The effects of harvesting on the reproductive and population biology of the New Zealand Littleneck Clam (Austrovenus stutchbury) in Waitati Inlet

Abstract

There has been considerable debate between local commercial harvesters of Austrovenus stutchburyi and local recreational and traditional fishers of Waitati Inlet as to how commercial harvesting affects biomass, the size structure of the remaining populations and the recruitment of new individuals to the population. This study focused on the direct impact of harvesting on biomass and density levels, the consequent shifts in length frequency distributions and the reproductive biology of Austrovenus stutchburyi.

Three study areas within Waitati Inlet were selected and each area divided in half, forming two study sites on each area. After initial biomass and density estimates were made, 60% of the biomass from one site on each area was removed. A year after the completion of this harvesting treatment further biomass and density estimates were made. Length frequency analyses were also undertaken prior to and following the harvesting treatment. Monthly reproductive analyses were made during the study period with condition indices, maturity indices and oocyte area analyses being utilised.

Density levels were found to recover to 87% and biomass 81% of initial estimates on treatment sites. An increase of 1% in density and 6% in biomass was evident on control sites. The distribution of length frequencies altered on treatment sites from a unimodal to a bimodal pattern with a cohort of young clams having been recruited to the population. On control sites a unimodal pattern was retained over the study period. No major changes in the onset of gametogenesis or spawning was found on treatment sites although clams were found to spawn longer and more intensely than those on control sites.

These results indicate that Austrovenus stutchburyi is quite resilient and able to recover from severe stock depletion in a relatively short period of time. This ability is likely to be due to the decrease in intraspecific competition brought about by harvesting and the consequent liberation of both space and food resources. This allows for an increase in growth rates and higher settlement levels within treatment populations, leading to a recovery firstly in density then followed by biomass.