Why are trout, pikes, and smelts common in cooler Northern Hemisphere waters?

Trout, pikes, and smelts are common in the cooler waters of the Northern Hemisphere because they are members of the superorder Protacanthopterygii, a group of fishes that evolved to thrive in these specific environments.12

Specifically, species of the family Salmonidae—which includes trouts—evolved in clear, cold, well-oxygenated water and are highly sensitive to water quality. Their ability to utilize an anadromous life cycle, where they spawn in fresh water but migrate to the sea for feeding and maturation, has allowed them to expand their range significantly into colder regions, particularly following the recession of glaciers.31

Pikes (order Esociformes) also inhabit these cooler northern regions, though they are restricted to fresh water throughout their life cycle. Their broad distribution is believed to have been established when direct freshwater connections existed between major drainage basins across Asia and North America. Meanwhile, the distribution of smelts in these areas is associated with relatively recent geological events, with many species remaining anadromous or marine.1