Historical population size of the harbour seal,Phoca vitulina,in the Delta area,SW Netherlands

Seals in the Dutch Delta area have been subject to hunting pressure for centuries, promoted by a bounty system which generated a sort of hunting statistics. Hunting mortality is used to estimate historical population size. Based on ranges for most likely net recruitment rates, corresponding population trajectories are back calculated from an assessed population size of 350 seals in 1960. It is concluded that the size of the harbour seal population in the Delta area in 1900 will have been close to 11 500 animals. Significant loss of habitat has occurred due to closing off parts of the larger estuaries and the enlargement of the entrance to the harbour of Rotterdam in the 1960s and early 1970s. It is estimated that about 4000 harbour seals could inhabit the remaining Delta area under tidal influence. This outcome, based on retrospective population analysis, will be an important reference in defining management objectives for the recovery of the harbour seal population in the Delta area, which amounted to 18 seals in 1992.