Intertidal shellfish monitoring in the northern North Island region, 2017–18

New Zealand’s coastal marine communities include species that are targeted in recreational and customary fisheries, including shellfish collections of intertidal bivalves. Two of the main target species in these fisheries in northern North Island are cockles (tuangi/tuaki, or littleneck clam, Austrovenus stuchburyi) and pipi (Paphies australis). Both species support non-commercial fisheries across a range of coastal environments, such as beaches, harbours and sheltered bays and estuaries.

Their populations are monitored by regular surveys commissioned by Fisheries New Zealand (and its predecessors), which focus on different northern sites across the wider Auckland region, Northland, Waikato, and Bay of Plenty. 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.

The present assessment presents the most recent data in the northern survey series, including the 2017–18 fishing year. The current survey included the following northern sites (in alphabetical order): Bowentown Beach, Cockle Bay, Little Waihi Estuary, Marsden Bank, Mill Bay, Okoromai Bay, Pataua Estuary, Raglan Harbour, Tairua Harbour, Umupuia Beach, Whangateau Harbour and Whitianga Harbour.

Cockle populations were present at 11 of the 2017–18 sites, excluding Marsden Bank. Their total population sizes ranged from a low 7.78 million (CV: 25.18%) cockles at Mill Bay to the highest estimate of 852.27 million (CV: 9.28%) cockles at Whangateau Harbour. Other northern sites with comparatively large cockle populations were Pataua Estuary with 406.39 million (CV: 11.78%) individuals and Raglan Harbour with 109.16 million (CV: 7.11%) cockles.

Corresponding cockle densities were high at a number of sites, with a maximum density of 2008 (CV: 6.25%) cockles per m² at Bowentown Beach, followed by Raglan Harbour and Pataua Estuary, where their densities were 1508 (CV: 7.11%) and 1467 (CV: 11.78%) cockles per m², respectively. Except for Mill Bay and Little Waihi Estuary, cockles also occurred at relatively high densities at other sites, ranging between 263 (CV: 15.24%) cockles per m² at Okoromai Bay to an estimated density of 922 (CV: 9.62%) cockles per m² at Tairua Harbour.

There was a general scarcity of large cockles (≥30 mm shell length) in northern populations, and this size class was only present at low densities at most sites; only Umupuia Beach and Cockle Bay had population densities that exceeded 100 large individuals per m². Large cockles were absent at Whitianga Harbour, and varied in density between two and 44 large cockles per m² at the remaining sites.

Pipi were present at nine of the survey sites, where their estimated abundance varied from 0.48 million (CV: 32.31%) pipi at Bowentown Beach to 95.12 million (CV: 12.93%) pipi at Whitianga Harbour. Their total population densities varied across sites from low estimates of seven (CV: 35.38%) pipi per m² at Pataua Estuary to 1637 (CV: 12.93%) pipi per m² at Whitianga Harbour.

Few of the pipi populations included large individuals (≥50 mm shell length), and their abundance and density estimates were generally low where they were present. Abundance estimates for this size class varied from 0.10 million (CV: 24.15%) large pipi at Raglan Harbour to 5.44 million (CV: 64.08%) individuals in this size class at Little Waihi Estuary. Their highest densities were at Little Waihi Estuary, Whitianga and Tairua harbours, with estimates between 30 (CV: 64.08%) large pipi per m² and 54 (CV: 21.56%) large pipi per m² at these sites; it was one individual (or less) per m² at the remaining sites.

Although there was some variation in sediment properties across the northern sites, areas inhabited by cockles were generally low in organic content and in the proportion of sediment fines (silt and clay; <63 μm grain size). Most sites were characterised by fine and medium sands (grain sizes >125 to >250 μm), but also contained some sediment in coarser fractions including gravel (>2000 μm).