Tiakina Kia Ora: Protecting Our Freshwater Mussels

Matauranga (traditional ecological knowledge) built up by Whanganui iwi during their long association with the Whanganui River provides information on local biota and anthropological changes to the river. This matauranga records a decline in one local species, the kakahi (Echyridella menziesii (Gray, 1843)). Reasons suggested for this decline include alterations to flow and desiccation following a hydropower scheme, sedimentation, domestic and agricultural pollution, gravel extraction and channel modification. Decline was confirmed by a survey of historic kakahi beds: decline was evident at 16 (73%) of 22 sites. Of those 16 sites, there were 7 sites where decline was so severe that the population had been extirpated. Of the 15 historic beds where kakahi are still extant, four (27%) were remnant populations. Evidence of recruitment was found at only four (27%) of the 15 extant populations, or 18% of the total number of sites searched. Effect of suspended sediment concentrations ranging from 5.5 to 1212 mg.L-1 on ka kahi feeding behaviour and physiology was explored. Both filtration rate and rejection rate increased with increased sediment load (from 1.62 mg.h-1 to 190.88 mg.h-1 and from 0.62 to 201.53 mg.h-1 respectively) but clearance rate decreased with sediment increase (from 0.42 to 0.20 L.h-1). Behaviour was unaffected, with kakahi filtering on average 78% of the time. As particulate organic matter increased, clearance rate decreased and filtration rate increased. Filtration rate declined with increasing % organic matter. Kakahi can continue feeding under very high sediment loads for short periods. Much remains uncertain about kakahi, from their early biology to reasons for decline. Restoration options were explored using an adaptive management framework within which different hypotheses can be trialled in an experimental manner. This proved difficult due to confounding factors. However, given the established link between vegetation clearance and sedimentation, an initial restoration focus which evaluates catchment revegetation and its impact on kakahi survival and growth is suggested.