(Photo : Wikimedia Commons) The twaite shad (Alosa fallax), a herring family member, was once abundant in British rivers, with thousands of fish spawning upstream in the spring. Overfishing, deforestation, habitat loss, and migratory path obstruction have all contributed to population declines. Hybridization between species is more common in species that have been disturbed by humans. The estuarine period, or the time they spend in estuaries swimming from spawning grounds to sea, is expected to last up to a year and a half in A. fallax. Specific heterogeneity and juvenile survival in the estuarine phase are not taken into consideration in the calculation.