Tuna - willow as habitat for tuna

Pioneer farmers started planting willows to stabilise river banks which had been stripped of native vegetation, a river control practice continued by many regional councils.

Introduced willow species (Salix spp.) are now a dominant component of the vegetation in areas developed for pastoral agriculture.

Over the last decade, concern has grown about the willows' invasive natural spread along many New Zealand waterways, including wetlands. While they are seen as important for bank stabilisation and shade, densely-willowed areas can reduce water transport capacity, productivity, natural biodiversity and access for recreation. Due to their ability to grow from displaced twigs and branches, willows often grow where they aren't wanted.

Consequently, water resource managers are often faced with the dilemma of having to make a decision about whether to remove or to retain willows along streams, and willow removal and poisoning programmes are now in place in some areas.

Willows on the margins of streams and rivers are known to provide habitat that is often used by both longfin and shortfin eels. This is important in both an ecological and a commercial sense, as the removal of riparian willows - without replacement with a suitable alternative - from streams and rivers could adversely affect both the amount of habitat available to eels, and the number of large eels available to fishers.

To date, few studies have explicitly examined the importance of willows for fish in our waterways.

A preliminary NIWA scoping study investigating the weights of eels in willowed and non-willowed reaches of three waterways (Shag River, Tiritea Stream, and Wakapuaka River) found that:

• Longfins > 220 g were much more abundant in willowed compared to non-willowed reaches.
• Longfins < 220 g were of roughly similar abundance and density in willowed and non-willowed reaches.
• No shortfins > 220 g were caught in non-willowed reaches, and only moderate numbers were found in willowed stretches.
• Shortfins caught in the three study streams were predominantly < 220 g, and these were five times more abundant in the willowed reaches.

The main reason that riparian willows appear to provide such significant habitat for eels is that, being a nocturnal species, eels utilise willows during the day to avoid light.

Eels are generally known as secretive and "cover-loving", and large eels in particular are frequently associated with overhung banks and dense instream cover such as logs and debris.

For juveniles, features of the substrate are very important, but larger eels require more complex cover like debris clusters and undercut banks and the availability of such cover will largely determine the density of large eels.

In most braided rivers, almost all such cover for large eels is provided by willows.