Intertidal shellfish monitoring in the northern North Island region, 2023–24

Recreational and customary fishing is culturally important in New Zealand, including the take of marine species in coastal environments. Two of the main target species in these non-commercial fisheries are cockle (Austrovenus stutchburyi) and pipi (Paphies australis), which occur throughout the country, often in close proximity to urban centres. This accessibility means that both species are vulnerable to overexploitation, and their presence in coastal environments makes them also susceptible to other human impacts.

To monitor cockle and pipi populations in northern New Zealand, Fisheries New Zealand initiated regular population assessments in the early 1990s. Following refinements and the spatial extension of this programme, the intertidal surveys currently assess bivalve populations at a range of sites in Auckland, Northland, Waikato, and Bay of Plenty. The surveys collect demographic data and document population trends across the northern North Island regions.

This study presents the findings from the most recent survey in the northern North Island monitoring series, providing data from the 2023–24 fishing year. The survey sites were (in alphabetical order): Bowentown Beach, Little Waihi Estuary, Mangawhai Harbour, Marsden Bank, Ngunguru Estuary, Raglan Harbour, Ruakākā Estuary, Tairua Harbour, Te Haumi Bay, Te Mata Bay (Waipatukahu), Umupuia Beach, and Whangateau Harbour. At five of the sites, Marsden Bank, Ngunguru Estuary, Te Mata Bay, Umupuia Beach, and Whangateau Harbour, existing fishery restrictions prevent the take of cockles and pipi.

Nine of the 2023–24 sites contained cockle populations, which varied in total abundance and density, depending on the site. The highest total population abundance was at Whangateau Harbour with an estimated 606.56 (coefficient of variation, CV: 10.68%) million cockles. The smallest total population estimate of 27.60 (CV: 4.65%) million cockles was at Bowentown Beach. At the same time, the Bowentown Beach cockle population had the highest density with 1843 cockles per m². Two other sites had similar high-density estimates (>1000 individuals per m²), Ngunguru Estuary and Raglan Harbour. In contrast, the lowest total cockle density was at Umupuia Beach, estimated at 132 (CV: 19.47%) cockles per m².

At the same time, Umupuia Beach was the only site with a high proportion and relatively high density of large individuals (≥30 mm shell length). At this beach, large cockles made up over 50% of the population, and occurred at an estimated density of 71 (CV: 22.23%) large cockles per m². Other cockle populations contained few large individuals, and their density estimates were markedly lower, typically with high uncertainty (including CV values above 40%).

Length-frequency distributions documented the prevalence of medium-sized cockles (sizes >15 mm and <30 mm shell length) across the northern populations. This size class largely determined the population size structure with varying proportions of recruits (≤15 mm shell length) contributing to the population overall. In 2023-24, the proportion of recruits varied between 19 and 63% at most sites (i.e., excepting Umupuia Beach), reflecting a marked influx of individuals to existing cockle populations.

Pipi populations were surveyed at ten of the northern sites. Their total population sizes ranged from 0.45 (CV: 15.66%) million pipi at Marsden Bank to 121.15 (CV: 13.48%) million individuals at Ruakākā Estuary. The highest density estimate was also at Ruakākā Estuary with 5857 individuals per m², largely caused by a significant recruitment event. The only other site with high pipi densities was Te Mata Bay, where this species exceeded 1000 individuals per m². The lowest density estimates were at Whangateau Harbour and Raglan Harbour, at 20 individuals per m² and 27 individuals per m², respectively.

Few of the pipi populations contained notable numbers of large pipi (≥50 mm shell length), and estimates for this size class frequently had high uncertainty. The only site where estimates for large pipi had low uncertainty (i.e., a CV of less than 20%) was Te Mata Bay. At this site, over 10% of the total population consisted of large individuals, and their estimated density was 171 (CV: 12.79%) large pipi per m².

The general scarcity of large-sized pipi was also evident in the length-frequency distributions, which were dominated by the medium pipi size class (>20 mm and <50 mm shell length), forming a strong cohort in unimodal populations. Exceptions to this population size structure were three of the four Northland sites, where substantial recruitment led to the prevalence of recruits (≤20 mm shell length) in the populations. At Te Haumi Bay, Ruakākā Estuary, and Marsden Bank, the proportion of recruits varied from 75 to 87%. At Marsden Bank, the strong recruitment indicated some population recovery, following the loss of the pipi population in 2021-22.

Sediment in the cockle strata was characterised by a low organic content (less than 3%) and varying proportions of grain size fractions. The most prevalent grain sizes were fine sand (grain size >125 μm), followed by medium and very fine sands (>250 and >63 μm). A number of sites had relatively high proportions of sediment fines (silt and clay; ≤63 μm grain size), which may impact suspension-feeding cockles through increased sediment resuspension. These sites were Ngunguru Estuary, Raglan Harbour, Tairua Harbour, Te Haumi Bay, and Umupuia Beach, where average proportions of sediment fines were between 8 and 10%. Individual samples at Umupuia Beach exceeded 60% of sediment in this grain size fraction.

Sediment data were also used to explore the relationship between sediment grain size fractions and cockle abundance, based on principal component analysis. General patterns from this data exploration indicated that cockle abundance was generally associated with fine sand size fractions, but less so with sediment fines.

Cockle population data were also used in geostatistical models to examine spatio-temporal trends in predicted cockle densities at the survey sites. This analysis allowed the identification of high-density areas within cockle strata, and assessment of changes in these “hotspots” over time. At most of the survey sites, the locations of high-density areas were comparable between the total population and large cockles, although hotspots were typically more restricted for the latter populations. At some sites, Umupuia Beach, Tairua Harbour, and Te Haumi Bay, distinct high-density areas in early surveys became smaller and less pronounced over time. In contrast, three sites were characterised by areas of high cockle densities that persisted over time, Bowentown Beach, Ngunguru Estuary and Raglan Harbour.