A photographic larval fish counting technique: Comparison with other methods, statistical appraisal of the procedure and practical use

The problems involved in counting small or fragile live aquatic organisms have not yet been entirely overcome at hatcheries. We describe a counting method based on photographing a whole population and show it to be four to ten times more precise than the usual weighing or hand counting methods. A simple photographic system was designed to fit the requirements of the method. For large populations, formulae are given to calculate population and sample sizes according to the precision required and using stratified random sampling of fractions of each photograph, No statistically significant differences were observed when single populations were counted by several operators. The mean human counting error was about 1%, Actual counts of Dicentrarchus labrax fry (mean weight 30 mg) are used to illustrate the statistical developments. From one sample representing 17.5% of the total population, three inexpert operators were able to quantify a population of about 120 000 fry, with +/-1.7% precision, in 3 h.