Distribution and abundance of toheroa in Southland, 2013–14

The surf clam toheroa (Paphies ventricosa) is endemic to New Zealand, where it inhabits the intertidal zone on exposed sandy beaches in North and South Island regions. Toheroa are of great cultural importance, and have a long history of supporting customary, recreational and commercial fisheries. Owing to substantial population declines, toheroa are currently only targeted in limited customary fisheries.

The main toheroa populations in Southland are at Oreti and Bluecliffs beaches, where they have been regularly monitored since the late 1960s, including Ministry for Primary Industries (MPI) surveys. The present report documents the most recent MPI survey in Southland, conducted in 2013–14 at Oreti, Bluecliffs, and Orepuki beaches. The latter beach supports a small toheroa population that has not been part of the regular surveys before, but was the focus of a population study in 2008.

The primary aim of the survey was to estimate the population of toheroa with shell lengths ≥40 mm. The total population estimate for the population of these toheroa at the three Southland beaches was 1 489 000 (95% CI: 1 036 000–1 941 000) individuals in 2013–14. Oreti Beach continued to support the largest toheroa population in Southland, with an estimated population size of 1 395 000 (95% CI: 951 000–1 840 000) toheroa ≥40 mm shell length at this beach. Of these toheroa, an estimated 1 005 000 (95% CI: 644 000–1 367 000) individuals were ≥100 mm shell length. In addition, there were estimated to be 2 052 000 (95% CI: 755 000–348 000) juvenile toheroa (<40 mm shell length) at Oreti Beach. The population of larger toheroa at Oreti Beach appeared to be stable, with a similar estimated number of toheroa at ≥100 mm shell length to that in a previous survey in 2009 (980 000, 95% CI: 780 000–1 200 000); although this previous survey covered a smaller area of the beach, excluding the area known as the reserve. In contrast, the mean estimate of the number of juvenile toheroa (<40 mm shell length) was around one third the estimate from the 2009 survey (6 030 000, 95% CI: 3 056 000–9 005 000).

A marked change at Oreti Beach was that gravel was noted in many of the quadrats (23.8% of all quadrats, with no gravel or stones recorded in 2009). The increase in the occurrence of gravel may be associated with the decrease in juvenile toheroa. This notion was supported by statistical modelling of the number of toheroa within each quadrat at Oreti Beach, which indicated a negative association between counts of juvenile toheroa and the presence of gravel. A negative association was also found between juvenile toheroa and ghost shrimp burrow density. There was no association between large toheroa and these habitat characteristics.

Toheroa ≥40 mm shell length were present at high densities at the southern end of the beach, close to the entrance of New River Estuary, and also about 11–14 and 15–16 km along the beach; they were present at low densities in other areas. Across the beach, individuals in this size class were predominantly in the mid- to low-intertidal zone. Juvenile toheroa also showed the highest concentration in the southern part of Oreti Beach, but their spatial distribution extended across the entire intertidal zone, including the upper intertidal area in this and other parts of the beach. The size-frequency distribution of the Oreti Beach population was bimodal, with a distinct cohort of juveniles and a second cohort of individuals at shell lengths ≥80 mm. There were few toheroa at intermediate sizes. The decrease in juvenile toheroa at Oreti Beach may be related to episodic recruitment. Nevertheless, if the number of juveniles remains low, it would in turn lead to lower recruitment to the adult population. In recent years, Bluecliffs Beach has been eroded, and the toheroa population there was small, with a total estimate of 65 000 (95% CI: 0–150 000) toheroa ≥40 mm shell length in two disjunct areas, including the western bay. The number of smaller juvenile toheroa at Bluecliffs Beach was not estimated. Although the sample size was small, the spatial distribution of toheroa at Bluecliffs Beach appeared to extend across most of the intertidal zone, from about 30 m distance from the toe of the dune to the low tide mark. The size frequency distribution was unimodal, with a strong cohort of juveniles and few individuals at intermediate and large sizes.

The toheroa population at Orepuki Beach was also small, with an estimated 28 000 (95% CI: 10 000–46 000) toheroa ≥40 mm shell length at this beach. The population at this beach was dominated by juveniles, with 365 000 (95% CI: 211 000–519 000) toheroa <40 mm shell length. Toheroa were distributed along the entire survey area at Orepuki Beach, with the distribution of large toheroa restricted to the mid- and low-intertidal zone. In contrast, the spatial distribution of juvenile toheroa included the entire intertidal extent of the beach, other than the upper intertidal zone (within 50 m of the toe of the dune) in the northwestern part of the survey area. Toheroa at Orepuki Beach had a unimodal size distribution dominated by a strong cohort of juveniles, with few individuals at ≥40 mm shell length.