Arctocephalus forsteri

Figure 1. Group of New Zealand Fur Seals Image from https://keepcalmandjastravel.files.wordpress.com/2018/10/nz-fur-seals-featured-image.jpg

Introduction

The New Zealand Fur Seal (Arctocephalus forsteri) can be found all around the coastline of New Zealand
Primary Source Article: Geographic and temporal variation in the condition of pups of the New Zealand fur seal (Arctocephalus forsteri): evidence for density dependence and differences in the marine environment (Bradshaw et al. 199)

Taxonomy

Kingdom: Anamalia

Phylum: Chordata

Subphylum: Vertebrata

Class: Mammalia

Order: Carnivora

Suborder: Caniformia

Family: Otariidae

Genus: Arctocephalus

Species: Arctocephalus forsteri

Figure 2. Seal pup Image from https://www.scottvsphoto.co.uk/wp-content/uploads/2019/02/DSF7372.jpg

Background

They are warm-blooded animals that respire via lungs
They breed mainly in the South Island and also on neighboring islands such as Chatham Island and Bounty Islands (Taylor et al. 1995)
New Zealand Fur Seals are sexually dimorphic animals, which means the males are larger than the females
Male fur seals weigh about 120-185kg, while females are about 40-70kg (Bonner 1994)
Females become sexually mature after 5 years, while males continue to grow until about 8-10 years (Dickie and Dawson 2003, McKenzie et al. 2007)
They are carnivores and feed on small birds (such as penguins), fish, and cephalopods
They sexually reproduce and give birth to live pups
Live in temperate, rocky areas in New Zealand and South Australia

Data Analysis

Scientists wanted to determine the affect of geographic and temporal variation on the condition of New Zealand fur seal pups in South Island, New Zealand. (Bradshaw et al. 1999)
The experiment was carried out over the course of 3 years and they investigated 16 breeding colonies in 1996, 19 breeding colonies in 1997, and 20 breeding colonies in 1998. (Bradshaw et al. 1999)
Conditions were measured for 5,917 seal pups, but 44.3% of the pups measured were recaptures from the summer making the total number of pups 6,856. (Bradshaw et al. 1999)
The difference in densities between seasons depend on the year because conditions are never the same during a season each year (Bradshaw et al. 1999)
Body measurement was used to help calculate the condition of the pups (Bradshaw et al. 1999)
Pup condition was estimated by comparing all pups in the years of the study using least-squares linear regression of the log_e-transformed measurements of length vs mass

Figure 3. Map of South Island, New Zealand showing the distribution of the colonies sampled

Figure 4. Distribution of the all season year condition index per pup over all years and seasons studied (1996-98)

Figure 5. Individual-season condition index and the all season-year condition index (combined CI) regressions of loge(mass) vs loge(length) for female and male pups captured.

Conclusion

The results of the study show that the variation in pup condition is partially density dependent (Bradshaw et al. 1999)
For example in 1999 El Nino, which was a natural disaster, caused a decrease in pup density. (Bradshaw et al. 1999)
The scientist were able to come to a conclusion that geography and temporal variation do have an impact on the condition of seal pups (Bradshaw et al. 1999)
In the winter, there are less resources such as food which can cause a decrease in the body mass of the pups and can even lead to death (Bradshaw et al. 1999)

Figure 6. Mean seasonal pup densities (pups/100 m2 ) per year. Sample sizes (i.e. number of colonies) in parentheses

Figure 7. Correlation parameters (r2 and P) for the Mo/Mp (combined CI, all-season CI, individual-season CI) calculations of pup condition vs time of capture. Maximum r2 values for each condition index in bold

Figure 8. Two male fur seals fighting. Image from Sue Wilson

References

Bradshaw, C. J., Davis, L.S., Lalas, C., Harcourt, R.G. 1999. Geographic and temporal variation in the condition of pups of the New Zealand fur seal (Arctocephalus forsteri): evidence for density dependence and differences in the marine environment. Journal of Zoology. 1-11.

Wilson, Sue. New Zealand Fur Seal, 2008, www.pinnipeds.org/seal-information/species-information-pages/sea-lions-and-fur-seals/new-zealand-fur-seal.

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