TY - JOUR

T1 - Risk for overexploiting a seemingly stable seal population

T2 - influence of multiple stressors and hunting

AU - Silva, Willian T. A. F.

AU - Bottagisio, Elio

AU - Harkonen, Tero

AU - Galatius, Anders

AU - Olsen, Morten Tange

AU - Harding, Karin C.

PY - 2021

Y1 - 2021

N2 - Conservation efforts have mainly been focused on depleted species or populations, but many formerly reduced marine mammal populations have recovered to historical abundances. This calls for new management strategies and new models for ecological risk assessment that incorporate local density dependence and multiple environmental stressors. The harbor seal metapopulation in Swedish and Danish waters has increased from about 2500 to 25,000 over the past 40 yr. Trend analysis based on aerial survey data and somatic growth curves indicates that the population is close to carrying capacity. We performed a population viability analysis based on realistic life history parameters and investigated a range of potential scenarios caused by future stressors. If the population is able to resume its high intrinsic rate of increase at about 11% annually, when pushed down below carrying capacity, it can also sustain additional mortality such as modest hunting and infrequent epizootics. However, if xenobiotics will cause even a slight reduction in average fecundity, the population becomes significantly more vulnerable. In the absence of epizootics, and given full reproductive capacity, hunting of a few hundred animals annually is not harmful to the long-term persistence of the population. Nevertheless, a slight decrease in growth potential, for example, caused by exposure to endocrine disruptors, makes even limited hunting risky. Our study shows how an apparently stable and abundant marine mammal population can be close to a point of rapid population decline. Thus, careful monitoring of population size, growth rate, health, and exposure to xenobiotics as well as recording of the age and sex structure of the hunt is required to avoid repeating the history of overexploitation and another population collapse.

AB - Conservation efforts have mainly been focused on depleted species or populations, but many formerly reduced marine mammal populations have recovered to historical abundances. This calls for new management strategies and new models for ecological risk assessment that incorporate local density dependence and multiple environmental stressors. The harbor seal metapopulation in Swedish and Danish waters has increased from about 2500 to 25,000 over the past 40 yr. Trend analysis based on aerial survey data and somatic growth curves indicates that the population is close to carrying capacity. We performed a population viability analysis based on realistic life history parameters and investigated a range of potential scenarios caused by future stressors. If the population is able to resume its high intrinsic rate of increase at about 11% annually, when pushed down below carrying capacity, it can also sustain additional mortality such as modest hunting and infrequent epizootics. However, if xenobiotics will cause even a slight reduction in average fecundity, the population becomes significantly more vulnerable. In the absence of epizootics, and given full reproductive capacity, hunting of a few hundred animals annually is not harmful to the long-term persistence of the population. Nevertheless, a slight decrease in growth potential, for example, caused by exposure to endocrine disruptors, makes even limited hunting risky. Our study shows how an apparently stable and abundant marine mammal population can be close to a point of rapid population decline. Thus, careful monitoring of population size, growth rate, health, and exposure to xenobiotics as well as recording of the age and sex structure of the hunt is required to avoid repeating the history of overexploitation and another population collapse.

KW - conservation

KW - hunting

KW - Phoca vitulina

KW - population dynamics

KW - strategy

U2 - 10.1002/ecs2.3343

DO - 10.1002/ecs2.3343

M3 - Journal article

VL - 12

JO - Ecosphere (Washington, D.C.)

JF - Ecosphere (Washington, D.C.)

SN - 2150-8925

IS - 1

M1 - 03343

ER -