REDUCED POPULATION GROWTH OF GRAY SEALS AT SABLE ISLAND: EVIDENCE FROM PUP PRODUCTION AND AGE OF PRIMIPARITY

Pup production on Sable Island, Nova Scotia, has been increasing exponentially since the early 1960s and by 1997 Sable Island was the largest gray seal colony worldwide. Using an aerial photographic survey, as in previous years, we estimated pup production in January 2004 to determine if this exponential rate of increase had continued. A total of 33,268 pups was counted on the color positives. When corrected for the proportion pups missed on the imagery (1.106 for the 12th; 1.527 on the 13th), the proportion of pups that died prior to the survey (0.031), and the proportion of pups born before the survey (east colony 0.966, west colony 0.962), estimated total pup production was 41,500 with SE = 4,381. The 2004 estimate indicates that pup production on Sable Island has continued to increase, but suggests that the rate of increase ( r ) may have declined (0.070 compared to previous 0.128). Females from the 1998–2000 cohorts were about 16 times less likely to give birth for the first time at age 4 yr and more than twice as likely at age 6 yr compared to those in the mid-late 1980s. The new estimate of pup production and observed changes in age of primiparity provide the first indication of changes in vital rates of this population. However, additional estimates of pup production and vital rates are needed to confirm this conclusion and to investigate the underlying mechanisms.     

GROWTH RATE AND SHEDDING OF VIBRISSAE IN THE GRAY SEAL, HALICHOERUS GRYPUS: A CAUTIONARY NOTE FOR STABLE ISOTOPE DIET ANALYSIS

Stable isotopes have become powerful tools for gathering information on food webs in marine ecosystems. The method is based on the concept that the ratio of Nitrogen-14 to ¹⁵N (or Carbon-12 to ¹³C) in the tissues of animals is directly related to the ratio found in their diet. Vibrissae provide a time series of stable isotope data as tissue is laid down sequentially over time. Here we examine the growth rate of 283 mystacial (muzzle) vibrissae of four gray seals, Halichoeruas grypus , over a five-month period to investigate their applicability for stable isotope diet analysis. The individual vibrissae did not grow at a constant rate during the study, Fifty-nine actively growing vibrissae were modeled to quantify the growth pattern using a three-parameter von Bertalanffy curve, with the parameters estimated using non-linear mixed-effects models. This model incorporated the inherent serial correlation of these data. The growth rate was 0.024 cm/d (95% CI = 0.019–0.030), the asymptotic length differed significantly by location ( F _3,56=9.64, P < 0.001), but no significant trend was found with muzzle location ( F _3,56= 0.15, P = 0.93). The Δlength/Δtime between each measurement was calculated and most of these data fell at or near zero growth (median = 0.04 cm/d, range = 0–0.78). Individual vibrissae were shed asynchronously and without any seasonal growth trend. This has serious implications for researchers attempting to extrapolate diet data from vibrissae. Because the growth is neither continuous nor synchronous, it will be a challenge to accurately identify the dates when the isotopes were incorporated into the tissue.     

EFFECTS OF MEAL SIZE ON OTOLITH RECOVERY FROM FECAL SAMPLES OF GRAY AND HARBOR SEAL PUPS

Recovered otoliths from pinniped feces provide valuable information on diet composition and prey size. We studied the effect of meal size on otolith recovery from the feces of one harbor and eight gray seal pups. Each of 11 experiments comprised a half-ration meal, a period of fecal collection, a 1.5-or double-ration meal again followed by a period of fecal collection. A significantly lower percentage of Atlantic herring otoliths were recovered from half-ration meals (25%± 12.5% in the harbor seal, 8.6%± 6.9% in eight gray seals) than from 1.5- or double-ration meals (62.5%± 3.1 % in the harbor seal, 32.8%± 23.5% in gray seals). Meal size also significantly affected the percentage of Atlantic cod otoliths recovered from gray seal feces (65.0%± 26.3% from half ration, 98.3%± 2.9% from 1.5 ration). For both size meals, recovered cod otoliths were more significantly eroded than herring otoliths. The development of correction factors to account for the effects of digestion will need to consider the distribution of meal sizes of free-ranging pinnipeds.