New NIWA research shows that adding logs into stream beds as part of waterway restoration could boost koura (freshwater crayfish) populations and speed up streams’ recovery.
Koura are prey for freshwater fish, and for humans. Historically they are an important part of Maori diets; they are also highly-valued in stream restoration projects.
Our research investigated the interactions between wood and koura (Paranephrops planifrons) in a natural ecosystem. We studied three environments in a native forest stream in the central North Island:
- a complex wood-rich environment
- a straight wood-less environment
- a winding wood-less environment.
Our findings included that koura like logs in streams, and that more, bigger koura are likely if certain conditions exist.
Koura like logs in streams
When horizontal logs crossed the stream, there were more varied habitats for the koura. The wood in the stream caused more diverse conditions of water depth, changed the speed of the water by creating pools and eddies, and promoted different materials along the bottom of the stream.
These different environments lead to more and bigger koura, compared to a straight reach of stream lacking wood. Undercut banks in woodless areas also supported abundant koura, but added wood seemed to boost the area of stream for the animals to live in by acting as a surrogate stream edge.
The prize: a koura or freshwater crayfish. (Kristel van Houte-Howes)
Where are the most, and the biggest, koura?
The highest numbers of koura caught during daylight hours were generally found when edge habitat combined with slow currents, proximity to cover, and depths of about 20-30 centimetres. This suggests the importance of wood for koura is not necessarily in providing direct cover, but perhaps indirectly, through creating a surrogate edge habitat with associated slow currents, proximity to cover, and pool formation.
The presence of the logs also influenced the size range of koura, boosting the larger-sized breeding animal numbers. The logs seemed to provide different habitats, including habitats for a wide range of sizes. This function of wood could be particularly significant for providing habitat for breeding populations in newly-restored streams.
Contact: Robert Davies-Colley, NIWA Hamilton ([email protected])
Full paper – New Zealand Journal of Freshwater and Marine Research 43 (3)
