Intertidal shellfish monitoring in the northern North Island region, 2015–16

New Zealand’s coastal habitats support a number of bivalve species that are targeted in non-commerical fisheries. In northern North Island, these bivalves include cockles (tuangi/tuaki, or littleneck clam, Austrovenus stutchburyi) and pipi (Paphies australis), which are the main target species in intertidal recreational and customary fisheries across different sheltered environments, such as beaches, estuaries and harbours. These coastal populations are potentially vulnerable to human activities, including fishing, and are regularly monitored in intertidal population surveys commissioned by the Ministry for Primary Industries. These surveys assess the status of the cockle and pipi populations in areas targeted by noncommercial fisheries at a number of sites in northern North Island, from the wider Auckland region to Northland, Waikato, and Bay of Plenty.

The present study continues the series of bivalve population surveys in the northern North Island region by presenting data from the 2015–16 fishing year. Sites surveyed in this study included (in alphabetical order) Bowentown Beach, Cheltenham Beach, Cockle Bay, Little Waihi Estuary, Marokopa Estuary, Ohiwa Harbour, Okoromai Bay, Pataua Estuary, Tairua Harbour, Umupuia Beach, Whangateau Harbour, and Whitianga Harbour. Data collected at each site included information of the population size and structure of cockles and pipi in areas targeted by fishing activities. The sampling in areas inhabited by cockles also included the collection of sediment data to assess the relationship between the abundance of cockles and sediment characteristics, including organic content and the proportions of sediment fines and gravel.

Cockle populations were present at ten of the 2015–16 survey sites, whereas only few individuals were present at the remaining two sites, Cheltenham Beach and Marokopa Estuary. Population estimates for this species varied across sites, ranging from the smallest total population size documented at Ohiwa Harbour of 23.01 million (CV: 14.33%) individuals to the highest population estimate of 742.44 million (CV: 7.02%) cockles at Whangateau Harbour. Other sites with comparatively large cockle populations included Pataua Estuary and Umupuia Beach, which supported an estimated 380.13 million (CV: 7.58%) and 98.88 million (CV: 15.93%) cockles, respectively. Cockle population densities in 2015–16 varied between 136 cockles per m² (CV: 8.48%) at Cockle Bay and 1799 cockles per m² (CV: 5.17%) at Bowentown Beach.

Large cockles (≥30 mm shell length) are considered to be primarily targeted in fisheries, and individuals in this size class were present at most sites. Five sites supported relatively high numbers of large individuals, including Pataua Estuary, Okoromai Bay, Cockle Bay, Umupuia Beach, and Whangateau Harbour, with abundance estimates between 4.89 million (CV: 29.68%) large individuals (at Pataua Estuary) and 45.43 million (CV: 18.77%) large cockles (at Whangateau Harbour). Nevertheless, their corresponding densities were only relatively high at two sites, with 128 large cockles per m² at Umupuia Beach, and 98 individuals per m² at Cockle Bay. At most sites, the large size class contributed few individuals to the total population.

Eight of the survey sites in 2015–16 contained pipi populations, with few or no pipi at Cheltenham Beach, Cockle Bay, Okoromai Bay, and Umupuia Beach. At sites that included pipi beds, total population estimates ranged from 0.15 million (CV: 16.6%) pipi at Bowentown Beach to the largest population size of 83.84 million (CV: 16.62%) pipi at Little Waihi Estuary. Estimated pipi densities were highest at Ohiwa Harbour with 1225 pipi per m² (CV: 12.1%), followed by 456 individuals per m² (CV: 16.62%) at Little Waihi Estuary, 333 pipi per m² (CV: 11.26%) at Marokopa Estuary, and 327 pipi per m² (CV: 15.64%) at Tairua Habour. Density estimates were lower at the remaining sites, with the lowest estimated density of 10 pipi per m² (CV: 16.6%) at Bowentown Beach.

Large pipi (≥50 mm shell length) were present in most populations (except at Marokopa Estuary), although their numbers were relatively low at most sites. The highest abundance estimate for large pipi was at Ohiwa Harbour with 3.70 million (CV: 18.37%) individuals, and their corresponding density at this site was also relatively high with 110 large pipi per m². The pipi populations at Little Waihi Estuary and Whitianga Harbour also included a relatively high number of large individuals with 2.35 million (CV: 43.62%) and 1.91 million (CV: 22.66%) individuals in this size class, and population densities of 13 and 31 large pipi per m², respectively. At the remaining sites, population estimates for large individuals were considerably lower.

In addition to assessing cockle and pipi populations across northern North Island sites, the current data collection also involved sediment variables, including sediment organic content, and the proportions of fines (<63 μm grain size) and gravel (>2000 μm grain size). These data were used to examine the relationship between cockle abundance and sediment properties. Cockle abundance decreased with increasing sediment organic content, proportion of fines and proportion of gravel at the stratum level, but these relationships were tentative. It is expected that increased replication (as more data are collected over time) will improve future assessments of the relationship between bivalve populations and sediment characteristics.