Caplin
The capelin or caplin (Mallotus villosus) is a small forage fish of the smelt family found in the North Atlantic, North Pacific and Arctic oceans. In summer, it grazes on dense swarms of plankton at the edge of the ice shelf. Larger capelin also eat a great deal of krill and other crustaceans. Among others, whales, seals, Atlantic cod, Atlantic mackerel, squid and seabirds prey on capelin, in particular during the spawning season while the capelin migrate south. Capelin spawn on sand and gravel bottoms or sandy beaches at the age of two to six years. When spawning on beaches, capelin have an extremely high post-spawning mortality rate which, for males, is close to 100%.Males reach 20 cm (8 in) in length, while females are up to 25.2 cm (10 in) long. They are olive-coloured dorsally, shading to silver on sides. Males have a translucent ridge on both sides of their bodies. The ventral aspects of the males iridesce reddish at the time of spawn.
Capelin are planktivorous fishes that forage in the pelagic zone. Studies analyzing diet in populations of Capelin in both the arctic marine environment as well as in west Greenland waters show that their diet consists upon primarily euphausiids, amphipods, and copepods. As capelin individuals grow, the composition of their diet changes. Smaller capelin primarily consume smaller prey (copepods) and shift their diet towards feeding on primarily larger euphausiids and amphipods as body and gape size increases. The sufficient distribution and abundance of these zooplankton is necessary for capelin to meet energy requirements for progressing through many stages of their life cycle. Capelin occupy a similar dietary niche as polar cod, which leads to a potential for interspecific competition between the two species.
As an r-selected species, capelin have a high reproductive potential and an intrinsic population growth rate. They reproduce by spawning and their main spawning season occurs in spring but can extend into the summer. The majority of capelin are three or four years old when they spawn. The males migrate directly to the shallow water of fjords, where spawning will take place, while the females remain in deeper water until they are completely mature. Once the females are mature, they migrate to the spawning grounds and spawn. This process usually takes place at night. In the North European Atlantic spawning typically occurs over sand or gravel at depths of 2 to 100 m (7–328 ft), but in the North Pacific and waters off Newfoundland most spawn on beaches, jumping as far up land as possible, with some managing to strand themselves in the process. Although some other fish species leave their eggs in locations that dry out (a few, such as plainfin midshipman, may even remain on land with the eggs during low tide) or on plants above the water (splash tetras), jumping onto land en masse to spawn is unique to the capelin, grunions, and grass puffer. In beach-spawning capelin populations, after the female capelins have spawned, they immediately leave the spawning grounds and can spawn again in the following years if they survive. The males do not leave the spawning grounds and potentially spawn more than once throughout the season. Beach-spawning male capelin are considered to be semelparous because they die soon after the spawning season is over. In ocean spawning capelin populations, it has been observed that both male and female capelin are semelparous and die after spawning. This difference observed between capelin populations shows that capelin are physiologically capable of an iteroparous or semelparous reproductive mode depending on spawning habitat.
Studies on two populations of Newfoundland capelin which spawn in two distinct habitats found a lack of evidence of genetic variability between beach and deep-water spawners. This provides support for the species being facultative spawners. Capelin may select optimal spawning location based on abiotic factors such as temperature range and sediment. The optimal temperature range for capelin eggs that leads to greatest hatching success and offspring quality appears when eggs are incubated between 5 °C and 10 °C. This optimal temperature range provides support that individual capelin are able to select spawning location based on temperature, as temperature is one of the most variable factors between beach and deep-water spawning habitats for capelin. There is also evidence that shows that temperature is not the only factor at play when it comes to selection of spawning habitat. When both habitats are simultaneously experiencing temperatures in the optimal range, capelin are found to spawn in both habitats. This may be an advantageous strategy that leads to increased fitness. Capelin have been observed to spawn at beaches when deep-water or subtidal habitat is lower than 2 °C and spawn in deep-water habitats when beach habitats temperature is consistently above 12 °C.
