Comparing Lethal and Non‐lethal Methods of Active Population Control for Harbor Seals in British Columbia

Pinniped populations around the world increased rapidly after hunting and culling during the nineteenth and twentieth centuries ended. Some believe that pinnipeds are now preventing the recovery of certain fish populations, and that controlling pinniped population abundance using lethal measures such as harvesting or by non-lethal means like contraception could recover fish populations. It is unclear, however, how effective and how long it would take for such methods of population control to bring numbers of pinnipeds down to target levels. We used sex- and age-structured population models to estimate how quickly harbor seal (Phoca vitulina) abundance in British Columbia, Canada, could be reduced by 50%, through combinations of lethal removals and sterilization of adult females. Models were fit to seal abundance, demographic, and harvest data collected between 1879 and 2014. Simulation modeling suggests reliance on contraception exclusively is unlikely to reduce the current harbor seal population (numbering similar to 100,000) by 50% within 25 years, and would result in more variable outcomes, compared to lethal removals. Contraception could be combined with harvesting to maintain a target abundance of harbor seals (although captive studies with harbor seals are needed to confirm the efficacy of contraception). Our simulation modeling approach provides a useful framework to assess how non-lethal measures could be integrated into policies that promote active population control of harbor seal numbers.