Temporal and geographical variation in body condition of common minke whales (Balaenoptera acutorostrata acutorostrata) in the Northeast Atlantic

Abstract

The common minke whale (Balaenoptera acutorostrata acutorostrata) is a boreo-arctic species, and the summer period is generally characterized by intensive feeding and consequently seasonal fattening at high latitudes. The fat deposited is stored as energy reserves for overwintering at lower latitudes where feeding is greatly reduced. It is therefore expected that their body condition on the summer grounds will reflect food availability during their most intensive feeding period and thus indicate how well the high-latitude ecosystems can support the populations. During the commercial catch operations on feeding grounds in Norwegian waters, body condition data (blubber thickness and girth) were collected from 10,556 common minke whales caught from 1993 to 2013. To investigate associations between condition and time/area, we applied the following three models: (1) multiple regression models with covariates, sex, year, latitude and longitude, to find significant coefficients of the covariate; (2) random effect models involving the random effects of variations by year or area and with sex as a fixed variable; (3) varying coefficient models, which were applied to investigate variation with year/area and to interpret covariate effects by visualizations. The significance of the estimated coefficients can be assessed by statistical tests. In conclusion, the total trend over the 2 decades of data available suggests a decrease in minke whale condition. However, this trend was most pronounced during the high summer season when we considered the seasonal effect over the annual sampling periods from April to September.