Storm Predator Version 3 Serial Number
Brandi Baylon
Coastal storms have increased in recent decades, affecting many species, including the South American sea lion (Otaria byronia). Reports of stranded sea lion pups are becoming common in Chile, presumably due to the increase in the frequency and intensity of coastal storms. To validate this assumption, a 10-year database was built by coupling wave generation and coastal propagation models to correlate pure wave parameters (significant wave height Hs, peak period Tp, normalized wave power Hs2 Tp) and wave parameters including the tidal level (maximum surface elevation η, modified wave power η2 Tp) with records of stranded pups in Cobquecura, the largest breeding colony in central Chile. The correlation between the number of pups stranded per day and wave parameters in the first half of January and the last half of February is poor, while they are stronger for the second half of January and the first half of February. The higher number of stranded pups coincide with coastal storms with normalized wave power values exceeding a threshold of 100 m2/s. Conversely, below this threshold there is wide dispersion between the number of strandings and wave parameters. Identifying wave parameter thresholds could be used to predict when newborn pups will be most affected by coastal storms, and thus help institutions to develop remediation techniques for animals at risk.
Most studies on climate change in marine environments have focused on the rise in temperature and changes in the availability of resources10,11. Other effects, such as the occurrence of coastal storms, have been overlooked, even though extreme events are expected to become more common over time, as they are associated with climate variability and global environment change12. A recent study showed that maximum monthly wave heights and the number of extreme events have both increased in central Chile in the last 60 years13. The majority of these events occur during the austral winter, but they sometimes occur in summer as a consequence of long swells generated in the north Pacific13. In general, the main focus of studies that have evaluated the impacts of coastal storms has been damage to infrastructure and urban areas14. The impacts on marine fauna that share coastal habitats with humans have received much less attention. In this context, it is relevant to know how extreme events affect coastal populations, and whether species can adapt to these changes15.
Iconic apex marine predators such as seabirds and marine mammals could be severely affected by variations and changes in the marine environment8. Storms affect marine mega fauna in different ways, such as habitat destruction and increased risk of stranding, with inshore species being particularly vulnerable16. Studies of shorebirds have demonstrated that increased storm intensity and frequency reduces the availability of habitats for nets establishment17. For pinnipeds (seals, sea lions and walrus), site selection is based on the proximity to favorable foraging areas and on the availability of terrain that allows access and egress during variable tidal heights18. For breeding colonies in particular, protection from wind and waves also plays a critical role in site selection, as pups have a better chance of survival in sheltered sites with favorable thermal protection19. Indeed, significant neonate mortality may result from unusual fluctuations in tidal height and storm surges because pups under three months of age cannot swim competently20. In Australia, Arnould & Littnan21 and Pemberton & Gales22 have reported that a considerable number of Australian fur seal (Arctocephalus pusillus doriferus) pups are left stranded when they are washed into the ocean by high swells and storms. However, to our knowledge no studies have monitored the effects of different intensities of coastal storms on pinniped species, with the identification of a threshold over which there are predictable effects on individuals and populations.
Considering an increase in the intensity of coastal storms in recent years, in this study we hypothesize that the number of stranded sea lion pups can be predicted by statistical wave parameters. To test this hypothesis, we compared the statistical parameters of coastal storms with the number of stranded SASL pups during the last decade in the largest breeding colony in central Chile.
A total of 541 stranded alive pups (i.e. pups swept away from the rookery and washed up on the beach), males and females, were counted and measured over a 10-year period (Table 1), most of them were registered between mid-January and mid-February (Fig. 1A). In general, the number of stranded pups increased over the 10-year period by around 10 pups per year (Fig. 1B). The percentage of total pups born in the colony that were stranded ranged from 0.7% (2013) to 11.6% (2018) (Table 1). Notably, the number of stranded pups represented more than 10% of those born in 2011, 2017 and 2018.
The impact of climate change on terrestrial mammals has been extensively examined in the literature. However, how marine mammals react to climate change, and specifically to the increase in the occurrence and intensity of coastal storms, have been seldom analyzed. The evidence indicates that rare extreme climatic events, like coastal storms, have become much more frequent and intense in the last decade, especially during the summer when SASL females give birth to their pups. Although different studies have noted that coastal storms affect pinniped pup survival22,31, to our knowledge this is the first study that assesses the effects of wave parameters on such phenomena. We discuss here how the increase in the number of pup strandings may affect population growth and the responses of SASL, and how this should be considered in management policies concerning this species in Chile. Although this study focuses on one species and one study area, climate change over the next 100 years will lead to increases in sea levels and the severity of storm surges in many places30, which in turn may affect different pinniped species around the world.
In general, the correlation between wave parameters and the number of pups stranded is notably stronger for the second half of January and the first half of February. This positive relationship may be because the number of SASL pups born in a colony increases as the month of January progresses, reaching a peak in the second half of January and the first half of February28. Thus, the probability of a pup being swept away from the rookery and washed up on the beach increases. As well, SASL mothers spend less time on shore as pup get older32,33,34, so pups are left alone for longer periods, making them more vulnerable to coastal storms. The low correlation between wave parameters and the number of stranded pups in the first fortnight of January may be because the number of newborn pups in the colony during this period is still very low. Similarly, the lack of any relationship between wave parameters and the number of stranded pups in the last two weeks of February could be because pups are bigger and heavier, and thus able to swim back to the colony by themselves29, which avoids animals being washed up on the adjacent beach.
Our results show that the number of pups stranded on the beach adjacent to the colony was especially high in certain years, exceeding 10% of total pup abundance in the colony in those years. Given that female SASL give birth to a single pup each year, a higher frequency of coastal storms that coincide with the breeding season can dramatically affect pups30, and ultimately may have a serious impact on the population. This in turn can impact juvenile recruitment and reduce the likelihood of recovery between events, thereby reducing reproductive output and ultimately causing population decline15,30. Long-term abundance monitoring, together with the estimation of demographic parameters of this population, such as age/class mortality and fecundity, provides critical information for SASL conservation and management. However, there presently are insufficient demographic data to reliably estimate the effects of pup mortality due to storm frequency and intensity on the SASL population dynamics in Cobquecura. We recommend that future studies focus on determining age-structured dynamics of the SASL population at Cobquecura in order to put the impact of pup strandings into context.
In the face of the increasing severity of storm surges, individuals in colonies may respond by either: (1) continuing to breed in areas that are becoming increasingly inundated by coastal storms, potentially causing higher pup mortality rates and affecting the colony's long-term demographics; (2) retreating to areas that are higher or further from the coast to avoid waves, leading to higher operational breeding densities; and (3) moving to other colonies or colonizing new areas. Many pinniped species, including the SASL, display a high degree of natal site philopatry36,37,38. The Cobquecura and other colonies in Chile have been used consistently as they afford the best habitats for pup survival, providing access to water for thermoregulatory requirements and close proximity to suitable feeding areas. These attributes suggest that SASL moving to other colonies will not happen easily or rapidly.
Moving to other colonies or colonizing new areas is another possibility for the species in order to reduce pup mortality. Grandi et al.37 found a proliferation of new SASL colonies and the transformation of some haul-outs to breeding sites over time in northern Patagonia, Argentina, as a response to population growth. A similar expansion to new breeding colonies has been reported for other otariid pinnipeds, such as Subantarctic fur seals (Arctocephalus tropicalis) [39], Antarctic fur seals (A. gazella) [39], and Steller sea lions (Eumetopias jubatus)40. However, dispersal to other established sites or the establishment of new breeding colonies can in turn lead to an increase in colony densities, which is associated with increased pup mortality41. SASL expand to other colonies that are close to the focal one37. In this sense, if the new area does not offer much protection against coastal storms, moving to other colonies nearby may not be an improvement.
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