Has the displacement of the capelin Mallotus villosus (Müller, 1776) feeding grounds induced a phenotypic response?

Capelin in the Iceland-East Greenland-Jan Mayen region has experienced a range shift over the last two decades potentially driven by climate change. The population now inhabits the east Greenland shelf during the late feeding season, instead of the north Iceland shelf as in the past. Spatial and temporal variation in phenotypic and life history traits such as body size, weight, length- and age-at-maturation, as well as body condition were used to comprehend the population response to environmental perturbations, using biological data spanning two decades. The findings showed that length-at-age, weight-at-age, body condition, and length-at-maturity increased over time, whereas age-at-maturity remained stable. A finer spatiotemporal modelling of length- and weight-at-age for each specific period, before and after the shift, showed that density-dependent effects were most prominent for all ages where the size and condition of organisms have improved over the years, likely because of reduced intra-specific competition. Temperature effects were more apparent for ages 1 and 2 where fish attain a smaller body size in warmer conditions. A positive relationship of length- and weight-at-age was apparent with net primary productivity. Along east Greenland shelf, the association of adult capelin with lower net primary productivity might be because of preferential prey selection that mature capelin feed on. By adjusting life-history traits to a new environment, the capelin population has exhibited a plastic response. A good understanding of the ecological processes that drive population response can prove useful for management in the future.