Running on empty? Freshwater feeding by spawning anadromous alewife Alosa pseudoharengus

Anadromous alewife Alosa pseudoharengus (n = 202; mean +/- s.d. fork length = 231 +/- 14 mm) were captured from 10 May to 27 June 2018 in an upper watershed lake on the Isthmus of Chignecto, Canada (45 degrees 57 ' N, 64 degrees 14 ' W). Thirty individuals (mean +/- s.d. fork length = 250 +/- 12 mm) were captured in an adjacent estuary downstream of a tide gate on 25 April 2018. Comparing estuarine to freshwater specimens, mean gonad mass and gonadosomatic indices in males and females decreased approximately 40% and 60%, and 31% and 50%, respectively. Individuals were characterized as pre-spawners in the estuary and spawners in the lake. Males maintained similar body condition throughout the spawning run whereas female condition decreased 9.4% between the estuary and lake. Stomach fullness decreased comparing estuarine and freshwater specimens, yet 93% of stomachs examined from individuals captured in the lake contained prey. Most males fed throughout all spawning stages (3%-17% empty stomachs), while all females fed during pre- and post-spawning stages and some fasted during spawning (11% empty stomachs). Cumulative prey curve never reached an asymptote, either weekly or for the entire sampling period, so freshwater diet may not have been completely described. Calanoid copepods (79.3%I-A) were a diet staple, with the secondary prey of mayfly nymphs (O. Ephemeroptera) consumed more by females (13.6%I-A) than males (6.2%I-A). PERMANOVA and PERMDISP analyses revealed significant dietary differences in freshwater were weekly and not due to dispersion effects, thus most likely due to feeding on various development stages of insect species. Our results challenge the long-held paradigm that anadromous A. pseudoharengus fast during the spawning migration.

Automated summary

The study revealed that anadromous alewife do not fast entirely during their spawning migration, primarily feeding on small crustaceans and other insects. Females consumed more mayfly nymphs in freshwater, while males fed more on calanoid copepods.