum NMFS-NWFSC-29. 313 pp.

Favorite, F. 1969. Fishery Oceanography. IV. Ocean salinity and distribution of Pacific salmon. Comm. Fish. Rev. 31: 29-32.

Francis, R.C., and Hare, S.R. 1994. Decadal-scale regime shifts in the large marine ecosystems of the Northeast Pacific: a case for historical science. Fish. Oceanogr. 3: 279-291.

Friedland, K.D., and Haas, R.E. 1996. Marine post-smolt growth and age at maturity of Atlantic salmon. J. Fish Biol. 48: 1-15.

Hinrichsen, R.A., J.J. Anderson, G.M. Matthews, and Ebbesmeyer, C.C. 1997. Effects of the ocean and river environments on the survival of Snake River stream-type chinook salmon. Project No. 89-108. Bonneville Power Administration, US Dept. of Energy, Division of fish and Wildlife. PO Box 3621, Portland, OR 97208. 111pp.

Hickey, B. M., L. J. Pietrafesa, D. A. Jay, and Boicourt, W. C. 1997 (In press). The Columbia River plume study: subtidal variability in the velocity and salinity field. J. Geophys. Res.

Mantua, N.J., Hare, S.R., Zhang, Y., Wallace, J.M., and Francis, R.C. 1997. A Pacific interdecadal climate oscillation with impacts on salmon production. Bull. Am. Meteor. Soc. 78:1069-1079.

Matthews, G. M., S. Achord, J. R. Harmon, O. W. Johnson, D. M. Marsh, B. P. Sandford, N. N. Paasch, K. W. McIntyre, and Thomas, K. L. 1992. Evaluation of transportation of juvenile salmonids and related research on the Columbia and Snake Rivers, 1990. Report to the U.S. Army Corps of Engineers, Contract DACW68-84-H0034, 51 p. (Available from Northwest Fisheries Science Center, 2725 Montlake Blvd., E., Seattle, WA 98112-2097).

National Research Council. 1996. Upstream: Salmon and Society in the Pacific Northwest. Committee on Protection and Management of Pacific Northwest Anadromous Salmonids, Board on Environmental Studies and Toxicology, Commission on Life Sciences, National Academy Press, Washington, D.C. 451 pp.

Pearcy, W.G. 1992. Ocean Ecology of North Pacific Salmon. Washington Sea Grant Program, University of Washington Press, Seattle, WA. 179 pp.

Peterman, R.M., B.J. Pyper, M.F. Lapointe, M.D. Adkison, and Walters, C.J. 1998. Patterns of covariation in survival rates of British Columbian and Alaskan sockeye salmon (Oncorhynchus nerka) stocks. Can. J. Fish. Aquat. Sci. 55: 2503-2517.

Pyper, B.J., R.M. Peterman, M.F. LaPointe, and Walters, C.J. 1999. Patterns of covariation in length and age at maturity of British Columbia and Alaska sockeye salmon (Oncorhynchus nerka) stocks. Can. J. Fish. Aquat. Sci. 56: 1046-1057.

Sherwood, C. R., D. A. Jay, R. B. Harvey, P. Hamilton, and Simenstad C. A. 1990. Historical changes in the Columbia river estuary. Prog. Oceanogr. 25: 271-297.

Simenstad, C. A., C. D. McIntire, and Small, L. F. 1990. Consumption processes and food web structure in the Columbia River estuary. Prog. Oceanogr. 25:271-298.

Simenstad, C. A., D. A. Jay, and Sherwood, C. R. 1992. Impacts of watershed management on land-margin ecosystems: the Columbia River Estuary as a case study, p. 266-306. In New Perspectives for Watershed Management - Balancing Long-term Sustainability with Cumulative Environmental Change. Edited by R. Naimen. Springer-Verlag, New York.

Weitkamp, L. A. 1994. A review of the effects of dams on the Columbia River estuarine environment, with special reference to salmonids. Report to US DOE, Bonneville Power Administration, Contract DE-A179-93BP99021, NOAA-NMFS, Coastal Zone Est. Stud. Div., NW Fish. Sci. Center, Seattle, WA. 148 pp.

Welch, D.W., A.I. Chirgirinsky, and Ishida, Y. 1995. Upper thermal limits on the oceanic distribution of Pacific salmon (Oncorhynchus spp.) in the spring. Can. J. Fish. Aquat. Sci. 52: 489-503.

Welch, D.W., Y. Ishida, and K. Nagasawa. 1998. Thermal limits and ocean migrations of sockeye salmon (Oncorhynchus nerka): Long-term consequences of global warming. Can. J. Fish. Aquat. Sci. 55: 937-948.

Wissmar, R.C., and Simenstad, C.A. 1998. Variability of estuarine and riverine ecosystem productivity for supporting Pacific salmon, p. 253-301. In Change in Pacific Northwest Coastal Ecosystems. Edited by G.R. McMurray and R.J. Bailey. NOAA Coastal Ocean Program. Decision Analysis Series No. 11.

Additional illustrations from this presentation

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