Contrasting responses of Arctic charr and brown trout to compensatory nutrient enrichment in an oligotrophicated reservoir

Many large lakes in northern Scandinavia have become oligotrophicated due to hydroelectric water regulation in the twentieth century, causing a loss of littoral habitat and negative consequences for ecosystem productivity, fish populations, and fisheries. Compensatory nutrient enrichment is a potential remediation method that has successfully been carried out in Canada and the US. Here we assessed the response of Arctic charr (<i>Salvelinus alpinus</i>) and brown trout (<i>Salmo trutta</i>) to nutrient addition in a whole lake experiment in Stor-Mjölkvattnet, Sweden, with nearby Burvattnet as a reference. Nitrate and phosphate were added for eight consecutive years. The study also included sampling the seventh year after discontinuation of nutrient addition, which allowed us to investigate how long nutrient enrichment would be effective on fish growth. Populations of Arctic charr and brown trout responded quickly and vigorously to the treatment, with approximately a doubling of the catch per unit effort. Nutrient addition had a consistent positive effect on charr length, weight, and condition at a given age, with a median response to nutrient addition (as measured by Shapley values) of 32 mm, 45 g, and 0.087 g cm⁻³ × 100. The response in length and weight was strongest in the age classes 4+ and 5+. The corresponding responses of trout were 13 mm, 32 g, and 0.044 g cm⁻³ × 100, respectively. Seven years after the enrichment had ended, charr at ages ≤6+ years were back to their previous state before treatment, i.e. slow growing and in bad condition. The older age-classes of charr (≥7+), however, were in good condition, suggesting that those fish, as young had experienced the excellent conditions prevailing in the last years of nutrient enrichment and largely kept this advantage. We conclude that compensatory nutrient addition is a useful method for restoring charr populations and reversible. Large water level fluctuations in northern hydropower reservoirs cause severe damage to littoral habitats, eventually causing lowered biological productivity including lowered growth rate and lower abundance of Arctic charr.Compensatory nutrient enrichment can mitigate the damage on native populations of Arctic charr by increasing plankton production and thereby restore charr growth to levels similar to before impoundment.After discontinuation of nutrient addition, the fish growth returns to low levels.Compensatory nutrient enrichment is thus an effective, yet reversible, management method for restoring charr fisheries in heavily damaged reservoirs. Large water level fluctuations in northern hydropower reservoirs cause severe damage to littoral habitats, eventually causing lowered biological productivity including lowered growth rate and lower abundance of Arctic charr. Compensatory nutrient enrichment can mitigate the damage on native populations of Arctic charr by increasing plankton production and thereby restore charr growth to levels similar to before impoundment. After discontinuation of nutrient addition, the fish growth returns to low levels. Compensatory nutrient enrichment is thus an effective, yet reversible, management method for restoring charr fisheries in heavily damaged reservoirs.

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