Science: Juvenile alewife migration influenced by observable factors

Alewives Alosa pseudoharengus are an anadromous species; each spring they migrate into coastal ponds, lakes and river systems to spawn.  Throughout the summer and into the fall, juvenile alewives migrate from freshwater nursery areas to the sea.  The timing of juvenile migration is influenced by anthropogenic and environmental factors.  Gahagan et al. (2010) conducted a study to evaluate the influence of certain environmental and endogenous variables on the timing of migration.  Sampling was performed on Bride Lake in East Lyme, Connecticut in 2006, when the first evidence of migration was detected in June and continued until August.  Migratory activity was evaluated by electronic fish counter and video on the stream used for migration between the lake and the sea.  Juvenile alewives were collected from the stream and the center of the lake by dip net; age and condition (a measure of length and weight) were measured.  Temperature, rainfall, stream discharge and lunar phase were also recorded.  The results of the study indicated that date, temperature, previous day’s rainfall and discharge were the best predictors of migration.  Most juveniles migrated in three distinct periods: late June, early July and late July.  Early in the season migration occurred approximately at dawn with an afternoon pulse and later in the season migration occurred at night.  Migrant length varied over the season but migrants were always longer, the same age or older, larger at age and in better condition than non-migrants.  Overall, the study indicated that migration of juvenile alewives is affected by a combination of environmental and endogenous factors.  Migration was initiated by transient decreases in water temperature, precipitation events and associated transient increases in steam discharge.  The greater size, age, size at age and condition of migrants over non-migrants suggests that growth and feeding success determined whether or not a fish migrated.  Also, seasonally varying environmental conditions had some influence on migration.  Fish migrated when stream flow was adequate.  The shift to night time migration was connected to low flow and water levels, and therefore thought to be a response to predation risk.  The seasonal timing of migration documented in this study was comparable to other reports but not necessarily applicable to every other site. 

Implications for anglers:  Any trout and salmon angler on Lake Michigan knows this simple rule of thumb:  find the bait fish and you’ll find the game fish.  When juvenile alewives move from the protection of their nursery grounds to deeper water, they become exposed to predation and game fish are likely to take advantage of the easy meal. Although alewives in Lake Michigan probably behave differently than alewives in the Atlantic Ocean, this study gives some conditions to look for.  After precipitation events or decreases in water temperature during late spring and summer, anglers would be wise to observe and take note of juvenile alewife movements.  Understanding what conditions initiate juvenile alewife migration and when should help anglers get their lines in the water at the right time.

Selected definitions:

Juvenile:  Period in the fish life-cycle between the larval stage and the adult stage, when individuals usually first resemble adults.   

Anthropogenic:  Derived from human activities.

Endogenous:  Resulting or originating from within an organism.

Gahagan, B.I., K.E. Gherard and E.T. Schultz. 2010. Environmental and endogenous factors influencing emigration in juvenile anadromous alewives. Transactions of the American fisheries society. 139: 1069-1082.