Intertidal shellfish monitoring in the northern North Island region, 2022–23

Culturally important marine species in New Zealand include cockles (Austrovenus stutchburyi) and pipi (Paphies australis), which are targeted in non-commercial fisheries throughout the country. Both species have a widespread distribution in intertidal soft sediments along New Zealand’s coastline, including populations close to urban centres. Given their easy accessibility to fishing in intertidal habitats, and possible impacts from other human activities, there is a potential risk of population declines and local depletion at some sites. To support the management of cockle and pipi populations, Fisheries New Zealand commissions regular (usually annual) assessments across the wider northern North Island region.
The monitoring programme commenced in the early 1990s in the Auckland area, and has since then been extended to include other sites in Fisheries Management Areas (FMAs) 1 and 9, adding Northland, Waikato, and Bay of Plenty sites to the survey series.

The current study provides population information for cockles and pipi at 13 sites included in the 2022–23 assessment. The survey sites in the current study were (in alphabetical order): Aotea Harbour, Cockle Bay, Eastern Beach, Grahams Beach, Hokianga Harbour, Kawakawa Bay (West), Ōhiwa Harbour, Okoromai Bay, Otūmoetai (Tauranga Harbour), Pataua Estuary, Waiotahe Estuary, Whangamatā Harbour, and Whangapoua Harbour. These sites represented a diverse range of intertidal sedimentary habitats, from open beaches to sheltered bays, estuaries, and large tidal inlets.

All of the northern sites contained cockle populations, which generally had relatively high abundance and density estimates. For all of the current estimates, the coefficient of variation (CV) was below the target percentage of 20%, except at Grahams Beach. At this beach, additional sampling effort did not lead to sufficient lowering of the CV below 20%. Cockle population sizes ranged from an estimated 4.42 million cockles at Grahams Beach (with a CV of 25.91%) to 303.16 million (CV: 11.48%) individuals at Pataua Estuary. The corresponding density estimates ranged from a low average of 17 cockles per m² also at Grahams Beach to 1206 cockles per m² (CV: 9.32%) at Whangamatā Harbour.

Across the 2022–23 sites, the cockle population size compositions were frequently dominated by medium-sized individuals (>15 mm and <30 mm shell length), whereas large individuals were scarce. This finding was similar to previous recent assessments of these sites, where cockle populations have usually been characterised by a unimodal cohort of medium-sized cockles. At the same time, their populations have been regularly augmented by an influx of recruits, and this size class contributed over 25% of the cockle population at the majority of sites in 2022–23.

Long-term fishing restrictions at Cockle Bay and Eastern Beach have restricted or prohibited the take of shellfish for a considerable period of time. Both sites were distinct from the remaining 2022–23 sites in supporting a comparatively large population of large cockles, which made up 20.6% and 27.8% of their current population, respectively. Nevertheless, total population estimates at Cockle Bay and Eastern Beach have undergone marked declines over the monitoring series, implicating factors other than fishing (e.g., habitat degradation) as impacting on the resident populations.

Eight of the 2022–23 survey sites contained pipi populations. At two of these sites, Grahams Beach and Waiotahe Estuary, the population estimates had high uncertainty; even with additional sampling effort, the CV values exceeded 20%.
Most of the pipi populations were small, with abundance estimates ranging from 1.47 million pipi at Ōhiwa Harbour to 39.27 million individuals at Hokianga Harbour. Their population densities were similarly low, with the lowest density estimate of 56 pipi per m² at Whangamatā Harbour (not including Grahams Beach and Waiotahe Estuary) compared with the highest density estimates at Hokianga Harbour of 390 pipi per m².

Similar to the current cockle populations, the majority of individuals within the pipi populations were in the medium size class (>20 mm and <50 mm shell length), and there were few or no pipi in the large size class at most sites. Notable exceptions were Pataua Estuary, Whangamatā Harbour, and Whangapoua Harbour, where large pipi made up at least 15% of the population. Recent recruitment to the pipi populations varied dependent on the site, with strong recruitment at Whangamatā Harbour and Hokianga Harbour, where the proportion of recruits was 25.05% and 31.40%, respectively; at Grahams Beach, 79.12% of the current pipi population consisted of recruits.

Sediment sampling in the cockle strata revealed that sediment at the northern sites was low in organic matter, at an average of less than 3% across sites.
The grain size composition of the sediment varied, but consisted primarily of medium and fines sands (grain sizes >125 and >500 μm). The proportion of sediment fines (silt and clay; ≤63 μm grain size), which have the potential to impact on suspension-feeding cockles, exceeded an average of 10% at Aotea Harbour and Cockle Bay. In addition, individual samples at these two latter sites and at Hokianga Harbour, Eastern Beach, Kawakawa Bay (West), Ōhiwa Harbour, Whangamatā Harbour, and Waiotahe Estuary (greatly) exceeded 10%.
Principal component analysis cockle abundance and sediment grain size fractions revealed no universal patterns for the association of total or large cockle abundance with particular grain size fractions.

In addition to survey-based estimates, the current study used geostatistical models to examine spatial patterns in predicted cockle densities over time. At all sites, except at Grahams Beach, the areas of high predicted cockle density were similar between the total population and large cockles, but the spatial extent was usually more restricted for the large cockle size class. At Grahams Beach, there have been no high-density areas throughout the survey series. Considering temporal patterns across the northern sites, there were no distinct shifts in predicted high-density areas over time, but there was an overall reduction in their spatial extent at Eastern Beach, Kawakawa Bay (West), Ōhiwa Harbour, Pataua Estuary, and Waiotahe Estuary. At Hokianga Harbour, Okoromai Bay, Otūmoetai (Tauranga Harbour), Whangamatā Harbour, and Whangapoua Harbour, high-density areas persisted throughout the time series.