How Industrial Chemicals are Driving Harbour Seal Mortality
Polychlorinated Biphenyls (PCBs) were industrial chemicals prized for their stability. That same stability makes them a persistent environmental poison. Though banned for decades, millions of tonnes remain, contaminating our oceans and accumulating in marine life. As top predators, harbour seals face the highest risk, becoming living records of this toxic legacy.
PCBs enter the marine food web and become dangerously concentrated in a process called biomagnification. At each step up the food chain, the concentration of PCBs increases, reaching its peak in apex predators like harbour seals.
Low PCB concentration
PCBs absorbed
Concentration rises
Concentration magnifies
Peak concentration
PCBs don't kill seals outright. Instead, they cause chronic, sublethal damage to the body's most critical systems, making seals vulnerable to other threats like disease and starvation.
PCBs weaken the immune system, reducing a seal's ability to fight off viral and bacterial infections. This turns manageable illnesses into deadly epidemics.
PCBs interfere with the thyroid hormone system, disrupting metabolism and energy regulation. This makes seals less resilient to food shortages.
Mothers transfer a lifetime's worth of accumulated PCBs to their first-born pups through their milk, leading to high rates of infant mortality and slowing population recovery.
PCB contamination varies significantly by region, reflecting historical industrial activity. Seals in areas like Puget Sound and the Wadden Sea carry dangerously high toxic burdens compared to those in less industrialized waters.
Mean ΣPCB concentrations (µg/g lw) in harbour seal pups or adults from various regions. Data reflects different time periods and highlights industrial hotspots.
The devastating Phocine Distemper Virus (PDV) epidemics in Europe highlight the role of PCBs as a threat multiplier. While not the direct cause, PCB-induced immunosuppression likely increased the severity of the outbreaks, leading to mass mortality.
1988 PDV Epidemic
~23,000
Harbour Seals Died
2002 PDV Epidemic
~30,000
Harbour Seals Died
While global bans have led to a significant decrease in PCB levels since the 1980s, the rate of decline has slowed. Legacy PCBs in sediments continue to contaminate the food web, keeping levels in many seal populations above the threshold for adverse health effects.
Illustrative trend of PCB concentrations in Puget Sound harbour seal pups, showing a sharp decline followed by a plateau.