Intertidal shellfish monitoring in the northern North Island region, 2019–20

Citation

Berkenbusch, K., & Neubauer, P. (2020). Intertidal shellfish monitoring in the northern North Island region, 2019–20. New Zealand Fisheries Assessment Report, 2020/38. 103 p. Retrieved from https://fs.fish.govt.nz/Page.aspx?pk=113&dk=24836

Summary

Recreational and customary fisheries throughout New Zealand target a wide range of marine species, including intertidal bivalves, such as littleneck clam (or cockle, tuangi/tuaki, Austrovenus stuchburyi) and pipi (Paphies australis). Concerns about their populations in northern North Island have led to regular surveys of cockle and pipi beds that are targeted in non-commercial fisheries in this region. The present study continued the northern North Island survey series, assessing cockles and pipi in the wider Auckland, Bay of Plenty, Northland and Waikato regions in 2019–20. The sites included in the present survey were (in alphabetical order): Bowentown Beach, Cockle Bay, Eastern Beach, Grahams Beach, Little Waihi Estuary, Pataua Estuary, Raglan Harbour, Tairua Harbour, Umupuia Beach, Waiōtahe Estuary, Whangateau Harbour and Whitianga Harbour.

All of the 2019–20 survey sites contained cockle populations, which varied in their abundance and density, depending on the site. The largest cockle population was at Whangateau Harbour, where their abundance estimate was a total of 887.67 million (CV: 10.72%) cockles, followed by Pataua Estuary, with an estimated 362.52 million (CV: 12.71%) cockles in 2019–20. The smallest cockle population was at Grahams Beach, where the total number of individuals was estimated to be 11.40 million (CV: 19.89%) cockles. Grahams Beach also had the lowest population density at 43 cockles per m2, compared with the highest density at Raglan Harbour of 1716 cockles per m2. Other sites with relatively high cockle densities (>1000 individuals per m2) were Bowentown Beach, Pataua Estuary, Tairua Harbour and Whitianga Harbour.

Within the cockle populations, large individuals (30 mm shell length) were generally scarce, and their abundance and density estimates typically had high uncertainty. Their highest density estimate was 98 cockles per m2 at Umupuia Beach, followed by density estimates at Eastern Beach and Cockle Bay of 77 and 75 cockles per m2, respectively. At the remaining sites, this size class was absent or only occurred at low densities (i.e., <30 individuals per m2).

At Waiōtahe Estuary, the cockle population exhibited a substantial decline in 2019–20, caused by the loss of medium-sized individuals since the preceding survey in 2016–17. This estuary has been exposed to considerable faecal bacteria contamination through dairy farm runoff in 2017. It is possible that this habitat degradation led to the recent decline in the resident cockle population.

Pipi populations were surveyed at nine of the sites, and their population sizes ranged from 0.29 million (CV: 15.16%) pipi at Bowentown Beach to 142.30 million (CV: 13.35%) pipi at Little Waihi Estuary. The latter estuary also supported their highest population density, with 849 pipi per m2; the other three sites with high pipi densities (i.e., >100 individuals per m2) were Waiōtahe Estuary (672 pipi per m2), followed by Tairua Harbour (309 pipi per m2) and Whitianga Harbour (163 pipi per m2). All other pipi populations occurred at densities of 41 pipi per m2 or less.

Little Waihi Estuary was the only site in 2019–20 with notable numbers of large pipi (50 mm shell length), and their density estimate in this estuary was 93 (CV: 18.74%) large pipi per m2. The next highest estimate was 16 (CV: 19.02%) large pipi per m2 at Whitianga Harbour. Large pipi estimates at the remaining sites were markedly lower (i.e., less than seven large individuals per m2) and had high uncertainty.

Sediment sampling in the cockle strata revealed sediments that were generally low in organic content (i.e., <5%) and consisted of varying proportions of sands. The proportion of fines (grain size <63 μm) was typically small, but exceeded 10% at several sites, with up to 69% of the sediment recorded in this grain size fraction in 2019–20 (at Umupuia Beach).