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

Highlights

• Modified tide gate management for improving instream habitat was trialled.

• Reintroduction of tidal fluctuations improved habitat conditions for native fish.

• Interaction with other local environmental stressors complicated outcomes.

• Optimising tide gate management regimes can reduce negative ecological effects.

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.

Introduction

Connectivity plays a critical role in defining the structural and functional attributes of riverine ecosystems (Fullerton et al., 2010, Lake et al., 2007). Fragmentation and disconnection of habitats as a result of water resource development is a key factor in the loss of biodiversity of freshwater ecosystems (Dudgeon et al., 2006, Rolls et al., 2013). Disconnections in rivers are particularly damaging because the dendritic structure of river networks restricts dispersal potential, with important consequences for population persistence (Fagan, 2002). Consequently, there is a need for improved understanding of how instream infrastructure, e.g. tide gates, weirs and dams, impacts on aquatic communities and how their effects can be mitigated and managed.

The impacts on fish communities of large barriers, such as hydroelectric dams, have been well documented (e.g. Williams, 2008, Morita et al., 2009) and attention is now increasingly turning to the potential effects of dam removal (Bednarek, 2001, Mclaughlin et al., 2013, Gangloff, 2013). However, smaller obstructions, such as weirs culverts and tide gates, are the most frequently encountered fish migration barriers in many catchments (Gibson et al., 2005, Warren and Pardew, 1998). Consequently, there has been a recent increase in efforts to design, test and implement cost-effective solutions for overcoming these small scale barriers (e.g. Franklin and Bartels, 2012, David et al., 2014, Newbold et al., 2014). Cote et al. (2009) and Rolls et al. (2014) demonstrated that barriers located near to the river mouth can have a large impact on fish with a diadromous life history, i.e. fish that need to migrate between marine and freshwater environments in order to complete their life cycles. Tide gates are used widely throughout the world to facilitate drainage of lowland areas and provide flood protection to valuable agricultural land and human infrastructure. Understanding the effects of tide gates on the movement of fish and their habitats is essential as they are often the first barrier that many upstream migrating diadromous fishes will encounter.

There are many types of tide and flood gates, but most tide gates operate as a one-way valve, allowing water to drain out, but preventing tidal water ingress. This process is generally driven by the relative water levels on the upstream and downstream sides of the tide gate, with the gate closing during the incoming tide and opening during the ebb tide. This results in significant alterations to the hydrological regime, connectivity and instream habitats in adjacent stream reaches. While some effort has been put in to the development of “fish friendly” tide gates, the physical effects on the environment have received relatively little attention.

Intuitively, when tide gates are closed they act as a physical barrier to fish migration. This has been demonstrated in paired studies of gated and un-gated stream systems where fish communities upstream of gated systems are impoverished relative to the un-gated systems (Pollard and Hannan, 1994, Kroon and Ansell, 2006, Doehring et al., 2011). Case studies of adapted tide gate management have also been used to demonstrate that when tide gates are opened, the upstream passage of target fish species is increased (Mouton et al., 2011, Boys et al., 2012). However, as well as directly restricting fish passage, tide gates can indirectly affect fish behaviour and community composition through alterations to the instream environment, particularly upstream of the gates. By blocking the normal bi-directional movement of water the upstream hydrological regime is altered; the velocity, turbulence and pattern of freshwater drainage is modified; the normal gradual transition in salinity is disrupted; and the stagnation of freshwater above the tide gates can result in sedimentation and increased water temperatures (Pollard and Hannan, 1994, Kroon and Ansell, 2006, Halls et al., 1998). This is likely to interrupt rheotactic and olfactory cues for migrating fish species (e.g. Knights and White, 1998), but also modifies estuarine and lowland stream habitats such that they may no longer be suitable or available for those species that would normally utilise them (Kroon and Ansell, 2006) and may impact on the migration of diadromous species.

Complete removal of existing tide gates is rarely feasible in the short-term because of competing flood mitigation and land use interests. However, in many cases it is likely that the structure and/or operation of tide gates can be modified to enhance connectivity for migratory species and reduce the severity of impacts on instream habitats. A number of studies have illustrated the benefits of adapted tide gate management for enhancing fish passage. For example, Mouton et al. (2014) showed that glass eel (Anguilla anguilla) migration was improved at an estuarine barrier as a consequence of increased barrier opening during tidal rise and Boys et al. (2012) observed improvement in both fish and crustacean communities following floodgate remediation. However, there has been little consideration given to mitigating the impact of tide gates on instream habitat.

This study describes the results of a 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 (see Section 2.1 for further details of the tide gates at the site). 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. Secondarily, the physical barrier effect of the tide gates on fish community structure was also considered. The tide gates involved in the trial were located upstream of the saline limit and therefore impacts on salinity were not considered. The results of the trial are now being used by local catchment managers to identify where modified tide gate management can be used to reduce the environmental effects of flood mitigation infrastructure at other locations.