The 1982 ODFW Coho Salmon Management Plan identified production goals for wild coastal coho. Because of a number of factors, including unfavorable marine survival, these production levels have never been realized. Much new information is now available about the factors affecting production of coho salmon. For example, extensive habitat inventory data are now available, a model has been developed to estimate coho salmon smolt capacity from habitat data, spawner numbers are estimated using statistically valid methods, and there is a better understanding of the effects of variability in climate on salmon production. New understanding of the interactions between freshwater and marine survival of coho salmon is of particular interest to the development of realistic production goals for wild fish.

Research has demonstrated that the quality of freshwater habitat (particularly over-winter habitat) has a direct influence on freshwater survival rate. Habitat and population modeling has demonstrated that to be equally productive, salmon inhabiting a stream with poor quality habitat will require a higher rate of marine survival than salmon inhabiting a stream with good quality habitat. As a result of these interactions, marine survival plays a dominant role in determining the productivity and sustainability of coho salmon populations.

The modeling predicts that extended periods of low marine survival, particularly combined with high fishery exploitation rates, cause extirpation of coho salmon from all but the best freshwater habitats. In fact, this is exactly what is observed today. A prolonged period of poor marine survival has occurred for coho off Oregon since the late 1970s. Harvest during this period was also excessive until the most recent years. Random sampling of coho spawner abundance indicates that very few stream reaches have large spawner populations, and that most stream reaches have few or no spawning coho salmon.

Thus, the concept of a single production goal has become obsolete. The concept of production potential is more appropriate. Production potential is the estimated number of adult salmon that might be expected from a population under a particular set of natural environmental circumstances. When estimating production potential, both the quality of the freshwater habitat and the probable levels of marine survival must be considered. Production potential and range of coho salmon abundance within a basin would be expected to expand and contract as marine survival increases and decreases.

The estimates of production potential presented in this chapter were developed based on actual measurements of habitat in individual stream reaches made during the period 1990-95 and two assumed levels of marine survival: 3% and 5% (see Table 1). Therefore, two tiers of freshwater habitat would be capable of supporting coho production, corresponding to the two levels of marine survival.

Table 1. Estimated Production Potential of Current Habitat for Coho Salmon in Oregon Coastal ESUs.

All estimates of production potential were derived with the assumption of having fully seeded freshwater habitat, and should be viewed as potentially achievable levels of production based on current habitat condition. For the transborder ESU that includes southern Oregon and Northern California, estimated production potential was calculated for the Rogue Basin only. Production potential for coho salmon is thought to be very small in other Oregon streams in this ESU.

Because estimates of potential production are based on modeling of freshwater habitat capacity, which relies heavily on winter habitat conditions, these estimates may be optimistic in some cases - especially for areas where high summer water temperatures may occur such as the Umpqua and Rogue basins. Temperature may be a more severe constraint than winter habitat on populations in some streams in these basins and limit production below the maximum levels estimated (see Table 1). Consequently, current estimates of potential production should be viewed as giving general guidance. Undoubtedly, this guidance will be revised in the future as population models are improved and more habitat data are collected.

To assess the status of a population relative to its potential, it is necessary to consider its history of relative marine survival. Potential production levels vary as marine survival changes. Thus, because marine survival for the last two decades has been poor and escapement has been reduced by over-fishing, attaining the production potential of the higher levels of marine survival will occur only after achieving adequate spawner abundance in the poor habitat that currently has few, if any, spawners. Achieving adequate spawner abundance in these poorer habitats may require that several generations experience improved marine survival and that the more restrictive harvest controls proposed in this plan are implemented (see Fish Management Measures in Chapter 17B).

For current habitat conditions, the modeling predicts that wild coho salmon production could range approximately from 168,000 in 800 miles of habitat to 430,000 in 2,100 miles of habitat in the Oregon Coastal ESU. Spawner needs are in the range of 126,000 to 235,000. Similarly, production potential for the Rogue Basin ranges approximately from 7,000 to 29,000 with spawner needs of 5,000 to 14,000.

Recent Population Trends

Since 1990, coho salmon spawner populations in the northern Oregon Coastal ESU have been estimated using statistically-designed, stratified random surveys. From 1950 to 1990, populations were monitored using standard survey sites. Whereas the standard surveys provided an index of abundance from year-to-year, the new methods provide actual population estimates. In the Rogue Basin, population estimates are made from ratios of unmarked fish to marked hatchery fish collected in a seining operation at Huntley Park in the lower river.

The populations estimated for each major coastal basin since 1990 are listed in Table 1. Abundance of spawners in the Coos and Coquille basins have been relatively strong since 1992, the first year of substantial harvest reduction on the south coast. In 1995, and especially in 1996, spawning populations in the coastal lakes, and the Umpqua, Siuslaw, and Yaquina basins have also seen substantial increases in abundance. The preliminary estimates for 1996 show a significant increase in total abundance for all but the northern third of the Oregon Coastal ESU. The stronger spawner returns in the southern two-thirds of the ESU do not appear to be due to better habitat quality. For example, based on our habitat modeling, the Coos and Coquille basins have habitat of similar quality to that in the Nehalem and Nestucca basins, respectively, yet had a 15- to 20-fold greater density of spawners in 1996.

Total production of the coastal populations can be estimated by dividing escapement estimates by one minus the exploitation rate. Figures 2 and 3 show these estimates for the Oregon Coast ESU and for the Rogue Basin, respectively. In five of the last seven years, coastal basins as a whole have been producing coho salmon at about one-half of their estimated potential, given the poor marine survival conditions. The primary reason for this reduced production is lack of adequate spawners. Figure 2 clearly demonstrates the effects of high exploitation rates during periods of poor marine survival. In hindsight, exploitation rates of 50% to 70% experienced by some coastal coho stocks as late as the early 1990s were clearly too high given the poor marine survival conditions experienced by the fish. As harvest has decreased, spawner abundance has increased. Since 1990, there has been a fairly steady increase in spawner abundance despite a generally flat trend in total production. Spawner-to-spawner ratios have ranged from 1.2 to 2.6 during the past four years. Estimated spawner abundance has increased by about four-fold in two generations (1990-96).

In the Rogue basin, the population has been much more variable (Figure 3). However, marine survival of hatchery fish has greatly improved since 1994. Abundance of wild spawners has also increased dramatically.

Prospects for the Future

The improved survival of coho salmon in the Rogue Basin and in the south and mid-coast basins in recent years are hopeful signs. We know that climate is cyclic and strongly influences marine survival. We have been in a poor survival cycle since 1977. The improved marine survival of coho from the Rogue Basin since 1994, and apparent improved survival in mid-coast basins in

1996 may be precursors of better survival in the near future. Climatologists predict a return to a wetter climate similar to that experienced in the 1960s, a period of good survival conditions for Oregon coho salmon.

So, what trends in production are populations likely to experience in the future? One possible answer has come from a simple model of projected populations that would result from the proposed OCSRI Harvest Strategy. If we consider two assumptions -- that marine survival will remain poor (average 3%), or that marine survival will improve (average 5%) -- we can develop a range of possible outcomes. Starting with the average spawner abundance of 50,000 fish for 1993-95, the model predicts an expected rebuilding trajectory for spawner abundance in the Oregon Coast ESU at 3-year intervals (Figure 4) while following the proposed harvest strategy. The typical cycle of Oregon coho salmon populations is three years from spawner-to-spawner. Thus, the values for 1998 in Figure 4 represent populations in 1996-98. Based on the model, the spawner population would be expected to be between 56,000 and 94,000 after one generation, and between 75,000 and 216,000 after four generations. The estimated returns for 1996 fall near the upper boundary of this predicted range after one generation.

A second answer to the question of where populations are likely to go in the future comes from a more sophisticated habitat-based, life cycle model. This model is based on the reach-level habitat data used to estimate production potential and incorporates a range of probable variation in survival at each life stage. The model also includes factors for straying of spawners, multiple spawning periods, sex ratio, redd failure, and loss of genetic fitness at low population size. Populations were modeled for ten generations using average marine survival rates and for 33 generations using a cyclic pattern of marine survival. (A detailed description of the model and results are presented in ODFW Attachment 1.)

Populations in the Yaquina, Coos, and Tillamook basins were analyzed using the model. These basins have the best, an intermediate level, and poorest habitat in the northern Oregon coast ESU, respectively. Results suggest that future population abundance will be heavily influenced by marine survival and by exploitation rate when marine survival is low. Only the habitats with high productivity remained viable when marine survival was low. Therefore, distribution and abundance of fish was a function of long- and short-term variability in marine survival and long-term patterns of habitat quality. Within a reach, populations were resilient, unless numbers dropped to a level where demographic risk factors became more important than density dependent population dynamics. Persistence of populations in a basin during periods of poor marine survival depended on the highest quality reaches.

The model predicts that there is a high probability of persistence of coho populations in all major basins of the northern Oregon coast ESU for the next century if habitat condition remains as it is today. The model was also used to predict risk of extinction assuming future declines in habitat quality ranging from 10% to 60%.

Based on these analyses, the model predicts that there would be a substantial increase in the risk of extinction (population of < 50 spawners occurring at any time) in basins with poor quality habitat, such as the Tillamook if habitat quality over the next century declines by 30-60%. This would probably also be the case in the Nestucca, and Coquille, based on an evaluation of habitat quality (see Figure 1). Similar declines in the quality of habitat in the remaining major basins in the northern Oregon coast ESU would not result in an appreciable decrease in the probability of persistence of coho salmon populations in those basins. However, decreased habitat quality would result in smaller populations.

Summary

• Actual measurements of stream habitat were used as the basis for estimating production potential for coho salmon in coastal basins.

• Production potential is a more appropriate concept than production goals because it varies greatly over time, depending on marine survival conditions.

• The population model predicts that productive habitat in the Oregon Coast ESU ranges from about 800 miles when marine survival is 3% (the estimated average for the last decade) to about 2,100 miles when marine survival is 5%.

• Estimated total production of coho salmon from the northern Oregon coast ESU has been relatively constant since 1990, whereas spawner escapement has increased significantly. Estimated escapement in 1996 is about four times the estimated escapement in 1990.

• Population modeling suggests that if marine survival continues at the level of the past decade, slow but steady rebuilding of coho salmon spawner populations should be expected, but there will be few or no harvest opportunities. However, if marine survival improves by two-thirds, spawner abundance should rebuild rapidly to levels near full seeding within four generations, even after providing increased harvest opportunities allowed under the proposed OCSRI harvest strategy.

• A sophisticated population model suggests that populations have a high probability of persisting over the next century if habitat quality remains in its present condition. If significant declines in habitat quality occur, persistence will be jeopardized in basins with poor habitat, although population abundance will decline in all affected basins.

• Improvement in habitat quality is expected through implementation of the Northwest Forest Plan on federal lands and through the OCSRI measures on state and private lands. These improvements will increase the expected production of coho salmon at various levels of marine survival and will help ensure persistence of coho salmon during periods of poor marine survival, particularly in basins with poor habitat.