Comparison of bathyscope and snorkelling methods for iwi monitoring of kākahi (Echyridella menziesi) populations in the shallow littorals of Lake Rotorua and Rotoiti

ABSTRACT

Kākahi (Echyridella spp.) are freshwater cultural keystone species in New Zealand; they represent a valuable mahinga kai (species used as a food) resource and have high cultural significance to Māori, particularly to the Te Arawa and Ngāti Tūwharetoa iwi (tribal groups). Population densities of kākahi are thought to have declined in many locations in New Zealand, and accurate assessment methods are thus required to monitor changes in populations. A bathyscope method is currently used by iwi to monitor kākahi populations in lakes Rotorua and Rotoiti. It is unclear, however, how accurate this method is for determining kākahi abundance changes compared to underwater methods such as snorkelling. Comparisons at seven sites in Lake Rotorua and three sites in Lake Rotoiti show that significantly fewer kākahi are observed using a bathyscope relative to snorkelling (p < 0.0001). On average, snorkelling estimated 2.1 times more kākahi than the bathyscope method in Lake Rotorua, and 2.5 times more in Lake Rotoiti. We conclude that under suitable conditions, snorkelling is generally more effective than surface visual assessments using a bathyscope. Nevertheless, the bathyscope method is inexpensive, safe and simple, ideal for use by iwi and community groups for long-term monitoring of kākahi populations.

KEYWORDS:

• Aquascope
• freshwater mussels
• biomonitoring
• cultural keystone indicators
• Hyriidae

Acknowledgements

We thank Te Arawa Lakes Trust, NIWA and the Bay of Plenty Regional Council for supporting this project. Niwa Nuri and Joe Butterworth assisted in the field. Astrid Schwalb and two anonymous referees made comments that improved our manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This study was supported by two Ministry of Business Innovation and Employment (MBIE) funded programmes: (1) Cultural Keystone Species: Co-management and Restoration (C01X1616) and (2) Project 3.5: ‘Managing biosecurity threats to freshwater taonga invertebrates in Aotearoa/New Zealand’ (C09X1501 ), which is a Biological Heritage component of the National Science Challenges programme.