Intertidal shellfish monitoring in the northern North Island region, 2024–25

Recreational and customary fisheries in New Zealand target a wide range of marine species, including intertidal bivalves in coastal environments. Two important target species for these non-commercial shellfish collections are cockle (Austrovenus stutchburyi) and pipi (Paphies australis), which are widely distributed in sedimentary habitats, and often easily accessible. In areas with considerable fishing activites, their populations are vulnerable to overexploitation, in addition to anthropogenic impacts from landuse changes.

In northern North Island, cockle and pipi populations are regularly monitored by Fisheries New Zealand across the Northland, Auckland, Waikato, and Bay of Plenty regions. This monitoring was initiated in the early 1990s, with its subsequent spatial extension and refinement of survey methods providing consistent information of cockles and pipi targeted in non-commercial fisheries across a diverse range of sites and environments. The survey data provide estimates of abundance and density for both species, and information of the population size structure. For most of the survey sites, multiple surveys since the early 2000s have resulted in extensive time-series data, allowing the assessment of population trends over time, and inter-regional comparisons. In addition, the subsequent adoption of sediment sampling in the cockle beds has provided an indication of benthic habitat quality since 2013–14.

Presented here are the survey results from the northern bivalve assessment conduced in the 2024–25 fishing year. The survey focused on 11 sites across northern North Island (in alphabetical order): Cockle Bay, Kawakawa Bay (West), Mill Bay, Ōhiwa Harbour, Ōkahu Bay, Okoromai Bay, Otūmoetai (Tauranga Harbour), Pataua Estuary, Waiotahe Estuary, Whangapoua Harbour, and Whitianga Harbour. Ōkahu Bay in Auckland was surveyed for the first time as part of this monitoring programme.

Cockle populations were present at all of the 2024–25 sites, and their population estimates varied from the smallest population of 10.22 million (coefficient of variation (CV): 17.05%) cockles at Ōhiwa Harbour to the largest population of 292.13 million (CV: 14.29%) individuals at Pataua Estuary. Ōkahu Bay had a relatively small cockle population and was the only site where the CV of the estimates exceeded 20%.

Most sites supported cockle populations at high densities, i.e., of several hundred individuals per square metre. The lowest density of 148 cockles per m² (CV: 11.18%) was at Waiotahe Estuary. Both Pataua Estuary and Whitianga Harbour had comparatively high densities at 1031 cockles per m² (CV: 14.29%) and 1617 cockles per m² (CV: 9.71%), respectively. Regardless of population density, none of the populations included a notable number of large cockles (defined as ≥30 mm shell length), and their population estimates had generally high CV values. A number of cockle populations were characterised by strong recruitment, indicated by a relatively high proportion (i.e., over 20% of the total population) of recruits (≤15 mm shell length). These populations were at Kawakawa Bay (West), Ōhiwa Harbour, Ōkahu Bay, Okoromai Bay, Otūmoetai (Tauranga Harbour), Pataua and Waiotahe estuaries, and Whitianga Harbour. At Ōkahu Bay and Whitianga Harbour, over 80% of the population consisted of recruits.

Pipi populations were assessed at six of the northern sites, at Ōhiwa Harbour, Otūmoetai (Tauranga Harbour), Pataua Estuary, Waiotahe Estuary, Whangapoua Harbour, and Whitianga Harbour. The abundance estimates for this species were generally similar across sites. The smallest population was at Whitianga Harbour, with an estimated abundance of 13.93 million (CV: 9.68%) pipi. In comparison, Waiotahe Estuary supported the largest pipi population, with an estimated 95.89 million (CV: 17.99%) individuals. All population estimates had CV values of less than 20%. Pipi densities were generally high (i.e., several hundred individuals per square metre), except at Pataua Estuary, where the density estimate was 60 pipi per m² (CV: 11.47%). The maximum density estimate was at Ōhiwa Harbour with 1110 pipi per m² (CV: 7.62%). Large pipi (≥50 mm shell length) were scarce or absent across all of the sites. Similar to some of the cockle populations, three of the pipi populations had a high proportion of recruits (≤20 mm shell length); at Ōhiwa Harbour, Pataua Estuary, and Waiotahe Estuary, recruits made up between 23 and 50% of the total pipi population.

Sediment sampling in the cockle beds documented a low organic content, which was below 3.4% on average across all sites. At most sites, the sediment granulometry was largely determined by fine sand (grain size >125 μm) and either medium or very fine sands (>250 and >63 μm, respectively). The proportion of sediment fines
(<63 μm) varied across sites, and was comparatively high (i.e., on average above 7%) at Cockle Bay, Kawakawa Bay (West), Mill Bay, Ōkahu Bay, and Waiotahe Estuary. Individual sediment samples at these sites contained a high proportion of sediment fines, ranging from 19% at Mill Bay and Waiotahe Estuary to 69% at Cockle Bay.

Applying Principal Component Analysis to explore cockle abundance in relation to sediment grain size indicated a general pattern of higher abundances associated with fine sand fractions at several sites.

For sites with multiple assessments, spatial and temporal trends in predicted cockle densities were also assessed through geostatistical modelling. The outcomes of this modelling allowed the tracking of high-density areas (“hotspots”) over time. At Cockle Bay, Ōhiwa Harbour, Okoromai Bay, Pataua Estuary, Waiotahe Estuary, and Whangapoua Harbour, these hotspots of the total population were mostly determined by the predicted densities of large cockles. At two of the sites, Ōhiwa Harbour and Waiotahe Estuary, the hotspots did not persist over time, following the disappearance of large cockles at both sites. Mill Bay, Otūmoetai (Tauranga Harbour), and Whitianga Harbour had extensive high-density areas of all cockles, and large individuals were generally scarce throughout the survey series. At Otūmoetai (Tauranga Harbour), there was a notable reduction in the spatial extent of high-density cockle areas over time. In constrast, the high-density area of total cockles at Whitianga Harbour showed a spatial extension throughout most of the cockle stratum in the current survey. At Kawakawa Bay (West), cockle hotspots were highly localised, and showed a general reduction in spatial extent over time.