MARINE MAMMAL SCIENCE, 33(3): 830–846 (July 2017) © 2017 The Authors Marine Mammal Science published by Wiley Periodicals, Inc. on behalf of Society for Marine Mammalogy This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. DOI: 10.1111/mms.12404
Onset and duration of gray seal (Halichoerus grypus) molt in the Wadden Sea, and the role of environmental conditions JESSICA SCHOP,1 GEERT AARTS, ROGER KIRKWOOD, JENNY S. M. CREMER, SOPHIE M. J. M. BRASSEUR, Wageningen University and Research – Wageningen Marine Research, Ankerpark 27, 1781 AG Den Helder, The Netherlands.
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. Key words: peak molt, gray seals, Halichoerus grypus, haul-out, population monitoring, annual life cycle, phocid seals, Dutch Wadden Sea, North Sea.
Most mammals and birds undergo molt, during which hair or feathers, respectively, are renewed. This is an energetically costly and potentially risky activity since it reduces the time available for other activities, such as foraging or migration and may increase predation (Boily 1995, Portugal et al. 2011). The annual molt in phocid seals involves the shedding of year-old fur and the growth of a new pelage. To enhance this process, seals spend more time on land (Boily 1995). Consequently, a higher proportion of the population is on land during molt than at other times of the year (Daniel et al. 2003, Cronin et al. 2014, Brasseur et al. 2015). Given the potential costs of molting, seals could be expected to time this with the most favorable environmental conditions within the year.
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Corresponding author (e-mail:
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Understanding how the molt process is triggered and when maximum numbers of seals can be expected to haul-out is important when conducting surveys aimed at assessing abundance and trends in pinniped populations (Thomas 1996, H€ark€onen et al. 1999). However, the timing of the onset (start date) and duration of the molt can vary from year to year changing the proportion of the population that is hauled out on a particular day of the year (Adkison et al. 2003). Thus, an understanding of the variation in timing and duration of molt is important to accurately interpret molt period, population surveys. An over-arching factor controlling when seals molt is photoperiod. Mo et al. (2000) experimentally manipulated the photoperiod experienced by captive harbor seals (Phoca vitulina) and discovered that molt was triggered by changing (in this case decreasing) day length. Within the molt period, though, there is considerable individual variability in timing. In part, this is related to age, sex and internal/ endocrine processes (Bogdanowicz et al. 2013). The annual molt occurs sequentially through age-sex classes: first juveniles molt, then adult females, and finally adult males (Boulva and McLaren 1979, Thompson and Rothery 1987, Daniel et al. 2003, Cronin et al. 2014). Endocrine hormones associated with the reproductive cycle, such as cortisol and thyroxin, are implicated in the age and sex differences (Riviere 1978, Ashwell-Erickson et al. 1986, Boily 1996, Yochem and Stewart 2002). Variation between individuals in the timing of molt can also be influenced by individual condition (influenced by prey capture success), sea temperature, and ambient air temperature also influencing molt duration (Bissonnette 1935, Ling 1970, Mo et al. 2000, Yochem and Stewart 2002, Paterson et al. 2012). High external temperatures, for example, will increase the peripheral blood irrigation to epidermal cells, which theoretically could lead to an acceleration of the molt process in phocids seals (Boily 1995). Accordingly, annual variations in external factors such as prey availability and environmental conditions (particularly sea and air temperature), may cause annual changes to the onset date and duration of molt at the population level. Gray seals (Halichoerus grypus) are phocid seals that undergo an annual molt in spring, rather than autumn as do most phocids. The molting season starts approximately two months after breeding (Bonner 1981). The gray seal’s breeding season in the Wadden Sea peaks in December and molt occurs between March and May (Reijnders et al. 1995, Brasseur et al. 2015). Population trends in the Wadden Sea are partly based on the surveys conducted during molt, when gray seals are most abundant (Brasseur et al. 2015), however, no specific study on the onset and duration of gray seal molt exist. The aim of this study was to determine whether there was annual variation in the onset of molt by gray seals in the Wadden Sea, and, if so, whether environmental conditions could drive the variation. The study comprised daily monitoring of captive gray seals, to identify phases of molt, and an analysis of these molt phases in wild gray seals that were photographed during aerial surveys in seven consecutive years. The times of onset of the molt were compared with local environmental conditions and the North Atlantic Oscillation Index, which is derived from climate and sea temperature across the seals’ foraging range. To interpret how variation in the onset and duration of molt could manifest in the survey data, simulations were run to model how such changes manifested in seal numbers on land.
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Material and Methods Molt in Captive Gray Seals The visible onset and duration of molt was recorded in nine captive gray seals, eight females and one male. Visible molt is obvious in these seals as old hairs are dull cream and brownish colored, whereas new hairs are gray, black, and white colored. Each animal was photographed when out of the water daily between 24 February and 15 June 2010. The photographs provided a means of defining phases in molt and recording the duration of each phase for each seal. Six of the seals were based at the Dolfinarium in Harderwijk, the Netherlands. The other three were held at Ecomare on the Island of Texel, the Netherlands. At the Dolfinarium, seals were housed in a fresh-water pond with a sandy haulout site. The water was filtered through sand, treated with a flocculent to maintain clarity, and buffered to stabilize the pH. The seals were fed three to seven times a day with thawed herring (Clupea harengus), mackerel (Scomber scombrus), and sprat (Sprattus sprattus). Added to the fish, as a supplement, were five salt pills (of 1 g sodium chloride) per day and a vitamin tablet once a week (AkwaVit, containing 400 mg vitamin E and 100 mg vitamin B1). The male was given a contraceptive (9.4 mg suprelorin SC, since 2008) before each mating season, November–December. The five female seals at the Dolfinarium had not received contraceptives for the previous 2 yr. The three female seals at Ecomare were held in a concrete seawater-filled tank with access to an above-water shelf. The seals were fed thawed herring twice a day supplemented with a vitamin tablet (AkwaVit) once a week. In aerial photos of wild seals, the first indications of molt, uncovering small patches of new hair and thinning of old fur around the eyes and ears, would not be detected. To enable direct comparisons of molt onset between wild and captive seals, initial subtle changes in the captive seals were defined as “pre-visible-molt” (Table 1). Once larger patches became visible, clearly showing new silver-gray hair, the animals were considered to be in “visible molt”. The visible molt was divided into an early and a late phase (described in Table 1). Finally, the “post-molt” was defined as being when the seals’ pelage was completely silver-gray. The molt phases were adapted from Badosa et al. (2006), Cronin et al. (2014), and Daniel et al. (2003). These studies describe pre-visible-molt, (visible) active-molt, and post-molt. The two visible molt phases in the present study correlate with those defined as “early molt” and “late molt” by Stutz (1967). Note that a clear start and end date can only be defined in the two phases in the visible molt (early-molt and late-molt). Molt in Wild Gray Seals Wild gray seals at haul-out sites between the Wadden Sea islands of Vlieland and Terschelling (53°21.50 N, 5°3.40 E to 53°16.10 N, 5°16.90 E) were classified using the defined molt phases (Table 1, Fig. 1). Up to several thousand gray seals haul-out in this area during the molt period (Brasseur et al. 2015). Aerial photographs from 22 surveys over seven years between 2004 and 2010 were analyzed (Table 2). Aerial surveys were conducted from a single engine, fixed wing, aircraft that flew between 160 and 200 km/h at approximately 150 m (500 ft) above the sandbanks where the seals hauled-out. The oblique pictures generally had a distance between 500 and 1,000 m to the seals. Flights
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Table 1. Molt phase classification criteria for gray seals. The pictures are of different captive females at the Dolfinarium in Harderwijk, the Netherlands. Molt phase
Captive
Description
Pre-visible-molt
Brown and fading pelage with a dull appearance; hair may start to shed, but new hair is not clearly visible.
Visible molt: Early-molt
Pelage shows both old hair and new hair; patches of new hair clearly visible on the head, tail and the flippers. The whole torso is covered with brown fur. The old pelage is >50% present.
Visible molt: Late-molt
Pelage shows both old hair and new hair; new hair is visible along the dorsal and ventral side of the torso; patches of old hair are present on the flanks; new hair dominates (>50%).
Post-molt
Brown patches have disappeared; pelage patterns are clearly visible; pelage is silver and has a smooth appearance.
were conducted between two hours before and two hours after low tide, and were aimed at low tides that occurred between 1000 and 1600 local time, to ensure sufficient daylight (Reijnders et al. 2003). The surveys were only performed if rainfall in the preceding 24 h period was
