Author:J Boubee, E Williams, J Richardson
Source:Auckland Regional Council
Of the 35 indigenous freshwater species currently recognised in New Zealand, 18 are diadromous and undergo migrations between fresh and saltwater as a necessary part of their life cycle. Apart from the degradation of adult habitats, one of the most significant causes of the decline in freshwater fish populations in New Zealand is the construction of structures such as dams and culverts that prevent fish from accessing otherwise suitable habitats. Management of the numerous freshwater resources has so far focused on avoiding, remedying, or mitigating the impacts of contaminants, physical activities and abstractions. However, these initiatives are significantly undermined if the resident aquatic biota do not have access to the resource.
The distribution of freshwater fish in the Auckland Region was analysed using data recorded in the New Zealand Freshwater Fish Database. In total, 15 indigenous and eight introduced fish species have been recorded in the Auckland Region. The majority of the indigenous species (13 species) are diadromous and fish migration barriers are therefore expected to have a major influence on fish distribution in the Auckland Region. Potential migration barriers like waterfalls, rapids, chutes and debris jams are natural, however the majority of instream obstructions are anthropogenic. These include badly positioned or undersized culverts, fords, dams and diversion structures, weirs (including flow measuring weirs), diversion channels, bed erosion control devices, and stream bed modifications.
This report provides guidance for the construction and retrofitting of in-stream structures to allow the upstream passage of fish. Although primarily aimed at road crossing culverts, solutions for the numerous low head weirs, artificial channels and dams present in the Auckland Region are also discussed.
Auckland Regional Council technical publication TP131
NIWA Client Report ARC 90229