Onset and duration of gray seal (Halichoerus grypus) molt in the Wadden Sea, and the role of environmental conditions

Abstract

Surveys of gray seals (Halichoerus grypus) during the molt period, when they are abundant on land, can be used to monitor changes in population size, but accurate interpretation of results requires an understanding of the molt process and how it may vary between years. This study investigates variability in onset (start date) and duration of visible molt by gray seals in the Wadden Sea, and the influence of environmental conditions on the onset. Molt was monitored in nine captive seals and observed molt phases were applied to wild seals over seven annual molt periods between 2004 and 2010, monitored using aerial photography. The onset of visible molt varied significantly between years, for example it differed 28 d between 2008 and 2009. Model selection by AIC retained one environmental variable that correlated with molt onset; however, its effect was inconsistent within the molt season and did not explain some of the apparent observed annual variation. Hence, the main causes of interannual variability in the onset of molt remain unclear and warrant further study. Researchers should account for annual variability in the onset of molt when interpreting survey results based on molt counts.

Molting phenology of a lacustrine ringed seal, Pusa hispida saimensis

“…The effects of day length on the initiation and progress of molt have been shown in numerous mammal and bird species in high latitudes (e.g., Beltran et al, 2018). In pinnipeds, photoperiod (Condy, 1979; Mo et al, 2000; Schop et al, 2017) and the amount of solar radiation (e.g., Moulton et al, 2000), which is highly connected to photoperiod, have also been shown to control the overall timing of the molt.…”

Convergence of biannual moulting strategies across birds and mammals

“…On the other hand, tropical residents and species that are subject to consistent annual daylength may rely heavily on non-photoperiodic cues such as temperature and rainfall [124]. Amphibious mammals such as pinnipeds apparently use a combination of cues for moult onset, including endogenous rhythms, changes in photoperiod, sea temperature, air temperature and body condition [126]. In turn, moult onset cues decide how species respond to global change; for example, migratory birds that depend on photoperiod cues for moult onset are expected to respond with less phenotypic plasticity than those cued by temperature [127].…”

Phenology of harbor seal pupping and the influence of weather on pup counts, investigated by UAV

“…This could be related to more variable temperatures at higher latitudes or that the relatively mild temperatures in the study area (mean of 14.4°C in June; https://www.dmi.dk) are high enough that fluctuations have little effect on the thermoregulation of the pups or that we did not have sufficient statistical power to detect an effect. Air temperature has been observed to influence seal haul‐out behavior during the molting season, as the epidermis needs elevated temperatures for follicular regeneration (Schop et al, 2017). As an example, temperature has proved to be important during the harbor seal molting season at latitudes similar to our study area, such as the Baltic Sea (Galatius et al, 2021).…”