Large-scale irrigation projects are being undertaken in the Mediterranean region, and agricultural practices are being intensified, but their impacts on instream biota remain poorly documented. The present study assesses the effects of an irrigation scheme on fish community structure in Mediterranean streams. Fish assemblages were sampled in unregulated intermittent streams over 6 years concurrently with the development of a new irrigation project in southwestern Iberia, and the biotic variation was separately related to increases in human activity and environmental gradients. Overall, fish assemblages were influenced by reservoir proximity (likely functioning as a source of non-native species), the proportion of irrigated lands, average water depth and wetted width (likely related with habitat permanency), and elevation. Results indicated that human actions linked to the irrigation project significantly shaped fish assemblage changes, after accounting for the effect of environmental variables. The proportion of irrigated lands in the catchment was related to some fish metrics (proportion of less tolerant specimens, proportion of invertivores and proportion of non-native specimens, except G. holbrooki). Our results support the thesis that the development and exploitation of irrigation schemes impact fish assemblages and reinforce the need to ensure that the ongoing agricultural intensification is compatible with the ecological integrity of Mediterranean intermittent streams.
As rainfall becomes scarcer or more erratic, we rely more on irrigation systems for water provision. Impacts of irrigation canals such as the barrier effect on wildlife movements are poorly documented. Although canal culverts and overpasses can be used by wildlife, little is known about their crossing patterns to guide barrier effect mitigation efforts. Over 7 years, we recorded medium-sized carnivore crossings by video-surveillance through 30 culverts and 28 overpasses in a large irrigation project in south-central Portugal. We examined the influence of the structures' features and landscape context on the likelihood of canal crossing. Culvert crossings were positively influenced by the proportion of nearby montado, a high nature value farming system. Overpass crossings were more likely in areas away from paved roads and with more nearby wetlands. Overpasses increased the crossing rates by about 11 % relative to culverts and both were crossed more often in landscapes with evenly distributed land uses. In the project area, 20% of the montado has recently transitioned to irrigated agriculture, and wetlands have increased by 43%. It is therefore plausible that the increase in the crossing rate of overpasses relative to culverts will be accentuated. Our study produced the first evidence of a contrast in crossing rates among irrigation canal crossing structures. We have shown that the landscape can be a driver of animal crossings but irrigation projects can in turn be transformative of the landscape. Broadly, the fact that the deployment of irrigation canals may favor some land uses over others creates a conundrum that needs careful consideration when planning barrier effect mitigation interventions.
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