Ecosystem Sciences Silver Creek Fish Habitat Inventory Report.

Silver Creek Fish Habitat Inventory

A key indicator of a stream’s health is the quality and quantity of habitat. Silver Creek is known for its first class trout fishery. Because a detailed inventory of Silver Creek’s fish habitat has never been performed, one of the tasks Ecosystem Sciences Foundation took on was the identification, qualification and mapping of aquatic habitat in Silver Creek on The Nature Conservancy’s Preserve. Ideally, all of the fish habitat from the headwaters to the creek’s confluence with the Little Wood River would be identified and mapped; however, funding limited the habitat inventory to the stretch of river on the Preserve.

The following maps, Figures 1 through 4, illustrate the results of the habitat survey. The stream was floated from Stocker Creek to Kilpatrick Pond and dam. Deep runs and pools were surveyed by snorkeling. Habitat inventories focus on the primary types of fish habitat necessary for all trout life stages, including spawning and incubation, early rearing, young-of-the-year, juvenile and adult habitat. Each life stage has specific habitat requirements that are critical, including gravel size, sediment conditions, instream cover, escapement, pools and run depths.

A habitat inventory is essential in order to define the principal limiting factor(s) on a fishery. For example, while spawning habitat may be extensive in a stream, a lack of early rearing habitat might be the cause of a small adult population because young-of-the-year and juvenile trout are susceptible to predation. In addition to habitat limitations, water quality can also affect a fishery and adversely impact fish production and size.

In the case of Silver Creek, summer temperatures and sediment deposition have been cited as having the potential to impact the trout fishery. What has not been identified to date is the extent to which habitat and habitat availability factors into the health of the stream’s fishery.

The conclusions that can be drawn from the fish habitat inventory (and shown in the maps) are:

· With the exception of Kilpatrick Pond, there are no stream reaches that do not contain some critical trout habitat feature. Spawning habitat with clean gravels is distributed throughout the stream. Early rearing and young-of-the-year habitat is juxtaposed with adult holding/rearing habitat, so that Silver Creek exhibits a mosaic of trout habitat in all reaches.

· Fish production is exceptionally high throughout. This is because benthic invertebrate (animals with no backbone that live on the bottom of the stream) production occurs in all reaches in substrate and aquatic vegetation, providing an unlimited food base. Incubation and egg-hatch appears to be very high and redds (depressions on the stream bottom in beds of gravel where fish eggs are deposited) are little affected by sediments. In fact, successful incubation requires a certain degree of sediments to ensure an adequate protective cap develops over the redd.

· Sediment deposition is greatest at the confluence of tributaries and agriculture drains into Silver Creek. These are isolated sites of extreme sediment depth and probably contain sediments deposited years ago as a legacy of cattle grazing throughout the watershed.

· Sediment deposits are relatively thin in other stream reaches. The gravel areas covered by thin layers of fine sediments pose no problem to large trout building redds; they can easily swipe away the fines as the egg nest is dug. These sediments are also too thin to limit benthic invertebrate production.

· Pools, especially deep meander pools, are scoured of sediments. As stream flows enter outside bends, the flow velocity picks up, which not only forms the pools but prevents sediment accumulation. Without these physical processes, all of the pools in Silver Creek would have vanished long ago under legacy sediments.

· Upper reaches of the stream from Stocker Creek to the Grove Creek confluence are heavily canopied and banks stabilized with riparian vegetation. The middle and lower reaches of Silver Creek within the Preserve, in contrast, are widened because of past livestock grazing and lack the riparian habitat of the upper reaches. However, this does not significantly degrade instream trout habitat.

· Reed canary grass (Phalaris arundinacea) is encroaching in many places along the stream. Typically, reed canary grass begins building platforms on what were once undercut banks. In many places, undercut banks have been lost due to winter conditions exacerbated by sediment inputs (to be discussed in an upcoming blog). Reed canary grass easily becomes established on these disturbed sites. The long term threat from reed canary grass is in those naturally shallow channel reaches where the plant builds platforms and encroaches into the channel year-by-year.

· Fish passage is generally not an issue within the Preserve boundaries, although a beaver dam about one-quarter mile above the Stocker Creek road does inhibit trout movement into the upper watershed streams (Cain, Mud, Stocker, etc.). TNC staff have removed the beaver and pulled down some of the dam, but the dam continues to have a backwater effect that accumulates sediments. The dam needs to be brought down to the stream water surface elevation to encourage sediment movement and more efficient trout passage. Except for this beaver dam, the only other impairment to trout migration on Silver Creek is the Kilpatrick Dam.

· Sediment movement into Silver Creek is clearly through the agriculture drains and tributaries. The massive deposition areas occur at these confluences and the solution is to attenuate, to the extent possible, sediment inputs from agriculture lands adjacent to the tributaries and from the irrigation ditches. There are also a few minor places within the Preserve that could generate sediment inputs from overland flow in the spring.

· The only place in Silver Creek that does not support high quality trout habitat and benthic invertebrate production is in Kilpatrick Pond. Legacy sediments combined with annual inputs of new sediments have rendered this reach of the stream all but unusable by all trout life stages except adult holding. Angling and catch effort continues to be high in the pond, mostly below the bridge and the Preserve boundary, because adult brown and rainbow trout move downstream in response to density-dependent competition, i.e., competing for space with one another.

Results from this fish habitat inventory indicate that the Silver Creek fishery not only lacks an identifiable habitat limiting factor, but habitat throughout the stream supports all life stages for all fish species. Silver Creek is a legendary fishery precisely because of the habitat quality found throughout the stream. Although physical habitat and the food base is not limiting, the fishery is adversely affected by elevated summer temperatures and sediment inputs. As temperature and sediment conditions worsen in time, it can be expected that these conditions will impose a limiting factor(s) on the fishery.