Biomass and distribution of five species of surf clam off an exposed west coast North Island beach, New Zealand

In 1992, the total biomass of five species of surf clam, between depths of 1 and 9 metres below chart datum, along 27.5 km of west coast North Island, New Zealand, open sandy coast was conservatively estimated as being 1.032 t. Approximately 50% was Dosinia anus, 19% was Mactra discors. 16% was Paphies donacina, 14% was Mactra murchisoni, and 3% was Spisula aequilatera. This biomass equates to approximately 375 t km^-1 of coastline, and the potential harvest from this standing crop is correspondingly limited. The only other common macrofaunal species taken was a sand dollar, Fellaster zelandiae. The numerical density of each surf clam species varied with water depth, each having an optimal range A trend of increasing density along the surveyed coast (increasing with latitude) was found with F. zelandiae, which suggests that recruitment processes are not always uniform or random along the coast at the scale of this study. A less marked trend was also found with P. donacina. No trends of density with latitude were found with D. anus. M. discors, M. murchisoni, or S. aequilatera. Good recruitment was evident from length frequency distributions in all species except P. donicina. It is suggested that the smaller size classes of this species live in shallower water than that sampled. Small S. aequilatera were more common in shallow, inshore waters than offshore, whereas the reverse pattern of size with depth was exhibited by the two Mactra species and by D. anus. P. donacina showed no trend of mean length with depth. All species exhibited highly positively skewed histograms of frequency of abundance within dredge tows, indicating that they all had highly aggregated distributions The relative 'patchiness' or degree of clamping for each species appeared to be positively related to its the relative abundance. Target fishing for single species is unlikely to be possible on a commercial basis owing to the high degree of overlap in their distributions and the high levels of patchiness for all species. The facts that the five species have different productivity levels and a wide range of growth rates, combined with the inability to target for particular species, have implications for the management of the resource. The sand dollar, F. zelandiae, can be so abundant that in some areas it could clog fishing gear during prolonged tows and become a severe nuisance to commercial fishing. Any fishery based on this coast would best be managed as a mixed species fishery, and the variability in available biomass suggests that a constant fishing mortality harvesting strategy (such as current annual yield) would be more appropriate than a constant catch strategy (such as maximum constant yield).