Long-term acoustical observations of the mesopelagic fish Maurolicus muelleri reveal novel and varied vertical migration patterns

Abstract

We studied the temporal dynamics in the vertical distribution of Maurolicus muelleri scattering layers (SL) by examining continuous acoustic recordings over a 15 mo period in Masfjorden, Norway, complemented by intermittent sampling campaigns. The data revealed known patterns as normal diel vertical migration (DVM), midnight sinking between dusk and dawn, and periods without migrations, as well as novel behaviours consisting of early morning ascents, reverse diel vertical migrations, and interrupted ascents in the evening. During the first autumn of the study, adult fish modified their normal DVM behaviour by suspending their migration in the evening, yet ascending toward the surface in the later part of the night to reach upper layers during dawn. This behaviour was not observed during the second autumn of the study. By mid-to end of November (1st autumn), adult fish had suspended the nocturnal ascent entirely, and in the subsequent period until the end of January, a fraction of the population rather performed limited reverse migrations, slightly shifting their vertical distribution upwards during the first part of the day. From January to March 2008, fish interrupted their evening ascent at apparently random intervals and returned to deeper waters, instead of completing a full ascent to the surface. Our study underlines the value of long-term recordings, with the results suggesting that M. muelleri has the capability of changing its behaviour in response to ontogeny and internal state (satiation and hunger) as well as to external stimuli. KEY WORDS: Diel vertical migration · DVM · Scattering layers · Maurolicus muelleri · Ontogeny · Stationary hydroacoustics Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 441: [241][242][243][244][245][246][247][248][249][250][251][252][253][254][255] 2011 from the surface (Neilson & Perry 1990, Baliño & Aksnes 1993, Widder & Frank 2001, Cohen & Forward 2009, Staby & Aksnes 2011.Several hypotheses for vertical migrations on a population level have been put forward. Eggers (1978) proposed that the adaptive value of vertical migrations was to minimise predation risk by avoiding predators. The predator avoidance hypothesis was later extended to include the optimisation of foraging in the anti-predation window at intermediate light intensities at dusk and dawn (Clark & Levy 1988, Scheuerell & Schindler 2003. The bioenergetic efficiency hypothesis states that organisms distribute at temperatures which maximise growth (Wurtsbaugh & Neverman 1988, Giske & Aksnes 1992, Bevelhimer & Adams 1993, while the foragingopportunity hypothesis predicts that the depth distribution of the predator overlaps with depth changes of its prey (Levy 1990a, Neilson & Perry 1990. On an individual level, the hunger-satiation hypothesis assumes that vertical movements of individuals are triggered by a change in their hunger or satiation state (Pearre 2003).Mueller's pearlside Maurolicus muelleri Gmelin 1789, a small mesopelagic fish (standard le...

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