Marine mammal culling programs: review of effects on predator and prey populations

Abstract

Culling is widely practised as a means to reduce predation effects of terrestrial carnivores, birds and marine mammals in many parts of the world. Of marine mammals, coastal pinniped species have usually been the target of culling programs, but dolphins and a large odontocete have also been culled. We reviewed the published literature on marine mammal culling programs to evaluate the extent of their efficacy as a fisheries management measure. Changes in species' distributions and abundance demonstrate that culling programs can be very effective at reducing predator density. Several conclusions from experimental studies of terrestrial mammals and birds may also apply to marine mammal control. Firstly, predator removal generally increases productivity and population size of target prey populations, but not always. Secondly, culling programs typically involve a large proportional reduction (>50%) in predator populations. Thirdly, the effects of culling are typically dependent on continued control, and in the absence of control the population rapidly returns to pre‐culling density. This underscores the need for predator removal to be a long‐term management strategy. Fourthly, culling predators often has non‐intuitive and unintended consequences for target species and for other predator and prey species. Marine mammal culling programs rarely have measurable objectives with respect to prey populations, and their success has not been evaluated. Culling marine mammals is controversial because of the following: (i) they are high‐profile charismatic megafauna; (ii) many populations are recovering from a period of over‐exploitation while others remain threatened or endangered; and (iii) the scientific evidence needed to justify a cull is usually highly uncertain. Marine mammal culling programs should be based on scientific analysis with stated and measurable objectives to be evaluated during planned follow‐up monitoring.

Genomic signatures of population bottleneck and recovery in Northwest Atlantic pinnipeds

“…The bottleneck event (in models 3 and 4) was parameterized to reflect recent human exploitation and bounties. We used a uniform prior between 100 and 5,000 for the timing of the bottleneck ( t b ) to span exploitation by Native Americans and Europeans starting up to 9,000 years ago and peaking in the 17th and 18th centuries (Lavigueur & Hammill, 1993), as well as the government‐sponsored bounties that followed (Bowen & Lidgard, 2013; Lelli et al., 2009). We used a uniform prior between 1 and 100 for the timing of recent recovery ( t r ) following the end of the bounties and beginning of local and federal protections.…”

“…Low availability of continental shelf habitat in the North Atlantic during the last glacial maxima suggests gray and harbor seal populations may have been small (~15,000–21,000 gray seals) until approximately 12,000 years ago when habitats expanded following glacial retreat and the populations accordingly increased (Boehme et al., 2012). Historical records thereafter suggest an abundance of seals in the Northwest Atlantic in the 16th century when European explorers arrived (reviewed in Lavigueur & Hammill, 1993), but by the mid‐20th century, subsistence hunting, government‐sponsored bounties (Bowen & Lidgard, 2013; Lelli, Harris, & Aboueissa, 2009), and commercial exploitation (Mowat, 1984) had drastically reduced both seal populations. At that point, gray seals were considered rare in both eastern Canada and the Northeast United States (Davies, 1957), and harbor seal pupping colonies had been extirpated south of Maine (Katona, Rough, & Richardson, 1993).…”

Molecular prey identification in Central European piscivores

“…In contrast, piscivores with high population densities such as cormorants, herons or pinnipeds have long been perceived as food competitors by humans (Duffy ; Gosch et al . ) resulting in shootings, culling and repellent measures (Boudewijn & Dirksen ; Bowen & Lidgard ). Any management of piscivores aiming at either protecting or regulating these animals depends on a sound understanding of their feeding ecology.…”

Evaluating the effect of a selective piscivore fishery on rockfish recovery within marine protected areas

“…Culling predators has been proposed both to increase productivity and yield of exploited prey (Flaaten and Stollery, 1996;Bowen and Lidgard, 2013) and to help conserve endangered prey species (Beamesderfer et al, 1996;Williams, 2014). The basic principle of culling is that reducing predator populations reduces natural mortality rates, which in turn increases the population growth rate (Reynolds and Tapper, 1996).…”

“…When the management goal is simply to maintain the current level of population productivity, rather than to enhance it, reducing natural mortality through culling predators means that populations may be able to sustain higher harvest rates, irrespective of whether the population is a target species. However, it must be emphasized to consider that the success of culling programmes has been highly variable (Reynolds and Tapper, 1996;Bowen and Lidgard, 2013) and rests on a thorough understanding of the ecosystem in which the predator-prey interaction exists (Yodzis, 2001).…”