Intertidal shellfish monitoring in the northern North Island region, 2025–26

The collection of seafood is an important recreational activity and of significant cultural importance in New Zealand. This non-commercial fishing includes the hand-picking and gathering of intertidal bivalves, such as cockle/tuangi (Austrovenus stutchburyi) and pipi (Paphies australis). Both species are widely distributed throughout the country and are key target species in customary and recreational fisheries. Their accessibility and frequent proximity to urban centres make cockle and pipi populations vulnerable to overfishing, particularly in coastal areas exposed to other human impacts.

To support their sustainable management, cockle and pipi populations across northern North Island are part of a long-standing monitoring programme by Fisheries New Zealand. The monitoring focuses on populations targeted by non-commercial fisheries across a range of intertidal environments in the wider Auckland, Northland, Waikato, and Bay of Plenty regions. Regular and frequent (e.g., biennial) monitoring at most sites, and the use of consistent survey methods since the 1999–2000 fishing year, have led to an extensive time series of population data for these northern bivalve populations. For cockles, the addition of a sediment sampling programme in 2013–14 (refined in 2015–16) also provides baseline information of benthic habitat quality.

The present assessment provides the survey findings for the 2025–26 fishing year, presenting bivalve population data for 11 northern North Island sites (in alphabetical order): Bowentown Beach, Eastern Beach, Grahams Beach, Hokianga Harbour, Little Waihi Estuary, Mangawhai Harbour, Marsden Bank, Ngunguru Estuary, Umupuia Beach, Whangamatā Harbour, and Whangateau Harbour.

Ten sites contained cockle populations (excluding Marsden Bank), and abundance and density estimates varied greatly across sites. The smallest cockle population was at Grahams Beach, where their abundance was estimated at 6.91 million (coefficient of variation, CV: 19.38%) cockles, and their average density at 26 cockles per per m². The most abundant population was at Whangateau Harbour, with an estimated 506.88 million (CV: 11.53%) individuals.The corresponding density estimate was 457 cockles per m², while the highest density estimate was at Bowentown Beach, with an estimated 1362 (CV: 6.15%) cockles per m². Density estimates were similarly high at Ngunguru Estuary and Whangamatā Harbour, exceeding 1000 individuals per m². All cockle population estimates had CV values below 20%, except for Little Waihi Estuary, where additional sampling effort did not succeed in lowering the CV below this target value.

Pipi were present at eight sites, Bowentown Beach, Grahams Beach, Hokianga Harbour, Little Waihi Estuary, Marsden Bank, Ngunguru Estuary, and Whangamatā and Whangateau harbours. Pipi abundance estimates ranged from 0.17 million (CV: 15.76%) pipi at Bowentown Beach to 115.37 million (CV: 12.77%) pipi at Little Waihi Estuary. The pipi population at Little Waihi Estuary also had the highest density estimate of 695 pipi per m². Density estimates were also high (i.e., several hundred individuals per m²) at Hokianga Harbour, Marsden Bank, and Ngunguru Estuary. The CV values associated with the estimates were above 20% at Grahams Beach and Whangateau Harbour, in spite of increased sampling effort aimed at lowering the uncertainty of the estimates. At both sites, pipi were highly localised. At Grahams Beach, the low abundance of pipi and their patchy distribution (of mostly recruits) led to high uncertainty around the population estimates. At Whangateau Harbour, pipi were confined to a designated pipi bed, but missing from the overall sampling extent, leading to the high CV value for the population estimate overall.

Sediment collected in the cockle beds at each site was characterised by a low organic matter content, with averages ranging between 0.8 and 2.6% across sites. The sediment grain size composition was largely determined by fine and medium sands (grain sizes >125 and >250 μm), which varied in their relative proportions depending on the site. Although the average proportion of sediment fines (<63 μm) was small, this grain size fraction exceeded 10% in individual samples at Bowentown Beach, Hokianga Harbour, Umupuia Beach, and Whangamatā and Whangateau harbours.

The relationship between cockle abundance and sediment grain size composition was investigated with principal component analysis. For cockle sites with discernible patterns, the association of cockle abundance depended on the site, and there was no universal pattern across sites or surveys.

Geostatistical models were used to assess spatio-temporal patterns in predicted cockle densities at each of the cockle sites. This visual assessment allowed the tracking of high-density areas (or “hotspots”) over time. At Eastern Beach and Umupuia Beach, hotspots were determined by large cockles, which diminished in recent surveys. A reduction in predicted total cockle densities and their spatial extent was apparent at Hokianga Harbour and Ngunguru Estuary. In contrast, at Little Waihi Estuary, Mangawhai Harbour, Whangamatā and Whangateau harbours, there was little change in total densities and in spatial patterns over time.