Estimating growth, size-dependent mortality, and tag loss in a mark-recapture study: demography of wavy turban snails in Southern California, USA

Abstract

Tagging studies are often used to measure survival and growth in wild populations. However, issues such as low returns of tagged animals, tag loss, and handling mortality can affect the precision and accuracy of demographic estimates if not accounted for. We conducted a mark-recapture study to measure the survival and growth of juvenile and adult wavy turban snails Megastraea undosa. Our study highlights several methodological advances, including a method for estimating a single growth function by analyzing the joint changes in length and weight of tagged animals; a simple method for estimating rates of tag loss when some animals are double tagged; and a way to estimate handling mortality by comparing encounter histories of animals immediately after tagging and after several weeks in the field. Tag loss was moderate (≈15% over a 100 d period) and handling mortality was substantial during the fall (≈55%), but negligible during the spring. If we did not account for tag loss and handling mortality, our estimate of survival would have been severely underestimated (≈46% lower over a 100 d period). After accounting for such effects, our study revealed that survival probably increased sharply with body size, but survival did not differ among seasons. These snails are slow-growing, mature after approximately 6 yr, and can routinely grow to >10 yr old. There is an emerging fishery for this species, and because our results suggest that the oldest snails have a very high reproductive value, measures that protect these large individuals are likely to be beneficial for the resilience of these populations.