Follow the light—diurnal and seasonal variations in vertical distribution of the mesopelagic fish Maurolicus muelleri

Abstract

Previous 1 d studies (Giske et al. 1990, Sarsia 75:65-81; Balino & Aksnes 1993, Mar Ecol Prog Ser 102:35-50; Rasmussen & Giske 1994, Mar Biol 120:649-664) of the mesopelagic fish Mauro- licus muelleri have suggested that their vertical distribution changes as though they were following a constant light intensity, sometimes termed the 'isolume'. Here we investigated whether such behav- iour is consistent under varying light conditions and over an extended period. We analysed acoustic observations of ontogenetically varying scattering layers (SLs) versus surface irradiance from selected days during a 9 mo period (January to September 2008). On the majority of days, a strong correlation (r 2 > 0.90) between upper SL depth and surface irradiance was observed. The SLs of M. muelleri suggest that this species has a restricted range of preferred light intensities. This tendency was found regardless of season and migration phase, i.e. during dawn descent, dusk ascent and day- time. The irradiance estimated at the top of the upper SL for consecutive days, regardless of migra- tion phase, varied on average by less than 1 order of magnitude, while the average monthly estimates for the descent, ascent and daytime periods varied from 0.004 to 0.39 µE m -2 s -1 , 0.08 to 2.35 µE m -2 s -1 and 0.02 to 0.68 µE m -2 s -1 , respectively. During their ascent, fish experienced up to 9 times higher light intensities than during the descent. Our study suggests that the vertical migration of M. muelleri emerges because of a vertical habitat selection that can be characterised by the preference of a restricted range of light intensities and that these intensities may be state and age dependent.