On a Sunday morning in March 2013, I spotted three dead California sea lion pups and two dying pups on the beach just south-east of the cliffs in Point Dume, Malibu. I called the Marine Mammal Care Center for help but they were overwhelmed and could not assist the pups. Two years later in April of 2015, I saw two dying sea lion pups just north-west of the cliffs. Someone called the CA Wildlife Center, and a representative came and put up signs that warned people to stay away and then left, saying there was no room for them.
During both years the National Oceanic and Atmospheric Administration (NOAA) declared an Unusual Mortality Event (UME) for California sea lions (Zalophus californianus). In 2013, more than 1600 California sea lions were stranded alive along the Southern California coastline and admitted to rehabilitation facilities in January, February, March, and April. Seeing some strandings is normal, particularly from mid-April to mid-May, when most pups are weaned and begin foraging on their own. Pups remain with their mothers for the first 10-11 months of their life and become independent around May of the year after their birth. This coincides with the peak upwelling period in the California Current System (CCS) when primary productivity is at its maximum. If the upwelling fails, we may witness an UME. That’s what happened in 2009—a huge mortality event took place between May and August, a period characterized by the strongest negative upwelling observed in 40 years, resulting in uncharacteristically warm sea surface temperatures.
But 2013 was unique because most of the strandings occurred in the first four months of the year and were located mainly in Southern California (see Figure 1).
This pattern not only repeated itself in 2015, but worsened. The strandings in 2015 occurred along the same stretch of coastline as they did in 2013, mostly from Santa Barbara through San Diego Counties. But the total strandings during the first five months of 2015 totaled 3340 (see Figure 2), more than doubling the 1262 strandings during the same period in 2013. Moreover, the strandings in 2015 was more than ten times the average stranding level for the same five-month period from 2004 to 2012.’
The UME status allowed for the establishment of a panel of experts to determine the cause of the mortality. What the investigation found in both years was “a change in the availability of sea lion prey, especially sardines, a high-value food source for nursing mothers. Sardine spawning grounds shifted further offshore in 2012 and 2013, and while other prey were available (market squid and rockfish), these may not have provided adequate nutrition in the milk of sea lion mothers supporting pups or for newly-weaned pups foraging on their own.”
Mark Lowry, a National Oceanic and Atmospheric Administration biologist, has collected sea lion scat from San Nicolas and San Clemente islands each season since 1981. Through the years, he has identified 133 different species, seven of which are commonly found in sea lions’ diets. The common prey are market squid and six kinds of fish—anchovies, sardines, two different kinds of mackerel, short belly rockfish, and pacific hake.
Of these forage, sardine and anchovy are fat- and calorie-rich while rockfish and squid are fat- and calorie-poor. Lowry, who has completed his analyses of 2013 samples as well as the 2015 winter and spring samples, said in February 2016 that in 2013 and 2015, anchovy and sardine pretty much dropped out of their diet. As a result, the sea lions’ diet became incredibly diversified. Evidence of common fish include market squid, some hake—though hake is also down from what they normally eat, and short belly rockfish. “But mostly they’re targeting a group of non-common squid and octopus and non-common fish,” Lowry said. The spring 2015 samples showed the highest levels of non-common cephalopod—squid and octopus—and non-common fish he had seen in a long time.
Of the 127 non-common species found in their diet, 40-60 different species were seen in samples taken from San Nicholas Island in 2015. At San Clemente Island, where Lowry collects fewer samples, 40-45 non-common species were found. Two kinds of mackerel, Jack and Pacific, have increased at San Clemente but not at San Nicholas Island.
In April 2015, Lowry told the Los Angeles Times that he had found “mystery stuff — gooey bits of substance you’d expect from a diet of jellyfish or tube worms.” Though yet to be verified, his analysis now shows they were pyrosomes—free-floating colonial tunicates which are closely related to salp. “That’s very unusual,” Lowry said. “It’s the first time I’ve ever seen those. What that says is that sea lions can’t find what they normally eat, so they’re eating whatever they come across.”
In a report published by Royal Society Open Science, Sam McClatchie, Supervisory Oceanographer for Southwest Fisheries Science Center, predicted the temporal sequence of pup weights between 2004 and 2013 based on two variables: the relative abundance of sardine and anchovy, and the relative abundances of rockfish and squid. When rockfish and squid are more abundant than sardine and anchovy, sea lion mortality increases. The six most common items in their diet are shown below with their corresponding calorie and fat content.
Sharon Melin, a wildlife biologist with the NOAA Fisheries National Marine Mammal Laboratory, tracks sea lion pups’ weight gain and growth, and correlates them with inferred diet of mothers. From an analysis of scats from females collected from breeding sites between June and September over eight years, she defined four different diet types and what years the types were dominant.
- Diet 1: low percentage of market squid and a high percentage of Pacific sardine (2002, 2005).
- Diet 2: dominated by market squid and Pacific hake (2000, 2001, 2010, 2011).
- Diet 3: comprised mostly of northern anchovy and Pacific sardine (2003).
- Diet 4: dominated by market squid and rockfish (2009, 2012).
Average pup weights tended to be heavier in years represented by Diets 1 and 3, average in years with Diet 2, and the lightest pups occurred in 2009 and 2012 with Diet 4.
During three weeks of fieldwork on San Miguel Island in April 2015, Melin observed the sea lions traveling farther and diving deeper to find food. That’s generally not a problem for males since they can go wherever the food is. After the breeding season, some males migrate north as far as Alaska. But a mother is tied to the rookery island until her pup is weaned almost a year later. Females generally remain within 90 miles (150 km) of the breeding rookeries.
California sea lion females give birth to a single pup between May and June in large rookeries on offshore islands along California and Baja Mexico. The main US breeding rookeries are located on the Channel Islands and California sea lion pups are born on the islands of San Miguel, San Clemente, San Nicholas and Santa Barbara. Once a mother begins foraging for food, a sea lion pup will usually nurse, on average, every third day. While she’s away collecting food, she produces milk while her pup waits on shore and fasts. When she returns, she locates her pups with a “pup attraction call,” which is established shortly after birth when the mother and pup call to one another and the pup imprints on its mother’s distinct call. They also recognize each other by scent.
Lactating female California sea lions consume approximately 11% of their body mass in food per day. In addition, because lactating females are usually also pregnant during nine months of the 11-month lactation period, a diet that is insufficient to support both lactation and gestation may result in the resorption of the fetus or a premature birth.
In December 2014, Melin’s team satellite tagged twelve adult female-pup pairs. What they found was while many stayed around the island through much of December, by January they started seeing females taking off and not returning, which was indicative of their pups dying. So out of the original twelve, by March only six were still coming to San Miguel Island, and they were consistently diving to 400, 500, 600 meters.
This diving requires extraordinary physiology. To dive this deep, sea lions’ lungs collapse at about 225 meters down and then re-expand at the same depth upon ascent. This technique not only staves off decompression sickness, by keeping nitrogen out of the bloodstream, but also reduces the amount of oxygen delivered from their lungs to their bloodstream—preserving the oxygen within the sea lion’s upper airways. When they head back to the surface, the preserved oxygen re-expands into the lungs and prevents the sea lion from blacking out in the shallows.
“To have half of our sample diving into those kind of depths is unusual,” Melin said in a NOAA Fisheries podcast last April by which time only four females-pup pairs remained. “These females that were doing this deep diving, they were going very deep and they were going out into very deep water. They were also the four females whose pups are still alive, and we were able to capture their pups. And although their pups were not giant, healthy pups, they were alive.”
Lowry’s 2015 winter and spring findings concur. Short belly rockfish, one of the common fish showing up in his samples, are generally found deeper in the water column than anchovies, sardines, and mackerel. Furthermore, a lot of the non-common fish that were found live on or near the bottom of the ocean, like flat fish and different species of rock fish, as well as cusk eels—a group of marine bony fishes.
A downward trend for sardine abundance has been going on for a decade, and over a large area (five degrees of latitude). McClatchie’s report, which uses data from the Southwest Fisheries Science Center’s Rockfish Recruitment and Ecosystem Assessment Survey (RREAS), shows that during the decade from 2004 to 2014, market squid and rockfish increased, while sardine and anchovy fell to very low numbers off central and southern California, and around San Miguel Island in northern California where sea lions breed.
According to the Pacific Fishery Management Council (PFMC), which develops regulations for fisheries in the U.S. off the West Coast, the sardine biomass (the estimated weight of a stock of fish) reached a peak of 1.3 million metric tons in 2006, then dropped to an estimated 97,000 metric tons in 2015, a decline of more than 90% and significantly below the 150,000 metric ton threshold for this directed fishery. In April of 2015, the Pacific Fishery Management Council announced the closure of the 2015-16 U.S. Pacific sardine fishery, beginning July 1.
Sardine populations rise and fall naturally, cycling as ocean temperatures shift. But, says Tim Essington, a University of Washington professor of aquatic and fishery sciences, “Fishing makes the troughs deeper.” In a paper published in March, Essington showed that overfishing worsens the magnitude and frequency of the cyclical declines of sardines and other forage fish, such as anchovies.
His March 2015 study showed for the first time that fishing likely worsens population collapses in species of forage fish, including herring, anchovies and sardines. To prevent complete collapse, the study recommended the use of risk-based management tools that would track a fishery’s numbers and halt fishing when they dipped too low. This strategy has the potential to not only ensure more fish for the sea lions, but long term stability in the fishing industry. Using simulations of this management strategy, researchers determined that by suspending fishing when a population falls below half of its long-term average, 64 percent of collapses could be prevented and average catch would be reduced by only two percent in the long term. Based on scientific recommendations by the Lenfest Forage Fish Task Force, Oceana suggests increasing the threshold for pacific sardines from 150,000 metric tons to at least 640,000 metric tons.
At the April 2015 PFMC meeting, assessment author Dr. Kevin Hill presented a Southwest Fisheries Science Center analysis of what the sardine population might look like in the absence of fishing (see Figure 3). While it is clear that, with the lack of recruitment, the population would have declined greatly even in the absence of fishing, Hill’s analysis showed the population would have been four times higher in 2015 without fishing; that is, approximately 400,000 metric tons (purple line, ‘no fishing’) versus the current estimated 96,688 metric tons (blue line ‘2015 update’). Moreover, sardine harvests exceeded Maximum Sustainable Yield levels during the decline.
The U.S. population of California sea lions is currently estimated to be 300,000 animals, all on the Pacific coast. From an estimated population of about 10,000 animals in the 1950s, their numbers have grown rapidly since 1972 when the Marine Mammal Protection Act was implemented. Lowry, who also conducts annual aerial surveys of California’s pinniped populations, told the LA Times that the sea lion population is increasing at a rate of about 5.1% per year.
Unfortunately, we now have a large sea lion population coupled with a decline in food quality near breeding colonies. Until high-quality food increases again in the Southern California Bight, pup emaciation may become the norm.
Originally published in Catalina Marine Society’s OceanBights, p. 4
Melin SR, et al, Unprecedented mortality of California sea lion pups associated with anomalous oceanographic conditions along the central California coast in 2009, California Cooperative Oceanic Fisheries Investigations Reports. 51: 182-194.
NOAA Fisheries, West Coast Region, 2015 Elevated California Sea Lions Strandings in California: FAQs
Mark S. Lowry, Research Biologist, Marine Mammal and Turtle Division, Interview February 22, 2016
Louis Sahagun, Scat may contain clues to marine mammals’ Southern California deaths, Los Angeles Times, May 13, 2016
Sam McClatchie et al, Food limitation of sea lion pups and the decline of forage off central and southern California, Royal Society open science, DOI: 10.1098/rsos.150628. March 2, 2016
NOAA Fishwatch, Pacific Mackeral
Sharon Melin et al,California sea lions: An indicator for integrated ecosystem assessment of the California current system, California Cooperative Oceanic Fisheries Investigations Reports. 53: 140-152. 2012
NOAA Fisheries, Transcript: Sea Lion Strandings – The View from the Rookery, April 13, 2015
Rich Press, NOAA Fisheries Science Writer, NOAA Fisheries, Transcript: Sea Lion Strandings – The View from the Rookery, April 30, 2015
Birgitte I. McDonald, Paul J. Ponganis, Lung collapse in the diving sea lion: hold the nitrogen and save the oxygen, Biol. Lett. 2012 8 1047-1049; DOI: 10.1098/rsbl.2012.0743. November 12, 2012
Pacific Fishery Management Council, Council Votes to Close 2015-2016 Pacific Sardine Fishery, April 13th, 2015
T.E. Essington et al, Fishing amplifies forage fish population collapses. Proceedings of the National Academy of Sciences of the USA. 2015;10.1073/pnas. April 6, 2015
Lenfest Forage Fish Task Force, Little Fish, Big Impact, April 2012
Oceana, Letter to Ms. Eileen Sobeck, Assistant Administrator for Fisheries, May 21, 2015, Agenda Item B.1.b Supplemental Open Public Comment 2
Washington Department of Fish and Wildlife, Conservation, California Sea Lion Fact Sheet
Emaciated California sea lion pup, D. Gordon Kelly, Westward Beach, Point Dume, Malibu, April 12, 2015
Figure 1, Annual California Sea Lion Strandings: NOAA Fisheries, West Coast Region, 2015 Elevated California Sea Lions Strandings in California: FAQs, Figure 1
Figure 2, California Sea Lion Pup and Yearling Strandings: NOAA Fisheries, 2013-2016 California Sea Lion Unusual Mortality Event in California, California Sea Lion Strandings
Figure 3, Sardine Stock Biomass: K.T. Hill et al, NOAA Fisheries, SWFSC-FRD, Assessment of the Pacific Sardine Resource in 2015 for USA Management in 2015-16, Fishery Impacts, Slide 12