Feast, Famine & the Maternal Instinct

Is maternal attendance a reliable indicator of environmental change?

For young sea lions, survival is an early lesson in life. Long before reaching their formidable adult size and assuming their position as apex predators atop the marine food web, young sea lions must first endure the critical first few months of life. Wholly dependent on mother’s milk, they are left unattended on shore while the mother forages for food. Under normal ocean conditions, prey are abundant and foraging trips are short. But in times of famine, a prolonged foraging trip by the mother can mean starvation for her pup.

In the case of South American sea lions (Otaria flavescens) along the coast of Peru, ocean conditions are occasionally complicated by El Niño, a cyclical variation in ocean climate that can profoundly affect stocks of their principal prey, Peruvian anchovy. Despite significant research into their causes and effects, El Niño and its sister event La Niña remain poorly understood and unpredictable.

With a view to determining whether physical changes in ocean climate are reflected in the diet and maternal behavior of South American sea lions, a group of researchers observed nursing sea lions on Peru’s Ballestas Islands over five consecutive breeding seasons between 1998 and 2002. The Ballestas Islands are home to one of Peru’s largest breeding concentrations of South American sea lions, and the study spanned two significant events in ocean climate: the strong El Niño of 1997-98 and the moderate La Niña that followed from 1999-2001.

Over the course of the study, researchers Karim H. Soto, of the University of British Columbia (UBC) and Instituto del Mar del Peru (IMARPE), Dr. Andrew W. Trites (UBC) and Milena-Arias-Schreiber (IMARPE) monitored the movements of marked nursing females to and from the rookery. They also analyzed fecal (scat) samples to determine the prey base of sea lions in the area, and assessed sea-surface temperature and fisheries data obtained from IMARPE.

Maternal attendance

“The strong 1997 to 1998 El Niño resulted in increased surface temperatures and a deepening of the thermocline, which drastically reduced primary marine productivity [plankton],” the authors write. “This in turn affected the distribution and abundance of anchovy and other potential prey for sea lions. In contrast, the moderate 1999 to 2001 La Niña was characterized by cold surface temperatures, shallow thermoclines and high marine productivity, which resulted in favorable prey conditions for the sea lions.”

According to the researchers, there appeared to be a strong relationship between diet, maternal attendance and the availability of prey for South American sea lions in the Peruvian marine ecosystem.

“Acute prey shortage during El Niño resulted in females increasing the length of their foraging trips and decreasing the time they spent onshore with their pups,” they write. “In contrast, shorter times at sea and longer times onshore were observed during the favorable conditions of La Niña when preferred prey (anchovy and squat lobster) were more abundant near the rookeries.”

In most cases, maternal attendance appeared flexible enough to accommodate changing ocean conditions. But during the strong El Niño of 1997-1998, which saw a severe prey shortage, the researchers observed an increase in pup mortality when females spent more time at sea searching for prey and did not return frequently enough to nurse their pups.

Careful analysis of numerous scat samples over the study period revealed that sea lions principally consumed anchovy and squat lobster during the 1999-2001 La Niña event. When these prey were scarce, however, as they were during the El Niño-affected years of 1998 and 2002, the sea lions fed on a variety of prey and may have foraged at deeper depths than usual. Based on this finding, the researchers say, sea lion diet may be used as a reliable indicator of the relative distribution and abundance of marine resources such as anchovy.

There is no doubt that the fluctuations in ocean climate brought about by El Niño and La Niña are extreme when compared to those in other ecosystems, such as the North Pacific Ocean. But understanding the behavioral response of sea lions to these changes can help scientists to understand how sea lions have evolved to cope with rapid changes in the ocean around them. This information could prove invaluable in connecting the complex strands of the marine food web, and in understanding how the ever-changing ocean can shape the life history of its apex predators.

14 June 2006

Publication:
Changes in diet and maternal attendance of a South American sea lions indicate changes in the marine environment and the abundance of prey. Soto, K., A.W. Trites, and M. Arias-Schreiber. 2006. Marine Ecology Progress Series 312:277-290. abstract Behavioural observations were made of South American sea lions Otaria flavescens in Peru to determine whether changes in their diet and maternal attendance patterns reflected physical changes in the marine environment and alterations in the abundance and distribution of prey. The study was conducted during the breeding season between 1998 and 2002, which was a period that encompassed a strong El Niño (1997–1998) and a moderate La Niña (1999–2001). Observations revealed strong linkages between maternal attendance patterns and the abundance of prey and oceanographic features close to the rookeries. Acute prey shortage during El Niño resulted in females increasing the length of their foraging trips and decreasing the time they spent onshore with their pups. In contrast, shorter times at sea and longer times onshore were observed during the favourable conditions of La Niña when preferred prey (anchovy and squat lobster) were more abundant near the rookeries. Pup mortalities increased when females spent more time at sea searching for prey and did not return frequently enough to nurse their pups. A larger diversity of prey species (particularly of demersal fishes) was consumed during El Niño when anchovy and lobster were less available. Females appeared to adjust their diets and maternal attendance patterns in response to annual changes in the abundance and distribution of prey. These observations suggested that diet and maternal responses reflect interannual fluctuations of the unpredictable Peruvian upwelling ecosystem, and implied that South American sea lions may be good indicators of relative changes in the distribution and abundance of marine resources.