Modified tide gate management for enhancing instream habitat for native fish upstream of the saline limit

Abstract

Tide and flood gates are used widely throughout the world to facilitate drainage of lowland areas and provide flood protection to valuable agricultural land and human infrastructure. However, these structures can impact on aquatic communities by disrupting connectivity and altering physical habitat conditions. Complete removal is rarely feasible in the short-term because of the competing flood mitigation and land use interests. However, in many cases it is likely that the structure and/or its operation can be modified to enhance connectivity for migratory species and reduce the severity of impacts on instream habitats.This study describes the results of a short-term trial investigating the effects of modified tide gate management on instream habitat and fish communities in a small tidal stream in the North Island of New Zealand. The study site was located upstream of the saline limit and thus does not directly address this potential limitation on managing the effects of some tide gates. The main objective of the study was to understand whether improvements in tidal flushing could mitigate the negative effects of tide gates on upstream habitats, thus improving their suitability for native fish communities.The trial demonstrated that in impacted lowland river reaches, the reintroduction of limited tidal exchange upstream of tide gates reduced negative impacts on instream habitat by restoring hydrological variability, increasing minimum dissolved oxygen concentrations and potentially also reducing water temperatures. However, it was also shown that the recovery may not be uniform and can be dependent on interactions with other stressors. The trial illustrated the potential for using modified tide gate management to mitigate the environmental effects associated with their operation, and to restore habitat conditions so that they are more favourable for the persistence of native fish species.