Intertidal shellfish monitoring in the northern North Island region, 2013–14

Coastal environments throughout New Zealand contain a number of bivalve species that are valued in recreational and customary fisheries. In sheltered, sedimentary habitats, these bivalves include cockles (Austrovenus stutchburyi) and pipi (Paphies australis), which are both common in a range of embayments, estuaries, and harbours. Throughout northern North Island, cockle and pipi populations are often the main target for recreational and customary fisheries. To monitor the sustainability of these non-commercial fisheries, regular population surveys have been commissioned by Ministry for Primary Industries to assess the cockle and pipi populations in areas targeted by non-commercial fisheries in the wider Auckland region, Northland, Waikato, and Bay of Plenty.

This study presents the findings from the most recent cockle and pipi survey in the northern North Island region, conducted in the 2013—14 fishing year. The sites included in the current survey were Cockle Bay, Grahams Beach, Little Waihi Estuary, Marsden Bank, Okoromai Bay, Pataua Estuary, Tairua Harbour, Umupuia Beach, Waikawau Beach, Waiotahi Estuary, and Whangateau Harbour. In addition to assessing the cockle and pipi populations, the current survey collected sediment data to provide baseline information on possible causative factors for population change (sediment organic content and grain size) across the survey sites.

Cockle populations were present at nine sites, and population estimates ranged from 4.41 million individuals at Grahams Beach to 545.24 million cockles at Whangateau Harbour. Cockle population densities varied between the lowest value of 16 cockles per m² at Grahams Beach and the highest estimate of 1317 cockles per m² at Pataua Estuary. A number of populations were characterised by small-sized individuals (<30 mm shell length), and most sites had declining trends in the number of large-size cockles over time. Only two sites, Cockle Bay and Umupuia Beach contained high numbers of large individuals in the cockle population, although the total number of cockles at Cockle Bay was markedly lower in the present survey than in the previous assessment in 2012—13.

Pipi populations were present at seven sites, and estimated population sizes varied from 3.23 million pipi at Marsden Bank to 113.46 million pipi at Little Waihi Estuary. The corresponding population densities ranged from 21 pipi per m² at Marsden Bank to 941 pipi per m² at Waiotahi Estuary. Numbers and densities of large pipi (defined as ≥50 mm shell length) were low at five of the seven sites that contained pipi, at ≤1 individual per m². At the remaining two sites, densities of large pipi were slightly higher, with 5 and 27 pipi per m² at Tairua Harbour and Little Waihi Estuary, respectively.

Population estimates for pipi at Marsden Bank reflected a continued decline in 2013—14, following a marked reduction documented in the previous survey in 2012—13. At Waikawau Beach, the pipi population has disappeared since the preceding survey in 2005—06, coinciding with a change from a sedimentary habitat to a beach characterised by large gravel and pebbles.

The present study also provided broad-scale information of sediment properties at the survey sites, including organic content and grain size. Most sites were characterised by low sediment organic content (i.e., <5%) and a small proportion of fines (silt and clay, < 63 μm grain size), although the latter was high in some samples, most notably at Cockle Bay and Umupuia Beach (up to 42 and 77%, respectively).