156 
The number of caribou present in the Prud- 
hoe area varied daily depending on the extent of 
immigration into and emigration out of the area. 
Thus, in 1972 between eight and 1,500 animals 
were observed on any single day in the study 
area, and in 1973 between one and 130 animals. 
In both years maximum numbers occurred on 
days of insect harassment, and caribou were 
observed to move into the area. Lowest numbers 
were observed in insect-free periods, and fre- 
quently caribou were tending to disperse out of 
the area. 
In 1972 groups harassed by insects would 
move towards the coast; under severe harass- 
ment, large herds would assemble, particularly 
on the sand dunes and the sand or gravel banks 
of the river deltas. Herds moving under insect 
harassment would sometimes swim the Sagavan- 
irktok or Kuparuk rivers. 
In summer 1973 maximum numbers ob- 
served again corresponded with days of insect 
harassment (Fig. 2), but the large ‘‘invasions’’ 
did not occur. High water levels in the Sagavan- 
irktok and Kuparuk rivers conceivably could 
have inhibited movement into the study area in 
the upper reaches of the rivers. Large herds 
Harassment 
Severe ry la 
F | 
(0) 
1600 
1400 1972 
3 1200 h 
32 
S 600 
2 400 . 
o 3 
= 
3 200 
Zz x! 2 
| 
fe) co NS rn Feo er TN fat 
Severs Harassment 
9 EMild_ A_Pao Wa V2a 
2 200 1973 
5 Oo Pot! 
Oo 25 30 5 10 15 20 25 30 S 
June July August 
Fig. 2. Relationship of caribou numbers and 
intensity of insect harassment at Prudhoe Bay. 
Caribou numbers were obtained from a census 
of caribou within binocular range of the road; 7, 
2, 3; aerial surveys between coast and Franklin 
Bluffs. 
under insect harassment were observed 35 km 
inland on the moderate heights of the Franklin 
Bluffs (Gavin, pers. comm.). 
Population composition sample counts in 
both years were taken to be representative of 
the animals present in the Prudhoe Bay area, but 
not necessarily representative of any whole 
population, such as the Porcupine Herd, the 
Arctic Herd, or a “‘resident’’ Prudhoe Bay popu- 
lation. 
For 1972 a calf:female ratio of 67:100 was 
estimated. This estimate was based on an ob- 
served ratio of 51.2 +4.0 calves per 100 females 
plus yearlings (counts of 941 calves/1,831 fe- 
males + yearlings in 26 herds of size greater than 
24 females + yearlings per herd) and an estimat- 
ed yearling: female ratio of 24:100 (counts of 
22 yearlings/90 females). A male:female ratio of 
101:100 was based on an observed ratio of 77 
males per 100 females plus yearlings (counts of 
1,103 male/1,429 females + yearlings). The near- 
ly equal sex ratio was the result of a large herd 
of more than 1,000, of which 60% of animals 
(excluding calves) were males, which were classi- 
fied as they moved into the area under severe 
insect harassment in 1972. 
From the fewer animals present in 1973, 
three separate counts of at least 120 animals 
found the calf:female ratio to be 31:100, the 
yearling:female ratio 25:100, and the male: 
female ratio 37:100. 
A comparison of the herd proportions re- 
corded at Prudhoe Bay with previous estimates 
of herd proportions is shown in Table 1. The 
proportion of calves in caribou herds at Prudhoe 
Bay (16-23% of the herd) is similar to the calf 
percentage (15-26%) noted for major counts on 
the Arctic and Porcupine Herds. 
When the herd proportion of calves is adjust- 
ed to a calf:female ratio, the data suggest that 
the survival of calves to the end of July 1973 at 
Prudhoe Bay was low compared with either the 
results for 1972 at Prudhoe Bay, or for the 
Arctic and Porcupine Herds in June. Moreover, 
the calf:female ratio of the Porcupine Herd in 
October 1972 (31:100), following a known high 
rate of mortality of calves in July and August, 
was the same as that for the Prudhoe Bay 
animals in 1973. Survival of yearlings was appar- 
ently similar for both years of the Prudhoe Bay 
counts and was slightly higher than that for the 
Porcupine Herd. 
