Limiting Factors For Salmon's Return -
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Old 12-06-2007, 06:04 AM   #1
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Default Limiting Factors For Salmon's Return

While doing some research I found a site that has some useful fact. It always amazes me, the more I read, the less I actually know about what makes the fish tick.

Flow Regimes as a Limiting Factor

While the specific flow requirements of salmonids vary for each species, local populations of salmon and steelhead have additionally evolved the necessary physiological and behavioral characteristics for them to survive the dynamic flows encountered during each phase of their life history at a specific time of the year. Flow regimes are one of the most important drivers of habitat structure at micro-, reach, and riverscape scales. During critical stages while eggs incubate, young fry forage or drift, and adults struggle to return to spawn, the relationship between flow volume and velocity demonstrates the importance of a variable flow pattern over daily, seasonal, and annual time periods that is critical for the long-term persistence of salmonid populations. A critical review by Bjornn and Reiser (1991) summarized a majority of the known literature concerning flow regimes and their relationship to salmonid habitat.

Predation, fishing, and disease are pressures facing most local salmon and steelhead populations, yet the potential difficulty of finding preferred spawning habitat is also a significant limiting factor. The volume and velocity of water within a watershed is often a cue for initiating spawning migration, and is the fluid boundary which limits redd construction. Culverts present difficulties for adult and juvenile salmonids migrating upstream, though the extent to which this is a barrier to passage often depends upon a particular species leaping ability. Flows can be problematic in streams and rivers even when the watercourse does not pass through culverts. A shift in the peak flows may strand salmonids in the wrong portion of a watershed preventing them from reaching spawning habitatand potentially subjecting them to predation while waiting for additional flows to pass further into the watershed or forcing them to spawn in lower reaches where egg survival is limited. Although there appears to be little observational data about minimum depths necessary for passage, a minimum depth of 12 cm for trout, 18 cm for steelhead and coho salmon, and 24cm for Chinook salmon are considered necessary for passage (Bjornn and Reiser 1991).

FishXing ( is an awesome resource, in both English and Spanish, which allows the user to evaluate and design fish passable culverts.

The timing and velocity of flows during spawning are critical characteristics of spawning reaches, and poor flow conditions can limit the survival of eggs and alevins. In locations like Butte Creek, redd imposition may limit the survival of earlier spawning Spring Run Chinook salmon, which are an important genetic component of this distinct stock of California Chinook salmon. With adequate management of flows released from upstream hydroelectric facilities, the quantity of spawning gravels available to Butte Creek Spring Run Chinook could be increased and egg survival potentially increased. Most salmonids need water depths of at least 15cm, though this is also variable and dependent on spawner density and possible upwelling and hyporeic flows. Smith (1973) described the depth and velocity characteristics of 1,170 redds of 10 species of salmonids, including all the species in California. Regardless, flows need to be sufficient to not limit the velocity of oxygen-rich water through the stream’s hyporeic zone. Kondolf (2000) suggested a unified approach for assessing the impacts of sediment during the critical spawning, incubation, and emergence stages and incorporated a life-stage specific evaluation of gravel and flow requirements.

Flow and water depth are also critical determinants of rearing habitats for fry, parr, and adult residents and together comprise the amount of habitat available to salmonids in streams. High flows can be responsible for side-and off-channel habitat formation, which provide critical micro-habitat for rearing. Flow regime is one of the factors dictating salmonids emigration from freshwater to the ocean. While long-term flow increases are likely necessary to support a steady rate of out-migration of populations far from the ocean, even small short-term increases can be an important stimulus for coho and Chinook salmon. Beechie et al. (1994) determined that 73% of summer habitat loss and 91% of winter habitat losses for a coastal Washington population of coho salmon were associated with hydromodifications associated with agriculture and urban lands. NMFS (PCSRF 2005) ranked degraded freshwater habitats and flows as moderate limiting factors to coho salmon recovery. The return of a dynamic, natural flow regime will be critical to minimize the threats of these limiting factors. Roni et al (2002) reviewed strategies for restoration and suggested a tiered watershed approach that first restored connectivity, then returned natural hydrologic and geologic variability and processes, before finally focusing on instream restoration that is necessary for salmonid recovery.

Addressing Sediment as a Limiting Factor

Restoring riverine habitats to conditions favorable for supporting salmonids through their reproductive and smoltification life stages as adults, eggs, and juveniles is critical to our necessary response for recovering aquatic biodiversity. The acceleration of fine-sediment delivery and storage in coastal rivers and inland tributaries supporting salmonids drastically decreases egg survival, foraging success, and juvenile growth while increasing injury. This problem is extreme in northern California where about 59% of watersheds are impaired by sediment. The literature is full of excellent studies that experimentally and empirically characterize the detrimental impacts of fine sediment of salmonids, and some of these are further highlighted here.

In 1985, Berg and Northcote, experimentally demonstrated changes in behavior of juvenile coho salmon to short-term pulses of suspended sediment. During these exposure tests, individual fish left their territories and had irritated gills. Additionally, reaction distances to prey, capture success rates, and prey ingestion rates declined during periods of higher turbidity. More recently, Lake and Hinchdescribed that the type of fine sediment was linked to fish stress and mortality in juvenile coho salmon. In an experimental comparison of distinct types of suspended sediment, natural fluvial sediments caused fish stress and mortality at much lower concentrations than found with anthropocentrically derived “extremely angular” suspended sediment. The behavioral modifications of juvenile coho salmon during a critical period may limit recovery opportunities where watershed condition is limited by fine sediment. Fine sediment enters streams from numerous pathways, and should be managed within and beyond the riparian zone.

Sediment also directly impacts the aquatic food chain in numerous streams creating habitats less favorable to salmonid survival A recent study completed at UC Berkeleyfound an increase in fine sediment deposition shifted the macroinvertebrate community towards burrowing taxa, which are less available for prey, and increased metabolic costs in fishes associated with greater activity and intraspecific competition. A linear response between increased deposited fine sediment and decreased juvenile steelhead growth suggests there is no threshold below which increased fine sediment delievery and storage is harmless, and the impact of sediment on stream food webs directly reduces salmonids ability to grow and survive. This study also suggested that any reduction in fine sediment could produce immediate benefits for salmonid restoration.
Embeddedness is a measure of fine sediment in spawning gravel. It is a common metric used by the Regional Water Quality Boards and National Marine Fisheries Services to identify sediment as a limiting factor in restoring spawning gravels. The desired condition for a recovering watershed to support greater salmonid spawning habitat is found where there is an increasing trend of locations where gravel and cobbles are ≤ 25% embedded. While emphasis is often placed on reducing direct inputs of sediment into streams, Opperman et al.found that coarse-scale quantification of watershed land was significantly related to measurement of embeddedness. In watersheds with agricultural and urban footprints, often characterized by restricted riparian areas, there was often not a direct decrease in fine sediment and reduction of sediment is best accomplished by envisioning reduction throughout the entire watershed. While reach-specific riparian protection offers numerous benefits to salmonids via cover, nutrient, temperature buffer, this type of protection has limited benefits for overall reduction of sediment.

Many restoration projects offer multiple benefits to salmonids during their various freshwater life history stages and holistic projects managing sediment from indirect and direct sources is critical in restoring salmon to viable population sizes in may coastal streams and inland rivers. Properly functioning roads, stormwater systems, and sufficient riparian buffers are necessary in many watersheds to adequately reduce fine sediment entering streams. SRF continues to highlight actions to reduce sediment using best management practices for Road upgrading and instream bioengineering projects, and will feature a number of sessions centered on water quality, water quantity and salmonids at the upcoming 2006 conference. If you are more interested in quantifying, monitoring, and assessing fine sediment in relationship to salmonids a number of great agency resources exist including:

Flosi, G., S. Downie, J. Hopelain, M. Bird, R. Coey, and B. Collins, 2004. Updated California Salmonid Stream Habitat Restoration Manual. 3rd Ed. Department of Fish and Game, Inland Fisheries Division.

NOAA Fisheries- Southwest Region. 2004. Sediment removal from freshwater salmonid habitat: guidelines to NOAA Fisheries staff for the evaluation of sediment removal actions from California streams.

Shawn Buller
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