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

Abstract

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.