Influence of turbidity and passage rate on the efficiency of an infrared counter to enumerate and measure riverine fish

This study aimed to ascertain the influence of turbidity and migration rate on the count accuracy and size determination of an automatic infrared fish counter. The effect of turbidity on enumerating silver perch (Bidyanus bidyanus) migration rates was insignificant when compared to the inability of the infrared counter to deal with large numbers of migrating fish. The infrared counter underestimated counts by 5684% at moderate migration rates (12 fish h-1) and by 6282% at the highest migration rate (120 fish h-1). When multiple fish were simultaneously passed through the counter, the software detected them as a single fish and overestimated fish length. Fish passed through the unit ranged from 340 to 520 mm but the infrared counter estimated the range to be 140780 mm, with the lengths of a high proportion of individuals being underestimated. Most issues of inaccuracy appeared to be software-related and could be overcome with further software development. Further assessment of the applicability of the unit to enumerate fish migration, at high migration rates, should then be considered.