A SIMPLE FIELD METHOD FOR STEREO-PHOTOGRAPHIC LENGTH MEASUREMENT OF FREE-SWIMMING FISH - MERITS AND CONSTRAINTS

A simple field method to correct for bias in stereo-photographic under water measurements is presented and its usefulness to improve the accu racy of length estimates of free-swimming fish is tested. The calibrat ion is based on the inclusion of stereo exposures of a slate of known dimensions at the beginning and end of each measuring session. and it allows correction for non-parallel optical axes, spherical aberration of the lenses, and for other sources of bias that may vary between ses sions. A precision of +/-3% for replicate length measurements of a 30 cm slate is obtained at distances between 0.7 and 2.0 m. This compares well with reported values that are obtained with more sophisticated ( laboratory) methods. However, application of this calibration does not significantly improve the accuracy of stereo measurements of the leng th of free-swimming fish (+/-2.5 cm or 7-11% of actual fish length), T he limitations are caused by difficulties in recognizing identical ext reme points at the body of fish that are photographed under field cond itions. By averaging three or more replicate measurements the accuracy is improved to +/-1 cm (2.7-4.5%). We conclude that effort should be aimed at increasing sample size, rather than at improving equipment an d correction procedures, when measuring free-swimming fish in their na tural habitat.