Scientists: Ocean acidification due to climate change could weaken salmon sperm - Ocean acidification is crippling salmon and sea urchin reproduction

Rising carbon dioxide levels are making oceans more acidic, and this shift is now threatening the reproduction of salmon and sea urchins. Scientists have discovered that the same chemical process controls sperm movement in both species—and acidification is disrupting it. Without intervention, declining pH levels could make it harder for these animals to reproduce at all.

Salmon and sea urchins rely on mass spawning, where males and females release eggs and sperm into the water for fertilisation. For sperm to swim and reach the eggs, they need a signalling molecule called cAMP. This molecule is produced by an enzyme that only activates in higher pH conditions.

Ocean acidification, caused by increasing CO₂ in the atmosphere, lowers seawater pH. As the water becomes less alkaline, the enzyme struggles to trigger cAMP production. Without enough cAMP, sperm stay motionless, preventing fertilisation entirely.

The research team first observed this pH-dependent mechanism in sea urchins before confirming it in salmon. They now suspect similar processes may exist in other marine species. Beyond direct effects on reproduction, acidification also weakens sea urchins' ability to form shells, harming larval survival along European coasts like the North Sea and Atlantic. Salmon face indirect threats too, as disrupted food webs reduce prey populations, lowering egg viability and juvenile growth—particularly in Norwegian fjords.

Unlike mammals, where cAMP is regulated by bicarbonate rather than pH, these marine species are far more vulnerable to chemical changes in their environment.

If ocean acidification continues, the decline in pH could further reduce cAMP production, crippling sperm motility in salmon and sea urchins. This would shrink populations by making reproduction far less successful. The findings also suggest that other marine animals relying on similar mechanisms may face the same risks.