Intertidal shellfish monitoring in the northern North Island region, 2016–17

Coastal environments throughout New Zealand support bivalve populations that are targeted in recreational and customary fisheries. In northern North Island, the main fisheries species in sheltered, sedimentary habitats are cockles (tuangi/tuaki, or littleneck clam, Austrovenus stuchburyi) and pipi (Paphies australis). Populations of both species are often easily accessible in intertidal areas, and occur close to urban centres, exposing them to considerable fishing pressure at some locations. The latter has prompted concerns about population declines and local depletion of some northern North Island cockle and pipi populations. To monitor their populations, the Ministry for Primary Industries (MPI) commissions regular (usually annual) surveys, which focus on a range of estuaries, harbours and inlets in the wider Auckland region, Northland, Waikato, and Bay of Plenty. The present assessment presents the most recent data in this monitoring series, including the 2016–17 fishing year. The sites included in this survey were (in alphabetical order) Aotea Harbour, Eastern Beach, Grahams Beach, Kawakawa Bay (West), Mangawhai Harbour, Ngunguru Estuary, Ruakaka Estuary, Otumoetai (Tauranga Harbour), Te Haumi Beach, Waiōtahe Estuary, Whangamata Harbour, and Whangapoua Harbour.

All of the 12 survey sites contained cockle populations, which varied in abundance and density across sites. The smallest population was at Grahams Beach, with an estimated 17.09 million (CV: 21.82%) cockles, compared with the highest population abundance at Kawakawa Bay (West) of 261.21 million (CV: 13.84%) individuals. The cockle population was also relatively large at Eastern Beach, with the current estimate of 176.91 million (CV: 13.05%) individuals at this site. The corresponding population densities ranged from 64 cockles per m2 at Grahams Beach to high densities at Ngunguru Estuary and Whangamata Harbour of 1461 cockles per m² and 1125 cockles per m², respectively. Cockle densities were also comparatively high at the remaining sites, with most sites supporting populations densities of over 400 individuals per m2.

The cockle populations included few large individuals (≥30 mm shell length), and this size class was absent at three sites, including Aotea Harbour, Grahams Beach, and Ruakaka Estuary. At the other sites, large cockles were present but only a small part of the total population. Their highest abundance estimate was 18.33 million (CV: 36.42%) large cockles at Kawakawa Bay (West), followed by 15.07 million (CV: 17.38%) large cockles at Eastern Beach. At the remaining sites, abundance estimates of this size class were 4.00 million (CV: 24.6%) individuals or less. Their highest density was 67 large individuals per m² at Eastern Beach.

Ten of the northern survey sites supported pipi populations, with no pipi beds at Aotea Harbour or Eastern Beach. Their population estimates varied, with the highest abundance estimates at Waiōtahe Estuary and Te Haumi Beach, including 166.25 million (CV: 18.36%) and 101.49 million (CV: 24.80%) million pipi, respectively. A number of sites supported high-density populations, with particularly high estimates of 1388 pipi per m² at Waiōtahe Estuary, and 1008 pipi per m² at Ruakaka Estuary. Densities were also high at Ngunguru Estuary, Te Haumi Beach, and Otumoetai (Tauranga Harbour), ranging from 453 pipi per m² to 889 pipi per m². Pipi densities were lowest at Kawakawa Bay (West) with an estimated mean of 6 pipi per m², and ranged from 33 to 99 pipi per m² at the remaining sites.

Eight of the populations included large pipi (≥50 mm shell length), but their numbers and densities were generally low (about one million individuals or less), with the highest abundance estimate at Whangamata Harbour of 3.87 million (CV: 20.49%) individuals in this size class. Their corresponding density at this site was 50 pipi per m², with only one higher density estimate of 89 large pipi per m² at Whangapoua Harbour. Their densities were markedly lower at the other sites. Owing to their low abundance, large pipi contributed few individuals to the population, except at Whangamata Harbour, where they constituted about half of the present population.

Sediment sampling in areas inhabited by cockles provided baseline information of sediment properties, including the organic content and grain size composition.