Marine biologists have been observing something off the coast of Sydney that would have seemed unlikely a generation ago. Sydney Harbour is now frequently home to tropical fish, which are colorful, warm-water species that are typically found further north, nearer the Great Barrier Reef. Pushed south by water temperatures that their biology can no longer withstand, they are arriving on what some researchers refer to as a one-way trip. The animals that inhabit Australia’s east coast are reacting to one of the planet’s fastest-warming marine areas in the only way they know how. They’re heading out.
This is not a singular incident. It’s happening in the waters off the coast of Scotland, the North Atlantic, the Pacific, and the fishing grounds that provide food for billions of people. According to research compiled by ClimateCheck, over 80% of marine life on Earth is currently changing its migration, feeding, or breeding patterns as a result of warming ocean waters. Some ocean species have already moved up to 600 miles from their previous location just a few decades ago, and they are moving ten times faster than land species. In the colloquial language of marine science, the rate of change is concerning.
It’s worthwhile to consider the numbers underlying all of this. Since 1850, the average ocean temperature has increased by 0.88 degrees Celsius. This may seem insignificant, but for cold-blooded fish, even a small degree can mean the difference between a habitable and uninhabitable environment. Up until the middle of 2024, the North Atlantic broke sea surface temperature records for 421 days in a row. From May 4, 2023 to May 4, 2024, sea surface temperatures set new worldwide records every single day. In addition to absorbing an estimated 90% of the excess heat produced by greenhouse gases, the oceans are currently warming twice as quickly as they did in the 1960s. This burden is becoming more and more apparent in what’s happening beneath the surface.
| Topic | Rising Ocean Temperatures & Fish Migration Patterns |
|---|---|
| Average Ocean Temperature Rise Since 1850 | 0.88°C (1.5°F) |
| Ocean Warming Rate vs 1960s | Twice as fast |
| % of Marine Life Changing Behavior | More than 80% |
| Speed of Ocean Species Migration vs Land | 10 times faster |
| Maximum Distance Species Have Relocated | Up to 600 miles from former locations |
| North Atlantic Record-Breaking Streak | 421 consecutive days of temperature records broken |
| Global CO₂ Emissions (2023) | 37.4 billion tonnes (record high) |
| Ocean Oxygen Decline Since Mid-20th Century | ~2% |
| Plankton Reduction Threat | Up to 26% decline possible under continued warming |
| Alaska Snow Crab Fishery | Collapsed — partly attributed to ocean floor heatwave |
| UK Fish Trend | Growing faster but reaching smaller adult sizes |
| Black Sea Bass Population Shift | ~2 degrees of latitude northward |
| Key Research Bodies | NOAA, CSIRO, University of Glasgow, Wildfish, IFAW |
Reference Links: Rising Ocean Temperatures and Marine Life — ClimateCheck Impact of Rising Temperatures on Wild Fish — Wildfish
The most obvious effect is that species that require cool water migrate poleward. Whiting and herring are heading toward the polar regions where they will spawn. Previously avoiding northern waters, Atlantic cod are now establishing themselves there. Along the US East Coast, black sea bass have moved their population center about two degrees northward in latitude. Pacific tuna populations have been moving eastward, away from the fishing grounds of Pacific Island nations that rely on them economically. This shift poses serious problems for nations whose entire fishing industries are centered around the former locations of those fish. Small-island commercial fishing may be the most sensitive industry in the world to the geographic distribution of fish, and at the moment, that distribution is changing more quickly than fishing agreements can keep up.
However, there is more to the migration story than just direction. It’s also about timing, and it’s difficult to overestimate the cascading effects of timing errors in the ocean. Many fish species time their reproductive cycles to coincide with plankton blooms, which are short, rich windows when microscopic organisms rise close to the surface and supply the food that young fish require to survive. Plankton abundance is decreased by warmer water because it keeps deep, nutrient-rich cold water from combining with warmer surface water. Recruitment rates decline when fish reach their customary spawning locations only to discover that the bloom has either already peaked or has not yet occurred. The cycle is completed by fewer fish. There are fewer births. Within a few decades, population dynamics that took centuries to develop begin to fall apart.
Salmon are especially good at telling this tale. They leave their river habitats to spawn, sometimes traveling thousands of miles to reach the ocean, only to return. Because of their extreme sensitivity to temperature, they need more oxygen than the oxygen-depleted warming water can supply because warmer water speeds up their metabolism. In UK waters, Atlantic salmon are currently endangered. Whales, seabirds, bears that eat salmon, and even the trees along riverbanks that take up nutrients from salmon carcasses are all impacted when fewer fish make it to spawning grounds. The failure of a fish to migrate is also, subtly, a failure of a forest.
Of all the consequences, the body-size story is arguably the least obvious. Because higher metabolic rates require more oxygen and warmer water contains less of it, fish in warmer waters grow faster as juveniles but reach smaller adult sizes. Fish populations in the UK are already exhibiting this pattern: larger juveniles and smaller adults. Fish markets and dinner plates are directly impacted economically. Smaller fish produce less per catch, put pressure on fishing quotas that are adjusted for animals of varying sizes, and alter the calculations of commercial viability for fishing fleets with narrow profit margins.
This has a species-level component that is equally concerning. Polar and sub-polar regions are being overrun by species that were never a part of those ecosystems, while tropical regions are losing biodiversity as species flee rising temperatures. Sharks are competing with established predators for food as they venture farther north. Due in part to heatwave conditions on the ocean floor that severely reduced crab populations, Alaska’s snow crab fishery, one of the most valuable in the world, effectively collapsed in recent years. Even scientists who had been closely monitoring the warming trend were taken aback by the abrupt collapse.
As all of this builds up, it seems like the discussion of fish migration has been too narrowly framed as an environmental issue, despite the fact that it is also a geopolitical, economic, and food security issue. Fishing rights agreements that have been negotiated over many years are based on the historical habitats of fish. These agreements become sources of conflict rather than cooperation as species move across jurisdictional boundaries. The economic effects of tuna shifting away from waters that are essential to their fishing industries are already being felt by nations like Kiribati and the Maldives.
The exact limit on these modifications is still unknown. Marine migration patterns are predicted to be severely disrupted under mid-to-high emissions scenarios for the remainder of the century, with polar regions absorbing invasive species more quickly than they can adapt and tropical regions continuing to lose biodiversity. The ocean lacks a rapid healing mechanism. The heat already absorbed by the oceans would continue to alter marine ecosystems for centuries even if carbon emissions stabilized this year.
All of this is unknown to the fish. As cold-blooded animals have always done, they are merely tracking temperature gradients in search of environments that suit their biology. The issue is that the conditions are changing more quickly than the biology can adjust, and the water no longer exactly matches the map they have been using for millions of years.
