March 15, 2017
You are what you eat. That’s the basic philosophy behind monitoring the diets of wild animals. In the case of Steller sea lions, we can’t see what they are eating since they feed at-sea, so we have to investigate their diet in other ways. The most common way to find out what Steller sea lions are eating is to — wait for it — collect and analyze their feces. That’s right, we go onshore to sea lions sites, where they have hauled out and pick up after them. Here’s a picture of a group of us ready to collect scat.
Collecting scat is just like cleaning up after your dog, if your dog weighed over 2,000 pounds! We use gallon size bags for samples. A good collection of many scat samples from one site might weigh 60 lbs.
What can we learn about their diet from analyzing their scat? Well, quite a bit. Sea lions can swallow many fish, squid and octopus whole. Fish bones and squid and octopus beaks from their mouth parts, gradually make their way through the sea lion’s digestive tract. We can isolate these hard parts in the scat to identify individual prey species from just a few bones.
In the lab, the scats are washed in sieves and the hard debris left behind is cleaned and studied further under a microscope. Each solid bit is compared to other parts kept in our reference collection of known sea lion prey species. As you can imagine, it takes a lot of skill and time to make identifications from small fragments.
Aside from cataloging hard parts, we can also analyze prey DNA found in the soft parts of the scat. This technique is especially good at detecting prey that don’t have very many hard parts (invertebrates like squid and octopus), fish that are very large and may not always be consumed whole (like Pacific cod), and fish that have small and fragile bones (like smooth lumpsucker) that may not make it through the digestive track undamaged. But DNA studies are expensive, so, we use this technique to supplement the traditional studies of hard parts, such as a recent description of the late winter diet of sea lions in the Aleutians.
Collecting scat is just like cleaning up after your dog, if your dog weighed over 2,000 pounds!
Steller sea lions target fish that aggregate near the bottom of the ocean or in the middle the water column. While sea lions can dive a lot deeper than humans, they aren’t considered deep divers compared to other marine mammal species. Most of the prey they target live on the continental shelf in waters less than 650 feet deep — that’s still a little deeper than the Space Needle in Seattle is tall.
Steller sea lions in the Aleutian Islands have a diverse diet. In the Aleutians, there are almost 300 species of fish alone, plus dozens of species of squid and octopus. Steller sea lions consume about 50 different species, usually whatever is the most abundant fish in the area.
Here are some of the most common prey items we see in the Aleutian Islands:
Atka mackerel is one of the most abundant yearlong resident fish in the Aleutians Islands, and it also sustains the largest commercial fishery in the region. During the summer, Steller sea lions feed on enough Atka mackerel to make up more than half of their diet. In winter, the fish make up about a quarter of their diet.
Like Atka mackerel, Pacific cod are year round residents and since they grow up to 43 inches long and weigh up to 37 pounds they are probably the biggest fish that sea lions eat in the Aleutian Islands. Pacific cod are consumed more often in winter than summer. In winter, Pacific cod make up about one quarter of the sea lion diet.
Sculpins are bottom-dwelling (demersal) fish that are widely distributed on the continental shelf. Steller sea lions eat mostly red Irish lords based on our DNA analyses. They eat sculpins mostly in winter, when they make up about 15 percent of their diet.
Salmon aren’t considered year round residents of the Aleutian Islands. Steller sea lions eat mostly pink and sockeye salmon based on our DNA analyses, and they eat salmon mostly in summer.
These funny little guys are an enigma. Not much is known about their abundance and distribution, but Steller sea lions eat them far more often in winter (10 percent of their diet), when lumpsuckers aggregate to spawn, than in summer. Lumpsuckers are named for their pelvic fins that have evolved to form an adhesive disk that enables them to latch onto rocks in areas with a lot of current, like the Aleutian Islands.
Squid and octupuses:
Cephalopods are a group of invertebrates that includes both squid and octopus. Based on DNA evidence in scat, we know Steller sea lions more commonly prey on giant Pacific octopus. Cephalopods make up about 10 percent of the sea lion diet in winter.
I have been studying Steller sea lions since 1990 with NOAA Fisheries Alaska Fisheries Science Center in Seattle. My primary research interests are sea lion population dynamics, demographics, and interactions with commercial fisheries. I’ve also worked on fish during my career with NOAA, particularly species eaten by sea lions, like Atka mackerel, walleye pollock (you may know them as fish sticks and imitation “krab”), and Pacific cod. I graduated from Bucknell University (B.A. Biology, 1976) and College of William and Mary (M.S. Marine Science, 1982), and started my science career in 1982 at Rutgers University as a Research Associate. At Rutgers, I worked at the Haskin Shellfish Research Laboratory in Bivalve, NJ (down the road from Shellpile… you can’t make this up) studying the shells of mollusks living in habitats ranging from freshwater lakes and streams to deep-sea hydrothermal vents. I even had the opportunity to go down in the Alvin submersible!
*Images of Aleutian Island sea lion prey were borrowed from other NOAA Fisheries programs.