History

Like most rivers in northern Colchester County, Nova Scotia, Waughs River begins in the uplands of the Cobequid Hills and drains northward toward Northumberland Strait. The Falls is the appropriate name given to a series of cascades where the river descends the edge of the plateau onto the flat lowlands along the coast. The lower river supports Atlantic salmon, brook trout and (introduced) brown trout.

Historically, the largest of the cascades at The Falls was probably a permeable barrier which some salmonids could ascend by jumping from pool to pool. However, a private dam built in 1867 transformed The Falls into an absolute barrier to fish passage. Although the dam has long been removed, reshaping of the channel to accommodate the dam continued to effectively prevent fish passage. Consequently, while there is an abundant population of landlocked brook trout in the upper river, no Atlantic salmon have lived there for 140 years.

All this changed in 2009 when the Department of Fisheries and Oceans built a fish ladder at The Falls.  The 20-m Alaskan Steepass ladder was installed to allow trout and salmon to ascend the otherwise impassable cascade. The new ladder permits migratory fish to use about 10 km of the upper river, including abundant prime habitat, that was previously inaccessible. 

Brown trout were observed using the fish ladder within days of its completion. Atlantic salmon began using the upper reaches during the 2010 spawning run. The population of salmon in the upper river is also being augmented with hatchery-reared fish. The North Colchester River Restoration Association, a local volunteer group in collaboration with DFO, incubated fry from 40 000 eggs for release into the upper river in 2010. The eggs were taken from spawning salmon collected below The Falls, where spawning activity tends to be concentrated.

Graduate student Heather MacDonald standing atop the fish ladder during high flow in November 2010.  She is standing on a leaf-covered grate over a holding pool, designed to give the fish a rest as they ascend the ladder.  The lower segment of the ladder extends down to the right in the photograph, while the upper segment runs upward behind Heather.  The Falls lies to the immediate right of the photograph.

The Project

The return of Atlantic salmon to the upper Waughs River may re-structure the fish community if brook trout begin competing with young salmon. Depending upon the outcome of this interaction, other aspects of the river ecosystem may also change, particularly benthic invertebrates and energy flow.  The fish community may influence leaf litter decomposition, an important energy source, by consuming litter-feeding invertebrates or changing their behaviour. This aspect of fisheries ecology has received little attention.

Upper Waughs River, above The Falls.  Abundant bankside cover, well-oxygenated water and cobble bottom creates fine rearing habitat for Atlantic salmon.

Graduate student Heather MacDonald studied the effects of immigrant Atlantic salmon and brown trout on upstream fish communities and ecosystem processes for her M.Sc. Thesis.  She counted redds (the pits in the stream bottom that salmon dig to hold their eggs) and used electro-fishing and seining to assess fish populations (with help from DFO).  Trout populations were not reduced by competition or predation by salmon, at least in the first few years after the fish ladder was installed.  However, native brook trout appear to favour upper reaches where salmon are fewer.

Salmon redds in upper Waughs River demonstrate that the fish are ascending the falls, but it proved difficult to show that they are using the fish ladder.  Heather tried marking migrating Atlantic salmon with PIT (passive integrated transponder) tags and then mounting a detector at the top of the ladder.  But catching enough fish to tag proved impossible.  In 2011, we installed underwater cameras at the top of the ladder, only to have them knocked out by a flood.  We can only infer that salmon use the ladder because of the redds upstream. 

The resting cage at the top of the fish ladder.  The bars on the upstream end are intended to prevent logs and other floating debris from clogging the ladder.  The metal grate on top provides cover from predators.  The ladder descends at the left rear of the picture.

Benthic invertebrate populations were assessed using Hess samples.  The effect of fish on litter decomposition rates were assessed using the "litter bag" method. Known quantities of red maple leaves were enclosed in mesh bags and pinned to the river bottom to simulate natural leaf packs. Coarse-mesh bags (1-cm mesh size) permit litter-feeding organisms (shredders) to enter, while fine-mesh bags (0.25 mm) exclude them.

Placing litter bags in Waughs River, above The Falls.  The bags are pinned to the bottom with spikes and then anchored with a cobble on top.

In the absence of other factors, litter decomposition rates from coarse-mesh bags tend to be higher than from fine-mesh bags because of the activity of leaf-feeding insects.  If fish alter the activity of these insects or reduce their numbers, a reduction in the rate difference between coarse-mesh and fine-mesh bags should become apparent as fish populations increase above The Falls.

Analysis of litter-bag data suggests no effect of fish on decomposition rates.  There were clear differences in mass loss rates between fine-mesh bags and coarse-mesh bags, as expected, and decomposition in the second year (2011) was significantly slower than in 2010.  Nevertheless, we could find no effect of fish presence on mass loss rates.  Surprisingly, there doesn't even seem to be a correlation between the number or size of leaf-feeding invertebrates and decomposition rate, in contradiction of findings in the literature.  Further analysis of these puzzling data are underway.