Environmental and Endogenous Factors Influencing Emigration in Juvenile Anadromous Alewives

We analyzed migration of juvenile anadromous alewives Alosa pseudoharengus at Bride Lake, a coastal lake in Connecticut, during summer 2006 and found that migration on 24‐h and seasonal time scales was influenced by conditions of the environment and characteristics of the individual fish. To identify environmental cues of juvenile migration, we continuously video‐recorded fish at the lake outflow and employed information‐theoretic model selection to identify the best predictors of daily migration rate. More than 80% of the approximately 320,000 juvenile alewives that migrated from mid‐June to mid‐August departed in three pulses lasting 1 or 2 d. Pulses of migration were associated with precipitation events, transient decreases in water temperature, and transient increases in stream discharge. Diel timing of migration shifted over the summer. Early in the season, most migration occurred around dawn; late in the season, migration occurred at night. To identify individual characteristics associated with migratory behavior, we compared migrating juveniles collected as they were exiting Bride Lake with nonmigrating juveniles collected from the center of the lake. Migrants were a nonrandom subset of the population; they were on average 1–12 mm larger, were 2–14 d older, had grown more rapidly (11% greater length at age), and were in better condition (14% greater mass at length) than nonmigrant fish. We infer that the amount of accumulated energy has a positive effect on the net benefit of migration at any time in the migratory season.