Tag Archives: Sharks in Alaska

Pacific Sleeper Shark (Somniosus pacificus)

January 31, 2021Fish, Kodiak WildlifeKodiak Wildlife, Pacific Sleeper Shark, Robin Barefield, Sharks, Sharks in Alaska, Somniosus pacificus, Steller Sea Lionsadmin

The reclusive, deep-dwelling Pacific sleeper shark (Somniosus pacificus) remains an enigma to biologists. Once believed to be sluggish bottom-dwellers that scavenged or fed on small fish and slow-moving prey, researchers now think Pacific sleeper sharks play a pivotal role in the North Pacific’s food web. Biologist Lee Hubert with the Alaska Department of Fish and Game studies Pacific sleepers, and he has learned the sharks spend little time on the bottom but instead move continually through all depths and are stealth predators of fast-moving prey.

I can understand how Pacific sleepers earned their reputation as sluggish sharks. We occasionally catch immature Pacific sleeper sharks when we halibut fish, and when reeled up to the boat, they look dead and move very little. I was surprised to hear these sharks are voracious predators. This shark’s ability to remain still, though, is one of the reasons it is such a successful predator. When it glides through the water, barely moving its body, it minimizes hydrodynamic noise, allowing it to elude acoustic detection by its prey.

Pacific sleeper sharks dive to depths exceeding 6500 ft. (1981 m). They typically remain deep during the day and then move to the surface at night, where they feed under cover of darkness. These sharks probably have poor eyesight, but they are extremely sensitive to electromagnetic fields. They can detect even minute electrical signals, such as the beating of an animal’s heart or its diaphragm’s movement. The shark does not need vision to detect these signals and attack its prey. Its dark grey body and stealth movements make it an efficient predator under the cloak of darkness.

The mouth of a Pacific sleeper shark is large and acts as a vacuum to inhale prey. Fish, such as salmon and cod, can be swallowed whole, but the shark uses its teeth to aid in eating larger prey items. Its upper jaw has small, sharp conical teeth used to hold the prey, while the teeth in the lower jaw interlock, forming a serrated blade used for slicing. A Pacific sleeper shark’s bite resembles the shape of a three-quarter moon.

Because they make little noise when traveling, a sleeper shark attacks with little warning. It might slowly swim up underneath a seal resting on the surface and attack the seal’s midsection, inflicting a fatal wound. Researchers know Pacific sleeper sharks eat fish, squid, octopuses, and marine mammals, but they are still trying to discern how much impact these sharks have on the their ecosystem. The number of Pacific sleeper sharks has increased dramatically in the North Pacific since the 1980s. Because they live very deep much of the time, it is difficult for biologists to estimate their population size. Still, in many areas where commercial fishermen caught few sleeper sharks in the 1970s, they now catch many.

Investigators are particularly interested to learn how many marine mammals Pacific sleeper sharks kill and eat. Pacific sleepers can grow to twenty feet (6.1 m) in length and weigh more than 8000 lbs. (3600 kg). They grow nearly as large as an adult orca, and recent evidence suggests these sharks might eat endangered Steller sea lions, especially sea lion pups.

In a 2014 study, biologists inserted “life-history transmitters” into the abdomens of 36 juvenile Steller sea lions.These transmitters record temperature, light, and other properties during the sea lions’ lives. When a sea lion dies, the tags either float to the surface or fall out onshore and transmit the data by satellite to researchers. After 17 of the original 36 tagged sea lions died, researchers noted that 15 of the transmitters indicated that predators killed the sea lions. Usually, when a predator kills a sea lion, the tag is ripped from the body and floats to the surface, recording a rapid temperature change and exposure to light. Three of the transmitters relayed data that suggested a very different type of predator, though. They recorded an abrupt drop in temperature, but they did not float to the surface and sense light, indicating that tissue surrounded them. The apparent explanation is that a cold-blooded animal, such as a shark, had eaten them. Other than sleeper sharks, great white and salmon sharks are the only other candidates living in the area near where the sea lions died. But both great white sharks and salmon sharks have counter-current heat exchangers in their bodies, giving them higher body temperatures than those recorded. Biologists think a Pacific sleeper shark is the only predator in the area that is large enough to eat a sea lion and has a body temperature as low as those recorded.

Pacific sleeper sharks live in polar and sub-polar regions year-round. They range from Baja California north to the Bering, Chukchi, and Beaufort Seas and the Okhotsk Sea off Japan. Biologists know little about Pacific sleeper shark reproduction and only recently learned they give birth to live young. Their social structure is also unknown, but researchers have photographed them feeding together in large numbers on whale carcasses.

Pacific sleeper sharks probably have a lifespan of more than forty years. Their tissue is toxic to humans and believed to be toxic to many other animals, so they have few natural predators except perhaps for other sharks.

Robin Barefield is the author of four Alaska wilderness mystery novels, Big Game, Murder Over Kodiak, and The Fisherman’s Daughter, and Karluk Bones. Also, sign up below to subscribe to her free, monthly newsletter on true murder and mystery in Alaska, and listen to her podcast, Murder and Mystery in the Last Frontier.

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Salmon Shark (Lamna ditropis)

January 12, 2021Fish, Kodiak Wildlife, UncategorizedAlaska wildlife, Lamna ditropis, Lamnid Sharks, Robin Barefield, Salmon Shark, Sharks in Alaskaadmin

I am always thrilled when I see the dorsal fin of a salmon shark protruding from the water as it swims near the surface. I love seeing any apex predator, but sharks conjure an air of mystery and fear. I wonder if the shark is chasing prey or if it is just watching and waiting for a fish to make the fatal mistake of swimming into its strike range.

Salmon sharks (Lamna ditropis) are some of the fastest fish in the ocean, and their high metabolism makes them voracious eaters. Salmon sharks are closely related to great white sharks, makos, and porbeagle sharks. Because their body shape so closely resembles a great white shark’s shape, people sometimes mistake salmon sharks as juvenile great whites.

Like other species of lamnids, salmon sharks have a conical snout, dark, round eyes, and a keeled, lunate tail. The salmon shark and porbeagle shark can be distinguished from great whites and makos by their smaller secondary caudal keel below the primary keel at the base of the tail. While the porbeagle shark inhabits the Atlantic and Southern Pacific, the salmon shark lives in the North Pacific.

A salmon shark has a bluish-black to dusky gray back, fading to white on the stomach. It has long gill slits and large teeth. Salmon sharks can grow to over 10 ft. (3 m) in length, but they average 6.5 to 8 ft. (1.9 – 2.4 m). They can weigh more than 660 lbs. (300 kg). Females grow larger than males.

Like other lamnid sharks, salmon sharks manage to sustain elevated body temperatures, even in the cold North Pacific. Their core body temperature measures approximately 80°F (26.7°C). They maintain this warm body temperature because they have a counter-current heat exchanger of blood vessels, directing heated blood through their core and dark musculature. This elevated body temperature permits the shark to live and hunt in a wide range of depths and water temperatures. The warm blood flow allows their brain, eyes, and muscles to function at peak performance.

A salmon shark is a big marine animal with no fur or blubber to keep it warm. To maintain its body heat, it must consume a large amount of food each day. Like a great white shark or a mako, a salmon shark aggressively chases its prey and sometimes even explosively breaches out of the water while in pursuit. Salmon sharks feed on fish, squid, other sharks, seals, sea otters, and marine birds. A study done in 1998 determined that salmon sharks consumed twelve to twenty-five percent of the total annual run of Pacific salmon in Prince William Sound.

While salmon sharks are most abundant in the North Pacific Ocean near Alaska, they travel as far south as northern Mexico and the Hawaiian Islands. Researchers have recorded salmon sharks dives as deep as 2192 ft. (668 m). Although biologists do not entirely understand salmon shark migrations, they believe the sharks spend the summer in the northern part of their range, and then they migrate south to breed. In the western North Pacific, salmon sharks migrate to Japanese waters to breed, and in the eastern North Pacific, they migrate south to the Oregon and California coasts. Their migrations are complicated, though, and they segregate by size and sex. Their migrations also depend upon available prey species in various areas. Scientists have determined that although many salmon sharks migrate south in the winter, some remain in the Gulf of Alaska and Prince William Sound year-round.

Male salmon sharks mature at five years of age, while females do not reach sexual maturity until they are eight to ten years old. They breed in the late summer or early autumn. Embryos develop inside their mother for nine months until she gives birth to between two to five pups. The developing embryos consume any unfertilized eggs in the womb. The mother provides no parental care to her young after birth, and they must fend for themselves. Females usually produce a litter every two years.

Male salmon sharks have a maximum lifespan of 25 years, while females can live 17 years. Other sharks sometimes eat salmon sharks, but humans pose the biggest threat. In Alaska, no commercial fishery exists for salmon sharks, but some sport fishing companies specialize in shark charters. Salmon sharks are big, strong, aggressive fish, and they pose a challenge and thrill for sport anglers. Each angler is limited to two salmon sharks per year. Salmon shark meat reportedly tastes similar to swordfish.

In my next post, I’ll describe another species of shark common in Alaska. Pacific sleeper sharks were long ignored as large, sluggish fish, but research over the past few years suggests Pacific sleeper sharks might play an essential role in the North Pacific’s food chain.

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Pacific Spiny Dogfish Shark (Squalus suckleyi)

December 20, 2020Fish, Kodiak WildlifeDogfish Shark, Pacific Spiny Dogfish, Pacific Spiny Dogfish biology, Robin Barefield, Sharks in Alaska, Sharks near Kodiak Island, Spiny Dogfish, Squalus suckleyi, Wildlife of Kodiak Islandadmin

One of the most abundant sharks globally, Pacific spiny dogfish belongs to the family Squalidae (the dogfish family). Pacific spiny dogfish range from the Bering Sea to Baja California to Japan and the Korean Peninsula. They are most common off the west coast of the U.S. and British Columbia. Dogfish are typically bottom dwellers and inhabit depths from shallow coastal waters to 4,055 ft. (1,236 m). They prefer water temperatures ranging from 44.6° F to 59° F (7-15° C).

Pacific spiny dogfish are small, streamlined sharks. Males can grow to 3.3 ft. (1 m), while females measure a maximum length of 4 ft. (1.2 m). A dogfish has a distinctive snout, large eyes, and a flattened head. The body has a cylindrical shape. The top half is dark gray with scattered white spots, and this color fades to light gray or white on the underneath side of the fish. The teeth of a dogfish have sharp edges, but they are specialized for grinding instead of tearing. Dermal denticles comprise the scales of a dogfish. These denticles are the same rigid material found in their teeth, and they make the skin very tough.

A dogfish does not have an anal fin, but it has two dorsal fins, with a spine in front of each fin. These spines are venomous, and the shark uses them as protection against potential predators, such as other sharks or humans. The dogfish employs its two dorsal fins in different ways. The first dorsal fin helps it maintain stability while swimming, and the second dorsal fin provides thrust. The large caudal fin (or tail) allows the shark to maneuver quickly and efficiently through the water.

A dogfish has five gills on either side of its body, but unlike bony fish, a dogfish does not have gill covers. To breathe through these gills, the shark must remain in constant motion, so it either must continually swim or rest in a current where water rushes past its gills. A dogfish has an adaptation called spiracles, aiding it to breathe in calm water. These specialized gills, located behind the eyes, allow the shark to breathe when resting or eating.

Dogfish earned their common name from fishermen who observed them hunting in packs like dogs. Schools of hundreds of dogfish swim close together during the day, hunting herring, capelin, other small fish, squid, octopus, and even jellyfish. The dogfish uses its teeth and not its spines when feeding. It uses its spines for protection. Scientists think dogfish eat less in the winter when they migrate to great depths. They are preyed upon by larger sharks, seals, orcas, and some larger fish.

Spiny dogfish can live 100 years, and females do not reach sexual maturity until they are approximately 35 years old. Males can reproduce at an average age of 19. Males internally fertilize females in October or November. Dogfish are ovoviviparous, meaning females give birth to live young, and they have a gestation of nearly 24 months, the longest of any vertebrate. They give birth to up to 22 pups, and the newborns range in length from 8 ½ to 12 inches (21.6 – 30.5 cm).

Pacific spiny dogfish stocks remain stable and are carefully managed. In some areas of the world, a commercial market exists for dogfish, and they are considered a good food fish, but they are not yet in demand as a food source in the United States.

We usually catch a few dogfish each year during our sportfishing trips, but this past summer, we landed as many as 20 per day while halibut fishing. Dogfish are tricky to release because while you try to get the hook out of its mouth, the shark attempts to whip its body into a position to stab you with one of its venomous spines. I was not pleased to encounter so many dogfish this past summer, but more importantly, I wondered why we were catching so many dogfish. I speculate that the dramatic decrease in the Pacific cod population led to an increase in small fish species typically eaten by cod. Dogfish probably are exploiting an opening in the food chain. Will their presence affect the abundance of other fish species in this region of the North Pacific? Only time will tell.

Happy holidays, and I wish us all a nicer, brighter 2021!

Mystery Newsletter

Sharks in Alaska

November 30, 2020Fish, Kodiak WildlifeDogfish Shark, Pacific Sleeper Shark, Robin Barefield, Salmon Sharks, Sharks in Alaskaadmin

Our guests are often surprised to learn sharks feed and swim in the frigid North Pacific. We catch spiny dogfish sharks when halibut fishing, and large salmon sharks terrorize commercial salmon gillnetters by ripping enormous holes in their nets when stealing fish from the mesh. Pacific sleeper sharks also live in Alaska’s waters, but sleeper sharks remain elusive, and biologists do not know much about their biology, diet, and habits. These three species fascinate me, and I think you will enjoy learning about their longevity, reproductive biology, and the mechanisms each species employs to stay warm in the frigid waters surrounding Alaska. I will cover each shark in detail in future posts, but for now, let me give you an overview of sharks in Alaska.

The three shark species I listed above are the most common but not the only sharks trolling the North Pacific. Over the past several years, ocean waters in the region have warmed, encouraging other shark species to venture into these nutrient-rich areas. Great white sharks began exploring Alaska in the 1970s, but recent, more frequent sightings suggest an increasing number of great whites have discovered the North Pacific’s fertile feeding grounds. Most of these visiting sharks only stay during the warm summer months, but researchers believe a small percentage find enough to eat to keep them in Alaska year-round.

Great whites are related to salmon sharks. Like salmon sharks, great whites have a highly developed countercurrent heat exchange mechanism that allows them to maintain a body temperature several degrees warmer than the ambient temperature. Sharks in this family represent some of the few species of endothermic fish in the ocean. Unlike their cold-blooded cousins, great whites and salmon sharks can produce bursts of speed to chase down prey, even in frigid ocean temperatures.

In recent years, Alaskans living and working in the far north regions of the Bering Strait and the Chukchi and Beaufort Seas have reported sightings of marine mammals with unusual wounds. Researchers noted that several ice-associated seals and Steller sea lions in the area suffered injuries from an uncommon predator.

Reports of seals with amputated flippers alerted biologists because killer whales have pegged teeth and don’t cause a slicing-type laceration. In some instances, scientists noted penetrating stab wounds and circular bite marks. Flesh torn by sharp, triangular teeth convinced researchers they were looking at the bite marks of a very large shark. Are the warming water temperatures and melting sea ice inducing great white sharks to travel further north where they can find a bounty of sea mammals to eat?

When great white sharks reach adulthood and grow very large, they seem to prefer eating marine mammals over fish, probably because marine mammals have a high energy-rich fat content. Observers have watched great whiles kill beluga whales in Cook Inlet, and biologists suspect they may even take walruses in the Bering Sea and the Arctic Ocean. Are great whites, at least in part, responsible for the diminishing numbers of Beluga whales? If the number of great whites increases in Alaska, will they affect other marine mammals’ population densities? Much more research is needed to answer these questions.

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I’ll take a closer look at my nemesis, the spiny dogfish shark, in my next post. We used to catch one or two of these nasty little critters a year during our summer fishing trips, but we caught as many as twenty per day this past summer. Is this increase in spiny dogfish a trend, or was this past year only an anomaly?