Week 3 - Puget Sound Biophysical Connections to Salmon

Required Readings

Si Simenstad, Alan Mearns, Steve Jeffries

January 20, 1999

Estuarine, Nearshore - Si Simenstad

Contaminants and Pollution - Mearns

Predators - Steve Jeffries

Outline

I. Estuarine and nearshore "gap" in our appreciation of the transition between freshwater and ocean
II. Diversity of Pacific salmon "approach" to estuarine and nearshore ecosystems
A. Species contrasts
B. Life history diversity
III. Evidence for estuarine and nearshore "dependence"
A. Reimers Sixes River estuary
B. Levings, Macdonald et al. 1983-85 Campbell River estuary manipulation experiments
C. Outcome isn’t fixed, it’s more of a lottery
IV. Functions that enhance salmon production at the "fringe"
A. Foraging
1. high densities of non-evasive prey
2. constant availability (non-migrating)
B. physiological transition
1. low velocity mixing zones; often extensive tidal freshwater zones
2. freshwater-brackish plumes
C. refuge from predation
1. turbidity
2. geomorphic structure (extensive intertidal flats, tidal channels, low gradient beaches, LWD)
3. vegetative structure (emergent marsh, eelgrass)
V. Production of juvenile salmon is dependent on unique structure and processes of the "fringe"
A. continuum of habitats
B. estuarine and nearshore circulation maintaining important habitat elements
1. substrate
2. structure
C. high direct and indirect productivity, especially compared to pelagic production of main basin
VI. Salmon at fringe supported by detritus-based food web
A. sources of food web are both external and internal
1. watershed contributions are nontrivial
2. trapping and retention of detritus important characteristic
B. high, compounded, overlapping productivity
C. "detritus cycling mill" sustains complex food web
1. physical disintegration
2. biological contribution to disintegration
3. biogeochemical decomposition promoted by fringe environments provides important value to detritus
D. salmon illustrate very discrete food web linkages
1. must respond to transitions in prey resources
2. often illustrate "optimum foraging" on bioenergetically-rich prey
3. suggests both carrying capacity and food web integrity limitations on salmon ecology, if not production
VII. Conundrum of understanding and managing salmon
A. concentrated human impacts
B. management missing at landscape scale
C. scientific denial of ecotone importance?

Abstract

In both science and society, we have often been recalcitrant in acknowledging the role that estuaries and nearshore ecosystems play in the evolution of Pacific salmon species, life histories and ecology. Among the diverse salmon species and life history types, many have evolved extensive but distinct approaches to utilizing estuarine and nearshore habitats in transition from their natal watersheds to the Pacific Ocean. This is readily apparent for genetically distinct stocks of ocean-type chinook and chum salmon (and perhaps even ocean-type coho) that demonstrate strongest affiliations with the estuarine and nearshore ecosystems of Puget Sound.
While we once considered estuaries to be "sinks" of extensive salmon mortality, it has become more apparent that they often form one among many evolutionary strategies that these salmon use to ‘hedge the bets’ against the vagaries of environmental variation. In fact, the production of many juvenile salmon in Puget Sound watersheds may be on average just as dependent on the estuarine and nearshore phase of their life history as on the freshwater phase. Compared to the open waters of the Puget Sound basin, the constricted estuarine and nearshore along the "fringe" provides more critical functions for salmon, including physiological transition, optimal foraging and refuge from predation. The productive capacity of Puget Sound to support salmon is also based primarily on the fringe, in part because of vast difference in quantity, quality and timing of production. Estuaries and nearshore ecosystems both trap organic matter from watersheds but also generate prodigious quantities over varying time scales. Rather than being consumed directly in a simple grazing (herbivorous) food web, the fringe of Puget Sound is sustained by complex food webs based on detritus, i.e., dead and decomposing organic matter. Until the make the full transition to the open waters of the Sound, juvenile salmon and many other aquatic organisms exploit these detritus-based food webs almost exclusively. Thus, the dynamic physical and geochemical processes of estuaries and nearshore environments that sustain the "detritus cycling mill" are just as important as the inherent biological processes.
Not surprisingly, the fringe of Puget Sound poses a major conundrum for science and management. Impacts from watersheds converge at the estuarine interface with the Sound, and development imposes cumulative effects on critical estuarine and nearshore processes. Management of salmon and the accumulating impacts, however, are ill prepared to deal with the landscape scale of the processes and problems. Even science has difficulties grappling with the importance of the Puget Sound fringe to salmon, challenged by the scale and complexities of dynamic processes affecting salmon passing through and along ecotones. The fact that the proposed ESA listings target ocean-type chinook and summer chum, perhaps the two salmonids most "dependent" on estuarine and nearshore ecosystems and processes, indicates that we cannot ignore the fringe of Puget Sound any longer.