DYNAMICS OF SALMONID POPULATIONS ASCENDING THE COLUMBIA AND SNAKE RIVERS DAMS

Alan Berryman
Departments of Natural Resource Sciences and Environmental Science
Washington State University, Pullman WA 99162
Berryman@mail.wsu.edu

 

The Columbia River drainage is one of the major watersheds in the western USA and supports a diverse community of andromonous fish (see Table). The Columbia River system is also the major source of hydroelectric power in the Pacific Northwest and is also used intensively for irrigation and navigation. To these ends, numerous dams have been constructed on the Columbia and its major tributary, the Snake River. Starting with the main stem of the Columbia we have Bonneville Dam constructed in 1938 about 150 river miles from the mouth, then The Dalles at river mile 191 in 1960, John Day at river mile 216 in 1971, McNary at river mile 292 in 1957, Priest Rapids at mile 397 in 1961, Wanapum at mile 416 in 1964, Rock Island at mile 453 in 1933, Rocky Reach at mile 474 in 1961, Wells at mile 516 in 1967, Chief Joseph at mile 545 in 1961, and finally Grand Coolee at mile 597 in 1942. All dams except Grand Coolee have fish ladders so that it is still possible for anadromous fish to travel almost 600 miles from the ocean. The only free-flowing section above Bonneville is a 50-mile section below Priest Rapids dam called the Hanford Reach. As we will see, this reach is a major spawning refuge for the remnants of the fall chinook runs.

The Snake River joins the Columbia about 50 miles above McNary Dam. The first dam on the Snake is Ice Harbor at river mile 10, completed in 1962. Then came Lower Monumental at river mile 42 in 1969, Little Goose at mile 70 in 1970, Lower Granite at mile 107 in 1975. These dams flood the Snake River to the cities of Lewiston, Idaho and Clarkston, Washington. They are fitted with fish ladders so that anadromous fish can ascend the middle Snake and its tributaries. However, ascent of the upper Snake is terminated by the high Hell’s Canyon Dam at mile 247 completed in 1967, while the North Fork of the Clearwater River, one of the major tributaries of the Snake, was blocked by the high Dwarshak Dam in 1973. To mitigate losses of salmon and steelhead spawning areas, numerous of fish hatcheries have been constructed throughout the Columbia basin, particularly during the 70s and 80s.

During their passage over the dams on the Columbia River and its tributaries, salmon and steelhead are counted and classified from fish-ladder observatories. Hence, there exists a continuous inventory (time series) of fish ascending the Columbia River for 61 years (1938-1998) and the Snake River for 37 years (1962-1998). This report summarizes the dam count data obtained from StreamNet (http://www.streamnet.org) and Columbia River Dart (http://www.cqs.washington.edu/dart).  

Counts at Bonneville, the first dam on the Columbia River, estimate the total number of all salmonids entering the system each year for 61 years (unfortunately chinook counts from 1938-1960 are missing from the StreamNet files) (Figure). General trends are best viewed on the logarithmic scale (Figure). The first thing to notice is that stocks of chinook, sockeye and steelhead do not show any dramatic changes over the 60 years. On the other hand, coho counts increased dramatically between the years 1962 and 1964. We will discuss this in more detail later. Annual variations in stocks are best viewed on the arithmetic scale (Figure). The only common pattern seems to be a general increase in all species in the mid 1980’s. For example, chinook returns reached their highest level in 60 years in 1986 and 1987. Steelhead returns reached their highest numbers in 1985 and 1986, and coho in 1986. Sockeye, on the other hand, peaked in 1985 but declined in 1986.

The final destination of salmon was calculated by subtracting counts at the beginning of a section of river from those counted at the end of that section. For instance, fish remaining in the Lower Columbia are given by the Bonneville count minus the McNary count. These fish are assumed to have spawned or died prior to spawning in the Lower Columbia River. Other destinations are the Hanford Reach (McNary count – Priest Rapids count – Ice Harbor count), the Snake River destination (Ice Harbor count) and the Middle Columbia destination (Priest Rapids count). On occasion the calculation of destinations resulted in a negative number due to errors in counting. When this happened, the number was set to unity to accommodate the use of logarithms. When many very small numbers and/or negative numbers were calculated within a given destination, it was assumed not to be a significant destination.

 

THE CHINOOK COUNTS

Chinook salmon are the most abundant species crossing Bonneville dam, averaging around 350,000 fish annually (Figure). Even in low years, more than 200,000 chinook usually cross Bonneville while in high years over 500,000 fish can be counted. In addition, chinook returns to Bonneville have remained remarkably steady over the last 36 years (Figure).

Calculation of chinook salmon destinations indicates that about 50% of the fish remain in the lower Columbia and that the pattern of fluctuation follows very closely the Bonneville counts (Figure). It is also clear from this figure that, although all chinook populations increased during 1986-87, it was the Hanford Reach that contributed most to this unusually productive period. This graph also shows that the upper Columbia and Snake River stocks have declined consistently since 1969.

The annual chinook run is normally divided into three components – the spring, summer and fall runs – according to the time they enter the river (Figure). This graph shows that all three runs increased over the period 1960-69, but that the spring and summer runs have declined while the fall run has remained steady or even increased in the years that followed.

Destination analysis was performed on the three chinook runs. As almost identical patterns were observed in spring and summer chinook, these two runs were combined (Figure). The most significant feature of this graph is the steep decline of the spring and summer run at Bonneville. As most of the spring and summer chinook ascend the upper Columbia and Snake Rivers, we calculated the number missing in the lower Columbia by subtracting the fish crossing Priest Rapids and Ice Harbor dams from those counted at Bonneville (shown as a thin black broken line in the Figure). Notice that, although quite variable, the number of missing fish has declined from around 70,000 in the late 60s and early 70s to 20,000 or less thereafter. This decline in missing fish roughly parallels the collapse of the commercial spring chinook fishery at about the same time (graphs of the commercial fishery can be seen at http://www.orst.edu/instruction/anth481/sal/colriv.html#spch). The second thing to notice is that the most rapid decline in spring and summer chinook was in what was originally the most productive region, the Snake River drainage (Figure). The third thing to notice are the deep depressions, especially in the Snake River returns, in 1965, 1974-75, 1979-84, and 1994-96.

In contrast to the spring and summer runs, the fall chinook run has increased, particularly on the Hanford reach, the last free flowing stretch of the middle Columbia River (Figure). This figure clearly shows that it was fall chinook returning to the Hanford Reach in 1985-87 that were responsible, in large part, for the unprecedented chinook runs during this period. It is worth noting that there were depressions in the fall chinook runs in 72, 77-83, and 92-93 which do not necessarily coincide with declines in the spring-summer run .

 

THE COHO COUNTS

The most noticeable feature of the coho time-series for Bonneville dam is the dramatic increase in the number of returning fish from 1960 to 1967 (Figure). Before this time annual coho returns over Bonneville averaged around 7,000 fish per year. During the period of increase (1962-67), counts increased to around 70,000 and then gradually declined over the next thirty years. An interesting question is, what caused the dramatic 10-fold increase in coho numbers from 1960-67? Sockeye and chinook returns also had less spectacular increases over this time but steelhead counts showed no obvious increases until the late 1970s (Figure). One possibility is that the increase in commercial salmon counts reflects a reduction in harvests during the early 1960s. After 1967, coho counts declined gradually, with the exception of a very high return in 1986 when more than 130,000 coho ascended Bonneville dam (Figure).

Destination analysis indicates that, except for a brief period from 1965-1975, almost all coho remain in the lower Columbia River (Figure). The logarithmic plot indicates that a small run of around 3,000 fish is being sustained on the Hanford Reach but that coho returning to the upper Columbia and Snake Rivers have declined drastically (Figure). The general conclusion seems to be that, although the extinction of coho in the upper Columbia and Snake rivers seems imminent, a small population is maintaining itself in the lower Columbia and along the Hanford Reach. There is also some indication that populations may be recovering recently above Priest Rapids and Ice Harbor dams (Figure). Whether this recovery is a permanent or temporary will have to await further data.

Our analysis indicates that coho salmon were never a major component of the Columbia River salmonid complex, at least during the last 60 years. The exception is a brief period of abundance from 1965-74 and the large return of 130,000 fish in 1986. Even though there has been a decline in coho numbers in recent years, populations are still higher than in the years before most of the dams were built on the Columbia and Snake rivers. Finally, it is important to determine what caused the spectacular increase in coho numbers from 1962 to 67 and again in 1986. An answer to this question could help restoration efforts as well as the management of coho populations in the Columbia system and other western rivers.

 

THE SOCKEYE COUNTS

The first noticeable feature in the sockeye time-series at Bonneville Dam is the gradual decline in total numbers of fish over the 60-year period; from an average of over 100,000 fish in the 1940’s to less than 50,000 in the 90’s (Figure). The second thing to notice are the four short but catastrophic declines in 1945, 1961, 1978, and 1994-5 (Figure). These collapses seem to occur at 16- and 17-year intervals

Destination analysis of the sockeye counts indicate that almost all sockeye salmon head for the middle Columbia River (averaging around 50,000 fish per year and sometimes exceeding 100,000). A small but rapidly declining population (rarely exceeding 1,000 individuals) ascends the Snake Rivers dams (Figure). Sockeye ascending the Snake mainly spawn in the vicinity of Redfish Lake, on the Salmon River drainage, and have been classified as an endangered species. Additional destination analyses were performed on sockeye in the middle Columbia River. Most Columbia River sockeye spawn either in the vicinity of Lake Wenatchee, on the Wenatchee River, or Lake Osoyoos, on the Okanogan River. The number of sockeye reaching the Osoyoos destination were estimated from counts over Wells Dam while the remainder were assumed to go to the Wenatchee destination (Figure). Over all years an average of 32,000 fish headed for Lake Osoyoos and 26,600 for Lake Wenatchee. These numbers remained relatively steady until 1993 when a decline seems to have started. Particularly ominous is the deep depression in sockeye heading for Lake Osoyoos in 1994, with less that 2,000 fish climbing Wells Dam.

 

THE STEELHEAD COUNTS

Steelhead are the second most abundant salmonid crossing Bonneville Dam, averaging around 150,000 fish annually prior to 1982 and over 200,000 thereafter (Figure). The rise in steelhead numbers in the early 1980s is coincident with the increasing production of hatchery fish. Like some of the other salmonids, phenomenal steelhead returns occurred in the mid-eighties.

Destination analysis of steelhead returns indicates that most of the fish remain in the lower Columbia or run up the Snake River (Figure). A small population of about 10,000 fish consistently return to the middle Columbia to spawn in the Wenatchee and Methow Rivers. The Snake River basin is a major steelhead destination. The time series indicates that the Snake River run declined from around 100,000 fish in the early 1960s to the disastrous return of only 12,000 fish in 1974. This period of decline coincided with the building of four low dams on the mainstem of the Snake and high dams in Hell’s Canyon and on the North Fork of the Clearwater River. Significantly, Dwarshak Dam on the North Fork blocked passage to the spawning grounds of the B-run, a famous strain of large steelhead that usually makes up about 35% of the Snake River run. To mitigate this loss, a large fish hatchery was built at the mouth of the North Fork and it was this, plus several other new hatcheries, that probably led to the rebound of steelhead in the Snake River to the present run of about 100,000 fish per year (Figure). However, the run was mainly composed of wild fish in the 1960s but is now 90% hatchery fish. This has lead to the wild Snake River steelhead being listed as threatened. In fact the B-run, with its major spawning grounds under Dwarshak pool, should probably be considered endangered.


I would like to acknowledge the assistance of Francois Pary in obtaining and analyzing the data. Francois is a student from France who did an internship with me on the analysis of Columbia River salmonid fluctuations during the summer of 1999.