Summary of first daily ring formation in otoliths of freshwater fishes in the continental United States

ABSTRACT Daily ring counts in young-of-the-year fishes are important for estimating important vital rates, such as growth, mortality, and timing of hatch. To accurately estimate some of these rates, the timing of the first daily ring must be estimated accurately. Variation in the timing of the first daily ring can be attributed to many factors, including biology of the species and experience of laboratory personnel. The amount of variation and the degree of differences, however, have not been quantified, hindering the utility of daily ring information to provide accurate estimates of spawning and hatching times. We conducted a review of studies for freshwater fishes in the continental United States to quantify variation in daily ring validation studies as it relates to timing of the first ring. We found 40 studies representing 12 orders, 15 families, and 35 species. Most studies investigated rings in the sagittae, although the lapilli and asterisci were also used for a few species. Variation in the timing of the first ring formation was evident, but not consistent among otolith types or groups of fishes. The first daily ring in sagittae varied from 31 d before hatch to 150 d after hatch. First daily ring formation in lapilli was consistent within families but formed before hatch in some families of fish and after hatch in other families. The first daily ring in asterisci were near universally formed after hatch, with the exception of one species of sturgeon (family Acipenseridae). Only three of the nine species where replicate studies existed were found to exhibit consistent first ring formation timing. Such findings suggest that differences among laboratories and personnel may play a larger role than differences among species or populations when inconsistent first ring formation timing results occur. For most species, error surrounding differences in timing formation is about 1 week, except for Salmoniformes, where error was up to a 150-d difference. Incorporating species biology along with uncertainty in temporal estimates based on otolith chronology would aid interpretation of results in field situations.