Salmon Sharks: The Anonymous Superpredator
The white shark (or great white) and mako shark are charismatic predators known worldwide in documentaries, feature films and literature. No less charismatic, but nearly unknown to the general public, is the salmon shark. This subarctic apex predator is in fact a cousin to the more famous white shark and mako, within the shark family Lamnidae. All species in this family share similar adaptations for hunting at great speed in cold water ecosystems.
Salmon sharks are the largest apex predator in the subarctic regions of Alaska and Western Canada, aside from the orca and an occasional white shark. They commonly grow to lengths of 250 cm and weigh at least 220 kg. Larger salmon sharks have been reported but these are unsubstantiated by official sources. Populations of salmon sharks are found in the western and eastern North Pacific. No shark has been documented moving between these populations but it is thought to occur. Salmon sharks segregate by sex with males and females living separately except to mate. Mating occurs in the late summer and early fall. The gestation is estimated at 9 months. Pups are fed in part by eating unfertilized eggs in the ovary prior to birth. Pups are then born in the spring to early summer and are a little under a meter in total length (60-65 cm precaudal length). There are up to 5 pups per litter. Salmon sharks appear to have a reproductive cycle that extends at least two years, with an extended rest and recovery period between pregnancies.
While older salmon sharks are specially designed to exploit feeding opportunities in cold, northern latitudes, the young sharks live in warmer, more southerly latitudes, such as along the coast of California. Salmon sharks undergo a major growth period each year between May and November, slowing some as January approaches. Growth undergoes a brief cessation or extreme slowing between January and March each year, before entering another period of more rapid growth in April or May.
Pups in the western North Pacific are known to grow to between 118-136 cm in their first year. They then grow by 10-15 cm per year up to four years of age, and reach their adult length by ten years. Salmon sharks in the eastern Pacific grow and mature more quickly. Sharks in both populations reach the same size at maturity, but the eastern sharks get there at a younger age. Female salmon sharks in the Eastern Pacific population reach sexual maturity between 6-9 years of age while those in the western population mature more slowly, reaching maturity at 8-10 years. Males mature more quickly reaching sexual maturity by about 5 years of age in the western Pacific. Maturity for males in the eastern population might occur as young as three years. Aging using vertebral rings has suggested that salmon sharks have a longevity of about 17-25 years. It must be noted that aging using bomb radiocarbon in white sharks has resulted in age estimates much greater than those using vertebral rings. Thus, it is also possible that aging using vertebral rings could underestimate the true longevity of salmon sharks.
Salmon sharks are opportunistic hunters, feeding on a variety of species of fish, crabs, squid and shrimp. Some of their documented prey species include salmon, rockfishes, sablefish, lancetfish, daggerteeth, lumpfishes, sculpins, mackeral, pollock, tomcod, herring, spiny dogfish, and Tanner crabs. The arrival and departure of migratory salmon sharks in Alaska matches the availability of salmon in these waters. It is widely held that salmon are a principal prey species, hence their English common name. Salmon sharks exploit the seasonal abundance of salmon which return to their natal rivers to spawn each summer and autumn. However, while the salmon provide a seasonally abundant food source, herring likely represent a more consistent prey species which is available broadly over the shark’s subarctic range and remains available throughout the year. The Russian name for this shark, appropriately, translates as the Herring Shark. In several areas of eastern Prince William Sound, in Alaska, deep canyons extend into inlets of the mainland (called Ports). Salmon sharks migrate into the deep water canyons early each summer. It is thought that the sharks hunt schools of herring, trapping them against the walls of the deep water canyons. Some observers have suggested the idea that salmon sharks hunt cooperatively to capture schools of herring. But, one must be careful to distinguish between a group of predators hunting independently while benefiting passively from actions of its cohorts, from true coordinated hunting behaviour.
Whereas the sharks hunt herring deeper in the water column, pursuit of salmon can occur near the surface. As a result, these predatory events can be spectacular to witness. Like white sharks and makos, salmon sharks are capable of spectacular breaches. Many attacks on salmon involve tight turns at the surface which appear as splashes or tail slaps when viewed from the surface. But, if the shark’s momentum carries it through the water surface, a spectacular leap results. A group of tourists in Port Fidalgo were out on a fishing trip years ago when a salmon shark leapt into the air in front of the boat. Across its jaws was a salmon that burst in two from the force of the impact. In 2005, I ventured to Port Gravina to see the sharks based on information that had been recently published from tagging data. The salmon shark population in Prince William Sound had been increasing in the preceding years and I witnessed intense hunting behaviour on salmon. One of the sharks breached at least three meters from the water that day. It was the kind of breach that one typically associates with a hooked mako. But, these smaller powerful sharks are capable of similar towering breaches.
During the autumn months, salmon sharks begin to disperse broadly through the north Pacific ocean basin. Sharks migrating away from Alaska and other areas of the west coast of North America reach areas as far as the waters north of Hawaii. The timing of departure from costal Alaskan waters is highly variable. In one large satellite tagging study, departures ranged from July 22 to March 8, with a median date of January 8. While moving south, the sharks travel from 25 to 103 km per day according to satellite tags, with an average of 68 km a day. The same tags showed that the average date at which they started back north was May 4. Female salmon sharks appeared to give birth during the southward migration. It is suspected that the mating for eastern Pacific sharks occurs in Alaska during autumn, based on bite marks from recent mating events.
The young appear to be birthed in the warmer waters off of California in the spring. Evidence for birthing in this area comes, in part, from stranding of young juvenile salmon sharks each year in California, mostly from the later summer to early fall. It has been shown that these young dead sharks fall victim to meningitis or meningoencephalitis caused by Carnobacterium. Following birthing, the newborns appear to swim northward into nursery areas as they grow. Salmon sharks are able to range further north as they increase in size. This is likely due to the ability of larger sharks to conserve heat due to a greater mass to surface area. This same phenomenon is seen in white sharks. In addition to the sex segregation mentioned earlier, salmon sharks also appear to segregate by size. The size segregation seems to be quite specific. Individuals within an aggregation are usually within 10-15 cm in size of one another.
The adult sharks also appear to hunt for prey and feed in the California current, off the west coast of North America. Further out to sea, in the warm Subtropical Gyre, still-pregnant females may invest time in the warmer waters while their embryos develop to full term. Mature female white sharks spend the months in this same general area while their growing fetuses develop. Studies show that while out to sea, squid account for 96% (by weight) of the prey consumed by salmon sharks. Interestingly, white sharks also appear to consume predominantly squid species while in pelagic phase. Along the coast, salmon species may be an important prey species along the southern habitat. The headwaters of some major spawning streams along Oregon and California during peak spring river flows. This timing makes the salmon available for predation by salmon sharks during the southern phase. In addition to the mature salmon returning to spawn, juvenile salmon are migrating from streams to the ocean in their second or third year of life, providing another prey source for the salmon sharks off the west coast of the mainland United States.
During the year, salmon sharks are exposed to a wide range of surface temperatures ranging from near freezing to as high as 26 °C. Tagging data suggests that the sharks seem to preferentially utilize depths with temperatures between 6 to 18 °C in Alaska and throughout their migration. Moreover, the sharks spend nearly 3/4 of their time in waters cooler than 10 °C. In southern latitudes, the salmon sharks are able to tolerate temperatures over 20 °C during their vertical movements but may use cooler water to mitigate the negative effects that warmer temperatures can have on the physiology of warm-bodied sharks. Thus, salmon sharks may occupy deeper depths in the southern reaches of their annual range as a form of thermal refuge from the warmer surface waters.
While in pelagic habitats, salmon sharks dive to deep water during the day and return to shallow depths overnight. This likely is due to foraging on squid and other animals that occupy deep water during the day and undergo a vertical migration to shallow water at night. One of the salmon sharks that bore a satellite tag dove beyond the staggering 1864 meter depth measurement limit of its tag. At such depths, there is no light, it is very cold and there is very little dissolved oxygen in the water for the sharks to extract through their gills. Like other deep divers, such as elephant seals, salmon sharks have very high concentrations of myoglobin in their muscles, allowing them to store oxygen. They also possess adaptations which permit greater oxygen transfer across the gills, increased oxygen transport capacity, and enhanced delivery of oxygenated blood to the tissues. All of these physiological adaptations help them to maximize the delivery and use of available oxygen. It is truly impressive that salmon sharks have shown the ability to tolerate the cold and low oxygen conditions of the deep for periods of up to nine hours at a time.
Many salmon sharks undergo long annual migrations. One shark was recorded traveling 18 220 kilometers over 640 days. However, not all salmon sharks make these long treks. Some sharks remain in the coastal waters of Alaska throughout the year. This means that they experience water temperatures down to 2 °C for months at a time. The same extraordinary adaptations that allow overwintering in near freezing water, permit all salmon sharks to exploit bountiful, cold water ecosystems that are beyond the physiological capability of many other predators. Like all sharks in the Lamnid family (including white sharks and makos), salmon sharks are warm bodied, maintaining a core temperature well above the surrounding water. However, while other Lamnids can keep their core temperatures elevated, those temperatures still vary with the surrounding water temperature. However, salmon sharks have taken this ability to such great specialization, that they have achieved near homeothermy, meaning the ability to maintain a higher and consistent core temperature despite outside conditions. Salmon sharks have been documented with stomach temperatures as high as 21.2 °C over ambient temperature and have been shown to maintain a specific stomach temperature independent of environmental temperature changes.
At the core of this specialization is an anatomical unit called the rete mirable (which means “miraculous net” in latin). Fundamentally, this involves blood vessels heading out from the core (typically arteries) running in close proximity with vessels carrying cold blood from the periphery (typically veins). Due to this proximity, heated blood from the core passes its heat to nearby veins returning from the cool periphery so that the heat is maintain in central body parts rather than being lost to cold surrounding water through the skin. Salmon sharks use these anatomical networks of blood vessels to warm their muscles, eyes, stomach and brain.
The salmon shark’s swimming muscles are highly evolved and similar to other fast-swimming fishes, and in particular to those of tuna. Two principal muscle types contribute to swimming and other motor tasks. The red, aerobic muscles are so-called slow twitch muscles. This means that they are used in slow, sustained tasks. The other muscle type is white muscle. The white muscle is capable of brief powerful bursts but fatigues quickly, The red muscle by contrast, produces less powerful contractions but its action can be sustained for a long time without fatigue. In most fish, the red muscle is situated in a lateral, subcutaneous location. However, in salmon sharks, as in tuna, the red muscle is located deep, next to the vertebral column, in a mid-body position. Salmon sharks have vascular heat exchangers (rete mirable) that warm the red muscle. The more superficial white muscle is also warmed by conductive transfer from the deep red muscle. Through this system, the salmon shark’s muscles are maintained at a temperature significantly elevated relative to the surrounding water.
The red, locomotor muscles power continuous swimming for the salmon sharks. These muscles are so highly specialized that the function in a manner similar to warm-blooded mammals. The red muscles of a salmon shark function only between 20-30 °C and are ineffectual at cooler temperatures. In fact, when warmed to just 10 °C above water temperature, the muscles only produce 20-50% of the power that is generated at 26 °C. These muscles maintain these elevated temperatures only by heat generated by the muscles themselves via the energy burned through contraction. The sharks have no means to heat the muscles independent of the muscle’s own contractions. Thus, salmon sharks must swim continuously in order to maintain ideal temperature. If the shark ceased swimming, and the muscle temperature were to fall below 20 °C (which is still 15 °C above the surrounding water), the red muscle would fail to function and might be incapable of recovery. This places the shark in a tenuous physiological position requiring constant swimming. Even if these sharks did not require constant swimming for heat generation, the muscles in their gills are too weak to pump water and maintain oxygenation without constant forward motion.
The white muscles of a salmon shark, by contrast, can retain the ability to generate high power at a wider range of temperatures between 10-26 °C. This is fortunate given the lateral positioning of the white muscle, near the skin. As a result of this position, there is a temperature gradient across the width of the white muscle between the warm inner fibres adjacent to the red muscle and the colder exterior near the skin. However, this gradient is between 18-20 °C, which is well within the tolerable limits for white muscle. The power generated by white muscle contraction gives the salmon shark a competitive advantage in hunting cold blooded fish in the subarctic and in the cold depths during southern migrations. In fact, the power generated by the salmon shark is powerful enough to produce the fastest speed recorded for any shark. The United States navy has reportedly recorded a salmon shark swimming at 80 km/hr. Consider where this places the salmon shark amongst the fastest and most agile predators in the sea:
Black Marlin 129 km/hr
Sailfish 110 km/hr
Salmon shark 80 km/hr
Striped Marlin 80 km/hr
Mako shark 74 km/hr
Bluefin tuna 70 km/hr
Blue shark 69 km/hr
Swordfish 64 km/hr
White shark 56 km/hr
Orca 56 km/hr
Like mako sharks and white sharks, salmon sharks also have extraordinary agility. The hydrodynamic benefits of its specialized scales (known as dermal denticles) allow a salmon shark to change direction near instantly and at high speed. The advantages conferred to the salmon shark from this raw speed and agility, in order to chase down prey in a rich, cold-water ecosystem can not be overstated.
However, successful predation in a subarctic environment requires more than speed and agility. The shark must track the prey, make quick decisions, and digest the captured animal. The salmon shark also has complex retia mirablia to warm the brain, eyes, and stomach. Salmon sharks have a well developed rete within an orbital venous sinus on each side of the cranium (the shark’s skull). Blood that was cooled as it passed through the gills and was exposed to cold ocean water then passes through the vessels of these retia before reaching the eyes or brain. Warm blood from the red swimming muscles passes through a vein providing a counter current heat exchanger to warm the cool arterial blood from the gills. Before passing through the orbital sinus, this blood warmed by the swimming muscle first bathes the brain in warm blood. Within the orbital sinus, arterioles divide and coil, provide a large surface area for heat exchange. The cold arteriolar blood flowing through these coiling vessels is bathed by warm venous blood flowing in the opposite direction in an open sinus, such that the opposing blood flow is separated only by the relatively thick arterial walls. In this way, heat exchange is maximized. The thick walls of the arterioles help prevent oxygen loss. This reduces heat transfer but this deficit is offset by the coiling of the vessels. In addition to warmth through the red muscle vein, the orbital sinus also receives heat from red, aerobic eye muscles, and possibly nearby aerobic jaw muscles. The internal carotid artery would normally provide a bypass for cooler blood to reach the brain, but in salmon sharks, this is greatly reduced. All in all, the provision of warmed blood to the eyes likely serves to improve the eye’s ability to provide rapid and sensitive discrimination of visual inputs in cold water, which helps with prey detection and tracking under challenging conditions. Similarly, a warmed brain is able to respond quicker versus the brains of the shark’s cold-blooded prey.
The stomachs of salmon sharks maintain a constant temperature between 25.0 to 25.7 °C, regardless of changes in the water temperature. This is as much as 21 °C above the environmental temperature. The temperature of the stomach faces challenges when the shark ingests cold sea water with its prey. The shark can actively maintain the stomach temperature by altering blood flow through vascular shunts in order to regulate heat gain and loss. For example, there is a hepatic sinus that allows blood to bypass a rete mirable located at the forefront of the liver. The anatomy of the hepatic sinus suggests that it can be opened and closed in order to control heat retention in the system.
One more cold adaptation should be highlighted. Cardiac tissue is prone to dysfunction at cold temperature. This is obviously is critical issue for survival in cold water. There is a protein called the sarcoplasmic reticulum Ca2+ ATPase, (SERCA2) which is important in the maintenance of calcium ion stores which for vital for beat-to-beat heart contractions. Salmon sharks have high expression of both SERCA2 and sarcoplasmic reticulum release channel proteins in the cardiac tissue. Activity of SERCA2 can be measured in salmon shark cardiac tissue at temperatures as low as 5 °C. This enhanced SERCA2 function in salmon shark hearts results in calcium re-uptake at rates that are an order of magnitude greater than blue sharks. Similar enhanced sarcoplasmic reticulum calcium re-uptake and SERCA2 expression helps to prevent cardiac dysfunction at low temperatures in hibernating mammals.
These heat regulation systems in the salmon shark allow it to function as a homeotherm. This is a scientific way to say that the salmon shark is “warm-blooded”. In other words, it can maintain and elevated and uniform body temperature independent of changes in ambient temperature. Makos and white sharks are also able to maintain an elevated core temperature but the salmon shark is the champion of this feat. The maximum elevation of stomach temperature over ambient temperature is 8.0 °C for makos and 14.3 °C for white sharks. The 21.2 °C recorded for a salmon shark demonstrates the significant advantage achieved by salmon sharks which allows them to live in near freezing conditions.
Unfortunately, this incredible animal, like too many charismatic marine species, lives under threat from human impact. The salmon shark is vulnerable to commercial and sport fishing at many points in its range. The salmon shark is a fast swimming and dynamic sport fish is as such has been prized by sport fishermen. It fights hard but is easily caught. The population of salmon sharks in Alaska is highly vulnerable to exploitation in a fishery. The females take up to 9 years to reach sexual maturity, become pregnant no more than every second year and have at most 5 pups per litter. Thus, the population is slow to rebound to losses through birth of new individuals.
On August 16, 2000, an aerial survey of Port Gravina estimated that there were 2000 salmon sharks visible at the water surface. Based on news of salmon shark aggregation, I visited Port Gravina in August of 2006. I hired a skiff and we headed up to Gravina. I allotted two days to explore the area. On the first day, the sight was incredible, much like had been described. There were seemingly salmon shark everywhere, hunting salmon. The sharks would corner the salmon against the steep canyon wall and attack them at the surface. Likely in part due to predatory competition, the action was intense with some spectacular breaches. Excited, I returned on the skiff the next day. When we arrived there was a fishing charter in Port Gravina. As we passed the charter boat, there were 6 salmon sharks hung over the side of the boat. Unlike the previous day, the water surface felt empty, with no evidence of salmon shark activity.
My visit to Port Gravina coincided with a relative frenzy in interest to sport fish salmon sharks. Not unexpectedly, anecdotal reports describe a crash in the salmon shark population in the years following. As it became harder to find the sharks, the demand to charter boats for shark fishing in Alaska faded away. In the years that have passed since, there appears to be a modest rebound in salmon shark populations in the Ports that mark the eastern limit of Prince William Sound. Year on year, we are seeing small but steady increases in the number and size of the sharks that return to their familiar haunts. It is encouraging to see these sharks make a comeback but when compared to the estimates of 2000 sharks visible in Port Gravina 20 years ago, the modern population would appear to be at least an order of magnitude less.
And still, the threat remains constant for the sharks. Interested parties in Alaska have tried to approach the state government and Alaska FIsh and Game with concerns regarding the salmon shark population and its vulnerability. However, the government has shown very little interest in protecting the shark. There are significant gaps in the data used for governmental stock assessments regarding all large sharks in Alaska. Any suggestion for greater protection would be suffer from the inadequate data to support any changes in management and face the immense power of the fishing lobby. At present, the sport fishing catch limit still allows for the killing of one salmon shark per person per day, one in possession, and two sharks annually. Thus, a fisherman could kill sharks on successive days. One can imagine that with the limited ability of salmon sharks to replace their population losses through reproduction, these catch limits allow the population to be wiped out quickly, at least in local aggregations.
Despite what one might hear, salmon sharks are not good to eat. The meat is generally difficult to correctly prepare and tastes rather foul (although this is subject to individual tastes). More importantly, like all large, apex sharks, salmon shark flesh is heavily laden with mercury and other heavy metals. This makes the meat dangerous and unhealthy to consume.
It often feels like only good fortune protects the sharks for now. They have been largely forgotten by sport fisheries in Alaska but this can change any day. It only takes fishermen talking amongst themselves to restore the desire to kill these animals. Greater public awareness of this incredible animal and appreciation of it as an Alaskan icon could be a path to better protection in the future. The emblematic status of the bald eagle certainly contributed to its recovery. A similar local pride of this special shark could go a long way to providing them the opportunity to recover towards historical levels and to survive into future generations.
To visit the full salmon shark gallery click here: Salmon Shark Gallery