Maternal transfer and long-term population effects of PCBs in Baltic grey seals using a new toxicokinetic-toxicodynamic population model

Abstract

Empirical evidence has shown that historical exposure of polychlorinated biphenyls (PCBs) to Baltic grey seals not only severely affected individual fitness, but also population growth rates and most likely caused the retarded recovery rate of the depleted population for decades. We construct a new model which we term a toxicokinetic-toxicodynamic (TKTD) population model to quantify these effects. The toxicokinetic sub model describes in detail the bioaccumulation, elimination, and vertical transfer from mother to offspring of PCBs, and was linked to a toxicodynamic model for estimation of PCB-related damage, hazard, and stress impacts on fertility and survival rates. Both sub-models were then linked to a Leslie matrix population model to calculate changes in population growth rate and age structure given different rates of PCB exposure. Toxicodynamic model parameters related to reproductive organ lesions were calibrated using published historical data on observed pregnancy rates in Baltic grey seal females. Our model showed that increased PCB exposure caused reduced fertility, decreased vertical transfer, and increased biomagnification. Compared to empirical data, the TKTD population model described well the age-specific bioaccumulation pattern of PCBs in Baltic grey seals, and thus, the toxicokinetic parameters, deduced from literature, are believed to be reliable. The model also captured well the general effects of PCBs on historical population growth rates. The developed model can be used to perform population viability analyses of Baltic grey seals with multiple stressors, also including by-catches and different hunting regimes. The model can also be extended to other marine mammals and other contaminants than PCB by adjustments of model parameter values and thus provides a test bed in silico for new substances.