In this paper, I review some of the factors that determine freshwater survival of coho salmon (Oncorhynchus kisutch) in Oregon coastal streams and discuss ongoing research that is evaluating how the quality of rearing habitat affects survival rates in fresh water. I also review marine survival rates for Oregon coastal coho salmon and discuss an experiment completed in the 1980s to determine if marine survival rates of hatchery reared coho salmon could be increased by releasing them in various locations in the ocean off the Oregon and Washington coasts.

In Oregon and throughout the Pacific Northwest, the freshwater life history of wild coho salmon has been studied extensively (Bustard and Narver 1975a,b; Moring and Lantz 1975; Peterson 1982a,b; Hartman and Brown 1987; Nickelson et al. 1992). Wild coho salmon in Oregon coastal rivers typically spawn in December and January in small tributary streams. Their offspring emerge from the gravels the following spring and rear throughout the summer and winter. In their second spring, the juveniles undergo physiological changes that prepare them for the marine environment. Migration out of the upper tributaries begins in March and April, and most fish enter the ocean by late May. Precocial males (jacks) return to the coastal rivers the same autumn, while adults return after spending a year and a half at sea.

Past studies in the Pacific Northwest suggest average freshwater survival rates (egg to smolt) of 2-3% throughout the region (Marshall and Britton 1990, Bradford 1995). Researchers have long recognized the influence in spawner density on the freshwater survival of coho salmon (Fig. 1). Recent research in Oregon coastal streams indicates the quality of freshwater habitat also influences freshwater survival rates significantly, particularly during the winter rearing period (Fig. 2).

Oregon has completed extensive aquatic habitat inventories in most Oregon coastal watersheds since the late 1980s. We have combined this data set with our understanding of the habitat quality/coho smolt production relationship developed from research streams to estimate freshwater survival potential for many of our coastal river basins. These physical habitat inventories indicate that habitat qualities, and therefore coho salmon freshwater survival rates, vary significantly between watersheds, and are generally lower than previously assumed (T. E. Nickelson, Oreg. Dep. Fish Wildl., 850 SW 15th St., Corvallis, OR 97333, Pers. commun., March 1996).

We have few direct measurements of ocean survival for Oregon's wild coho salmon, but ocean survival of coho salmon reared and released from most Oregon coastal hatcheries has dropped significantly during the last decade (Fig. 3) (Nickelson et al. 1994). Oregon coastal hatcheries north of Cape Blanco have averaged only 1.6% ocean survival over the past 10 years. These declines in marine survival have been correlated with various ocean parameters, including upwelling and nearshore temperatures (Fig. 4) (Nickelson 1986, Fisher and Pearcy 1988). While wild coho salmon stocks in Oregon may be surviving at two or three times the rate of hatchery stocks, some wild stocks will have trouble replacing themselves under present habitat conditions. Recent surveys of spawning adult coho salmon indicate that this is indeed the case for some Oregon coastal basins.

Ocean conditions appear to have negatively affected coho salmon survival of most Oregon coastal coho salmon stocks, particularly north of Cape Blanco. However, data from adult spawning surveys completed every winter on Oregon's central coast suggest that estuaries may also sometimes play an important role in determining adult survival. In the past 2 years, we have observed significant differences in the number of recruits per spawner in three central coast river basins (Siletz, Yaquina, and Alsea Rivers). The 1991 and 1992 broods from the Yaquina River showed a significant increase in spawners compared to the number of parental spawners (apparently in response to a substantial decrease in ocean harvest rate during 1994 and 1995). The Siletz and the Alsea Rivers (located approximately 30 km to the north and 18 km to the south of the Yaquina River, respectively) showed no improvement in adult returns for the 1991 and 1992 broods (Fig. 5). Such different return rates from river basins in such close geographic proximity suggest factors other than offshore ocean conditions may be influencing smolt survival. Differential smolt survival within estuaries would certainly produce the results recently observed in these three river basins and needs to be considered as an additional factor that may play an important role in adult production of Oregon's wild coho salmon stocks.

To determine if marine survival could be influenced by release location in the marine environment, experimental groups of hatchery reared coho salmon smolts were released into the lower Columbia River (below Bonneville Dam at RKm 234), the Columbia River estuary, and four marine locations off the mouth of the Columbia River from 1983 to 1987 (Solazzi et al. 1991). We chose release sites based on different oceanic features associated with the mouth of the Columbia River and its discharge into the Pacific Ocean. The marine release locations included 1) the mouth of the Columbia River, 2) the Columbia River plume water, 3) coastal water approximately 19 km north and 19 km offshore of the mouth of the river, and 4) oceanic water approximately 38 km offshore. Coho salmon smolts released at the upper end of saltwater intrusion into the Columbia River (Rkm 29) survived at significantly higher rates than fish released in fresh water below Bonneville Dam (Rkm 234). The coho salmon smolts released directly into the ocean at the four marine release sites did not survive at rates significantly different from those of the control groups released in fresh water near Bonneville Dam, and they yielded few clues to the location or source of most of the marine mortality. All groups showed interannual variation. Groups released in 1983 showed the poorest survival, resulting from the high ocean temperatures associated with the El Niño event that year (Johnson 1988). Groups released in 1985 experienced the lowest ocean temperatures during the spring, and all groups survived at their highest rate that year (Table 1).

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

Bustard, D. R., and D. W. Narver. 1975a. Aspects of the winter ecology of juvenile coho salmon (Oncorhynchus kisutch) and steelhead trout (Salmo gairdneri). J. Fish. Res. Board Can. 32:667-680.

Bustard, D. R., and D. W. Narver. 1975b. Preferences of juvenile coho salmon (Oncorhynchus kisutch) and cutthroat trout (Salmo clarki) relative to simulated alteration of winter habitat. J. Fish. Res. Board Can. 32:681-687.

Chapman, D. W. 1965. Net production of juvenile coho salmon in three Oregon streams. Trans. Am. Fish. Soc. 94:40-52.

Fisher, J. P., and W. G. Pearcy. 1988. Growth of juvenile coho salmon (Oncorhynchus kisutch) in the ocean off Oregon and Washington, USA, in years of different coastal upwelling. Can. J. Fish. Aquat. Sci. 45:1036-1044.

Hartman, G. F., and T. G. Brown. 1987. Use of small, temporary, floodplain tributaries by juvenile salmonids in a west coast rain-forest drainage basin, Carnation Creek, British Columbia. Can. J. Fish. Aquat. Sci. 44:262-270.

Johnson, S. L. 1988. The effect of the 1983 El Niño on Oregon's coho (Oncorhynchus kisutch) and chinook (O. tshawytscha) salmon. Fish. Res. (Amst.) 6:105-123.

Marshall, D. E., and E. W. Britton. 1990. Carrying capacity of coho salmon streams. Can. Manuscript Rep. Fish. Aquat. Sci. 2058:1-32.

Moring, J. R., and R. L. Lantz. 1975. The Alsea watershed study; effects of logging three headwater streams of the Alsea River, Oregon, Part I. Biological studies. Oreg. Dep. Fish Wildl. Res. Rep. 9:1-39.

Nickelson, T. E. 1986. Influences of upwelling, ocean temperature, and smolt abundance on marine survival of coho salmon (Oncorhynchus kisutch) in the Oregon production area. Can. J. Fish. Aquat. Sci. 43:527-535.

Nickelson, T. E., J. Nicholas, H. Weeks, and K. Kostow. 1994. Oregon coho salmon biological status assessment. Oreg. Dep. Fish Wildl., Corvallis, OR, 59 p.

Nickelson, T. E., J. D. Rodgers, S. L. Johnson, and M. F. Solazzi. 1992. Seasonal changes in habitat use by juvenile coho salmon (Oncorhynchus kisutch) in Oregon coastal streams. Can. J. Fish. Aquat. Sci. 49:783-789.

Peterson, N. P. 1982a. Population characteristics of juvenile coho salmon (Oncorhynchus kisutch) overwintering in riverine ponds. Can. J. Fish. Aquat. Sci. 39:1303-1307.

Peterson, N. P. 1982b. Immigration of juvenile coho salmon (Oncorhynchus kisutch) into riverine ponds. Can. J. Fish. Aquat. Sci. 39:1308-1310.

Solazzi, M. F., T. E. Nickelson, and S. L. Johnson. 1991. Survival, contribution, and return of hatchery coho salmon (Oncorhynchus kisutch) released into freshwater, estuarine, and marine environments. Can. J. Fish. Aquat. Sci. 48:248-253.

Table 1. The number of adult coho salmon caught in ocean fisheries that were originally released as smolts from various locations in the ocean, the Columbia River estuary, and in the Columbia River 234 km upstream. The number of adult returns has been adjusted to reflect a 40,000 smolt release for each group. See Solazzi et al. (1991) for additional information.

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