Several cues prompt outmigration:
Temperature
Lake productivity (eutrophic, mesotrophic, and oligotrophic)
Photo period
Fish age
Flow
Water body “size”

All these things mix and combine in multiple fashions to produce an outmigration event(s).  No, it doesn’t happen all at once.  Like the adults on their return the juveniles exit on a bell shaped curve.  Some might leave immediately because they’re close to the outlet as a larval stage fish and are unable to “fight” the current.  Others hang as long as possible to make the most of available food supplies (zooplankton).  High productivity lakes (eutrophic) and ponds tend to hold fish longer than low productivity lakes and ponds (oligotrophic).  Like Lake George in Canaan.  Very low production pond so the juveniles come out soon and small.  Sebasticook Lake they come out later and much larger than their Sebasticook cousins.

Most of the cues are predominantly abiotic but with biotic under-drivers.  i.e. Sebasticook Lake is highly productive.  Creates many large juveniles on outmigration.  This is driven by lake productivity.  Lake productivity is driven by available nutrients.  Available nutrients supply phyto/zoo plankton blooms.  So, P is present to feed phytoplankton which in turn is fed upon by zoo plankton etc. etc.  No P or reduced P and phytoplankton production goes down and subsequently zooplankton populations are lower, feed quality is poorer, grass is greener out of this water body.
Things can prompt earlier outmigration in ponds that would typically hold fish for longer periods.  Say a tropical depression roars through and dumps 8″ of rain all of a sudden and drops water temps markedly.  Production declines.  Fish split.

Nate Gray
 Maine Department of Marine Resources
 Sea-Run Fisheries and Habitat
 Scientist
 (207) 624-6344 Office
 (207) 446-8870 Cell
 nate.gray@maine.gov