Eggs from anadromous adults provide marine-derived nutrients to Atlantic salmon and brown trout parr in late autumn – observations from a Swedish coastal stream

In this paper we demonstrate that eggs from anadromous salmonines can constitute a large proportion of the dietary intake of individual juvenile Atlantic salmon Salmo salar and brown trout Salmo trutta during the spawning period in late autumn. The majority of fish older than 1 year residing on a spawning ground were found to have ingested eggs from spawning salmonines (Salmo sp.). Young-of-the-year brown trout were also able to consume eggs, at least down to a minimum fork length of 68 mm. The smallest Atlantic salmon found to have consumed eggs had a fork length of 79 mm. Larger fish were more likely to consume eggs, and consumed more eggs, than smaller individuals. The eggs of anadromous salmonines provide a rich source of marine-derived nutrients to the freshwater residing juveniles at the onset of winter. Our study highlights that these eggs may constitute an important energy input for juvenile salmonines in European anadromous populations, and could possibly influence winter survival rates and subsequent smoltification in the following spring.