How do temperature and salinity limit the distribution of clupeiform fishes?

The geographical distribution of clupeiform fishes is primarily limited by temperature and salinity.1

Temperature

Temperature plays a significant role in the global range of these fishes:
* Tropical Concentration: Approximately 70 percent of clupeiform species inhabit tropical waters.1
* Subtropical and Polar Limits: Only a few species visit subtropical regions, while more than 20 species are restricted to boreal and subarctic environments.1
* Hemispheric Distribution: Notably, very few species are found in the Southern Hemisphere.1

Salinity

Salinity acts as a physiological barrier that dictates where specific groups can survive and breed:
* Marine vs. Freshwater: About half of all living clupeiform species are strictly marine. Others are either anadromous—migrating between the sea and fresh water to breed—or are entirely restricted to freshwater environments.1
* Physiological Adaptation: Anadromous species require specialized physiological mechanisms to regulate their blood's osmotic pressure when moving between high-salinity ocean water and low-salinity rivers or lakes.2
* Transition Zones: Because this physiological conversion is not instantaneous, these fish often congregate in brackish estuaries to avoid the shock of sudden changes in salinity.2