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Research Document - 2013/021

Long-term Changes in Grey Seal Vital Rates at Sable Island Estimated from POPAN Mark-resighting Analysis of Branded Seals

By C.E. den Heyer, W.D. Bowen, and J.I. McMillan


Populations facing resource limitation are expected to exhibit changes in vital rates, such as reduced juvenile survival, delayed maturation and reduced adult survival. Population growth rate of grey seals at Sable Island, Nova Scotia, has been monitored from 1963 to 2010 by estimating pup production. Recently, the rate of increase in pup production has slowed to 4%/yr from 13%/yr prior to 1997. Periodically between 1969 and 2002, more than 7000 grey seals were uniquely branded at weaning. Sighting of branded grey seals has been conducted annually from 1983 to 2012 by means of 3-7 weekly censuses of the breeding colony. Here, we use the mark-resighting analysis of branded females to estimate i) juvenile survival (weaning to age 4), ii) adult survival and iii) age-specific pupping probabilities (ages 4 to 14) was used. Two groups of cohorts (1985-89 and 1998-2002) were analyzed separately to test for temporal changes in vital rates using the sightings of the previous group of cohorts to help estimate the probability of sighting. Sightings from 1987 to 1999 of the 1980s cohorts and from 2000 to 2012 of the recent cohorts provided estimates of juvenile survival and average age at first birth. The estimates of capture probability (p) from the POPAN model provide a lower bound for average reproductive rates at 53 to 78%. Estimated average apparent survival rates of adult females were 0.95 and 0.97 for the 1980s and the 1998-2002 cohorts, respectively. Estimated average age at first birth (over ages 4 to 14 years) has increased from 5.6 to 6.5 years between the 1980s and 1998-2002 cohorts. However, apparent juvenile survival decreased from 0.74 in the 1980s cohorts to 0.33 in the recent cohorts.

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