Vanishing Spring Spawners? Changing Stock Dynamics of Atlantic Herring in Newfoundland

Herring stocks in the northwest Atlantic are composed of both autumn and spring spawning fish, with the dominant reproductive strategy following a latitudinal gradient of autumn spawners to the south and spring spawners to the north.  This distribution is thought to be largely determined by environmental conditions, with spring spawners being more suited to colder climates. Over the past decade, a warming trend has led to a shift toward autumn spawner dominance in northern areas, including along the south and northeast coast of Newfoundland. These stocks which were once composed of 90% spring spawners now have proportions as low as 10%, due to declines in abundance and recruitment. These declines have been accompanied by behavioral changes, with spawning now occurring during the summer in many areas. This contradicts past work that suggested warmer winters would lead to larger gonads and earlier spawning times. A more likely explanation is that spawning cycles are linked to the annual spring phytoplankton bloom and resulting zooplankton abundance. With changes in temperature and plankton community dynamics in recent years, this could have led to a mismatch in the timing of spring spawning and the resulting decline in recruitment for that stock component in Newfoundland.