Intertidal shellfish monitoring in the northern North Island region, 2021–22

New Zealand’s coastal environments support a diverse range of marine species that are treasured in recreational fisheries and as kai moana. Their presence in coastal environments makes their populations vulnerable to human impacts, which may lead to localised population declines and depletion. In this context, regular monitoring of fishery species supports the management of their populations, ensuring long-term sustainability. For the recreational and customary take of intertidal cockles (Austrovenus stutchburyi) and pipi (Paphies australis), Fisheries New Zealand monitors their northern North Island populations through regular surveys in Fisheries Management Areas 1 and 9. This survey programme was first implemented in the early 1990s, and the survey methods became consistent in 1999–2000. Within this programme, the field surveys usually sample cockle and pipi populations at 12 northern sites each year and provide information about their abundance, density, and size structure in areas targeted by non-commercial fisheries.

Reported here are the findings from the northern North Island monitoring programme, updating survey data to the 2021–22 fishing year. The 16 sites in the 2021–22 assessment were (in alphabetical order): Bowentown Beach, Cockle Bay, Little Waihi Estuary, Mangawhai Harbour, Marsden Bank, Mill Bay, Ngunguru Estuary, Pataua Estuary, Raglan Harbour, Ruakākā Estuary, Tairua Harbour, Te Haumi, Te Mata and Waipatukahu (Te Mata Bay), Umupuia Beach, Whangateau Harbour, and Whitianga Harbour. Four of these sites (Mangawhai Harbour, Ngunguru Estuary, Ruakākā Estuary, Te Haumi) were intended to be sampled in the previous fishing year, but travel restrictions to counter COVID-19 prevented the survey of these sites in 2020–21.

Cockle populations were surveyed at 14 of the 16 sites, where they varied in their population size and density. Total abundance estimates varied from 8.04 million cockles at Ruakākā Estuary (where the coefficient of variation (CV) exceeded 20% at 25.94%) to 983.06 million (CV: 9.91%) individuals in Whangateau Harbour. The corresponding density estimates ranged from 141 cockles per m² (CV: 17.48%) at Little Waihi Estuary to 1774 cockles per m² (CV: 12.24%) at Ngunguru Estuary. Most sites supported relatively high cockle densities of several hundred individuals per square metre, but some of the high estimates were caused by a strong influx of recruits (<15 mm shell length). Apart from Ruakākā Estuary, where phase-2 sampling did not succeed in lowering the CV value below 20%, estimates at all other sites had CV values below this target percentage.

Only two of the cockle populations surveyed in 2021–22 included comparatively large numbers of large cockles (≥30 mm shell length); they were at Mangawhai Harbour and Umupuia Beach. Estimated densities of this size class were 79 cockles and 72 individuals per m² at Mangawhai Harbour and Umupuia Beach, respectively. Cockle populations at all other sites had notably lower densities of large cockles.

Pipi populations were surveyed at 11 of the northern sites, excluding Marsden Bank, which no longer contained any pipi in 2021–22. Population sizes of this species ranged from small estimates of 0.46 million (CV: 23.00%) pipi at Bowentown Beach to 166.06 million (CV: 14.54%) pipi at Ruakākā Estuary. The pipi population at Ruakākā Estuary also had the highest density estimate at 4061 pipi per m², compared with the lowest density of 13 pipi per m² at Pataua Estuary. The low total population estimates at Bowentown Beach, Little Waihi Estuary, and Pataua Estuary had high uncertainty with CV values >20%, in spite of additional phase-2 sampling.

There were few large pipi (≥50 mm shell length) included in the current populations. Instead, the pipi populations were largely determined by medium-sized individuals and recruits (<20 mm shell length). The highest density of large pipi was at Te Mata Bay, estimated at 83 pipi per m² (CV: 14.65%). Sediment samples from the cockle habitats across the northern sites indicated sediment that was low in organic content (<3.3%) and consisted predominantly of fine and medium sands (grain sizes >125 and >500 μm. The proportion of other grain size fractions varied across sites and included several sites with a relatively high proportion of sediment fines (silt and clay; <63 μm grain size).

In addition to providing population information, the current study explored the relationship between cockle abundance and sediment characteristics through principal component analysis. At most sites, there were no prominent patterns or temporal shifts in cockle abundances, most likely owing to relatively small changes in sediment granulometry over time. The exception was Umupuia Beach, which was characterised by a high proportion of sediment fines, particularly in recent surveys. At this site, total cockle abundance associated with all other (coarser) grain size fractions was high compared with sediment fines.

The current survey data were also used to update geostatistical models for predicting spatial and temporal patterns in cockle densities at the northern sites. Predictions of total cockle and of large cockle densities from the geostatistical models documented differences in spatio-temporal patterns between some of the sites. For example, the model outcomes highlighted the influence of strong recruitment events on the spatial distribution of predicted cockle densities and also illustrated the diminishment of high-density areas over time, dependent on the site.