A simple field method for stereo-photographic length measurement of free-swimming fish: merits and constraints

Abstract

A simple field method to correct for bias in stereo-photographic underwater measurements is presented and its usefulness to improve the accuracy of length estimates of free-swimming fish is tested. The calibration 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 sessions. A precision of t3% 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 length of free-swimming fish (22.5 cm or 7-11% of actual fish length). The limitations are caused by difficulties in recognizing identical extreme points at the body of fish that are photographed under field conditions. By averaging three or more replicate measurements the accuracy is improved to il cm (2.7-4.5%).We conclude that effort should be aimed at increasing sample size. rather than at improving equipment and correction procedures, when measuring free-swimming fish in their natural habitat.

Plastic growth of the herbivorous reef fish Sparisoma viride:field evidence for a trade-off between growth and reproduction

“…Daylength varies between a winter minimum of 11 h 25 min and a summer maximum of 12 h 51 min (see Fig. 1 in van Rooij et al 1995).…”

“…Although seasonal fluctuations in temperature and daylength are small, and despite the lack of a distinct spawning season, Sparisoma viride shows seasonal variation in condition (van Rooij et al 1995). Fish were in poorest condition in the season with longest days (May to July), while their condition was best when water temperature was highest (around October).…”

A practical guide to the use of consumer-level digital still cameras for precise stereogrammetric <I>in situ</I> assessments in aquatic environments

“…The advantages of stereo-imaging in quantitative in situ assessments have been described in various studies using either analogue (Van Rooij and Videler, 1996; Evans and Norris, 1997; Bräger et al, 1999; (Abdo et al, 2006;Fischer et al, 2007;Williams et al, 2010) stereophotography systems. The rapid change from analogue to digital systems within the last decade has furthermore significantly enhanced the potential of the method.…”

“…Both systems tested were built in the classical manner of stereogrammetry, as described in Klimley and Brown (1983) and Van Rooij and Videler (1996), with the optical axes pre-adjusted to be as parallel to each other as possible.…”

“…As in terrestrial ecology, stereophotography enables reference-free and non-invasive measurements of object coordinates in the 3D realm, thus eliminating the need for a parallel estimation of quantitative parameters such as distance or volume (e.g. Cullen et al, 1965;Rorslett et al, 1978;Van Rooij and Videler, 1996;Costa et al, 2006;Williams et al, 2010). Photographic assessments of time series of communities also allow the estimation of quantitative derivatives, including growth or productivity over time, using an empirically determined relationship between biomass and linear measurements (Williams, 1969;Rorslett et al, 1978;Fischer et al, 2007;Williams et al, 2010, Doya et al, 2014.…”