Distribution, abundance and biology of Group V humpback whales ...

Variations in the abundance of prey on feeding grounds
may be directly related to the year-to-year variation in
reproductive rates (Clapham 1996) and abundance on the
wintering grounds (Chaloupka and Osmond 1999). Wiley
and Clapham (1993) tested the hypothesis of whether or
not maternal condition affects the sex ratio of offspring.
Their investigation concluded that the sex ratio of calves
born to females in “superior” condition, or that had a calving
interval of three years, was biased toward sons.
7.4 Male reproduction and behaviour
Mature male humpback whales depart the feeding grounds
with mature females (Dawbin 1966). Humpback whale
courting behaviour includes elaborate songs sung by males
during their migration to (Charif et al.2001) and on wintering
grounds (Helweg et al.1998b). Helweg and Herman (1994)
found there was no difference between singing behaviour
during day and night in deep water, although Au et al.(2000)
found that in shallow water off Maui there was a change in
the occurrence of singing behaviour with time of day,
with a peak at night.
The song patterns produced by singing humpback whales
depend on where individuals live, with populations inhabiting
different ocean basins normally singing quite distinct songs
(Noad et al.2000). Singing whales have been used to
determine migratory routes (Charif et al.2001), demonstrate
interchange between populations (Helweg et al.1998)
and as an index of abundance (Noad 1998). Mobley et
al.(1988) played a variety of sounds to humpback whales
on wintering grounds and concluded that the different rates
of behavioural response were attributed to the behaviour
of sexually active males seeking to affi liate with sexually
mature females.
A study conducted by Frankel et al.(1995) supported
the hypothesis that song functions to maintain distance
between singers and McCauley et al.(1995) give a succinct
description of the song and hearing capabilities
of humpbacks.
Richardson et al.(1995) is a comprehensive review of
acoustics and how they relate to marine mammals. The
information presented includes specifi c references to
humpback whales. They concluded there is no research
to demonstrate that acoustics adversely affects humpback
whales. However, more recent research demonstrates
that humpbacks alter their behaviour in response to
anthropogenic noise (Miller et al.2000). Williams et al.(2002)
concluded that weakening or not enforcing the minimum
approach distances for the whale watching of orcas in
Canada would result in higher levels of disturbance.
Corkeron (1995) demonstrated that humpbacks in Hervey
Bay altered their behaviour when vessels were within 300m.
This and several other studies show that the behaviour
of humpbacks is altered by the presence of vessels but it
remains uncertain if the effect is detrimental (Bauer and
Herman 1986; Baker and Herman 1989; Bryden et al.1988).
8 Population estimates and rates
of increase
Reviews of international data indicate a strong recovery
in most studied humpback whale populations (Best 1993
and Clapham 1999b). Paterson et al.(1994) suggests that
the level of recovery of the Group V population, in the
post-whaling period, is due to the stock experiencing no
detrimental environmental factors.
8.1 Southern hemisphere population
estimates
During 1933–1939, the estimated total number of
southern hemisphere baleen numbered 220,000–340,000.
Approximately 10 percent of the southern hemisphere
stock of baleen whales was humpback whales, and the
ratio between the groups I–V was considered to be
1:1:2:3:3 respectively (Matthews 1957 cited Chittleborough
1965). Based on these estimates the Group V population
could be estimated at 22,000–34,000 individuals for
1933–1939 (Figure 8).
8.2 Group V population estimates
Figure 8 is a summary of the population estimates and
illegal catches by the Soviet fl eet for Group V humpbacks
during 1930–2005. Chittleborough (1965) estimates the
Group V pre-whaling population numbered approximately
10,000 individuals. By 1960, he estimated a population size
of only 500 individuals. In 1963, the International Whaling
Commission (IWC) imposed a ban on whaling, and in
1964 the Group V population was estimated at just 400
individuals. Until 1963, humpback population estimates
were derived by modelling the fi sheries-based catch per
unit effort data (Chittleborough 1965).
Chittleborough (1965) compared population estimates
derived by two methods and argues that for the estimate
to be correct using the DeLury method, a further 5000
humpbacks would have had to been fi shed during the years
1960 and 1961. Only 302 and 270 whales were reported
to the IWC for each of those years. Although there is some
uncertainty in these estimates, it is clear that during the late
1950s and early 1960s, the Group V population experienced
a signifi cant decline in numbers.
Recent data presented by Mikhalev (2000) supports
DeLury’s population estimate discussed by Chittleborough
(1965). Mikhalev (2000) presented data collected by two
Soviet vessels illegally whaling during the summer season
of 1959 in the Antarctic Area V. The Russian vessels
signifi cantly exceeded the quota for all countries by illegally
fi shing a total of 11,605 individuals from the Group V
population.
Chittleborough (1965) estimated that the Group V population
would take 36–63 years to regain its unfi shed status of
10,000 individuals. However in 1998, 33 years later, the
Group V population was estimated at 4000 individuals
(Paterson cited QDEH 1999). If a constant rate of increase
is assumed, the 1962 population estimate of 500 individuals
would have to have increased at a rate of approximately
8 percent a year to reach its unfi shed state in 36 years.
The rate of increase for the population to reach its unfi shed
state in 63 years is just 4·8 percent a year. However,
Mikhalev (2000) details the large-scale illegal Soviet catch
data for the late 1960s and illustrates that the catches of
humpback on their feeding grounds during 1963–1968
exceeds the population estimates derived by Chittleborough
(1965). The population estimates for this period are being
revised to account for the illegal catches by the Soviet fl eet
(pers. comm. P Corkeron 2002).
14 • Distribution, abundance and biology of Group V humpback whales Megaptera novaeangliae: A review • August 2002