Intertidal shellfish monitoring in the northern North Island region, 2020–21

Marine coastal invertebrates that are targeted in non-commercial fishing activities throughout New Zealand include littleneck clam (or cockle, tuangi/tuaki; Austrovenus stuchburyi) and pipi (Paphies australis). Both species are valued in recreational and customary fishing in this country, and popular target species for shellfish gathering on sheltered beaches, estuaries, and tidal inlets. Their presence in coastal environments makes both cockles and pipi vulnerable to human impacts, such as fishing pressure. Concerns about the sustainability of cockle and pipi populations have led Fisheries New Zealand to commission population surveys at different locations. In northern North Island, a regular (currently annual) survey programme was initiated in the early 1990s, which is focused on particular cockle and pipi beds that are targeted in shore-based recreational and customary fishing activities.

This assessment presents the most recent findings from the northern bivalve monitoring series, based on the survey conducted in 2020–21. The field sampling assessed bivalve populations at eight northern North Island sites, with access to four of the usual 12 survey sites hampered during the field sampling by travel restrictions through Auckland (in response to COVID-19). The eight 2020–21 survey sites were (in alphabetical order): Aotea Harbour, Kawakawa Bay (West),
Ōhiwa Harbour, Okoromai Bay, Otūmoetai (Tauranga Harbour), Te Mata and Waipatukahu (Te Mata Bay), Whangamatā Harbour, and Whangapoua Harbour. Te Mata Bay, on the west coast of Coromandel Peninsula was surveyed for the first time.

Cockles were present at seven of the eight survey sites, with no discernible cockle population at Te Mata Bay.
Cockle population sizes ranged from 12.61 million (coefficient of variation, CV: 19.44%) individuals at Ōhiwa Harbour to the largest population at Kawakawa Bay (West) of 200.93 million (CV: 12.1%) individuals. Cockle population densities exceeded 300 individuals per m² at all of the sites, with the highest density estimate of 884 cockles per m² at Whangapoua Harbour, on Coromandel Peninsula.
All of the current population estimates met the target CV of less than 20%.

The populations surveyed in 2020–21 contained few large individuals (≥30 mm shell length): this size class was absent at Otūmoetai (Tauranga Harbour), and their densities were low (i.e, less than 10 large individuals per m² at Aotea Harbour and Whangapoua Harbour. Their highest density was 43 large cockles per m² (19.09%) at Whangamatā Harbour.

Five of the current survey sites supported pipi populations. Total pipi abundance estimates varied between
7.15 million (CV: 10.26%) pipi at Ōhiwa Harbour and 49.01 million (CV: 7.34 %) pipi at Otūmoetai (Tauranga Harbour). The lowest density estimate was at Whangamatā Harbour, with 95 pipi per m², compared with the highest estimate of 1284 pipi per m² at Te Mata Bay. The associated CV of the population estimates was less than 20% at all sites.

Similar to the cockle populations, the pipi populations at the 2020–21 sites contained only a few large individuals (≥50 mm shell length). Across sites,
their density estimates ranged from 2 large pipi per m² at Otūmoetai to 119 large pipi per m² at Te Mata Bay.

The sediment sampling across the different northern sites documented intertidal cockle strata that were generally characterised by low sediment organic content and a small proportion of sediment fines (silt and clay; grain size <63 μm).
The prevalent grain size fraction was fine sand, followed by very fine sand (grain sizes >125 μm and >63 μm).