Bivalve abundance in relation to sediment properties across northern North Island

Intertidal cockles (tuangi, or littleneck clam, Austrovenus stutchburyi) are an mportant target species for customary and recreational fisheries throughout New Zealand. Their presence in coastal habitats, including sheltered bays and estuaries, makes these intertidal populations vulnerable to human impacts, including overexploitation and habitat degradation. The latter include runoff of terrigenous sediment and increased concentrations of suspended sediment, which have been linked to decreases in cockle abundance.

Across northern North Island, regular population surveys have focused on the spatial distribution, abundance and size structure of cockle populations targeted by non-commercial fishing. In view of the importance of sediment properties and their value as potential indicators of habitat quality, a sediment sampling component was added to the northern surveys in 2013–14; subsequent improvements to the sampling design have allowed formal analyses of cockle populations in relation to sediment characteristics.

The present study provides an investigation of the relationship between cockles and sediment grain size across northern North Island survey sites, based on data from five years of monitoring. The initial exploration of data was a principal component analysis, followed by the modelling of cockle population abundances as a function of different grain size fractions; a quantile regression was used to examine sediment-mediated effects on higher quantiles (>0.5) of observed abundances. To investigate which part of the cockle distribution responds to sediment grain size distributions, quantiles between 0.50 and 0.95 were modelled. The modelling was conducted for total cockle abundance and also for individuals in the large size class (i.e., individuals >30 mm shell length). The sediment grain size fractions included in the analysis were sediment fines (>63 μm grain size), very fine sand (>125 μm), fine sand (>250 μm), medium sand (>500 μm) and gravel (>2000 μm).

The findings from the analysis documented relatively small changes in sediment properties across sites over time: there was a small reduction in samples containing gravel, and there was also a small increase in medium to fine sands. Neither of these changes were sufficient to notably affect cockle abundance; the latter was highest in fine to medium sands. With most of the sediment consistently dominated by sand fractions, observed declines in cockle populations at some sites are likely related to factors other than sediment grain size characteristics. In general, the sediment habitat in the cockle strata appeared stable throughout the study period, and there were no marked increases in fines or gravel.

Overall, the proportion of sediment fines was generally below the threshold of about 10% that has been identified to limit cockle populations. Nevertheless, there was considerable variability in this grain size fraction at some sites, evident in individual samples where fines exceeded 20% of the sediment. In addition, some sites exhibited recent changes in sediment composition, with discernible increases in fine sand and sediment fines, even though these increases did not markedly affect cockle abundances; however, the abundance of the large individuals was low when the proportion of sediment fines exceeded 20%.

With multiple pressures potentially impacting intertidal cockle populations, information of sediment properties allowed the elucidation of this potential stressor in relation to population changes. With the current analysis indicating relatively few changes in sediment grain size composition over the study period, the findings indicate that changes in cockle populations are determined by other factors at the survey sites.