Why would salmon care about the weather?

Ann Gargett
Dept. of Fisheries and Oceans
Institute of Ocean Sciences, Patricia Bay
Sidney, British Columbia V8L 4B2
Canada

Nobody who reads newspapers can fail to notice the increasing concern about sustainability of many marine FISHERIES. While there are many factors which contribute to the size of marine fish stocks over time - predation, commercial and recreational fisheries, and habitat destruction, to name but a few -over the past several years, evidence has been accumulating to suggest that climate variability on decadal time scales is an additional and very strong influence on marine fish stock size.

FIGURE 1

One example is the variation of North Pacific SALMON stocks, represented in Fig.1 as the mean annual catch of the major fishing nations, which appears to be related to an index of the strength of the wintertime Aleutian Low pressure system which dominates atmospheric circulation in the wintertime North Pacific. The correlation between these time series is such that catch is generally high during periods of intensified winter low pressure in the Gulf of Alaska. While one must be wary of a correlation over what amounts to only one cycle, this is only one of several pieces of evidence pointing to some connection between the success of salmon stocks in the north Pacific and the state of the overlying atmosphere.

Things are not quite this simple however, because the connection appears to CHANGE PHASE between the northern and southernmost range of salmon habitat in the North Pacific. The anecdotal information shown here has been extracted from the trade magazine Pacific Fisherman by Nate Mantua, University of Washington. Other more quantitative evidence says the same thing . . . that when the northernmost ALASKAN stocks do well, i.e. in 1939 a time of generally STRONG Aleutian Lows, southern stocks in WASHINGTON / OREGON / CALIFORNIA do poorly - and vice versa.

To go beyond simple correlations, we need to answer

3 QUESTIONS:

FIGURE 2

The last question is a very important one to answer FIRST, because of the highly migratory nature of salmon - as shown in the monthly mean positions of young salmon which enter the ocean from Canada's Fraser River during the month of May.

FIGURE 3

Over the following several months, the young fish migrate counterclockwise around the rim of the Northeastern Pacific, remaining concentrated in coastal waters. Because many different pieces of evidence are beginning to suggest that survival rates are determined mostly during the first year of ocean life, this trajectory and its timing means that the factors affecting survival most likely operate in the coastal ocean. What might these factors be?

FIGURE 4

This picture reminds us that marine plants have neither root nor branch - unlike their terrestrial counterparts, they cannot use stems or branches to position themselves favourably in the sunlight, nor roots to tap the wealth of fertilizer which lies below the nutricline. Instead it is physical processes, operating in the upper ocean, which provide the nutrient and light environments which shape marine ecosystems from their base in primary production.

The important physical processes act at all scales.

FIGURE 5

At the LARGEST scales

---wind-driven upwelling in subpolar gyres positions the nutricline quite close to the euphotic zone - while subpolar gyres are consequently rich in nutrients, primary production can be light-limited at least part of the year at these high latitudes.

--- In subtropical gyres, light levels are higher and more constant throughout the year, but the nutrient supply is meager, because downwelling places the nutricline well below the euphotic zone.

Whether the nutricline is shallow or deep, the actual vertical transport of nutrients up into the lighted euphotic zone occurs through the action of a variety of SMALL-SCALE physical processes, generally lumped together under the name "turbulence". At the same time, turbulent processes modulate the light environment of phytoplankton, by moving them in the strong vertical gradient of near-surface light.

The turbulent processes which shape the nutrient and light environment of the upper ocean are strong functions of the STRATIFICATION there.

--- Strong vertical stratification severely inhibits the vertical motions associated with turbulence, and the associated vertical turbulent fluxes of nutrients.

--- Where water columns are weakly stratified, vertical excursions and nutrient fluxes may be large.

This general result can be used to form a very simple CONCEPTUAL SUMMARY of the interaction between physics, represented by water column stability, and biology, represented by primary production.

FIGURE 6

When stability is weak, production is low because although large vertical fluxes provide plenty of nutrients, large vertical excursions result in low average light.

Production is also low in conditions of high stability, where small vertical velocities lead to adequate light but fail to supply sufficient nutrients. BOTH light and nutrients will be adequate at intermediate stabilities, within an "optimal window" of stability.

Given the mean nutrient and light supplies which are characteristic of subpolar and subtropical gyres, it seems reasonable to assume that phytoplankton populations in these Northern and Southern gyres exist towards opposite ends of this window - the subpolar populations limited by light, the subtropical ones by nutrients.

Imagine now what happens if coastal water column stability INCREASES everywhere in the Northeast Pacific. As the environment of the N populations becomes more stratified, they move into more favourable conditions (as higher stratification brings higher light levels), while S populations move to less favorable conditions (as higher stratification lowers the nutrient supply). The opposite happens if stability decreases everywhere - S populations thrive and N populations struggle.

FIGURE 7

This can be directly translated to a mechanism for

--- out-of-phase variation in N and S fish stocks

--- linked to the strength of the wintertime Aleutian Low

IF:

---- there is a linear connection up a simple food chain, so that more phytoplankton means more zooplankton, means more fish survive

AND

---- the strength of the winter Aleutian Low is directly related to COASTAL stability over the full N/S extent of the eastern border of the N. Pacific

Fisheries biologists indicate that a simple food chain is reasonable for salmonids. The remaining question is whether there's a basis for a relationship between coastal stability and the Aleutian Low . . .

FIGURE 8

This figure illustrates the dominant characteristics of the wintertime atmosphere in the Northeast Pacific during periods of STRONG and weak Aleutian Low.

A STRONG Low brings strong flows of moist marine air up against the coastal mountains of ALASKA and northern BC. In the N, where stability is mostly determined by salinity, the coastal ocean becomes more strongly stratified through increased precipitation and run-off, both direct and stored. A STRONG Low also displaces the California High pressure area to the SE, bringing increased incidence of southerly winds to the CALIFORNIA coast. Here in the S, on the eastern edge of the subtropical gyre, coastal stability is most strongly influenced by upwelling - the increased incidence of southerly winds means less upwelling, hence again a more strongly stratified water column.

Exactly the opposite holds when the Aleutian Low is weak - water columns throughout the eastern coastal domain become less stable.

Thus I suggest the following answers to the questions of

FIGURE 9

The "optimal window" mechanism presented here may or may not be the correct explanation for why salmon care about the weather. Right or wrong however, a mechanism can be tested - and it is by focussed attempts to test specific mechanisms that we will learn more of what we need to know about the complex interactions underlying apparent correlations between the physical environment and marine fish stocks.