Ohau Point Seal Colony & the earthquake

A magnitude 7.8 earthquake near Kaikoura, New Zealand destroyed much of the State Highway 1 coastal road on either side of Kaikoura on 14 Nov 2016. These include huge landslips that also occurred on the coastal cliff face of the Ohau Point, burying a large portion of the Ohau Point Seal Colony.

The Ohau Point stream and waterfall, which is the only known place in the world where fur seal pups made annual visits up a stream, appears to have sustained a moderate amount of damage to the pool beneath the waterfall due to rockfall. However the walkway and the waterfall itself appear to be largely intact (Link to article).

ohau-earthquake

Direct impact of the earthquake: Fortunately the earthquake occurred after the weaning period of the yearlings (pups born in the summer of 2015/2016) and before the pupping season (late Nov – Jan). This means that most of the yearlings should have left the colony to learn to forage for themselves, and the pregnant females should also have been foraging before giving birth. The earthquake also hit around midnight; as NZ fur seals (including the seals of the Ohau Colony) have been reported to forage during the night (Fea, Harcourt & Lalas 1999; Krajewski & Sazima 2010), any direct casualties from the landslips can be assumed to have been minimal.

Short term impact (1 year): The parts of the colony (north and south ends of the Ohau Point colony) uncovered by landslips are likely to continue to be used by returning pregnant females for birthing. The area north of the stream mouth have also been used for pupping for the last 2-3 pupping seasons, and seem to have been less affected by the landslips. I expect competition for good birthing/resting spots will be more intense than previous years due to the significant coverage by the landslips, which are not likely to be seen as good haul-out sites (unstable, loose and small rocks). Some females driven out by the competition may migrate to the areas north and south of the main colony for birthing, which would fast-track the previously-observed growth of the Ohau Point Colony in terms of its length. Others may migrate to the Kaikoura Peninsula seal colony, which has seen a significant growth in pup numbers in the last 2 years. Male-male competition for territory would be higher than previous years due to the decrease in space. This is likely to result in more frequent and vicious fights, which may in turn result in increased female displacement and pup injuries.

The landslips have almost completely covered the steep rockfaces composed of large rocks and boulders on the northern half of the colony, which were used by females and pups for safety from high waves during winter storms, and provided shade for pups on hot days. The replacement of these rockfaces by unstable formation of small rocks may result in a higher pup fatality to storms as well as high temperatures, and increased movement of pups and adults onto the road and the carpark. If the coastal road is open to the public before winter, this may result in much higher vehicle-caused seal fatalities as well as increase in crash risk. The large landslips have blocked any above-land access between the north end and south of the colony, so there is likely to be restricted movement of pups between these areas.

Its unclear how the road clearing and recovery work will affect the colony. It has been announced that the rubble from landslips on the SH1 will be dumped into the sea, which will likely impact the tidal zone ecosystem (link to article). If the rubble in the Ohau Point colony is moved to neighbouring sections within the colony, it will further reduce the ideal haul-out habitat for the seals and may negatively influence the pup survival rates. The direct disturbance from road work is also likely to put adverse pressure on the pup growth and survival by stressing both females and pups (more energy spent on running away from humans/noise & pups get less time to suckle).

Long term impact: If the females that pup in the colony in the 2016/17 season experience offspring loss due to the decrease in favourable birthing/resting sites, coupled with the likely occurrence of overcrowding, they may deem the colony unfit for reproduction and move elsewhere for the following pupping season; resulting in a decline in returning females being evident from the 17/18 pupping season. However, to my knowledge, there is no evidence of decision-making in birthing site being influenced by weaning success in previous seasons in current literature. Current literature reports site fidelity in NZ fur seal females, where they annually return to the site where they previously gave birth.

The long term impact of the colony damage on the seal numbers may not be evident for another 5 – 6 pupping seasons. This is because many fur seal species are largely philopatric, meaning they return to their birth site to breed (Lento et al 1997), and females usually become productive at age 4 – 5 (Gentry 1987; McKenzie 2006). The potential decline in the rate of pup survival this year due to the habitat loss will influence the number of returning reproductive individuals once they reach their reproductive age.

Due to the significant decrease in habitat including the rockfaces which are crucial to pup survival during winter, the Ohau Point seal colony is likely to experience a population decline which may not recover to its previous level. The geographical Ohau Point ‘colony’ may be extended in the future to include the area north of the stream mouth, as increased pupping activity in the area is highly likely. However, this area does not appear to have as favourable pupping conditions as the pre-earthquake main colony, as it largely lacks in tidal pools (preferred for thermoregulation and crucial for pups to practice swimming) and  and boulders (for pups to hide under). Therefore, even if the seals from the main colony expand out to neighbouring areas, the total seal population of the Ohau Point area as a whole is highly unlikely to follow the pre-earthquake trend of annual increase.

 

 

References

Gentry, R. L. (1987, June). Status, Biology, and Ecology of Fur Seals. Proceedings.

Lento, G. M., Haddon, M., Chambers, G. K., & Baker, C. S. (1997). Genetic variation of southern hemisphere fur seals (Arctocephalus spp.): investigation of population structure and species identity. Journal of heredity88(3), 202-208.

Fea, N. I., Harcourt, R., & Lalas, C. (1999). Seasonal variation in the diet of New Zealand fur seals (Arctocephalus forsteri) at Otago Peninsula, New Zealand. Wildlife Research26(2), 147-160.

McKenzie, J. (2006). Population demographics of New Zealand fur seals (Arctocephalus forsteri).

Krajewski, J. P., & Sazima, I. (2010). Fish feed on faeces and vomits of New Zealand fur seals: marine mammals bring nutrients from open to inshore waters. Marine Biodiversity Records3, e81.

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