Vertical habitat use by whale sharks ('Rhincodon typus')

Abstract

The vertical habitat use has been the focus of research for fishes of the open ocean (e.g. sharks, tunas and swordfishes) for the last 30 years, following the development of satellite-linked tags. In Chapter 1 I reviewed recent studies in this field. These reveal that most fishes use surface waters (< 10 m) and the mixed layer and display patterns of vertical migration that correspond to the vertical movement of the sound scattering layer (SSL). This layer is an accumulation of invertebrates and small fishes that use depths of approximately 250-500 m during the day and < 200 m during the night. The match between movement patterns and the SSL suggests that foraging is the principal factor deriving vertical habitat use of fishes in the open ocean. A great variety of evidence suggests that these foraging patterns are constrained by physical gradients (notably temperature and oxygen) within the water column. Most analyses of time-depth records (TDRs) provided by satellite tags have divided time series on an arbitrary basis (e.g. day-night) or analysed the record as a continuous time series to detect cyclic patterns. In Chapter 2 I used a novel approach to the analysis of TDRs to detect specific behaviours of whale sharks ('Rhincodon typus') and to gain insights on vertical habitat use.