MEPS 227:25-42 (2002)  -  doi:10.3354/meps227025

Growth and seasonal fluctuations in size and condition of male Northwest Atlantic harp seals Phoca groenlandica: an analysis using sequential growth curves

D. Chabot1,*, G. B. Stenson2

1Institut Maurice-Lamontagne, Fisheries and Oceans Canada, PO Box 1000, Mont-Joli, Quebec G5H 3Z4, Canada
2Science Branch, Fisheries and Oceans Canada, PO Box 5667, St. John¹s, Newfoundland A1C 5X1, Canada

ABSTRACT: Pinnipeds have marked seasonal changes in body condition, accumulating energy reserves prior to the reproduction period, and depleting these reserves during the breeding and molting periods. This cycle is not well described in male harp seals Phoca groenlandica. We obtained morphometric measurements for 3167 Northwest Atlantic male harp seals sampled between 1979 and 1995. The part of the yearly cycle covered in this study (November to May) was divided into short blocks around median dates called Œperiods¹. For each period, a growth curve was calculated for body mass, sculp mass, core mass and standard length. These sequential growth curves were used to calculate changes in size-at-age between periods. In addition, seasonal changes in 3 condition indices (general, sculp and core condition), in blubber thickness and in the ratio of sculp to total mass were analyzed using locally weighted regressions. Male harp seals returned from their high-latitude feeding grounds heavier, in better condition, and with a thicker blubber layer than when they left the area in the spring. However, maxima in length, mass, condition and blubber thickness were not observed until the February 22 period (February 12 to March 4), or in the case of seals younger than 5 yr, sometime in March. Mass losses associated with the rut began between the periods February 22 and March 15 (March 5 to 25) and were most pronounced in seals older than 10 yr. About 66% of the losses in mass came from the core during this period. We estimated that males lost 39.4 kg during the rut (1.16 kg d-1 or 0.78% of initial mass d-1, 44% from sculp and 56% from core) and incurred energy expenditures of 840 MJ (24.7 MJ d-1). This corresponds to twice the standard metabolic rate (SMR), and suggests that males are feeding during the rut. All males lost mass in April, and again in May. They began to molt April 19 period (April 16 to 22). Fewer than half the seals had completed the shedding of old hair when the study ended, in the May 10 period. Rates of mass loss during the molt varied from 1 to 1.8 kg d-1, depending on the age of the seals. Minima of length, mass, condition and blubber thickness were observed in May. Molting appears to be a period of high energy expenditures (3 to 5 SMR) despite the low levels of activity observed at this time. This study revealed that length also changes seasonally: fat seals in February were longer than lean seals of the same age in late April.

KEY WORDS: Harp seal · Phoca groenlandica · Growth · Condition · Morphometrics · Males

Full text in pdf format