Physical and biological oceanographers have long recognized the various scales of spatial and temporal variability affecting ecological processes in the ocean (Denman and Powell 1984, Legendre and Demers 1984, Haury and Pieper 1987, Steele 1978, Mann and Lazier 1991, Dickey 1990). The purpose of this paper is to examine several of the spatial and temporal scales of variability that Pacific salmon (Oncorhynchus spp.) are exposed to during their early life in the ocean, and discuss how these may specifically affect the feeding ecology of these juveniles. The scales of variability encountered by an individual salmon may be broad in terms of spatial extent, from those affecting the few meters immediately nearby to those affecting much of the Pacific basin. Similarly, processes that occur on temporal scales ranging from a few minutes to several decades may be important. Some of the spatial and temporal scales of variability that may be important to the feeding of juvenile salmon are shown in Figure 1.
It is beyond the scope of this paper to examine in detail all of these physical factors. I will instead focus on three of these which may be particularly important. Included among these are long-term interannual cycles of variability which may be affected, such as that seen for El Niño events, by physical forcing mechanisms initiated well beyond the waters of the Pacific Northwest. I will also look at a mesoscale feature, such as the dynamics of the Columbia River plume, which may be important for only a few months and in only part of the oceanic range of a salmon species. Finally, I will examine small-scale features, such as Langmuir circulation cells and internal waves, and suggest how these may also be important to the feeding ecology of juvenile salmon.
Long time series, extending, for example, back to the early part of the century, are now available for several environmental and biological variables in the North Pacific (e.g., Beamish 1995). Several areas, notably the California Cooperative Oceanic Fisheries Investigations grid and Ocean Station P, have been intensely studied for several decades and provide some measure of the interannual variability occurring on a local scale (Brodeur et al. 1996). Superimposed upon this shorter term interannual variability is the longer scale environmental variability associated, for instance, with the El Niño/Southern Oscillation (ENSO) phenomenon. Although ENSO warming events are known to occur on the order of every 3-4 years, it is only every 25 years on average when an event is of sufficient strength to affect high latitude ecosystems. An event occurring during 1982-83 was one of the strongest and best documented on record (Mysak 1986, Pearcy and Schoener 1987) and had substantial negative impacts on adult salmon production (Pearcy et al. 1985, Mysak 1986, Johnson 1988). I will present some possible ways in which an ENSO event may affect juvenile salmon feeding and subsequent survival.
One of the most direct ways in which an El Niño may affect salmon may be in the suppression of normal upwelling conditions in coastal waters. Several studies have shown a strong relationship between the survival and growth of coho salmon (Oncorhynchus kisutch) and the upwelling intensity when they first entered the ocean (Scarnecchia 1981, Nickelson 1986, Fisher and Pearcy 1988). For example, mean monthly Bakun upwelling indices were far below normal during the summer of 1983, resulting in high surface temperatures and low chlorophyll a concentrations along much of the West Coast (Fiedler 1984, Brodeur and Pearcy 1992). Even when upwelling conditions were favorable, a strong and deep thermocline, caused by onshore and northward advection of warm water, prevented normal mixing and resulted in warm, nutrient- poor water being upwelled to the surface (Huyer and Smith 1985).
The reduced upwelling may have greatly affected the entire pelagic ecosystem off Oregon and Washington, resulting in less overall food and a shift in the diets of most pelagic predators, including juvenile and adult salmon, to less preferred prey items (Brodeur and Pearcy 1990, 1992). Although the mean stomach fullness of juvenile salmon was not significantly different than in other non-Niño years (Brodeur 1992), the prey composition was radically different, consisting of smaller and probably less nutritious zooplankton species instead of the euphausiids and larval and juvenile fishes normally consumed by juvenile salmon (Pearcy et al. 1985, Brodeur and Pearcy 1990). As Fulton and LeBrasseur (1985) have shown, a major northward shift in the Subarctic Boundary occurs during strong ENSO events (Fig. 2), which results in a smaller prey-size spectrum of available food organisms, which may not be used as efficiently by juvenile salmon.
Other ways in which an ENSO warming event may influence survival of salmon is by changing their migration and distribution patterns, making them more available or susceptible to predators (Mysak 1986). The intrusion of warm oligotrophic waters onto the shelf may limit the distribution of salmon to a much narrower nearshore zone. If the offshore abundance of alternate prey species is also reduced during an El Niño year, the concentrations of juveniles may attract avian and piscine predators (Pearcy 1988). A decrease in mean growth rate expected under low productivity would potentially increase the probability of salmon being consumed by a size-selective predator (Healey 1982).
Furthermore, if concentrations of prey resources are much lower, the salmon must expend a much greater amount of time searching for food at a greater energy expenditure while being able to spend less time avoiding predators. The higher temperatures also necessitate higher rations to sustain the increased metabolic rates. Juvenile coho and chinook salmon (O. tshawytscha) food consumption during 1983, based on bioenergetic modeling estimates, indicated that both species were food limited due to the higher rations and decreased food availability during ENSO conditions (Brodeur et al. 1992). During normal years, there appeared to be adequate food for juvenile salmon survival.
An example of a mesoscale feature affecting mainly Oregon and Washington coastal waters and lasting several months is the dynamic nature of the Columbia River plume. At the height of its development, this oceanic extension of the Columbia River extends well offshore and south to off central Oregon. According to Barnes et al. (1972), its outer boundary is delimited by the 32.5‰ isohaline. During the winter, the outflow from the Columbia River is much weaker and the winds are from the southwest which pushes the plume onto the Washington coast. During a period called the spring transition in April-May, the prevailing wind pattern switches to northerly winds and the plume moves south. The winds also promote inshore coastal upwelling which pushes the plume well offshore (Fig. 3), although the shape and orientation of the plume can be variable during the summer (Fiedler and Laurs 1990). During the period of maximum runoff, the equivalent height of fresh water averages about 1 m (Barnes et al. 1972).
There is some evidence that juvenile coho salmon originating from the Columbia River utilize the plume as a transitional "habitat" between the brackish estuary and high salinity oceanic water. Marked juveniles caught at sea appear to follow the plume southward, travelling at about the same speed as the alongshore currents (Pearcy and Fisher 1988). It is only late in the summer or during poor upwelling years when the plume breaks down that juvenile coho begin to move northward. The timing of events also appears to be important. There appears to be a strong relationship between the date of the spring transition and the subsequent survival of the year class of coho salmon (Zirges 1981). Presumably, fish which enter the ocean before or shortly after the transition encounter a turbulent and highly saline environment, whereas later migrants that enter into a well-developed plume would enter a relatively benign environment and could gradually adjust to oceanic conditions. Riverine plumes also tend to have higher concentrations of zooplankton prey relative to adjacent marine waters, especially along the plume frontal region, which may attract juvenile salmon (St. John et al. 1992).
The date of transition can be variable on an interannual basis, extending from late January to May. The approximate timing of this transition can be deduced by examining for abrupt changes in the daily Bakun upwelling index or sea level, although in some years the transition is not well demarcated using these methods. A direct although costly method would be to examine moored surface current-meter data. An alternate method might be to use satellite temperature and ocean color imagery which can delineate the plume from adjacent upwelling and oceanic water (Pearcy and Keene 1974, Fiedler and Laurs 1990). The use of satellites may be an important tool for future salmon fisheries managers (Fiedler et al. 1984).
In addition to the initial effects that the timing of the transition date has on the survival of salmon, the date of the transition between winter and summer regimes appears to be inversely related to the total cumulative upwelling occurring during any particular summer (Fig. 4). Although the long-term mean discharge of the Columbia River appears to be relatively constant, there has been a decrease over the last several decades in the volume of water exiting the Columbia River during peak outflow periods (Fig. 5) which may affect the thickness and offshore extent of the plume. The Fraser River system, which has less hydroelectric development and agricultural diversions than the Columbia River system, does not show a similar decrease in peak flow (Fig. 5). It is uncertain whether climatic-scale changes in precipitation patterns or anthropogenic changes (water storage and irrigation) is the main cause of this decrease, but it may possibly have serious consequences for salmon production in the Columbia River.
Small-scale variability refers to processes that may last only hours or even minutes and may affect the immediate surroundings of an individual juvenile salmon. Two such processes affecting surface waters are wind-driven vortical circulation such as Langmuir cells (Barstow 1983) and wave packets formed by internal waves (Owen 1981). Both of these processes form long rows of aggregated materials evident as slicks on the surface; the main difference being that the Langmuir cells are oriented in the direction of the wind while the internal waves do not necessarily have to be (Fig. 6). Because of their circulation patterns, both these processes have been shown to concentrate zooplankton in the neuston layer at densities several orders of magnitude above the surrounding surface and subsurface waters (Shanks and Wright 1987, Shenker 1988), which may in turn attract juvenile fish which feed on these concentrations (Kingsford and Choat 1986).
I have hypothesized (Brodeur 1989) that foraging juvenile salmon may utilize these food concentrations to some degree to minimize the search area needed to satisfy their daily energy requirements. The species composition found in neuston net tows bears a distinct similarity to those found in the stomachs of juvenile coho salmon. Some major food items, such as crab megalopae, are found in such high abundances in these aggregations that they may be visible from the deck of a ship (Shenker 1988). There is evidence that coho salmon reside in the upper 2 m of the water column at night (Pearcy and Fisher 1988) and may take advantage of these prey patches while minimizing predation upon themselves by visual surface predators such as seabirds. The dynamics of small-scale wind and wave motions may thus be linked to the feeding and hence survival of these juveniles.
Physical processes that occur on many different scales of variability appear to be important to the ecology of juvenile coho salmon. In addition to the three factors that are discussed here, other processes such as long-term environmental trends and regime shifts (McLain 1984, Francis and Hare 1994, Brodeur et al. 1996), upwelling intensity (Nickelson 1986), and coastal currents (Pearcy and Fisher 1988) may also be important. Upwelling intensity appears to be the most important factor controlling food production in the coastal zone for juvenile salmon. Both the duration of upwelling events as well as the absolute intensity over the entire upwelling season may be important (Cury and Roy 1989). Excessive or persistent upwelling-favorable winds may be detrimental to coastal primary production in that nutrient-rich waters will be continuously advected offshore (Small and Menzies 1981).
Clearly, comprehensive reviews of oceanographic factors affecting juvenile salmon, such as those by Tabata (1984) for sockeye salmon (O. nerka) off British Columbia and Pearcy (1992) for coho salmon off Oregon and Washington, should be desirable reading for any biologist intending to do research on ocean salmon survival. We can no longer afford to ignore the variability inherent in the marine system when studying the variability in ocean survival. Salmon biologists would be wise to interact with physical oceanographers or at least become cognizant of many physical processes likely to affect the ecology of salmon in the ocean.
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