Distribution and abundance of toheroa at Oreti Beach, Murihiku/Southland, 2016–17

Toheroa (Paphies ventricosa) is a large species of surf clam that is endemic to New Zealand. Its populations are found in North and South Island regions, where they are most abundant on exposed west coast beaches. This species is of cultural importance and treasured as kai moana, with toheroa populations historically targeted in extensive commercial, recreational and customary fisheries throughout the country. As these populations experienced substantial fishing pressure, marked population declines led to fishery restrictions and closures. In spite of these measures, toheroa populations have shown little recovery to date, and the only take currently permitted is under the customary permit system.

The main southern toheroa populations are found in Murihiku, Southland including Oreti Beach, which supports the largest toheroa population in this region. Toheroa at this beach has been regularly surveyed since 1969, providing information about population trends, with consistent survey methods in recent years (i.e., since 1998) allowing direct comparisons across recent surveys. The current study presents the most recent toheroa assessment at this beach, based on survey data collected in 2016–17. The assessment focused on the population abundance and size structure of toheroa, including population estimates for juveniles (<40 mm shell length) and toheroa ≥40 mm shell length at this beach.

Current abundance estimates for the toheroa population included 2 154 000 (95% CI: 1 602 000–2 706 000) individuals 40 mm shell length in 2016–17. Most of the toheroa ≥40 mm were at sizes that exceeded 100 mm shell length, with an estimated 1 279 000 (95% CI: 853 000–1 705 000) individuals in this size class, compared with 875 000 (95% CI: 655 000–1 095 000) medium-sized toheroa (40–99 mm shell length). In addition, the population contained a substantial number of juveniles, with an estimated 8 507 000 (95% CI: 3 082 000–13 932 000) toheroa that were smaller than 40 mm shell length. All of the current estimates reflected increases in the toheroa population, especially in the abundance of juvenile toheroa, which showed an almost four-fold increase from the preceding estimate of 2 052 000 (95% 755 000–3 348 000) juveniles in 2013–14. Since juvenile toheroa were first (systematically) included in the surveys in 1998, their abundance consistently declined across surveys, to the lowest estimate in 2013–14. The current study documents the first increase in the population of juveniles, indicating a strong recruitment event preceding the field survey in 2017.

The spatial distribution of toheroa varied along the beach, and was dependent on the size class. Toheroa 40 mm shell length were widely distributed along most parts of Oreti Beach, with particularly high concentrations at the southern end, close to New River Estuary. The southern area was also important for juvenile toheroa, with markedly fewer juveniles occurring in other areas. The significance of the southern end of the beach has been highlighted in previous surveys, and this area continues to be important for the toheroa population. At the same time, there were few toheroa at the northern end, past Waimatuku Stream. Across the beach, toheroa 40 mm shell length showed a general preference for the mid- and low-intertidal zones, whereas juveniles were more restricted to high-tide areas.

The population size structure was dominated by the high number of juveniles at Oreti Beach. Lengthfrequency distribution documented a strong mode of these small individuals, with a considerably smaller mode of large toheroa. At the same time, medium-sized individuals were scarce. The current population size structure is characteristic of the Oreti Beach population and consistent with previous findings.

The findings from the present survey document the persistence of the toheroa population at Oreti Beach, in spite of the low number of juveniles in the preceding survey in 2013–14. Furthermore, the first documented increase in juveniles since 1998 suggests strong recruitment, with potential to augment the existing toheroa population at this site. The southern end of the beach continues to be an important area, including recruiting juveniles and adult toheroa. With only small toheroa populations remaining at other southern sites, Oreti Beach continues to support the main southern population of this species.s