Stefan Schmutz is a professor at the University of Natural Resources and Life Sciences in Vienna, Austria, and heads the Institute of Hydrobiology and Aquatic Ecosystem Management. In this interview, he discusses the state of knowledge on aquatic species, and the challenges that hydropower developers face in addressing connectivity.

Why is aquatic species connectivity an important topic in relation to hydropower?

Hydropower impacts the conditions within the aquatic species systems, and therefore connectivity is an important consideration for hydropower developers. Fish migration is the most recognised problem, and so we need to ensure that hydropower is not blocking migration; we can do this by equipping dams with adequate fish passes.

Longitudinal continuity is a huge problem for fish migration, as is the lateral connectivity, meaning the exchange between the river, the floodplains and the tributaries. This is important for spawning migration, and also habitat shifts.

Can you describe the current state of knowledge on this topic worldwide?

Scientists have conducted lengthy studies of some species, from which we have collated a great deal of information. Salmon is one such example; we have considerable knowledge of their spawning and downstream migratory habits, based on data going back several decades.

The biggest challenge for all rivers is still downstream migration."

We also have obtained knowledge relating to potamodromous species, or medium distant migrants as they are sometimes called. These fish, unlike other species, do not always travel between the sea and fresh waters. Instead they migrate within the river system, often covering several hundred kilometres, and so they need free continuum for spawning migration.

There are a number of species, largely residing in tropical rivers, for which we have limited knowledge of their ecological requirements. For example, in the Mekong system there are more than 800 species, and only for a small proportion of these do we have precise information regarding their migratory routes, spawning locations and seasonal migratory patterns.

Consequently, there is a need for greater research on tropical species in order to better assess the impact of hydropower on their migratory habits.

What would you say are the most difficult challenges that hydropower developers need to overcome in order to avoid or minimise impact on river basins?

The classic solution to connectivity disruption is a fish pass. In the past, a great deal of concern was expressed over the efficiency of passes; however, today’s standards have helped to ensure that hydropower developers, on the whole, build passes which function well for the majority of species.

The exception is fish passes that are used in tropical or larger rivers – in this environment it is still a challenge to build highly effective passes that keep the ecological status of the fish communities in an acceptable condition.

The biggest challenge for all rivers is still downstream migration. In order to tackle these difficulties, we are researching the biological requirements of downstream migrating species and trying to acquire information regarding their seasonal migratory patterns.

Progress has been made in resolving the issues for some species, and once again salmon provides a good example; however, for many fish we just do not have the appropriate facilities to guarantee efficient downstream passage.

Are there any examples of good practice that you have seen which address issues relating to river basin impacts?

I can draw on a number of examples for small and medium non-tropical rivers where good practice has been applied. In Austria, for example, we have built more than 1,000 fish passes within the last decades.

Larger rivers provide greater challenges; however, there are still some very good recent examples. The fish pass at the mouth of the Elbe River in Germany – called the Geestacht fish pass – is probably the biggest and most expensive in the world, but the efficiency is really impressive. The fish count adds up to 500,000 per year, and this covers all important species expected in this location.

What do you hope will be the outcome of the session looking at connectivity for aquatic species at the 2015 World Hydropower Congress?

I hope that through the session, developers can see that there are many solutions for problems relating to connectivity, but also that there are still a number of unanswered questions that we will need to try and answer through further research.

One issue, that I have yet to mention, is that the building of fish passes will not mitigate all the impacts associated with the implementation of hydropower.

Building reservoirs can change sediment transportation, the flow conditions of the river, and sometimes even the river’s temperature, and these issues are not addressed by only looking at connectivity conditions.

Stefan Schmutz is speaking in a session on connectivity for aquatic species at the 2015 World Hydropower Congress. Find out more here.