Historical dynamics of the fur seal population : evidence of regulation by man?

The Australian fur seal (Arctocephalus pusillus doriferus) was severely over-exploited in the 18th and 19th centuries and until relatively recently its population had remained steady at well below estimated presealing levels. However, the population is now increasing rapidly (6%&ndash;20% per annum) throughout its range and there is a need to understand its dynamics in order to assess the potential extent and impact of interactions with fisheries. Age distribution (n = 156) and pregnancy rate (n = 110) were determined for adult females collected at a breeding colony on Seal Rocks, southeast Australia, in 1971&ndash;1972. Mean &plusmn; SE and maximum observed ages were 9.37 &plusmn; 0.41 and 20 years (n = 1), respectively. A stochastic modelling approach was used to fit an age distribution to the observed age-structure data and calculate rates of recruitment and adult survival. Annual adult female survival and recruitment rates between 1954 and 1971 were 0.478 &plusmn; 0.029 (mean &plusmn; SE) and 0.121 &plusmn; 0.007, respectively, suggesting that the population was experiencing a decline during the 1960s. The pregnancy rate increased from 78% at 3 years of age to an average of 85% between 4&ndash;13 years of age before significantly decreasing in older females (the oldest was 19 years of age). There was no significant effect of body mass or condition on the probability of a female being pregnant (P &gt; 0.5 in both cases) and the nutritional burden of lactation did not appear to affect pregnancy rates or gestational performance. These findings suggest that the low survivorship was due to density-independent effects such as mortality resulting from interactions with fishers, which are known to have been common at the time. The recent increase in the population is consistent with anecdotal evidence that such interactions have decreased as fishing practices have changed.<br /

Past and present distribution, densities and movements of blue whales <i>Balaenoptera musculus</i> in the Southern Hemisphere and northern Indian Ocean

1Blue whale locations in the Southern Hemisphere and northern Indian Ocean were obtained from catches (303 239), sightings (4383 records of =8058 whales), strandings (103), Discovery marks (2191) and recoveries (95), and acoustic recordings.2Sighting surveys included 7 480 450 km of effort plus 14 676 days with unmeasured effort. Groups usually consisted of solitary whales (65.2%) or pairs (24.6%); larger feeding aggregations of unassociated individuals were only rarely observed. Sighting rates (groups per 1000 km from many platform types) varied by four orders of magnitude and were lowest in the waters of Brazil, South Africa, the eastern tropical Pacific, Antarctica and South Georgia; higher in the Subantarctic and Peru; and highest around Indonesia, Sri Lanka, Chile, southern Australia and south of Madagascar.3Blue whales avoid the oligotrophic central gyres of the Indian, Pacific and Atlantic Oceans, but are more common where phytoplankton densities are high, and where there are dynamic oceanographic processes like upwelling and frontal meandering.4Compared with historical catches, the Antarctic (‘true’) subspecies is exceedingly rare and usually concentrated closer to the summer pack ice. In summer they are found throughout the Antarctic; in winter they migrate to southern Africa (although recent sightings there are rare) and to other northerly locations (based on acoustics), although some overwinter in the Antarctic.5Pygmy blue whales are found around the Indian Ocean and from southern Australia to New Zealand. At least four groupings are evident: northern Indian Ocean, from Madagascar to the Subantarctic, Indonesia to western and southern Australia, and from New Zealand northwards to the equator. Sighting rates are typically much higher than for Antarctic blue whales.6South-east Pacific blue whales have a discrete distribution and high sighting rates compared with the Antarctic. Further work is needed to clarify their subspecific status given their distinctive genetics, acoustics and length frequencies.7Antarctic blue whales numbered 1700 (95% Bayesian interval 860–2900) in 1996 (less than 1% of original levels), but are increasing at 7.3% per annum (95% Bayesian interval 1.4–11.6%). The status of other populations in the Southern Hemisphere and northern Indian Ocean is unknown because few abundance estimates are available, but higher recent sighting rates suggest that they are less depleted than Antarctic blue whales.</li

Growth and condition in Australian fur seals Arctocephalus pusillus doriferus (Carnivora : Pinnipedia)

Mass and length growth models were determined for male (n = 69) and female (n = 163) Australian fur seals (Arctocephalus pusillus doriferus) collected at a breeding colony on Seal Rocks (38˚31&prime;S, 145˚06&prime;E), Bass Strait, in south-east Australia, between February and November during 1970&ndash;72. Growth was best described by the logistic model in males and the von Bertalanffy model in females. Asymptotic mass and length were 229 kg and 221 cm for males, and 85 kg and 163 cm for females. In all, 95% of asymptotic mass and length were attained by 11 years and 11 years, respectively, in males compared with 9 years and 5 years, respectively, in females. Males grew in length faster than females and experienced a growth spurt in mass coinciding with the onset of puberty (4&ndash;5 years). The onset of puberty in females occurs when approximately 86% of asymptotic length is attained. The rate of growth and sexual development in Australian fur seals is similar to (if not faster than) that in the conspecific Cape fur seal (A. p. pusillus), which inhabits the nutrient-rich Benguela current. This suggests that the low marine productivity of Bass Strait may not be cause of the slow rate of recovery of the Australian fur seal population following the severe over-exploitation of the commercial sealing era. Sternal blubber depth was positively correlated in adult animals with a body condition index derived from the residuals of the mass&ndash;length relationship (males: r2 = 0.38, n = 19, P &lt; 0.001; females: r2 = 0.22, n = 92, P &lt; 0.001), confirming the validity of using such indices on otariids. Sternal blubber depth varied significantly with season in adult animals. In males it was lowest in winter and increased during spring prior to the breeding season (r2 = 0.39, n = 19, P &lt; 0.03) whereas in females it was greatest during winter (r2 = 0.05, n = 122, P&lt; 0.05).<br /

Causes of extinction of vertebrates during the Holocene of mainland Australia: arrival of the dingo, or human impact?

The arrival of the dingo in mainland Australia is believed to have caused the extinction of three native vertebrates: the thylacine, the Tasmanian devil and the Tasmanian native hen. The dingo is implicated in these extinctions because, while these three species disappeared during the late Holocene of mainland Australia in the presence of the dingo, they persisted in Tasmania in its absence. Moreover, the dingo might plausibly have competed with the thylacine and devil, and preyed on the native hen. However, another variable is similarly correlated with these extinctions: there is evidence for an increase in the human population on the mainland that gathered pace about 4000 years ago and was associated with innovations in hunting technology and more intensive use of resources. These changes may have combined to put increased hunting pressure on large vertebrates, and to reduce population size of many species that were hunted by people on the mainland. We suggest that these changes, which were quite dramatic on mainland Australia but were muted or absent in Tasmania, could have led to the mainland extinctions of the thylacine, devil and hen