Longfin Smelt

S. thaleichthys is characterized by its small size, not exceeding 20 centimeters in total length. It has a laterally compressed fusiform body, which means it is somewhat elongated and streamlined, and if viewed from above would be relatively thin. It is generally silver-white on the sides and ventral surface, with slightly darker coloration on the dorsal surface, ranging from olive-green/brown to pinkish. It has cycloid scales, and its lateral line is incomplete, only reaching just below the dorsal fin. Fins are soft-rayed, with 8-10 rays in the dorsal fin and 15-19 rays in the anal fin.The caudal fin is forked and homocercal (the two forks of its tail are the same size). S. thaleichthys possess an adipose fin. Their lower jaw protrudes beyond their upper jaw when compressed, and the base of their jaw reaches below the middle of the eye. Teeth are present on their tongue and both jaws. Most notably, their pectoral fins reach nearly the base of their pelvic fins, inspiring their common name "Longfin Smelt." There are few visible morphological differences between males and females, but males often have a small bump that protrudes their anal fin, and their anal fin is generally more angular, whereas females have a more rounded anal fin. Additionally, males are generally slightly smaller than females, but have longer fins.

S. thaleichthys is native to the Pacific coast of North America, its range stretching from the San Francisco Bay in California to the Gulf of Alaska. It is an anadromous species that spawns in coastal rivers but generally inhabits bays or estuaries. There are two known landlocked populations, one in Lake Washington (Seattle, WA, USA) and one in Harrison Lake (BC, Canada). While the Washington and California populations were once thought to be separate subspecies, more recent evaluation has shown that despite a lack of gene flow between populations, they are not significantly different and are therefore considered to be the same species. S. thaleichthys have a large salinity tolerance due to their habitat generally being estuaries or deltas in between salt and freshwater. They are able to inhabit marine, estuarine, or freshwater environments. They are open-water fish, inhabiting most of the range of the water column. They prefer shallow areas when they do inhabit marine environments, although that is also likely due to the necessity for proximity to an estuary or river system.

S. thaleichthys is a shoaling species, occurring in large schooling groups. Their lifespan is generally 2 years, but some individuals can live an additional year. Their diet is somewhat consistent across their range, but changes slightly based on location. It also shifts as they age. Juveniles <1 year old feed on chironomids (midges), copepods, and cladocerans (water fleas), with copepods making up 85% of their diet in the first 6 months of life. In the Lake Washington population, juveniles have a more diverse diet in summer months leading into the fall, and their food source shifts as they grow, beginning to include more cladocerans, mysids, and chironomids. During autumn, when they reach an age of about 6 months, their diet switches to mainly mysids, composing 50-70% of the diet for the rest of their lives. This percentage was historically higher, but in the Lake Washington population human development allowed for higher egg retention, which in turn increased smelt population and lowered mysid population due to increased predation. During winter months, amphipods and cladocerans are still important food sources, while the summer diet becomes more diverse as in the juveniles. S. thaleichthys generally exhibit feeding behavior in response to the day-night cycle, remaining slightly deeper in the day to shelter from harsher sunlight, and ascending at night to feed. …

S. thaleichthys is an anadromous species, spending most of its life in estuaries, deltas, or marine environments but traveling up coastal rivers to spawn. They congregate in large groups at the mouth of their chosen river before traveling upstream. The migration is relatively short, generally only a few kilometers upstream, and as a result spawning can sometimes occur in somewhat brackish water. Males generally migrate before females, and outnumber them at the spawning site. Once the population reaches its spawning destination, spawning occurs among the entire group, strictly at night due to sunlight sensitivity and predator avoidance. S. thaleichthys are broadcast spawners, so they expel their eggs and sperm into the water column, where they mix and fertilize. This process takes place over sand, pebbles, or aquatic plants, and the fertilized eggs stick to the substrate. Typically, individuals die after spawning, but some females have been known to survive another year. S. thaleichthys have a long spawning period, generally ranging from October to June. The Lake Washington population spawns from January to April. In general, spawning occurs at water temperatures of 7-14.5 degrees celsius. S. thaleichthys eggs are double membraned, and when the egg first contacts the water, the outer membrane ruptures, forming a stalk that it uses to attach to the substrate. …