The interplay between culling and density‐dependence in the great cormorant: a modelling approach

Abstract

Summary1. The population of great cormorants Phalacrocorax carbo sinensis breeding in northern Europe has increased from 5000 pairs 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 taken in several countries. 2. Since 1990, presumed density-dependent declines in demographic performance have appeared in cormorant populations. We employed an extended Leslie matrix model to study the interaction between culls and density-dependence in regulating breeding and autumn population sizes, with emphasis on evaluating the effects of culling. 3. 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. 4. We modelled six scenarios with varying assumptions about the strength of densitydependence 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. 5. 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. 6. We conclude that culls probably have had a limited effect on cormorant populations, but if carried out in a density-dependent way they 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, we recommend the adoption of an adaptive and co-ordinated management strategy across Europe. We also advocate the need to account for density-dependent mechanisms in general culling strategies.

Fish are not safe from great cormorants in turbid water

“…Although scientific evidence supporting these views is scarce (Engstrom 2001), cormorant culling has been resumed in several countries of the European Union, including France, where 17 000 non-breeding great cormorants are estimated to have been shot annually since the late 1990s (Frederiksen et al 2001). However, ecological research predicted that the European great cormorant population would self-regulate according to the carrying capacity of regional freshwater ecosystems (Suter 1995).…”

Towards optimized population control efficiency in space and time: A modelling framework adapted to a colonial waterbird

“…However, our modelling framework is flexible enough to allow additional sophistication of the model as well as more complex, real-life scenarios to be investigated in the future. For instance, future versions of the model could accommodate the possibility of density-dependent survival (Frederiksen and Bregnballe, 2000; but see also Frederiksen et al, 2001) and the possibility that cormorants, particularly first-time breeders, use public information to select a breeding colony (Henaux et al, 2007). More realistic scenarios may include irregular spatial grids with unequal spacing between active colonies, and the fact that different colonies may have different carrying capacities and/or be at different stages of their dynamics.…”

“…The first method is based on Frederiksen et al (2001). The breeding proportion of year 2 subadults and adults (b2 and b3, respectively) are density-independent (b(0) values) when the number of potential breeding pairs (pot pairs) is below a given threshold thr DDb, pot pairs being calculated as pot pairs = b2(0) × B2m + b3(0) × B3m, where B2m is the number of year 2 subadult males and B3m the number of adult males present.…”

Development of great cormorant population (Phalacrocorax carbo sinensis) in North-East France – synthesis of long term monitoring (1997–2008)

“…We can conclude that observed decrease is not linked with shooting in accordance with Marion (2003) and Paquet (2005). Frederiksen et al (2001) also conclude that culls probably have had a limited effect on cormorant populations, but if carried out in a density-dependent way they could stabilize numbers near a desired level. Moreover, we can emphasise that regulation by shooting only very partially influences the local numbers, the birds shot in this attractive zone being rapidly replaced by a constant flow of new wintering and migrating birds (Keller and Lanz, 2003).…”