Life-history and population dynamics of a short-lived Norwegian capelin stock in a changing environment

Abstract The population dynamics and diet of capelin in Balsfjord in northern Norway were studied and compared to earlier studies there and in other areas. The temperature in the capelin habitat in Balsfjord has increased ca. 1 °C from the 1980s to the study period (2019–2022). Results indicate that length at age 2 and mortality rates have slightly increased since 1981. The mortality rate of adult age 2 to 3 capelin in Balsfjord (ca. 1.8 year⁻ 1 ) was high compared to juvenile capelin in the Barents Sea. Capelin in Balsfjord primarily mature at age 2, which is 1–2 years earlier than the typical maturity age (3–4 years) in the Barents Sea. Production per biomass (P/B) values were calculated using a simulation model with a Monte Carlo filtering approach that also accounts for production at early stages. The input data for the model consisted of estimates of fecundity, mortality rates, body weights, maturity at age proportions and age-composition of the spawning stock. P/B-values for Balsfjord (ca. 2.2 year −1 ) were higher than for capelin from Barents Sea for the same time-period (ca. 1.7 year −1 ). These P/B-values exceeded most values used for capelin in ecosystem models. The diet of capelin in Balsfjord in 2019 and 2022 was dominated by copepods and euphausiids, similar to findings from 1978. The Balsfjord capelin exhibits high fecundity, early maturation and high mortality rates, likely approaching the limits of its phenotypic plasticity in response to relatively warm water and elevated mortality conditions.