COS 6-1 - Longer adult freshwater residence time related to higher reproductive output in an anadromous clupeid

Background/Question/Methods

Anadromous fishes hatch in freshwater, migrate to the ocean as juveniles, and return to freshwater to mate. Their unique life cycle makes anadromous fishes especially vulnerable to climate impacts. Anadromous fish spawning migrations have shifted earlier in recent decades, likely due to changes in climate and earlier spring warming. Climate change-induced temperature changes have also been associated with shorter time spent in freshwater spawning areas (freshwater residence time) for anadromous species like alewife (Alosa pseudoharengus). While changes in phenology have been documented, their impact on anadromous fish populations is not well understood due to the diversity of anadromous taxa in this­­ polyphyletic group. Studies of relationships between residence time and reproductive success in anadromous fishes have been largely limited to semelparous salmonid species. We present new information about how residence times relate to reproductive output in an iteroparous clupeid species, alewife. Alewife are anadromous forage fish that are native to the Atlantic coast of North America and support commercial fisheries and the broader ecosystem. We estimated freshwater residence time for spawning adult alewife by pairing a pedigree analysis with otolith-derived spawning date estimates.

Results/Conclusions

We developed a Generalized Linear Model to evaluate relationships between freshwater residence time, arrival date, length, sex, and reproductive success. We found that freshwater residence time varied widely, ranging from 1 to 64 days across our two study years (2014 and 2015). Longer residence time was associated with earlier arrival date. Sex and length did not predict residence time. Our model shows that longer residence time, along with being female, arriving earlier, and being larger, was also associated with higher reproductive success. Longer freshwater residence times likely allow alewife to develop more clutches, engage in additional mating events, and have greater access to well-suited nursery conditions. Projected shifts to earlier migrations, earlier departures from freshwater, and truncated migration periods may reduce reproductive output for alewife by limiting freshwater residence time and causing mismatches in mating and nursery conditions. While plasticity in freshwater residence time in alewife may allow them to adjust migration times and durations, lower reproductive output may still result if freshwater residence time is reduced.