Low-temperature-driven early spawning migration of a temperate marine fish

1. It is often assumed that the timing of annual migrations of marine fish to spawning grounds occurs with very little change over time. However, it is unclear how much migration is influenced by climate change in marine species that spawn at sea but spend most time in estuarine conditions, especially as thermal regimes in estuaries may differ significantly from those in the open sea. 2. Migration phenology was studied in a population of flounder, Platichthys flesus (L.) off south-west England using high-temporal resolution trawling data over a 13-year period. 3. Flounder migrated from their estuarine habitat to spawning grounds at sea some 1-2 months earlier in years that were up to 2degreesC cooler. Flounder arrived on the spawning grounds over a shorter time period (2-6 days) when colder than normal conditions prevailed in the estuary, compared to warmer years (12-15 days). This suggests that they were responding to low temperatures by exhibiting a more synchronous, population-level early migration. 4. The timing of migration was earlier when the largest differences in temperatures between near-estuary and offshore environments occurred, differences that were related significantly to cold, negative phases of the North Atlantic Oscillation (NAO). 5. Flounder migration phenology appears to be driven to a large extent by short-term, climate-induced changes in the thermal resources of their overwintering habitat. This suggests that climate fluctuations characterizing the NAO may have significant effects on the timing of the peak abundance of fish populations generally, which, in turn, may have implications for fisheries management.