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.

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Earlier spring warming and anadromous fish migrations prompted by climate change are linked to shorter freshwater residency. Impacts of phenological change on anadromous fish populations are poorly understood with limited studies focused on iteroparous non-salmonids. We assessed freshwater residence time and reproductive success in an iteroparous clupeid, alewife (Alosa pseudoharengus) using a pedigree analysis and otolith-based spawning dates from captured juveniles. The primary objectives were to 1) estimate adult spawning duration in a freshwater pond (freshwater residence time) and 2) evaluate adult freshwater residence time, arrival date, length, sex, and reproductive success across two years in one system. Estimated freshwater residence times varied widely (1-64 days), and longer residence times were associated with earlier arrival dates, higher reproductive success, and more mating events. Longer freshwater residence times may allow alewife to spawn with more mates, produce more gametes, and experience a range of spawning and nursery conditions. Plasticity in alewife freshwater residence time could support earlier and shorter migration periods, but may result in lower reproductive output if adults spend less time in freshwater ponds.

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