Why do freshwater clupeiform species often produce more eggs than marine species of similar size?

Freshwater clupeiform species typically produce more eggs than marine species of a comparable size as an adaptation to the higher mortality rates associated with riverine conditions.1

In these environments, eggs and larvae face various threats, including physical factors like wave action and sudden temperature drops, as well as biological risks such as predation by birds and other animals. Because freshwater and anadromous clupeiforms often lay adhesive eggs that develop on the bottom, they are particularly susceptible to predators, which necessitates higher fecundity to ensure the survival of the species. This increased egg production, combined with behaviors such as "repeated portion" spawning—where eggs mature in multiple batches rather than all at once—helps enhance the chances that at least some offspring will survive despite the perilous nature of their environment.1