Southern Clams

Sustainability

Early Harvest History

The littleneck clam has a long history of exploitation extending back probably to the very beginning of human settlement. In the later 14th Century at Purakanui, it was a significant minor food resource obtained by estuarine gathering (Anderson 1981), ranking after pipi (Paphies australis), but ahead of the mudsnail (Amphibola crenata). The species has widespread occurrence in middens throughout the country and is used as an indicator of estuarine gathering. Comparative analysis of growth zones in shells from midden and modern material has been used in attempts to provide information regarding the season of prehistoric occupation of certain sites, including some in the vicinity of Otago Peninsula (Coutts 1970, 1971, 1974,).

At several locations littleneck clams have status as "kai wairua", having particular spiritual significance in the mana and reputation of adjacent marae. This is true, for example, for dense beds of large littleneck clams near the entrance of Otago Harbour that have special significance for Te Runanga Otakau. Similar status applies to large beds at Port Levy, Banks Peninsula (Owen 1992, p.14).

There is an on-going traditional customary and recreational harvest throughout New Zealand. In the past twenty years there has been a growing commercial harvest of littleneck clams, the main sites being located on Snake Bank in the Whangarei Harbour, at Pakawau Beach in Tasman Bay, and at Papanui and Waitati Inlets in the vicinity of Otago Peninsula.

The Littleneck Clam Habitat

The New Zealand littleneck clam, more traditionally known as the New Zealand cockle, tuaki or tuangi, is the single most abundant large invertebrate animal found on intertidal sand flats in sheltered harbours and estuaries throughout New Zealand. This species, Austrovenus stutchburyi, is found only in New Zealand, including the Chatham Islands, and is common throughout the country. At many locations large beds of littleneck clam reach a maximum abundance around 4500/square metre, with a biomass of several kilograms.

The littleneck clam flourishes between the mid- and low water, where it is nourished by microscopic algae filtered from overlying water for several hours on each tide. As a suspension feeder, the clam derives its food by ciliary-mucoid capture of the microscopic, planktonic plant cells contained in a stream of water pumped through the mantle cavity. While the littleneck clam tolerates moderate salinity depression, the species is characteristically marine, and can survive low salinities for short periods, down to 10% seawater (~4‰ salinity) but cannot feed in less than 50% seawater (~18‰ salinity).

The littleneck clam is characteristic of sheltered harbours and inlets with extensive sandflats that are exposed at low water and flooded from the open sea by the rising tide. Its abundance is limited in estuaries that receive large volumes of fresh water inflow.

Stable fine sand sediments appear to be essential for littleneck clams to flourish. They are virtually absent from open coast beaches where mobile, well-sorted sands are inhabited by a variety of other bivalve species collectively known as surf clams. Littleneck clams are also absent from excessively muddy sediments.

Growth rates in littleneck clams are extremely variable, not only between sites, but also between tidal levels at any one site. Populations frequently contain significant numbers of individuals more than 20 years old, some with shell lengths greater than 60 mm. Sexes are separate and sexual maturity is thought to be attained at around 18 – 20 mm shell length. Gonadal development peaks in summer and prolonged spawning takes place over a protracted summer-autumn season in populations from Auckland to Dunedin.

Several animals prey on littleneck clams, including wading birds, fish, crabs and sea-stars. Each predator usually targets a particular size range of clam as preferred prey. Those clams that avoid being eaten have the potential to be long-lived, surviving for 25 years and more, reaching a size of >60 mm and a total weight > 90 g.

Littleneck clams have long been the subject of traditional harvest, their shells being found extensively in middens of varying age throughout the country. There is an on-going traditional customary and recreational harvest throughout New Zealand. In the past twenty years there has been a growing commercial harvest of littleneck clams, the main sites being located on Snake Bank in the Whangarei Harbour, at Pakawau Beach in Tasman Bay, and at Papanui and Waitati Inlets in the vicinity of Otago Peninsula.

Littleneck Clams Lifestyle

The littleneck clam is well adapted to life in the stable beach sediments of sheltered shores where they flourish from mid-tide level down to below low water of spring tides. The adult shell is heavy and plumply rounded, heart-shaped in section, and greyish white to rust brown, with prominent external sculpture of radial ribs intersecting with growth lines that are concentric with the outer shell margin.The interior of the shell is strongly marked with purple.

The heavy sculpture stabilises the clam in sand, where it burrows to a depth of only two or three centimetres. A pair of short conical siphons, fringed with small papillae, gives the animal the name of littleneck clam. One siphon draws an intake from the overlying water, and the other expels an exhaust stream. In the process of suspension feeding, particles that are entrapped from the water stream as it passes over the gills, are then transported to the mouth in a mucus sheet.

The short siphons must remain within reach of the sediment surface, though they are usually withdrawn during the low-tide period. The foot is strong and compressed, and smaller littleneck clams occasionally crawl along the surface, leaving a furrow usually less than a metre long. Clams normally remain immobile with the end of the shell often protruding.

Protruding clams become eroded and often develop a distinctive epibiotic community. Epibionts include the asexually reproducing anemone Anthopleura aureoradiata, sometimes with several attached to the shell of a single clam. The estuarine barnacle Elminius modestus, is common wherever salinities are depressed. Green algal staining provides a food source for a small limpet Notoacmea helmsi that lives by grazing the shell surfaces. Tubicolous amphipods often provide a tuft of parchment-like tubes on each shell. Spionid worms etch their U-shaped tubes into the shell surface of living clams, providing a refuge from which their feeding palps are extended into the overlying water.

Until quite recently the littleneck clam Austrovenus stutchburyi, was generally known as the common New Zealand cockle, and by its scientific synonym, Chione (Austrovenus) stutchburyi. The name littleneck clam is adopted here, partly because the name has growing use for marketing purposes, but also because the species is not a true cockle. The genus Austrovenus was erected by Finlay (1926) to accommodate this species, which had originally been placed in the genus Venus. The species has other scientific synomys. Marwick (1927) subsequently recognised Austrovenus as a subgenus within the genus Chione. Following detailed examination of both soft and hard part anatomy and morphology in five chionines, Jones (1979) concluded that "Austrovenus does not belong to the genus Chione, and may not belong to the [subfamily] Chioninae. It seems more likely that the similarity of Austrovenus and Chione in sculpture is the result of convergent evolution of very different lineages adapting to a shallow infaunal mode of life in shifting substrata in the intertidal and shallow subtidal zones."

The current taxonomic classification and nomenclature of the littleneck clam is as follows:

  • Class: Bivalvia
  • Order: Veneroidea
  • Family: Veneridae
  • Subfamily: Chioninae
  • Genus: Austrovenus Finlay, 1926 (Type species Venus stutchburyi Gray)
  • Species: Austrovenus stutchburyi (Gray, in Wood 1828)
  • Synonyms: Venus stutchburyit
  • Antigona stutchburyi
  • Chione stutchburyi
  • Common names: littleneck clam, cockle, tuaki / tuangi


Research History

Since 1982 Southern Clams has researched the impact of its wild-stock rotational harvesting system on habitats, refining its operations accordingly. As well as the intrinsic importance of sustaining the littleneck clams’ environment for future generations, the company’s livelihood depends upon it.

Decades of research and resource surveys have examined the impact of harvesting at Blueskin Bay and Papanui Inlet. The results show that clam beds harvested by Southern Clams tend to recover over 50% of their clam biomass a year after harvest. The impact on other species (macrofauna generally) is almost undetectable after 30 days, as is any effect on the sandy substrate of their habitat. Indeed, evidence shows that reducing the shellfish density by 60% (analogous to thinning carrots) has a positive impact on the growth and health of the remaining shellfish.

In his study of the effects of harvesting on littleneck clams in Waitati Inlet, Irwin (1999) showed a return in the space of one years to over 80 % of original biomass, in areas recovering from a 57% reduction through harvesting effort. This recovery was attributed in part to migration, in addition to growth and recruitment. In a follow-up elective study on this observation, McDonald (1999) concluded that migration occurs from high density areas surrounding areas affected by harvest.

Parasitic infestation has also been implicated in impairing the ability of littleneck clams to burrow, leaving them lying on the sediment surface and prone to predation by the parasite’s definitive oystercatcher host (Poulin et al.2000).

Experimental Studies

Experimental transplantation at different densities between low, mid and high shore levels were undertaken and maintained for 12 months in Otago Harbour (Dobbinson 1985, Dobbinson et al. 1989). Mid and high shore clams showed enhanced growth and condition, and lower mortality, when transplanted to lower shore levels. Lower shore clams transplanted to higher shore levels did not grow measurably and had lower condition and higher mortality. Mortality increased with height on the shore. On the basis of these results, though they were tempered by density effects, it was suggested that transplantation of smaller older clams from higher shore levels, to replace the low shore harvest, was a possible management strategy to counter problems of natural recruitment.

The effects of harvesting as a form of manipulation on biomass, length frequency and recruitment in remnant populations was undertaken in Waitati Inlet by Irwin (1999). Irwin found evidence for enhanced growth, recruitment and immigration following harvest removal of 55 – 60% of original biomass, though there was considerable spatial variation between his sites. (Irwin 2004)

A series of discretely separate littleneck clam populations are to be found in the stretch of coast from Blackhead in the south, to Shag Point in the north. South of Blackhead is a long stretch of coast extending to the Catlins Estuary, beyond Nugget Point, on which there are only minor populations to be found. The largest estuary in this area, that of the Taieri River, is unfavourable for littleneck clams by virtue of its intermittently high silt load and low salinity values. North of Shag Point the coast is comprised of sandy beaches, rocky shores and gravel, with only minor estuaries suitable for littleneck clams, extending as far as Banks Peninsula.

Planktonic larval life of the littleneck clam is to be measured in weeks (~3) rather than days or months which suggests that all the populations in the vicinity of Otago Peninsula are likely to a have significant exchange of larvae. This is particularly true from north of Cape Saunders to Shag Point because the counter-current eddy that forms in the northern downstream lee of Otago Peninsula under most wind and tidal regimes will strongly favour the retention of larvae in this area (Murdoch 1989, Murdoch et al. 1990).

Recognition of the from area Shag Point to Blackhead as a single management area would enable a sensible and potentially sustainable plan to be developed for the future harvest of littleneck clams.

Otago Harbour Research Project

Southern Clams has developed a five year research project In order to address concerns raised about the sustainability of clam harvesting in Otago harbour and its impact on marine ecology.

This is to be undertaken on a virgin resource in the middle banks of Otago Harbour.

It is to measure and record the baseline status of stocks in fauna and macrofloral ecologies, avifauna and bank morphology. It is then to record the impact on those systems from harvesting commencing in September 2009 to September 2014.

The experimental area comprises 180 hecatres, half of which is to remain untouched as controls. From the other half, 10% of the clam biomass is to be removed every year over five years. It is the intent of this experiment to identify any ecological impacts resulting from clam harvesting.

Listed below are all reports tabled to date. Anyone interested in receiving a copy of any of these reports are invited to submit their request by email: roger@nzclams.com

Baseline Survey June 2008: Clam (Austrovenus Stuchburyi) Resource and Habitat Survey in Otago Harbour

Research Proposal January 20009: Investigation into the Ecosystem Effects of Commercial Harvest of Clams (Austrovenus Stuchburyi) in Otago Harbour(COC3), Otago.

Otago Harbour Bathmetric Survey 1804 & 1805 – Hunter Hydrographic Services, 2009

Research Report, Ryder Consulting, January 2010: Investigation into the Ecosystem Effects of Commercial Harvest of Clams (Austrovenus Stuchburyi) in Otago Harbour(COC3), Otago.

Research Report, Derek Onley, May 2010: Report on Monitoring Birds in Relation to Clam Harvesting in Otago Harbour, Dec 2008- Mar 2010.