Population dynamics of the krill Meganyctiphanes norvegica (M. Sars, 1857) (Crustacea: Euphausiacea) in the Ligurian sea (NW Mediterranean sea). Size structure, growth and mortality modelling

Abstract

The seasonal abundance, population structure and dynamics of the euphausiid Meganyctiphanes norvegica were studied from samples taken year round in the NW Mediterranean Sea. Length-frequency distributions were generally bimodal: the first mode was interpreted as the born-in-the year cohort (0+) and the second as a combination of the older age classes (1+). To describe growth, a bilinear function provides the best fit. A growth rate of 0.052 mm day-1 (1.56 mm month-1) was calculated for the first part of the growth function and 0.0034 mm day-1 for the second part. A negative exponential mortality function was fitted, giving a mortality rate of 0.0034 day-1 (1.25 year-1). A numerical simulation of the size structure of the Ligurian population was performed and the validity of the model was checked using a multifactorial analysis. The demographic characteristics of the Ligurian population ware compared to those of other areas and were interpreted as a functional adaptation of M. norvegica to the southern part of the geographical distribution of the species.