Improving methods to accurately age rainbow smelt (Osmerus mordax)

Various methods of cleaning and aging rainbow smelt (Osmerus mordax) scales and of validating these ages are examined to establish standard protocols. State fisheries managers in Massachusetts, New Hampshire, and Maine are working collaboratively to create mortality estimates for rainbow smelt to address concerns about population decline. Reference collections of scale images are being developed to determine accurate age-length keys for the species, as existing keys have been applicable only to small geographic areas or short time periods.  Scales are cleaned using a 5% diluted solution of pancreatin to digest mucus followed by a 15 minute bath in a high-frequency sonic cleaner. Scales are viewed using the image analysis program Image Pro (V6.2) which drives a digital video camera mounted atop a dissecting microscope with transmitted lighting. Image Pro software and transmitted lighting enable enhancement of the image to emphasize true and false annuli.  The accuracy of aging rainbow smelt scales has yet to be validated. Validation of age one smelt can be accomplished by comparing otolith daily ring counts to scales. Older ages can be validated by oxytetracycline hydrochloride (OTC) marking during the spawning run at streams sampled annually. Validation of all ages is crucial to accurately determine mortality.