A comparison of the suitability of alizarin red S and calcein for inducing a nonlethally detectable mark in juvenile guppies

Mark-recapture studies are an important component of fisheries research. A diversity of marks is needed to meet the demands of experimental designs and to overcome species-specific variation in marking success. Suitable marks must not alter the viability of marked individuals, must be easy to detect, and must be retained for an appropriate period of time. I compared the effect of alizarin red S and calcein on the individual growth and mortality rates of guppies Poecilia reticulata via short-term experiments (<14 d) conducted both in environments where alizarin- and calcein-marked fish were allowed to interact with unmarked fish and in environments where fish were segregated by mark. Neither mark affected the growth or mortality of marked individuals. Both marks were easily applied, did not affect the appearance of fish, and could be detected on the skeleton of live, anesthetized fish or ethanol-preserved specimens without the additional preparation (or lethality) involved in detecting marks on otoliths. However, both marks were subject to fading with time and when fish were exposed to high water temperatures or to direct sunlight. Thus, pilot experiments should be conducted under field conditions before marks are used for long-term mark-recapture studies. I also present an inexpensive and portable technique for detecting alizarin that uses a green laser pointer as the excitation source. This alizarin detector performed as well as a portable calcein detector (Leips et al. 2001) and could easily be modified for improved performance. Alizarin red S and calcein fluoresce at different wavelengths, so both marks can be used simultaneously in studies examining multiple treatment groups or cohorts. The use of alizarin red S may be preferred to that of calcein because its red fluorescence is more easily distinguished from the autofluorescence of bone.