September 18, 2026
What Happens in the Ocean in Fall? Four Seasonal Changes Beneath the Waves
Estimated reading time: 0 minutes
BY: Uzezi Odharo
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The leaves are changing colour, days are getting shorter, and temperatures are cooling, which means one thing: it’s officially fall, but what happens in the ocean in fall?
While we can easily spot the signs of seasonal change on land, the changes happening beneath the surface are a little harder for us to notice. From whale migration and salmon spawning to phytoplankton blooms and Arctic sea ice formation, the ocean in fall is active.
Fall Migration in the Ocean
Every year, some of the most remarkable migrations in the ocean take place in fall. Pacific humpback whales that spend spring through late fall in western Canadian waters feeding on krill and small schooling fish migrate to warmer, sheltered waters off Hawaii, Mexico, and Central America.
North Atlantic right whales travel more than 1,600 kilometeres from their feeding grounds in eastern Canadian waters to calving grounds in the shallow, coastal waters off North Carolina, South Carolina, Georgia, and Florida.
Sea turtles aren’t left out either. Because they are cold-blooded, their body temperature is largely influenced by the water around them. Leatherbacks migrate to Atlantic Canada, especially around Nova Scotia and the Bay of Fundy, to feed on jellyfish before returning south in the fall to nesting grounds in French Guiana, Suriname, Grenada, the United States, and Trinidad.
Spawning
Cooler water and other environmental cues help signal an important stage in the life cycle of some salmon: spawning season. During the fall salmon run, many salmon leave the ocean, where they have spent years feeding and growing. These remarkable fish return to the freshwater streams and rivers where they were born to spawn.
Blooms
While the largest phytoplankton bloom occurs in spring, the ocean blooms again each fall on a smaller scale. In many parts of the North Atlantic, cooling surface waters mix with deeper, nutrient-rich water, triggering a seasonal phytoplankton bloom. Phytoplankton form the foundation of marine food webs and help absorb carbon dioxide from the atmosphere, making them an important part of Earth’s climate system. These blooms can sometimes be seen as a shift in the colour of the water. Some patches are so large that they can be seen from space, visible in satellite photographs.
Arctic sea ice freeze-up
Arctic sea ice goes through a seasonal cycle each year. In the fall, as sunlight fades and air temperatures drop, new ice begins to form at the pole. From there, the ice keeps building through the winter, with the total area it covers usually reaching its maximum extent by early March.
It’s the quietest event on this list, and one of the biggest: a vast layer of sea ice slowly spreading across the top of the world as winter returns to the Arctic. The annual freeze-up helps shape Arctic ecosystems and provides critical habitat for species such as polar bears, seals and walruses.
The Ocean in Fall: A Season of Transition
From whales beginning epic migrations to salmon returning home to spawn, fall marks a season of transition beneath the waves. While harder to spot than the turning leaves on land, the ocean is equally busy as it prepares for winter.
The ocean goes through several seasonal changes in the fall, four of the most notable are: large-scale animal migration south, fall spawning for some salmon, a second seasonal phytoplankton bloom, and the start of Arctic sea ice freeze-up as the polar ocean begins to harden for winter.
Shortening days and dropping water temperatures signal it’s time to leave cold, food-rich summer feeding grounds for warmer waters where whales can mate and give birth. North Pacific humpbacks, for example, leave Alaska and British Columbia for Hawaii, Mexico, and Central America.
Salmon spawning season in Canada generally runs all year including in the fall, peaking in September and October. For example, Pacific salmon runs in British Columbia rivers like the Fraser River peak between September and November, depending on the species and population.
Arctic sea ice begins forming each fall as sunlight decreases and air temperatures drop, typically starting in September and October. The ice continues building through winter, usually reaching its maximumby early March.
The leaves are changing colour, days are getting shorter, and temperatures are cooling, which means one thing: it’s officially fall, but what happens in the ocean in fall?
While we can easily spot the signs of seasonal change on land, the changes happening beneath the surface are a little harder for us to notice. From whale migration and salmon spawning to phytoplankton blooms and Arctic sea ice formation, the ocean in fall is active.
Fall Migration in the Ocean
Every year, some of the most remarkable migrations in the ocean take place in fall. Pacific humpback whales that spend spring through late fall in western Canadian waters feeding on krill and small schooling fish migrate to warmer, sheltered waters off Hawaii, Mexico, and Central America.
North Atlantic right whales travel more than 1,600 kilometeres from their feeding grounds in eastern Canadian waters to calving grounds in the shallow, coastal waters off North Carolina, South Carolina, Georgia, and Florida.
Sea turtles aren’t left out either. Because they are cold-blooded, their body temperature is largely influenced by the water around them. Leatherbacks migrate to Atlantic Canada, especially around Nova Scotia and the Bay of Fundy, to feed on jellyfish before returning south in the fall to nesting grounds in French Guiana, Suriname, Grenada, the United States, and Trinidad.
Spawning
Cooler water and other environmental cues help signal an important stage in the life cycle of some salmon: spawning season. During the fall salmon run, many salmon leave the ocean, where they have spent years feeding and growing. These remarkable fish return to the freshwater streams and rivers where they were born to spawn.
Blooms
While the largest phytoplankton bloom occurs in spring, the ocean blooms again each fall on a smaller scale. In many parts of the North Atlantic, cooling surface waters mix with deeper, nutrient-rich water, triggering a seasonal phytoplankton bloom. Phytoplankton form the foundation of marine food webs and help absorb carbon dioxide from the atmosphere, making them an important part of Earth’s climate system. These blooms can sometimes be seen as a shift in the colour of the water. Some patches are so large that they can be seen from space, visible in satellite photographs.
Arctic sea ice freeze-up
Arctic sea ice goes through a seasonal cycle each year. In the fall, as sunlight fades and air temperatures drop, new ice begins to form at the pole. From there, the ice keeps building through the winter, with the total area it covers usually reaching its maximum extent by early March.
It’s the quietest event on this list, and one of the biggest: a vast layer of sea ice slowly spreading across the top of the world as winter returns to the Arctic. The annual freeze-up helps shape Arctic ecosystems and provides critical habitat for species such as polar bears, seals and walruses.
The Ocean in Fall: A Season of Transition
From whales beginning epic migrations to salmon returning home to spawn, fall marks a season of transition beneath the waves. While harder to spot than the turning leaves on land, the ocean is equally busy as it prepares for winter.
The ocean goes through several seasonal changes in the fall, four of the most notable are: large-scale animal migration south, fall spawning for some salmon, a second seasonal phytoplankton bloom, and the start of Arctic sea ice freeze-up as the polar ocean begins to harden for winter.
Shortening days and dropping water temperatures signal it’s time to leave cold, food-rich summer feeding grounds for warmer waters where whales can mate and give birth. North Pacific humpbacks, for example, leave Alaska and British Columbia for Hawaii, Mexico, and Central America.
Salmon spawning season in Canada generally runs all year including in the fall, peaking in September and October. For example, Pacific salmon runs in British Columbia rivers like the Fraser River peak between September and November, depending on the species and population.
Arctic sea ice begins forming each fall as sunlight decreases and air temperatures drop, typically starting in September and October. The ice continues building through winter, usually reaching its maximumby early March.
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