Scientists have finally discovered the reason behind declining worldwide salmon stocks, and the answer will both surprise and alarm …

For the first time scientists have discovered that migrating salmon can detect mammalian predators by the scent of already digested salmon in wastewater, which allows the migrating fish to determine whether its safe to move upstream.

"It’s the predator’s diet – not just its own smell – that’s alerting the salmon," explains Dr. Laura Roberts from the University of Swansea, co-author of the report published in Animal Behavior.

Otters are common predators of salmon so it’s clearly useful to the fish to be able to sniff them out.

The smell acts as an early-warning system for the fish, even when they can’t see the predator. It lets them work out the potential risk of being eaten and balance predator avoidance with other vital activities like foraging and reproducing.

… and test dives performed just outside the mouths of historic salmon rivers have found all the missing salmon milling about waiting for someone else to chance the first dash upstream.

Females eventually give up and release their roe in one girdle busting spasm – and then they all rush back to the safety of the open ocean. Wastewater treatment hasn’t been able to remove the scent of the fillet you ate yesterday, and everytime you flush you’ve reinforced the notion that upstream is instant death.

Again it’s your fault … if you’d been man enough to walk back to the porta-potty – instead of using a nearby bush, them fish would’ve bowled you over with sheer numbers.