South American Sea Lions

A steady source of food and a stable living environment frequently determine whether or not a population thrives. In the case of South American sea lions Otaria flavescens, however, an unpredictable food supply driven by an unpredictable marine ecosystem is simply a fact of life. In order to understand how these populations respond to food shortages, a team of researchers recently examined the effects of a scarce food supply on female sea lions and their pups.

Researchers investigated this relationship in South American sea lions on Peru’s Ballestas Islands between 1997 and 2002, a period that encompassed the strongest El Niño on record and a La Niña event. Results of the study, which took place over six breeding seasons on one of Peru’s largest reproductive colonies, were recently reported by Karim H. Soto of UBC and the Instituto del Mar del Peru (IMARPE), Dr. Andrew Trites of UBC, and Milena Arias-Schreiber of IMARPE.

Upwelling along the Peruvian coastline supports a highly productive marine ecosystem with large anchovy, seabird and pinniped populations, but this productivity varies from year to year. The El Niño event of 1997-1998 produced anchovy Engraulis ringes of poor quality, in low numbers, and at deeper depths than normal. The scarcity of their most important prey meant less food for South American sea lions. In contrast, the La Niña event from late 1998 to 2001 supported healthy anchovy stocks and ensured ample food.

The 1997 El Niño had severe short-term effects on South American sea lion populations. All pups born during the height of the event died from starvation because lactating females spent too long at sea foraging for scant prey and did not return frequently enough to nurse their pups. The plummeting anchovy stocks may have also prompted physiological changes in the sea lions; birth rates dropped and abortion rates increased as food became scarce.

Despite an increase in anchovy stocks during the La Niña that followed, there was also a high number of pup deaths during the 1999-2000 breeding seasons that suggests that pup survival depends on more than just the availability of food. Notably, fewer than two-thirds of adult males survived the previous El Niño. The conspicuous absence of these dominant males during the subsequent La Niña meant there were few males to defend the breeding beaches. This resulted in high numbers of pups being killed by aggressive interactions with subadult males. In addition, a number of females inexplicably took and killed pups that were not their own. This abnormal behaviour started in 1999, and may have been influenced by a low pregnancy rate and low incidences of successful pupping immediately after El Niño. Although not all La Niña years had consistent environmental conditions, researchers noted a doubling in the number of pups born in 2002 compared to 1999, with births occurring later in the seasons that followed famine years, and earlier in the seasons that followed abundant years.

While it may be no surprise that more sea lion pups survived when more food was available, and vice versa, the study produced valuable new information about how female sea lions survive and recover from unexpected food shortages. But this research also carries a warning: the stronger and more frequent El Niño events that are predicted by global warning models could offer the sea lions little chance of recovery between future cycles. The long term survival of South American sea lions is still very much in question.

Publication:
The effects of prey availability on pup mortality and the timing of birth of South American sea lions (Otaria flavescens) in Peru. Soto, K., A.W. Trites, and M. Arias-Schreiber. 2004. Journal of Zoology 264:419-428. abstract Pup mortality and the timing of birth of South American sea lions Otaria flavescens were investigated to determine the possible relationship between fluctuations in prey availability in the Peruvian upwelling ecosystem and current and future reproductive success of sea lions during six consecutive breeding seasons. Our study from 1997 to 2002 encompassed the strongest El Nino on record and one La Nina event. Pup mortality ranged from 13% before El Nino to 100% during El Nino, and was negatively correlated with prey availability. Abortions were also more frequent when prey availability was low. However, pup mortality remained high following El Ni~no due to the punctuated short-term effects it had on population dynamics and subsequent maternal behaviour. Births occurred later in the season after years of low food availability and earlier following years of high food availability. The peak of pupping coincided with the peak of mortality in all years, and may have ! been the product of intensive competition between bulls at the peak of the breeding season. The stronger and more frequent El Ninos that appear to be occurring along the Peruvian coast may produce significant stochastic changes in future births and pup mortality, which may place the vulnerable South American sea lion population in Peru at greater risk.