Long-Term Effects on Somatic Growth, Life History Traits and Population Biology of Arctic Charr (Salvelinus alpinus) Following the Fish Culling Experiment in Takvatn

Abstract

Stunting in fish populations is a special case of density-dependent growth characterized by old slow-growing individuals with low recreational and resource use value. The phenomenon is commonly observed in lacustrine populations of Arctic charr (Salvelinus alpinus) in temperate regions. In Takvatn, a subarctic lake in northern Norway, stunted growth of Arctic charr was attributed to food shortage in the overcrowded fish population. A large-scale culling experiment removing 31 tons (666 000) of Arctic charr was carried out between 1984 and 1989. The culling resulted in a substantial reduction in the population density, with a decrease of almost 80% in the littoral zone by the end of the culling. The present study investigates the long-term changes in somatic growth and life history traits – age and size at maturity – of Arctic charr over four decades during and following the culling experiment and addresses how these responses are reflected at the population level. Somatic growth of Arctic charr was shown to increase substantially the initial years following the culling before stabilizing at an enhanced level over time, as a response to the increased food availability for the remaining charr. Furthermore, an investigation of the impact of increasing water temperature due to climate warming on somatic growth revealed no clear temperature effect, suggesting that the changes in population density plays the most significant role in explaining the growth enhancement observed in Takvatn. The study further demonstrated a shift in maturation schedule towards maturation at earlier age but larger sizes, likely as a compensatory response to the reduced population density. In addition, males and females appeared to respond differently to the density reduction the initial years after the culling started. Finally, the responses of the culling at the population level were manifested in a shift in the age- and size structure from a dominance of small-sized old fish pre-culling to a higher proportion of younger individuals as well as larger individuals. The new demographic structure has persisted for more than three decades, emphasizing its strong resilience. The present study highlights the importance of a detailed examination of individual-level responses to culling to increase our understanding of density-related implications on the demographic structure at the population level. Such insights, when applied to management practices, enable the assessment of population status and the establishment of fishery regulations grounded in scientific research.