Merten, Veronique J. (2016) The trophic ecology of the oceanic orangeback squid Sthenoteuthis pteropus (Steenstrup) in the eastern tropical Atlantic. (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 50 pp.

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Abstract

In the eastern tropical Atlantic, the orangeback flying squid Sthenoteuthis pteropus is an opportunistic short-living carnivore and among the fastest growing squids. It is one of the dominant members of the epipelagic nekton community. Due to its high plasticity and ability to tolerate environments with low oxygen concentrations it is supposed to better cope with a changing ocean than many other nekton organisms. So far our understanding of its trophic ecology is limited. The present study characterises the role of S. pteropus in the pelagic food web of the eastern tropical Atlantic by investigating its diet and how its feeding habits change during ontogeny and geographical distribution. During hand jigging 129 specimens of Sthenoteuthis pteropus, ranging from 15.5 to 47.5 cm (dorsal mantle length), were caught in the eastern tropical Atlantic in the course of three oceanic expeditions in 2015. Besides body mass and size, sex, maturity stages and stomach fullness were recorded. Stomach content analysis was combined with stable isotope analysis of muscle tissue (∂15N and ∂13C) to describe diet and current trophic position of S. pteropus. Stable isotope analysis of the gladius, the chitinous backbone of the squid, which acts as an archival structure, were also conducted to study fine-scale variation in squid trophic ecology and foraging migration. This study demonstrates that Sthenoteuthis pteropus mainly preys on myctophid fishes, but also on cephalopods and crustaceans. It shows a highly opportunistic and variable feeding behaviour and potentially competes with large predatory fishes for the same prey resource. The trophic position of S. pteropus increases by likely one trophic level with increasing mantle length. The reconstructed feeding chronologies of the gladii reveal high intra- and inter-individual variability in the squid’s trophic position and foraging area that cannot be recognized in the squid diet and muscle tissue. This complex life history strategy suggests a high plasticity and capability of S. pteropus to rapidly colonize new areas and niches made available and to adapt to new environmental conditions.

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