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

Previous 1 d studies (Giske et al. 1990, Sarsia 75:65–81; Baliño & Aksnes 1993, Mar Ecol Prog Ser 102:35–50; Rasmussen & Giske 1994, Mar Biol 120:649–664) of the mesopelagic fish Maurolicus 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 behaviour 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 (r2 > 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 daytime. The irradiance estimated at the top of the upper SL for consecutive days, regardless of migration 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.