The δ15N values of organisms are commonly used across diverse ecosystems to estimate trophic position and infer trophic connectivity. We undertook a novel cross-basin comparison of trophic position in two ecologically well-characterized and different groups of dominant mid-water fish consumers using amino acid nitrogen isotope compositions. We found that trophic positions estimated from the δ15N values of individual amino acids are nearly uniform within both families of these fishes across five global regions despite great variability in bulk tissue δ15N values. Regional differences in the δ15N values of phenylalanine confirmed that bulk tissue δ15N values reflect region-specific water mass biogeochemistry controlling δ15N values at the base of the food web. Trophic positions calculated from amino acid isotopic analyses (AA-TP) for lanternfishes (family Myctophidae) (AA-TP ∼2.9) largely align with expectations from stomach content studies (TP ∼3.2), while AA-TPs for dragonfishes (family Stomiidae) (AA-TP ∼3.2) were lower than TPs derived from stomach content studies (TP∼4.1). We demonstrate that amino acid nitrogen isotope analysis can overcome shortcomings of bulk tissue isotope analysis across biogeochemically distinct systems to provide globally comparative information regarding marine food web structure.

When offered a choice between open water and a ‘vegetated’ area of an aquarium (artificial sea grass), mysid (Paramesopodopsis rufa) swarms always selected open water. In contrast, juvenile and adult sea horses, Hippocampus abdominalis, which feed by ambush predation, always selected vegetated areas even when their mysid prey occurred in the adjacent clear water. Lunging predators, Australian salmon (Arripis trutta), always preferred open water to vegetated regions of the tank. Increasing vegetation density, i.e. from low to medium habitat complexity had a positive effect on capture success of juvenile and adult sea horses feeding on mysid swarms. However, number of attacks by juvenile sea horses, but not adults, was significantly reduced in higher density vegetation. Number of attacks by Australian salmon decreased as vegetation density increased but capture success, nonetheless, increased. Density of vegetation did not significantly affect the number of unsuccessful attacks by adult sea horses. Whereas proportion of unsuccessful attacks increased with vegetation density for juvenile sea horses, it decreased for juvenile salmon. This result is thought to be due to the fact that high vegetation densities disrupt structure of the prey swarm, itself a defensive formation, leaving a large number of more vulnerable small groups. Cohesive escape responses of a large swarm are likely to be a more effective defence against a lunging predator than against an ambush predator whose strategy is to use stealth to avoid provoking escape responses. Dense vegetation will militate against this strategy.

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