Abstract
The jumbo squid Dosidicus gigas plays an important role in marine food webs both as predator and prey. We investigated the ontogenetic and spatiotemporal variability of the diet composition of jumbo squid in the northern Humboldt Current system. For that purpose we applied several statistical methods to an extensive dataset of 3,618 jumbo squid non empty stomachs collected off Peru from 2004 to 2011. A total of 55 prey taxa was identified that we aggregated into eleven groups. Our results evidenced a large variability in prey composition as already observed in other systems. However, our data do not support the hypothesis that jumbo squids select the most abundant or energetic taxon in a prey assemblage, neglecting the other available prey. Indeed, multinomial model predictions showed that stomach fullness increased with the number of prey taxa, while most stomachs with low contents contained one or two prey taxa only. Our results therefore question the common hypothesis that predators seek locally dense aggregations of monospecific prey. In addition D. gigas consumes very few anchovy Engraulis ringens in Peru, whereas a tremendous biomass of anchovy is potentially available. It seems that D. gigas cannot reach the oxygen unsaturated waters very close to the coast, where the bulk of anchovy occurs. Indeed, even if jumbo squid can forage in hypoxic deep waters during the day, surface normoxic waters are then required to recover its maintenance respiration (or energy?). Oxygen concentration could thus limit the co-occurrence of both species and then preclude predator-prey interactions. Finally we propose a conceptual model illustrating the opportunistic foraging behaviour of jumbo squid impacted by ontogenetic migration and potentially constrained by oxygen saturation in surface waters.
Highlights
The ommastrephid jumbo squid Dosidicus gigas is the most abundant nektonic squid in the surface waters of the world ocean [1,2] and supports the largest cephalopod fishery
The eastern tropical Pacific is characterised by the presence of an oxygen minimum zone (OMZ) [17] and D. gigas is a part-time resident of the OMZ thanks to adapted behavior and specific metabolic characteristics [18,19]
We decipher one paradox in the jumbo squid diet: why do they hardly forage on the tremendous biomass of anchovy Engraulis ringens distributed off coastal Peru? We show that the shallow OMZ in this area could hamper the cooccurrence of jumbo squids and anchovies, impacting jumbo squid foraging behaviour
Summary
The ommastrephid jumbo squid Dosidicus gigas is the most abundant nektonic squid in the surface waters of the world ocean [1,2] and supports the largest cephalopod fishery This squid, endemic to the Eastern Tropical Pacific, is mainly distributed in the oceanic domain [3] over a wide bathymetric range [4]. The impact on exploited marine resources can be strong [4] and the broad trophic niche of jumbo squid is enhanced further by physiological abilities This squid can undertake extensive vertical migrations, up to 1200 m, foraging on deep, mid-water and surface organisms [2,7,14,15]. Jumbo squid vertical migrations impact the vertical energy flow, providing an efficient energy transport from the surface to deeper waters [7,15]
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