Large Woody Debris Structures and Their Influence on Atlantic Salmon Spawning in a Stream in Nova Scotia, Canada

Abstract

Although large woody debris from streamside vegetation has profound influences on channel morphology and habitat for salmonid fishes, it has often been depleted by channelization or deforestation of the riparian zone. We installed artificial structures mimicking naturally fallen trees in a third‐order stream in northern Nova Scotia from 1992 to 2004 to determine whether the structures enhanced spawning success of Atlantic salmon Salmo salar. In total, 250 digger logs (which mimic fallen tree trunks and stimulate formation of pools) and bank deflectors (which encourage meandering) were constructed in degraded reaches of Brierly Brook beginning at the downstream end. We monitored spawning each year by counting redds during the fall spawning season. Spawning rates (redd counts) in the whole brook increased exponentially for the first 4 years after restoration work began, from 43 in 1992 to 592 in 1996. By 1996, the entire length of Brierly Brook contained redds, and Atlantic salmon were using gravel that accumulated behind digger logs to build redds. Thereafter, redd counts remained high (502–605) but no longer increased, suggesting that restoration near the mouth of the brook removed an impediment to upstream migration. In 2004, reaches with artificial structures had significantly more redds (336) than reaches without the structures (280). In reaches with artificial structures, 48% of the redds were associated with gravel pool tails or the heads of riffles, 44% were near artificial structures, and 7% were near natural large woody debris. In reaches without artificial structures, almost 89% of the redds were associated with pool tails and the remainder were associated with natural large woody debris. Large woody debris, whether natural or artificial, appears to be an important source of spawning habitat for Atlantic salmon. Artificial structures mimicking naturally fallen wood are effective in the restoration of spawning habitat.

Wood placement in river restoration: fact, fiction, and future direction

“…If one assumes that success rates in excess of 75% or 80% are high, then the argument that there are high failure rates of placed wood is not supported by the literature. On the contrary, the literature suggests that we can expect the majority of placed wood to persist in its original location for a decade or longer for recent wood placement projects (e.g., MacInnis et al 2008;White et al 2011). The question remains: precisely how long do natural accumulations of wood persist?…”

Effects of artificial woody structures on Atlantic salmon habitat and populations in a Nova Scotia stream

“…Brierly Brook is a 20-km-long, third-order tributary of West River, draining an area of approximately 35 km 2 . The brook and its drainage basin are described in more detail elsewhere (MacInnis et al, 2008). The study site begins 7.6 km upstream from the mouth of Brierly Brook.…”

“…Digger logs were 15-20 cm in diameter, and straight trunks from streamside trees. We fixed the logs across the stream at a 308 angle to the perpendicular bisector of the stream banks, and held them in place with rocks and rebar, as described in MacInnis et al (2008). We built a cobble ramp on the upstream side of the log on a 2:1 slope using material from the side where the pool was to be formed.…”

“…We estimated the bankfull width of Brierly Brook before degradation from a regression of bankfull width against bankfull discharge (1 in 2 year daily average storm flow) for 30 undisturbed water courses in mainland Nova Scotia, as described by MacInnis et al (2008). We concluded that the degraded sections of the study site should have a bankfull width of 8 m. We then multiplied this number by 6 (because pools occur naturally every 5-7 stream widths) to determine the pool spacing, and therefore the restoration structure spacing of 48 m.…”

“…Apart from the Old Restored Site, which contained five artificial woody structures, before the study began there was almost no large woody debris anywhere in the study area. Several other reaches of Brierly Brook, outside the study area, were also restored over the study period (MacInnis et al, 2008).…”