Linking vertical movements of large pelagic predators with distribution patterns of biomass in the open ocean

Abstract

Many predator species make regular excursions from near-surface waters to the twilight (200 to 1,000 m) and midnight (1,000 to 3,000 m) zones of the deep pelagic ocean. While the occurrence of significant vertical movements into the deep ocean has evolved independently across taxonomic groups, the functional role(s) and ecological significance of these movements remain poorly understood. Here, we integrate results from satellite tagging efforts with model predictions of deep prey layers in the North Atlantic Ocean to determine whether prey distributions are correlated with vertical habitat use across 12 species of predators. Using 3D movement data for 344 individuals who traversed nearly 1.5 million km of pelagic ocean in [Formula: see text]42,000 d, we found that nearly every tagged predator frequented the twilight zone and many made regular trips to the midnight zone. Using a predictive model, we found clear alignment of predator depth use with the expected location of deep pelagic prey for at least half of the predator species. We compared high-resolution predator data with shipboard acoustics and selected representative matches that highlight the opportunities and challenges in the analysis and synthesis of these data. While not all observed behavior was consistent with estimated prey availability at depth, our results suggest that deep pelagic biomass likely has high ecological value for a suite of commercially important predators in the open ocean. Careful consideration of the disruption to ecosystem services provided by pelagic food webs is needed before the potential costs and benefits of proceeding with extractive activities in the deep ocean can be evaluated.

Keywords: bio-logging; bioacoustics; deep ocean; marine megafauna; movement ecology.

Fig. 1.

Use of the deep pelagic ocean is widespread among predators. (A) Daily satellite tag–based position estimates of all tagged species in the North Atlantic Ocean. (B) Individuals made extensive, often daily, vertical movements into the deep ocean. Mean daily maximum depth (across species) indicates that these species regularly move into at least the upper mesopelagic (yellow colors in B) in almost every grid cell but many traverse the meso- and into the bathypelagic, $>$1,000 m (blue colors in B). See SI Appendix, Fig. S1 for species-specific daily max depth.

Fig. 2.

Predators revealed a broad spectrum in how they use deep pelagic habitat. There are clear species-specific patterns in how tagged individuals use the deep ocean: from limited time at depth (e.g., blue marlin, A) to periodic “dives” from the surface to deep ocean characterized by less overall time at depth (e.g., Chilean devil ray, B, bigeye tuna, C, and blue shark, D) to consistent daily use of deep habitats (e.g., swordfish, E) to seasonal residency in the ocean twilight zone (e.g., basking sharks, F). See SI Appendix, Fig. S2 for all species.

Fig. 3.

Biotic and abiotic factors modulate deep pelagic habitat use. Mean deep scattering layer depths ($Z_{\mathit{DSL}}$) and thermal structure ($\mathrm{\Delta }$T) were used as explanatory variables in a model to predict daily 95th percentile of predator daytime depth distribution (predicted q95, $P_{q95}$) for comparison against $Z_{q95}$, the observed q95 (A). The distribution of model-derived $Z^{\prime }$ (model-predicted q95 relative to the predicted mean DSL depth; Eq. 1) varies as a function of DSL depths and water column thermal structure (B). Swordfish vertical habitat use, for example, is clearly influenced by DSL depth (as evidenced by the coupling of predictions and observations in A) but is largely unaffected by temperature as $Z^{\prime }$ remains unchanged over a $\sim$10 ${}^{°}$C range in $\mathrm{\Delta }$T (in B). In contrast, model-predicted q95 for blue sharks also indicates strong agreement with the observations (in A); yet, while $Z^{\prime }$ indicates the predicted q95 relationship to mean DSL depth is consistent for low values of $\mathrm{\Delta }$T (muted colors, $Z^{\prime }$$\pm$ 50), it becomes increasingly disconnected as $\mathrm{\Delta }$T indicates larger temperature gradients (saturated red, $Z^{\prime }$$\sim$$-$200).

Fig. 4.

Predator interaction with scattering layers highlights complex predator–prey dynamics in the deep ocean. Individual predator dive data (black) overlaid on backscatter from shipboard Acoustic Doppler Current Profilers, as a proxy for potential prey (color), for four example species (A–D) for which predator data could be adequately collocated to a nearby research vessel with high-quality acoustic data. White points overlaid on the predator dive data indicate tag-derived detections of bioluminescence events deeper than 200 m (presence-only; see SI Appendix, Supplemental Methods). The yellow color around 18:00 in panel (C) indicates noise in the acoustic data.