Generating Heterogeneity: Construction of Fish Hooks in the Humber River Induces Community Change

The diversity in morphology of fishes is a product of the competitive processes that necessitate niche partitioning (MacArthur and MacArthur 1961). For example, interspecific competition is mediated through differentiation in mouth orientation; fishes with ventral mouths feed in the lower water column, whereas fishes with dorsal mouths feed in the upper water column (Wikramanayake 1990). Variation in water velocity reduces interspecific competition through character displacement, where low velocity waters contain fish with deep bodies and shorter fins and shallow, long-finned fishes are characteristic of high velocity waters (Wikramanayake 1990). Habitat heterogeneity facilitates the maintenance of morphological variation though an increase in dimensionality and habitat availability (MacArthur and MacArthur 1961). However, industrialization and development can strip lotic systems of heterogeneity. The Lower Humber River (LHR) watershed is entirely developed (TRCA 2008), with the LHR flowing from the Peel Plain through the City of Toronto and terminating at Lake Ontario. The LHR is surrounded by massive urban infrastructure; highway overpasses, rail lines, water treatment facilities, and residential high-rises surround the area. The watercourse is substantially altered to protect this infrastructure. There is extensive channelization resulting in the elimination of levee wetland systems and emergent and submergent aquatic vegetation that characterize natural estuaries. The channelization of the river has produced a single habitat type within the LHR, where fine grain size material and soft sediments dominate the substrate and natural structure or refugia are absent. This reduction in heterogeneity subjects individuals to competition, predation and extirpation (Gause 1932).

Historical data from the LHR watershed documents the presence of 54 fish species; however, only 22 fish species were captured in 2004 aquatic surveys. Monitoring data indicates the LHR watershed only supports generalist, tolerant and cool-warmwater fishes, whereas habitat specialists and higher trophic species are absent from the watershed (TRCA 2008).

Construction of artificial habitat can regenerate heterogeneity in anthropogenically simplified habitats (Gore and Bryant 1988). Stream and river restoration is predicated on generating a diversity of available habitats. Current deflectors, dams, cover structures, and bank protection elements are regularly installed to provide diversity in abiotic conditions (Gore and Bryant 1988). Restoration of heterogeneity in the LHR was performed using standard river restoration practices; however, in 2006 the Toronto Region Conservation Authority (TRCA) implemented a novel restoration method to increase the diversity of habitats available for fishes (TRCA 2008). The TRCA constructed 2 fish hooks at the mouth of the Humber River to supply static, low velocity backwater areas in the Humber River. The fish hooks form 2 semi-enclosed areas along the east bank of the Humber River (Figure 1). The structures deflect and concentrate flows, entrain bedload sediments, encourage the establishment of emergent vegetation, and provide small eddy pools for habitat and primary production. The fish hooks are comprised of 3 layers: a 300-mm thick granular bedding stone, forming the base of the structure against the riverbed; a 600-mm thick riprap stone affixed to the base; and armor stone at the surface interface. The fish hooks are 30 m wide at the base and jut out 8 m into the river.

Figure 1.

Satellite image of the fish hooks in the Humber River, Toronto, Canada. Arrows denote fish hooks. Inset is a cross section of the fish hooks.

To determine the fish community response to the novel habitat, we assessed both pre- and post-fish hook communities by electroshock sampling. Between 1989 and 2010 at a site adjacent to the fish hooks (43°37′54.7568″, – 079°28′18.7388″), we performed boat electrofishing 25 times (6 pre-construction and 19 post-construction). We performed boat electrofishing along a 350-m transect sampled upstream and downstream, executing each complete transect over a period of 1000 s. We subsequently transformed the site-by-species data into a similarity matrix, which we produced with a Pearson’s Phi resemblance measure for multivariate presence/absence data was produced. From the dissimilarity matrices, we produced a 2-dimensional PCoA ordination to facilitate visual interpretation, where similarity among sites...