Chronic ocean warming drives 20% annual decline in fish biomass
Chronic ocean warming is driving a near 20% annual decline in fish biomass across large swathes of the Northern Hemisphere, according to a startling new study led by scientists at the Museo Nacional de Ciencias Naturales and the National University of Colombia.
It’s a sweeping study that goes some length to paint the complexity of the picture. This is a story more nuanced than a simple downward trend, but one that has found that extreme marine heatwaves – those now increasing in frequency and intensity – can temporarily obscure this long-term decline by triggering short-lived biomass surges in some of the ocean’s colder regions.
Drawing on an unprecedented dataset of 702,037 biomass change estimates from 33,990 fish populations recorded between 1993 and 2021, the team examined trends across the Mediterranean, the North Atlantic and the Northeastern Pacific. The scale of the analysis provides critical insight for fisheries management and marine conservation at a time when global food security is under mounting pressure.
Marine heatwaves do not affect all species equally. Instead, impacts hinge on what scientists describe as a species’ “thermal comfort zone” – the temperature range in which it can grow and reproduce most effectively.
When extreme heat pushes already warm-water populations beyond that zone, the consequences can be severe. In these regions, biomass can plummet by as much as 43.4%. Conversely, fish populations in cooler waters may temporarily flourish as temperatures rise, with biomass increases of up to 176%.
“Although this sudden increase in biomass in cold waters may seem like good news for fisheries, these are transient increases. If managers raise catch quotas based on biomass increases caused by a heatwave, they risk causing the collapse of populations when temperatures return to normal or when the effect of long-term warming prevails, because these are short-lived increases,” warns MNCN researcher Shahar Chaikin.
