Toheroa on Oreti Beach: management to minimize threats of local extinction

Abstract

Toheroa (Paphies ventricosum) is widely regarded as the national shellfish, yet since the early 1900s toheroa have been disappearing from the beaches of New Zealand. Despite 20 years of a ban on harvesting, the population of toheroa at Oreti Beach, Southland, shows little sign of recovery.

A particle size analysis was conducted on sediment from the inter-tidal zone of Oreti Beach. This involved taking samples every kilometre, at depths of 5, 10, 20 and 30 cm. In addition, vehicle surveys were conducted between the high and spring tide marks along the beach to indicate the distribution, density and overall health of toheroa at Oreti Beach.

Particle size analysis showed a trend of increasingly unsuitable sediment for toheroa towards the northwest end of Oreti Beach. This was especially evident close the Waimatuku Stream, were the majority of sediment particles were larger than 4 mm. Comparisons with the findings of a previous toheroa survey on Oreti Beach showed a strong mode of juvenile individuals in this area, with few adults. This indicates that sediment size could be an influencing factor in the population dynamics of toheroa and explain relatively high mortality amongst juveniles.

Surveys of vehicular traffic on Oreti Beach demonstrated a predominance of traffic within two kilometre of the entrance, and a concentration within 100 metres of the public access point. Vehicles were recorded primarily between the spring and high tide marks. Comparisons with the findings of a previous toheroa survey on Oreti Beach revealed a strong mode of juvenile individuals in this area, but only a few adults. This provided an indication that vehicle traffic could influence population dynamics by causing mortality in juveniles.

Other environmental influences, such as climate and stormy seas, are likely also to influence the population dynamics of toheroa on Oreti Beach.

On the basis of findings reported here and in previous surveys a management regime is suggested to ensure the survival of toheroa on Oreti Beach.