Larger body size and earlier run timing increase alewife reproductive success in a whole lake experiment

Environmental conditions can influence biological characteristics, such as phenology and body size, with important consequences for organismal fitness. Examining these fitness consequences under natural conditions through genetic pedigree reconstruction offers a lens into potential population responses to changing environments. Over 3 years (2013–2015), we introduced adult alewife (Alosa pseudoharengus), anadromous, iteroparous clupeids, into one Massachusetts (USA) lake to complete the first detailed examination of this species’ mating system and assess relationships among body size, reproductive timing, and seasonal reproductive success. We reconstructed pedigrees using 15 microsatellites and genotypes from all possible parents and samples of naturally produced offspring within 4 months of hatching. In each of the 3 study years, spawning adults had multiple mates and spawned multiple times. Larger females that arrived and were translocated earlier had higher reproductive success. Declining body size and altered migration timing, through an influence on reproductive success, can influence population vital rates and productivity over time.