Iliamna Lake is an inland body of water in southwestern Alaska, U.S. It is the second-largest freshwater lake situated entirely within the United States. It covers an area of 1,150 square miles (3,000 square km), being 80 miles (130 km) long and 25 miles (40km) wide, and contains several islands. Small communities of Alaskan natives pebble the shore, and the town of Iliamna lies on the lake’s northern side. Northeast lies the active Iliamna Volcano.
Iliamna Lake supports a thriving population of freshwater seals of the species harbor seals (Phoca vitulina). Seals are incredibly rare in freshwater systems as permanent residents, and the population in this lake is one of five populations of seals living in freshwater in the Northern Hemisphere. The approximately 400 individuals living in Iliamna Lake are recognized as a distinct population, as they genetically differ markedly from the nearest marine population.
The current population was likely established by marine seals swimming up the Kvichak River between 200 to 5,000 years ago, however local oral tradition supports that seals have been in the area for longer than people. The seals subsist on a diet of fish, preying largely on Salmonids but also smaller fish species. The seals pup mainly during June – July and typically give birth to a single pup, which the mother nurses for 4 – 6 weeks on milk containing up to 50% fat. In addition to the seals, the lake supports a rich fish community of which the most fascinating species are listed below.
Fish Species in Iliamna Lake
1) Arctic cisco (Coregonus autumnalis)
The Arctic cisco is a member of Coregoninae, a subfamily of Salmonidae (salmon) typically known as freshwater whitefish. The Arctic cisco is sometimes known by its Inupiat name qaaqtaq. The species has been important for native communities and is still commercially fished using gill nets. They can be difficult to distinguish from other ciscos, but can be identified from the least cisco, which they share the Iliamna Lake with, by the terminal mouth and very little pigmentation on the pelvic fins.
They are anadromous (meaning they migrate up rivers from the sea to spawn) and have a high salt tolerance compared to other coregonids. Their distribution varies in association with the movement of high-salinity water upriver, for example following strong winds, or when the salinity increases in fall following ice formation. They enter freshwater to spawn and return to the ocean immediately after. The young hatch in spring and their movements are highly dependent on current movements.
2) Arctic char (Salvelinus alpinus)
The Arctic char is a member of the Salmonidae family (salmon) and can be distinguished as a char by its light spots on a silvery background. The Arctic char has a dark back with a lighter belly and orange to bright red paired fins. The tail is somewhat forked. During the breeding season, both males and females become colorful. The males in particular attain a strong orange to red belly with intensely colored fins.
The Arctic char has a great capacity for blending into its surrounding environment. Their color can range from brown to slightly green, which is an adaptation to avoid predation. The adaptations to avoid predators are innate in this species. A study of juvenile Arctic char reared in a hatchery, and therefore not familiar with predators, tested their innate response to the smell of predators. In the test with brown trout (Salmo trutta), the juvenile Arctic char displayed strong anti-predator responses, such as freezing, when presented with water containing the odor of brown trout, showing that they are born with the recognition of this particular predator’s smell.
3) Arctic grayling (Thymallus arcticus)
The Arctic grayling is the species that the subfamily Thymallinae (graylings) is permanently associated with, also known as the type species of the genus. They have 17 or more fin rays in their dorsal fins and large scales with indented, embedded margins. The Arctic grayling occupy the same niche as the brook trout and the two species are seen as non-compatible, as the more aggressive brook trout appear to out-compete the Arctic grayling.
The Arctic grayling has a striking deep purple dorsal side, a gray to blue body with a slight pink iridescence, and a white belly. The sides are scattered with dark spots, although they lose some with age. The dorsal fin is greatly enlarged, and the pelvic fins are striped with orange or yellow. They move from the cold, open water to rocky creeks to spawn. The adults mainly feed on surface insects, although they also prey on fish and fish eggs, while the young feed on zooplankton.
4) Arctic smelt (Osmerus dentex)
The Arctic smelt is a member of the Osmeridae family (smelts). The family name stems from the Greek word ‘omse’, which means smell or smelly, a nod to the odor of the smelts, which is described as similar to the smell of cut cucumber. The Arctic smelt is anadromous and enters the local rivers to spawn when it reaches sexual maturity around 3 – 4 years old. In subsequent years they return to spawn and can live to the age of 11.
For many years, the Arctic smelt was treated as a subspecies or conspecific to the rainbow smelt native to North America, however morphological and mitochondrial genome data support the separation of the two into distinct species. A comparison of the mitochondrial genomes of rainbow smelt, Arctic smelt, and the European smelt revealed that all three species are closely related, but the level of divergence between them is high enough for them to be considered distinct.
5) Burbot (Lota lota)
The burbot is a member of the Lotidae family (cod-like) and the only member of the Gadiformes order (cod) that lives in freshwater. Therefore, it is also sometimes referred to as the freshwater cod and can be distinguished from other freshwater species by its distinct cod-like characteristics, such as the chin barbel. The burbot sports a yellow coat with a pattern of dark brown or black spots.
The burbot is an early spawner, spawning in the middle of winter or early spring before the ice melts. They typically spawn at night, and the adults leave the spawning grounds before day breaks. The male and female lie close together and release eggs and milt simultaneously. Afterward, the female beats her tail to scatter the eggs. The eggs sink to the bottom at random, and the adults provide no parental care.
The burbot was most recently assessed by the IUCN in 2012 and is listed on the IUCN Red List of Threatened Species as ‘Least Concern’, since the species survives in many subpopulations, has a large population size on average, and lacks major threats. The population trend over the last ten years remains stable but can be locally threatened.
6) Chinook salmon (Oncorhynchus tshawytscha)
The Chinook salmon is a member of the Salmonidae family (salmon) in the genus Oncorhynchus that contains Pacific salmon and trout, the name stemming from the Greek words for ‘lump’ (ὄγκος) and ‘snout’ (ῥύγχος) referring to the hooked snout the males develop in the mating season. The same is true for the chum salmon and coho salmon listed as the next species (as well as the pink salmon, the rainbow trout, and the sockeye salmon all present in this lake).
The Chinook salmon, also known as the king salmon, is famously the largest species in this genus. They can undertake great migrations, but they still return to their natal streams to spawn. A female can make and release eggs in multiple nests, and she generally guards them for as long as she is capable, although the Chinook salmon usually die a few days after spawning.
The Chinook salmon has not been assessed by the IUCN; however recent demographic changes are closely followed due to the commercial value of the species. These suggest that the average size and age of the populations are declining, and that the average size of the fish returning to spawn has consistently decreased. This is concerning, since the largest females lay the most eggs, and are therefore of great importance for the populations and viability of Chinook salmon fisheries.
7) Chum salmon (Oncorhynchus keta)
The chum salmon is a member of the Salmonidae family (salmon) in the genus Oncorhynchus. They can be distinguished from other salmonids by the lack of distinct black spots and a silvery blue color that becomes lighter towards the white belly. They spend most of their life in the ocean, as they migrate from their natal streams already as juveniles, only a few weeks or months after they hatch. They re-enter freshwater during an advanced stage of sexual development and initiate spawning behavior shortly thereafter.
The chum salmon develops striking breeding characteristics in the spawning season. Both sexes change color to a striped pattern of red and black stripes, but the males’ colors are stronger. In addition, the males develop long fangs. The fangs are thought to be important during the fight for females, which precedes the spawning behavior. Contrastingly, the females do not develop enlarged teeth in the spawning season, but they do compete for space to build their nests. The males fight for access to the largest females, as these lay the highest number of eggs, thereby letting the males father more offspring.
8) Coho salmon (Oncorhynchus kisutch)
The coho salmon is a member of the Salmonidae family (salmon) in the genus Oncorhynchus. They can be distinguished from other members of the genus by the lack of dark pigmentation along the gum line of the lower jaw, rough and widely spaced gill rakers, and a nearly straight lateral line. During spawning season, the fish turns a dark green around the head and back and bright red on the sides, a color change that is more pronounced in the males than the females.
The coho salmon is a semelparous species, which means that it spawns only once in its lifetime. Compared to other species also within its genus, the coho salmon has a short life cycle; three years elapse from the eggs being laid to that generation dying after spawning. In the spring of their second year, they migrate to the lake and in the fall of the following year, they migrate back to their parent stream to spawn. They will migrate as far up the stream as they are able to go, even in streams as shallow as 5 centimeters!
9) Dolly Varden trout (Salvelinus malma)
The Dolly Varden is a member of the Salmonidae family (salmon) in the subfamily Salmoninae (trout). The population in Iliamna Lake is a part of a non-anadromous, patchily-distributed Alaskan form that is not well-researched. They differ from the anadromous variant mainly in size (they are smaller), and they do not spend a part of their life at sea. They are opportunistic drift feeders that occupy small and harsh streams. Among Dolly Varden, dominance is a better predictor of feeding success than size difference between competing fish. They are considered important for sport fishing and subsistence.
The adult Dolly Varden swim upstream in the spawning season. The female digs a depression when she has selected a nest site, while the male courts her and attempts to win the competition against other males. The male and female release eggs and milt in the depression over several rounds before the female covers the fertilized eggs. Afterward, she may start building a new nest and the process is repeated. Both the male and female die shortly after spawning.
10) Humpback whitefish (Coregonus pidschian)
The humpback whitefish, also known as the Siberian whitefish or Arctic whitefish, is a member of the Salmonidae family (salmon) in the Coregonus (whitefish) genus. The humpback whitefish gets its name from the characteristic hump behind the head in adults. The adipose fin is characteristic of the family and is particularly pronounced in males. The juveniles have no parr marks. The back is dark, fading into silver on the sides. The adults feed on aquatic larvae, crustaceans, and mollusks.
The humpback whitefish prefers the lower reaches of rivers and large lakes and can be found in rivers during migration. In preparation for migration, they congregate in large schools and swim upriver until they reach their spawning sites. They typically spawn annually in the fall. Afterward, the adults seek out deep holes down the river to overwinter, where they stay until the ice starts to melt. According to the IUCN, which assessed the species in 2008, the humpback whitefish is listed as ‘Least Concern’.
11) Lake trout (Salvelinus namaycush)
The lake trout is a member of the Salmonidae family (salmon) in the subfamily Salmoninae (trout) together with the Dolly Varden. The lake trout is sometimes known as landlocked salmon but can be recognized as a trout or char by its light spots dotted on a dark green background. They have a white belly and a forked tail, and their fins (apart from the dorsal fins) can sometimes be orange. The young sport parr marks. During spawning the males develop a black lateral stripe.
The vast majority of lake trout populations feed on a wide variety of food items from freshwater sponges, crustaceans, and insects to small mammals. However, some populations have become ‘vegetarian’ and feed on plankton throughout their whole life.
Due to their high rank in the food chain, and their general taste for a broad diet, the lake trout is often used as a barometer for water pollution stemming from industrial compounds. This works because these compounds bioaccumulate at the top of the food chain. Therefore, scientists can test individual lake trout for specific compounds and track lake trout populations since water pollution quickly impacts this species and lowers recruitment.
12) Least cisco (Coregonus sardinella)
The least cisco is a member of Coregoninae, a subfamily of the Salmonidae family (salmon) known as the freshwater whitefish. The species is hard to distinguish from the Arctic cisco, but the lower jaw of the least cisco always protrudes beyond the upper jaw. The least cisco spawns in the fall just before or after freeze-up. In contrast to other whitefish, this species does not develop breeding tubercles, which are keratin-based skin nodules that, among other things, assist in species recognition and stimulation of females.
The species is viewed as a bellwether (a trend indicator) for Arctic lakes. It is an important prey resource for top predators like the northern pike, which means that changes in population numbers have implications throughout the food web. A recent study investigated the potential implications of future climate change on least cisco and found that the current expected trajectory of temperature increase will not reach a critical limit for this species before 2090 and that shorter, warmer winters will allow the species to grow faster, as less weight is lost during the hard winter months.
13) Northern pike (Esox lucius)
The northern pike is a carnivorous member of the Esocidae family (pike). They live solitarily and can be highly territorial. It is popular among anglers, as it grows to impressive proportions. However, the European individuals typically grow larger than the North American ones. The northern pike can be recognized by its long, flat snout and a mouth filled with large, sharp teeth. Also, note that they have patches of sharp teeth on their gill arches.
The northern pike is an important fish both commercially and as a gamefish. Additionally, it can often be seen in local cold-water aquaria. The total adult population size is unknown but expected to be very large and stable, and the species was listed as ‘Least Concern’ by the IUCN in 2011. Due to their behavior as ambush predators, they prefer vegetated areas in larger lakes.
The northern pike feeds continuously throughout the day, and the best season for pike fishing is from late spring to early winter. Ice fishing can also be successful.
14) Pink salmon (Oncorhynchus gorbuscha)
The pink salmon is a member of the Salmonidae family (salmon) in the genus Oncorhynchus. It is anadromous following a 2-year cycle. Therefore, the pink salmon in a particular lake might be known as an even-year or odd-year line. For example, an even-year line is deposited as eggs in 2020 and will return to spawn themselves as adults in 2022. The spawning begins in the fall, like many other salmon species, and hybridization between pink salmon and Chinook, coho, and sockeye salmon can occur. The spawning pink salmon males become greatly deformed with a hooked snout, gaping mouth, and a large hump forming in front of the dorsal fin.
The pink salmon is pelagic at sea and returns to freshwater to spawn. However, in contrast to many other salmon species, the pink salmon does not harbor a strong homing behavior. While some individuals do end up spawning in their natal stream, pink salmon have been observed spawning as far away from their natal stream as 640 km. Upon arrival at the spawning site, the female builds a depression with her tail. When she is finished, she drops into the depression followed by one or more males. The female often spawns on multiple occasions, building new nests upstream from the previous one.
15) Rainbow trout (Oncorhynchus mykiss)
The rainbow trout is a member of the Salmonidae family (salmon), which can be recognized by its adipose fin that is situated between the dorsal and tail fin. Their life cycle starts in a freshwater stream, where they live for the first two years, before migrating to a larger body of water. Here, they spend 2 or more years maturing, before migrating up the parent stream to spawn. The males do not develop a hump in the spawning season, which differentiates them from the pink salmon. However, their colors darken and become more intense, and the males develop a hook on the lower jaw.
As an adaptation to nutrient uptake, the rainbow trout and some other fish (e.g. lake trout) have pyloric caeca, a series of finger-like appendages. These excrete enzymes and increase the surface area of the digestive intestinal part right after the fish’s stomach. In the rainbow trout, these have been shown to be important for growth, as individuals with more caeca were better at converting food than individuals with less. The specific mechanism at play is still unknown, as the number of caeca might reflect initial growing conditions or contributing factors to food utilization.
16) Round whitefish (Prosopium cylindraceum)
The round whitefish is a silvery member of the Salmonidae family (salmon) in the Coregoninae subfamily (whitefish) with a dark dorsal side and brownish sides. It belongs to the Prosopium genus, a genus where only 6 species are recognized, and the round whitefish is the most widespread member. The name stems from the shape of the body, which is almost round if you make a cross-section. The dorsal side looks dark because each scale has darkly pigmented borders. The pectoral fins, and to some degree the pelvic and anal fins, have an orange hue.
Curiously, when it comes to spawning behavior, the round whitefish seems to move to the spawning grounds in pairs. Even in groups of more individuals, the fish seem to be paired up. The breeding pair develop breeding tubercles, about one per scale on the sides, but fewer on the female and the pectoral fins become more orange. They spawn in shallower waters from November to early December.
17) Sockeye salmon (Oncorhynchus nerka)
The sockeye salmon is a member of the Salmonidae family (salmon) in the genus Oncorhynchus. It occurs in two different forms, the anadromous sockeye salmon and the landlocked variant known as kokanee. Each kokanee population has developed independently from the anadromous form. However, the population in Iliamna Lake is anadromous and is the world’s largest population of sockeye salmon on record and will continue to be so, since the species returns to its natal stream to spawn. When they return, they wear iconic mating colors famously displayed in Disney’s ‘Brother Bear’. The head becomes bright green, and the body a striking red. The males develop a huge hump and a slightly hooked snout, and their colors are brighter than those of the females.
The sockeye salmon was most recently assessed by the IUCN Red List of Threatened Species in 2010 and listed as ‘Least Concern’. However, a study published in 2012 set out to make a more detailed evaluation of individual populations. The populations around Bristol Bay and in part Iliamna Lake make up 50% of the sockeye salmon biomass and are therefore incredibly important for the survival of the species. Using demographic and historic data, they found that the risk of independent populations varied greatly from ‘already extinct’ to ‘least concern’. According to this study, 17 of the populations they investigated are currently at risk of extinction.
