A number of statistical studies have demonstrated that the survival of coho salmon (Oncorhynchus kisutch) in the ocean is correlated positively with the strength of coastal upwelling (Fig. 1) during the summer of smolt out-migration (first noted by Gunsolus in 1978; reviewed in Pearcy 1992). The causal variable(s) which might account for the positive correlations are not known; in papers that discuss such correlations, it is often stated that summers having good upwelling represent "good ocean conditions" and summers having low upwelling represent "poor ocean conditions." The purpose of this paper is to attempt to improve the definition of the term "ocean conditions," using as a proxy the abundance of zooplankton over the continental shelf.

As an index of ocean conditions, two kinds of data are presented, the Bakun upwelling index and the speed and direction of winds measured at the Newport, OR South Jetty. The wind data are presented as progressive vector diagrams; data shown here have been published (Peterson and Miller 1975, 1976).

The biological data are from a series of zooplankton samples collected from June 1969 through 1972, and another series of samples collected during late-spring and summer of 1973, July/August 1977, and July/August 1978. The 1969-72 samples were collected as part of a survey of copepods and ichthyoplankton carried out by Sally Richardson, Bill Pearcy, and Charles Miller with the aid of Oregon Sea Grant funding. Results of this project have been published (e.g., Richardson and Pearcy 1977; Peterson and Miller, 1975, 1977). Most of the zooplankton samples were collected with a 20-cm bongo net fitted with 240-mm-mesh plankton nets; zooplankton were enumerated by species and developmental stage. A summary of all data from 1969 to 1972 is published in a technical report (Peterson and Miller 1976).

The zooplankton data are presented in two ways. First, data for all years for a given species are pooled in a series of illustrations that show the seasonal cycle of abundance. Second, the data are broken out by abundance, by year, and for July/August (since these are the months for which the most data are available).

Upwelling-favorable winds (i.e., winds which blow southward) usually begin to blow in mid-April and continue through early September off central Oregon. The winds are never continuous from north to south; rather the pattern is punctuated by periods of either calm winds or reversals during which winds may blow from the south and east. The intermittency of the winds generates what are known as upwelling events. Figure 2 shows an example of each of the two patterns usually observed off central Oregon in summer. During 1969, 1970, and 1973, the winds were predominantly from the north with very little onshore set to the wind. During the summers of 1971 and 1972, there was a greater preponderance of winds from the east. The wind data from all 5 years may be summarized as follows:

Read this table as follows: for 1969, if you multiplied wind speed by vector of direction, you would find that the wind "blew" a total of 14,000 km toward the south (i.e., from the north) and 2,000 km toward the east (i.e., from the west), from May through August. A comparison of south to east winds among years shows that winds were more easterly in 1971 and more southerly in the other years. The Bakun upwelling index is fairly well correlated with the southerly wind for all years but 1972.

Highest zooplankton abundances are found within the upper 20 m of the water column over the inner- and mid-shelf (Peterson and Miller 1975, 1976). The dominant species are copepods and they are found in distinct ecological zones, parallel to the coast. A near-shore group is made up of Acartia clausii, Centropages abdominalis, and Pseudocalanus mimus. The offshore group is composed of Acartia longiremis and Oithona similis. The life history stages of Calanus marshallae are distributed across both zones, with eggs and nauplii near shore and older juveniles farther from shore. Relationships between zonation of copepod species, hydrography, and mechanisms for population retention are discussed in Peterson et al. (1979).

The seasonal cycle of abundance is shown in Peterson and Miller (1977). Here, I show the climatological cycle (i.e., all data are pooled regardless of year) for 1969-73 and for 1978 for the dominant species. Considering first the small copepods (<1.5 mm length and <30 m g dry weight), the two species of Acartia have a common abundance pattern (Fig. 3); both populations begin to increase in numbers in early June; A. clausii reaches peak abundances in October, whereas A. longiremis peaks as early as mid-July. The main growth period for the copepods Pseudocalanus mimus and Centropages abdominalis is also June-October, with peak abundances in July. For the large copepod Calanus marshallae (adult length 3.5 mm and dry weight 350 m g), the population begins to increase in mid-February (Fig. 4). High numbers of adults can be seen throughout the spring and summer months.

Euphausiids are one of the chief prey of juvenile coho salmon (Peterson et al. 1982; Emmett et al. 1986; Brodeur 1989; Brodeur and Pearcy 1990, 1992). Though one does not sample the adults and older juveniles quantitatively with small bongo nets, one can learn something of the distribution and abundance of adults by the presence or absence of eggs and larval stages. Based on the egg data shown in Figure 5, adults begin spawning in mid-April and continue to do so through the summer. Apart from the nauplii which showed a strong peak in summer, the other life-cycle stages are about equally abundant in spring as in summer. The presence of high numbers of eggs from May through August suggests that adult euphausiids are abundant throughout this period.

The copepod data can be summarized in terms of total copepod biomass. Biomass was calculated by multiplying copepod numbers by weights of individuals. The table below summarizes these data along with abundances for euphausiid furcilia (juveniles) for data collected in July/August since those are the months for which the most data are available. These data can be compared to percent coho salmon survival:

The coho salmon survival data can be sorted into "good" years (1969, 1970, and 1973) and "bad" years (1971, 1972, 1977, and 1978). Using total biomass of small copepods, biomass of Calanus, or numbers of euphausiids as indicators of ocean conditions, I conclude that 1969, 1977, and 1978 were good years for zooplankton and 1971 and 1972 were poor years. There is poor agreement between these two sets of years, so no conclusions concerning relationships between zooplankton biomass and coho survival can be reached. Also, linear regression between copepod biomass, Calanus biomass, or euphausiid numbers (in July/August) and coho salmon survival were not significant. Thus it is unclear how upwelling is related to ocean conditions with respect to coho salmon survival (in terms of using zooplankton abundance or biomass as a proxy for ocean conditions). However, a problem with this analysis is that coho salmon survival and zooplankton in July/August were compared. If coho salmon survival is established during the first few weeks they are at sea, then ocean conditions during the peak of the upwelling season (July/August) may be unimportant. Perhaps our attention should focus on the April/May period.

Juvenile coho salmon enter the coastal ocean in April/May. This migration is not related to the seasonal cycle of copepod abundance since the seasonal peaks in abundance of the dominant (but small) copepods (Acartia, Pseudocalanus, and Centropages) occur in July and August. Not surprisingly, these small copepods are not prey for juvenile coho. The coho migration may be timed to the seasonal cycles of abundance of their major prey items. For their fish prey, sand eel (Ammodytes) larvae are most abundant in March/April (Richardson and Pearcy 1977), implying that juvenile sand eels are abundant in April/May, and smelt (osmeriid) larvae peak in May/June, with juveniles abundant in June/July. For the larger zooplankton, the observation that euphausiid spawning commences in mid-April implies that adults are present in the water column and are available to juvenile salmonids. Also, the largest copepod which is common off the Oregon coast in spring, Calanus marshallae, is abundant during this April/May window. Thus I suggest that salmon migrations may be timed to the presence or abundance of their preferred prey. If we are to find meaningful correlations between prey abundance and salmon survival, direct measurements may be required of the abundance of their preferred prey in April and May.

There may, however, be a proxy variable that can serve as a correlate. I hypothesize that the oceanographic feature that is driving food-chain dynamics in April/May (and that may be related to salmon mortality) is the timing of the onset of upwelling, the so-called "spring transition." This event is correlated with the appearance of euphausiids in continental shelf waters off central California (D. G. Ainley, Point Reyes Bird Observatory, 4990 Shoreline Highway 1, Stinsom Beach, CA 94970. Pers. commun., 1996), for example, and may be related to a redistribution of fish prey off Oregon since coastal circulation patterns change as upwelling is initiated. If the coho salmon migration is fixed in time, but the spring transition is variable, perhaps a classical "match-mismatch" set of events is important here. For example, if the spring transition is initiated later in the spring (e.g., in May) at the present time as compared to during the 1960s and 1970s, the coho salmon may be arriving at sea sooner than the time of greatest availability of their prey. Thus they may be encountering "poor ocean conditions" in April that are not related to strength of upwelling, but to the timing of the onset of the upwelling season.

There were no clear relationships between zooplankton abundance and coho salmon survival. This was due chiefly to the fact that the peak abundance of zooplankton occurred in July/August, several months after juvenile coho salmon enter the sea. Future work on possible relations between food availability and coho salmon survival must include intense sampling in the April-May period, and must target their chief prey—sand eels, smelt, euphausiids, and larger copepods. We did learn from the study of seasonal cycles of zooplankton, that euphausiids begin to spawn in mid-April, at the time of the spring transition. Since coho salmon arrive at sea at the same time, I suggest that future retrospective analyses of coho salmon and ocean conditions should look for correlations between data of spring transition and coho salmon survival. Moreover, work on relationships between date of spring transition, the possible movement of euphausiids onto the shelf, and the possible redistribution of juvenile fish prey (sand eels and smolt) at this time might provide a partial answer to the question of what is meant by the phrase "good vs. poor ocean conditions" with respect to coho salmon.

Brodeur, R. D. 1989. Neustonic feeding by juvenile salmonids in coastal waters of the Northeast Pacific. Can. J. Zool. 67:1995-2007.

Brodeur, R. D., and W. G. Pearcy. 1990. Trophic relations of juvenile Pacific salmon off the Oregon and Washington coast. Fish. Bull., U.S. 88:617-636.

Brodeur, R. D., and W. G. Pearcy. 1992. Effects of environmental variability on trophic interactions and food web structure in a pelagic upwelling ecosystem. Mar. Ecol. Prog. Ser. 84:101-119.

Emmett, R. L., D. R. Miller, and T. H. Blahm. 1986. Food of juvenile chinook, Oncorhynchus tshawytscha, and coho, O. kisutch, salmon off the northern Oregon and southern Washington coasts, May-September 1980. California Fish Game 72(1):38-46.

Gunsolus, R. T. 1978. The status of Oregon coho and recommendations for managing the production, harvest and escapement of wild and hatchery-reared stocks. Oregon Dep. Fish. Wildl. Processed Rep., 59 p. (Available from Oregon Dept. Fish. Wildl., P.O. Box 59, Portland, OR 97207.)

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Peterson, W., and C. Miller. 1976. Zooplankton along the continental shelf off Newport, Oregon, 1969-1972: Distribution, abundance, seasonal cycle and year-to-year variations. Oregon Sea Grant Program, Oregon State Univ., Publ. ORESU-T-76-002, 111 p.

Peterson, W. T., and C. B. Miller. 1977. Seasonal cycle of zooplankton abundance and species composition along the central Oregon coast. Fish. Bull., U.S. 75:717-724.

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