1) Does Outlet Hang Height Impede Upstream Fish Passage?

Weak Swimmers

Fig. 3. Difference (downstream-upstream) in catch per unit effort (#fish/second) of weak swimming fish species belonging to the families Gadidae (burbot), Cyprinidae (northern red belly dace, pearl dace, lake chub, long nose dace), Gasterosteidae (brook stickleback), Catostomidae (longnose sucker, white sucker), Cottidae (spoonhead sculpin), Percopsidiae (trout perch) at culvert stream crossings. A positive value indicates that at a site more fish of a particular species were found downstream of a stream crossing, while a positive value indicates higher presence of a species upstream of a stream crossing. 

In this study bridges were used as control sites. There is a slightly higher downstream catch rates of weaker swimming fish species as hang height at a culvert outlet increases. However, only approximately 7 percent of the variation in the data is explained by outlet hang height (R2=0.0663). Since this is a weak relationship, it appears that culvert hang height is not an extremely strong predictor of weaker swimming fish species catch rates.

Strong Swimmers

Fig. 4. Difference (downstream-upstream) in catch per unit effort (#fish/second) of strong swimming fish species belonging to the family Salmonidae (rainbow trout, mountain whitefish, arctic grayling) at culvert stream crossings. A positive value indicates that at a site more fish of a particular species were found downstream of a stream crossing, while a positive value indicates higher presence of a species upstream of a stream crossing. 

Again, bridges were used as a control to culvert stream crossing structures. Downstream catch rates decreased as outlet hang height increased in the stronger swimming Salmonids. Similar to the weak swimmers, there was a weak relationship between hang height and catch per unit effort, with approximately 14 percent of the variation being explained by outlet hang height (R2=0.1405). It is possible that culvert hang height is not the strongest predictor of strong swimming fish species catch rates.