The effect of rainfall and runoff of the Bio-Bio and Itata rivers on the landings of the clams Protrothaca thaca and Venus antiqua in central-south Chile

We analyzed the effects of river runoff (Bio-Bio and Itata rivers) and rainfall on annual landings of Protothaca thaca and Venus antiqua in central-south Chile (VIII Region) during 1975-1994. The environment-resource relationship was analyzed on two spatial scales: (i) whole VIII Region, and (ii) local level focusing on the closest main ports to the rivers mouth. Negative correlations were found between annual landings of P, thaca and V. antiqua and annual mean rainfall both at regional (r = -0.79; P<0.01) and local scale (Tome, r = -0.91, P<0.01; Coronel, r = -0.72, P<0.01). The lag time allowing the best correlation (r = -0.79; lag time = 3; P<0.01) between landings time-series and rainfall time-series corresponded to the time range needed by the clams to reach the minimum commercial size (i.e. 3 to 5 years). The annual mean rainfall explained 62% of the variability of the annual clam landings in the VIII Region (lag time = 3 years) incrementing to 92% when El Nino event 1982/1983 was discarded. These findings suggest that rainfall is an important factor in the survival of early life stages of the clams. A significant positive correlation was also found between the annual mean runoff of the Itata river and the annual landings of P. thaca and V. antiqua in the VIII Region with a lag time of 5 years. In addition, a significant correlation was found between annual clams landings in Tome and Itata's runoff during one of the principal spawning months (December) of P. thaca and V. antiqua. The lag time between these two time series corresponded to the reported clams commercial extraction age (5 years). In contrast, the annual mean Bio-Bio's runoff was not significantly correlated with the annual clams landings at any lag time both at regional or local level (Coronel). However, the monthly mean Bio-Bio's runoff was correlated to the annual landings of the VIII Region. The linear regression models between landing time-series of P, thaca and V.. antiqua in Tome port and the environmental time-series explained 72% (Itata's December runoff) and 83% (annual mean rainfall) of landings variability.