The interplay between culling and density-dependence in the great cormorant: a modelling approach.
The population of great cormorants Phalacrocorax carbo sinensis breeding in northern Europe has increased from 5000 pairs during around 1970 to c. 100 000 pairs in the late 1990s, leading to serious conflicts with fishery and aquaculture interests. Management action, including widespread culling, has been carried out in several countries. Since 1990, presumed density-dependent declines in demographic performance have appeared in cormorant populations. An extended Leslie matrix model was employed to study the interaction between culls and density-dependence in regulating breeding and autumn population sizes, with emphasis on evaluating the effects of culling. During 1979-92, the breeding population of great cormorants in northern Europe increased by 18% year-1, in accordance with observed life-cycle parameters before the appearance of density-dependent declines. Six scenarios were modelled with varying assumptions with regards to the strength of density-dependence in adult survival and the proportions of breeding cormorants. A series of cull estimates was also included. Scenarios with moderate or strong levels of density-dependence provided predictions that fit the observed numbers of breeding pairs, whereas scenarios without density-dependence in survival overestimated real population growth. The most well-supported scenarios indicated that the effect of culls at the present level (1998-99: 17 000 cormorants shot) was limited (<10% reduction at equilibrium). Increasing the annual cull to 30 000 still had a limited effect, whereas shooting 50 000 birds year-1 led to population extinction within 20-40 years. Shooting a fixed proportion of the population exceeding a threshold, through density-dependent culling, could eliminate differences among scenarios and stabilize the population. It was concluded that culls have probably had a limited effect on cormorant populations, but if carried out in a density-dependent way, could stabilize numbers near a desired level. However, a reduction in the number of cormorants may not lead to a similar reduction in conflicts, and actions to control damage rather than cormorant populations are likely to be more cost-effective. If culling is to be continued, the adoption of an adaptive and coordinated management strategy across Europe is recommended. The need to account for density-dependent mechanisms in general culling strategies is also advocated.