Use of passages across a canal by wild mammals and related mortality

From 1993–1998, we monitored a 24.1-km long concrete water canal in northern Spain for drowned mammals. Along the canal, 14 concrete bridges and 9 small bridges permitted crossing by humans and livestock. Our objective was to test whether those bridges could be used as passages for wild animals to prevent drowning. We used tracks recorded in sand on bridges to identify species passing. Of the bridges, 65% were used by wild mammals; the 9 small bridges accounted for 57% of crossings by wild animals. Wild and domestic canids (Canis lupus, C.l. domesticus, Vulpes vulpes) were the main users (85%), followed by wild ungulates (mainly wild boar, Sus scrofa) at 71% of the crossings. Proximity to a mountain slope did not increase crossing in deer species, but did not hinder wild boars, foxes and wolves from doing so. Up to 88.1% of the wild mammalian species selected passages that were close to scrubland or forest. Of the drowned animals observed, 70% were dogs and livestock. More roe deer (Capreolus capreolus) drowned than other species, and 73% drowned between April and October. Throughout the year, 6.5% of wild boars fell into the canal . Foxes were the main species crossing the canal, but accounted for only 2.1% of drowned species. We recommend that the following action should be taken to reduce drowning: (1) bridges that are simpler and rustic in design should be constructed and (2) water catchments should be dug into the forest to provide water, so that animals do not need to use the canal as a water source or need to cross it to reach the river.