Estimation of Population Size of Loach Misgurnus anguillicaudatus in an Experimental Paddy Field

―Validity of estimation values and methods involving how to capture, and the possibility of a simple estimation technique―

Abstract

Loach inhabiting paddy fields is an important species as a food resource for higher-level consumers, and the quantity of the resource is of great concern for environmental conservation. However, in ordinary paddy fields undergoing active farming, it is nearly impossible to validate the estimated population size. This difficulty occurs, simply because the true population value is unknown. We conducted an experiment to estimate population size by using a closed experimental paddy field where barriers were installed to prevent individuals' migration. At the onset there were no fish in the field. We released 1,185 loaches into the field so the number of individuals was considered nearly fixed. In the experiment, we selected 75 sampling points located at regular intervals to set the gear (wired trap) for an area of 300 m² and sampled 7 times with removal of captured individuals each time. We calculated population size based on all 75 points and on the 36 peripheral points of the sampling grid. Based on the results we obtained, it is suggested that application of such high-density sampling, repetitive removal, and application of DeLury's first model or the program CAPTURE have sufficient validity on the practical level. Further, our results suggest that approximating population size using limited data from peripheral sampling points may indicate the future possibility of a simple approach to population estimation.