The impact of predation by South Island Pied Oystercatchers (Haematopus finschi) on a protected shellfish, the toheroa (Paphies ventricosa), in northern New Zealand

Abstract

ABSTRACT In northern New Zealand, there is speculation that predation by a recovering shorebird, the South Island Pied Oystercatcher (Haematopus finschi), has caused the continuing decline of an endemic surf clam, the toheroa (Paphies ventricosa). Consequently, there have been requests by the public for these protected birds to be culled to protect and restore the toheroa. To better inform management and decision-making for these species we set out to understand the impact of oystercatcher predation on toheroa at Ripiro Beach in New Zealand. A study was conducted to establish the spatio-temporal overlap of toheroa and oystercatcher distributions, determine oystercatcher prey selection, and assess the impacts of oystercatcher predation on toheroa population size structure. Monthly surveys over a 12-month period indicated limited spatial overlap between the two species, and that most toheroa beds on Ripiro Beach are not subjected to oystercatcher predation. In areas where oystercatchers and toheroa co-occurred, oystercatchers consumed a variety of prey including polychaetes, small and medium-sized toheroa and a non-threatened surf clam (P. subtriangulata). While oystercatchers do feed on toheroa (and may be capable of locally reducing abundance of small toheroa in a population), feeding on larger individuals (>50 mm in length) was not observed. These results suggest that culling of oystercatchers would be of no benefit for the recovery of toheroa populations. It is likely that less obvious factors are impeding the recovery of toheroa in New Zealand. This study highlights the importance of understanding predator-prey interactions before implementing management actions such as lethal control.