Intertidal shellfish monitoring in the northern North Island region, 2014–15

In New Zealand’s sheltered coastal environments, bivalve species targeted in recreational and customary fisheries include cockles (tuangi/tuaki, or littleneck clam, Austrovenus stuchburyi) and pipi (Paphies australis), which both inhabit sedimentary habitats throughout the country. In the northern North Island region, cockles and pipi are the principal fisheries species in sheltered environments of beaches, harbours, and estuaries, where some populations are under considerable pressure from these non-commercial fishing activities. To monitor the northern cockle and pipi populations, the Ministry for Primary Industries (MPI) commissions regular population assessments in northern North Island, with survey sites distributed across the wider Auckland region, Northland, Waikato, and Bay of Plenty.

The present study documents the most recent bivalve survey in the northern North Island region, conducted in 2014–15. The sites included in this survey were (in alphabetical order) Aotea Harbour, Eastern Beach, Kawakawa Bay (West), Mangawhai Harbour, Mill Bay, Ngunguru Estuary, Otumoetai (Tauranga Harbour), Raglan Harbour, Ruakaka Estuary, Te Haumi Beach, Whangamata Harbour, and Whangapoua Harbour. At each site, the population survey focused on areas targeted by non-commercial fisheries to determine the abundance and population densities of cockles and pipi. The survey also involved the collection of sediment data (grain size and organic content) to provide broad-scale baseline information about some of the habitat characteristics that influence bivalve populations.

All of the 2014–15 survey sites contained cockle populations. Cockle population sizes and densities varied across sites, with total abundance estimates ranging from the smallest population of 16.66 million (CV: 9.56%) cockles at Mill Bay to the largest population of 109.56 million (CV: 4.95%) individuals at Raglan Harbour. Whangamata Harbour and Ngunguru Estuary also supported large cockle populations, with an estimated 104.53 million (CV: 6.59%) and 92.67 million (CV: 7.53%) cockles, respectively. Population densities were also variable, with relatively high density estimates at three sites, Ngunguru Estuary, Raglan Harbour, and Whangamata Harbour, ranging from 1372 cockles per m² at Whangamata Harbour to 1696 cockles per m² at Ngunguru Estuary. At the remaining sites, cockle densities were considerably lower, with the next highest estimate of 675 cockles per m² (CV: 8.77%) at Ruakaka Estuary. The lowest population density was 68 cockles per m² (CV: 16.59%) at Eastern Beach.

Most cockle populations were dominated by small and medium-sized cockles, with relatively low numbers and densities of large individuals (≥30 mm shell length). Furthermore, time-series comparisons across surveys (starting in 1999–2000) documented a general decrease in the population of large cockles, with only Eastern Beach reflecting a notable increase in this size class in 2014–15. In contrast, recruits (≤15 mm shell length) were abundant at the majority of sites, where they constituted a considerable proportion of the population (up to 53.82% at Ruakaka Estuary).

Pipi populations were present at 11 (of the total 12) sites in the 2014–15 survey, excluding Aotea Harbour, where only one individual was sampled. Most of the pipi populations were small, and abundances were only relatively high at three sites, Te Haumi Beach, Ruakaka Estuary, and Otumoetai (Tauranga Harbour), where pipi numbers ranged between an estimated total of 55.91 million (CV: 18.38%; Te Haumi Beach) and 92.59 million (CV: 5.59%; Otumoetai) individuals. The corresponding population densities at these sites were 438 pipi to 1207 pipi m² (at Te Haumi Beach and Otumoetai, respectively), compared with considerably lower densities at the remaining sites, including a maximum density of 90 pipi per m² (at Mill Bay).

There was a general scarcity of large pipi (≥50 mm shell length) in the 2014–15 populations, and this size class was absent at five of the sites surveyed. The lack or low abundance of large pipi at these sites was consistent throughout the survey series, especially in recent surveys (i.e., since 2005–06). At the same time, recruits (≤20 mm shell length) were present in all pipi populations in 2014–15, with up to 74.50% of individuals in this size class (at Eastern Beach).

At all sites, the sediment was characterised by a low organic content, which was less than 4%. The bulk of the sediment consisted of fine or medium sands (>125 to >250 μm grain size), with only a small proportion or no fines (silt and clay; <63 μm grain size). Only individual samples at Mangawhai Harbour, Ngunguru Estuary, Ruakaka Estuary, and Te Haumi Beach exceeded 10% in this grain size fraction, with the highest proportion of fines at 20.6% in one sample at Te Haumi Beach.