Water column structure defines vertical habitat of twelve pelagic predators in the South Atlantic

Abstract

Quantifying vertical distributions of pelagic predators elucidates pelagic ecosystem structure and informs fisheries management. In the tropical South Atlantic Ocean, the recently designated large-scale marine protected area around Ascension Island hosts diverse pelagic predators for which basin-specific vertical habitat information is minimal or absent. We used pop-up satellite archival tags to analyse vertical habitat use in 12 species (bigeye tuna Thunnus obesus, blue marlin Makaira nigricans, blue shark Prionace glauca, dolphinfish Coryphaena hippurus, Galapagos shark Carcharhinus galapagensis, oceanic whitetip Carcharhinus longimanus, sailfish Istiophorus albicans, silky shark Carcharhinus falciformis, swordfish Xiphias gladius, tiger shark Galeocerdo cuvier, wahoo Acanthocybium solandri, and yellowfin tuna Thunnus albacares) and quantify parameters (temperature, dissolved oxygen, diel cycles, lunar phase) known to constrain vertical movements. Predator depth distributions varied widely, and classification trees grouped predators into four clades: (i) primarily epipelagic; (ii) partial thermocline use; (iii) oscillatory diving with thermocline/sub-thermocline use; and (iv) extensive use of sub-thermocline waters. Vertical habitat differences were linked to thermal physiology and foraging ecology, and species-specific physical constraints from other ocean basins were largely conserved in the South Atlantic. Water column features defined species-specific depth distributions, which can inform fisheries practices and bycatch risk assessments and population estimates.

Diving into the vertical dimension of elasmobranch movement ecology

“…It is possible that the remotely sensed measure of SST used in the models was too coarse to describe the fine-scale effects on individual vertical movements. In addition, the effect of SST is modulated by subsurface physical variables, such as mixed-layer depth ( 21 ), which can be highly variable but could not be estimated here. Aligning vertical data with archived temperature records provides an alternative avenue to assess how elasmobranch movement in this plane will be influenced by changing climates.…”

“…Yet, an early study ( 18 ) and recent reviews have identified consistent vertical patterns among certain taxa, including diel vertical movement (DVM) and deep-diving behaviors ( 19 , 20 ). Furthermore, a recent synthesis of a regional predator assemblage in the South Atlantic revealed that dynamic species-specific space use was tied to water column structure ( 21 ). Direct comparisons of vertical space use among elasmobranchs, however, are often hampered by inconsistent reporting of depth data.…”

Global habitat loss of a highly migratory predator, the blue marlin (Makaira nigricans)

“…Ten environmental variables were selected to describe the habitat preference of blue marlin based on their importance in previous studies (Brodie et al, 2018; Carlisle et al, 2017; Madigan et al, 2021): SST, surface chlorophyll‐a (Chl‐a), sea surface height (SSH), oxygen at 100 m depth, total kinetic energy (TKE), mixed layer depth (MLD), bathymetry, rugosity and the standard deviation of SST and SSH (an index of frontal activity, Brodie et al, 2018) (Table S1). Oxygen at 100 m was chosen as a 2‐dimensional representation of oxygen depth limitation (Carlisle et al, 2017), which may influence the horizontal distribution of blue marlin.…”

“…Ten environmental variables were selected to describe the habitat preference of blue marlin based on their importance in previous studies (Brodie et al, 2018;Carlisle et al, 2017;Madigan et al, 2021):…”

Water column structure influences long-distance latitudinal migration patterns and habitat use of bumphead sunfish Mola alexandrini in the Pacific Ocean

“…It has been reported that many pelagic fishes tend to show avoidance responses to hypoxia zones with minimum oxygen concentrations < 2.5 ml L –1 39 . Although bumphead sunfish displayed a wide range in vertical movements, they preferred to stay in regions with high DO concentration (4–5 ml L –1 ) which is similar to other epipelagic predators, e.g., dolphinfish and billfish 40 – 42 . During northward migrations, bumphead sunfish spent most of their time in high DO environments where water temperature and mixing of eddies influenced DO concentrations 43 , 44 .…”