Intertidal shellfish monitoring in the northern North Island region, 2018–19

Coastal bivalve species such as cockles (tuangi/tuaki, or littleneck clam, Austrovenus stuchburyi) and pipi (Paphies australis) are widely distributed throughout New Zealand, where they support recreational and customary fisheries in different coastal environments. In northern North Island, regular monitoring of cockle and pipi populations that are targeted in non-commercial fisheries provides data on their abundance and population size structure. This monitoring programme assesses bivalve population across different northern sites each year, including estuaries, bays and large inlets in Northland, Waikato, Bay of Plenty and the wider Auckland region. These surveys collect data on the population status of cockle and pipi populations, and allow temporal comparisons of population trends across the northern North Island region.

This study presents the most recent survey findings, based on data from the 2018–19 fishing year. Northern sites included in this survey were (in alphabetical order): Aotea Harbour, Hokianga Harbour, Kawakawa Bay (West), Mangawhai Harbour, Mill Bay, Ngunguru Estuary, Ōhiwa Harbour, Otūmoetai (Tauranga Harbour), Ruakaka Estuary, Te Haumi Bay, Whangamatā Harbour and Whangapoua Harbour.

All of the 2018–19 survey sites contained cockle populations, with most sites supporting notable population sizes. Abundance estimates ranged from 3.46 million (CV: 12.25%) cockles at Ruakaka Estuary to 222.41 million (CV: 17.52%) cockles at Kawakawa Bay (West). The corresponding density estimates were between 88 cockles per mm² at Ruakaka Estuary and 1997 cockles per m² at Ngunguru Estuary. Density estimates were also high (>1000 individuals per m²) at Whangapoua, Mangawhai and Whangamatā harbours.

None of the northern sites contained notable numbers of large cockles (30 mm shell length), and this size class consistently contributed few individuals to the total population. The highest density estimates of large cockles were just over 30 individuals per m² at Mangawhai, Ōhiwa and Whangamatā harbours and Te Haumi Beach. This size class was near-absent at Otūmoetai (Tauranga Harbour) and Ruakaka Estuary, and the remaining sites had small abundance and density estimates (frequently with high uncertainty) for this size class.

Nine of the current survey sites supported pipi populations, with population sizes varying from an estimated 1.97 million (CV: 13.89%) pipi at Mangawhai Harbour to 91.64 million (CV: 17.84%) pipi at Ruakaka Estuary. Population densities for this species included a low estimate of 27 individuals per m² at Mangawhai Harbour to the highest density of 2333 pipi per m² at Ruakaka Estuary.

Large-sized individuals (50 mm shell length) were absent at two of the sites that supported pipi populations, namely Hokianga and Mangawhai harbours. At the remaining sites, large pipi were scarce, and density estimates varied from 0.45 million (CV: 24.34%) large pipi per m² at Te Haumi to 1.44 million (CV: 13.32%) pipi per m² at Whangapoua Harbour. Most of the estimates for large pipi had high uncertainty, with a coefficient of variation exceeding 20%.

Sediment sampling at each site documented that cockle strata were generally characterised by a low organic content and a small proportion of sediment fines (silt and clay; grain size <63 μm). Grain size compositions showed some variation across sites, but fine and/or medium sands (grain sizes >125 to >250 μm) made up the bulk of sediment across sites.