What If Gone

Species → Atlantic Cod

Atlantic Cod

Gadus morhua

Scotian Shelf and Northwest Atlantic stocks collapsed by more than…
Extinction risk Vulnerable
Trophic cascadeMarine environmentOverfishingPredator-prey cycleCatastrophic decline
Atlantic Cod (Gadus morhua)
Photo: Hans-Petter Fjeld, Wikimedia Commons (CC BY-SA 2.5)

What if it disappeared?

Impact Major
Over a decade

Once cod was removed en masse from the Scotian Shelf, the entire food chain beneath it tipped into cascade: the small pelagic fish it fed on proliferated, the zooplankton those fish feed on collapsed, and at the level below that, phytoplankton increased while available nitrates declined.

Documented
Detail

Frank, K.T., Petrie, B., Choi, J.S. et Leggett, W.C. (2005). Trophic cascades in a formerly cod-dominated ecosystem. Science, 308(5728), 1621–1623.

The study draws on several decades of monitoring data on the Scotian Shelf (Canadian Northwest Atlantic). Before the collapse, cod and other large groundfish dominated the ecosystem and kept populations of small pelagic fish (such as sand lance and herring) at a low level. Their massive removal through overfishing triggered a four-level trophic cascade — one of the rare documented cases of such a cascade at the scale of an open marine ecosystem rather than in a closed or coastal system.

More than 20 years

Despite the halt in cod fishing, the population has not rebounded — on the contrary, the now-abundant small pelagic fish began devouring cod eggs and larvae, reversing the predator-prey relationship and actively preventing its recovery.

Documented
Detail

Frank, K.T., Petrie, B., Fisher, J.A.D. et Leggett, W.C. (2011). Transient dynamics of an altered large marine ecosystem. Nature, 477, 86–89.

The study documents a precise mechanism explaining the lack of recovery despite more than two decades of moratorium: pelagic fish (herring, sand lance, capelin), grown abundant after the disappearance of their predator, now consume cod eggs and larvae en masse — reversing the predator-prey role that prevailed before the collapse. The authors call this a 'runaway' consumption dynamic that locks the ecosystem into its new state.

By extrapolation

The pelagic fish that block cod's return have themselves depleted their own zooplankton food supply and are in turn declining — which could, over time, loosen the lock and allow a gradual return toward a cod-dominated ecosystem state.

Extrapolated
Detail

Extrapolation à partir de Frank et al. (2011), qui documente le déclin des poissons pélagiques et la tendance à la hausse du zooplancton de grande taille sur la même période, sans confirmer que cette tendance se traduira par un rétablissement de la morue.

The authors note that a trend toward greater abundance of large-bodied zooplankton could signal a gradual return toward the ecosystem's earlier state, but they themselves describe it as a still-uncertain signal rather than a confirmed recovery.

What's missing to confirm it: No published follow-up since confirms whether this zooplankton rebound has translated into a measurable increase in cod biomass on the Scotian Shelf specifically — unlike the separate northern Newfoundland stock, where a recovery has indeed been measured.

Documentation

Last verified: August 14, 2026