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. 2021 Mar 24;11(9):4588-4604.
doi: 10.1002/ece3.7357. eCollection 2021 May.

Fine-scale oceanographic drivers of reef manta ray (Mobula alfredi) visitation patterns at a feeding aggregation site

Joanna L Harris  1   2 , Phil Hosegood  2 , Edward Robinson  2 , Clare B Embling  2 , Simon Hilbourne  1 , Guy M W Stevens  1
Affiliations

Fine-scale oceanographic drivers of reef manta ray (Mobula alfredi) visitation patterns at a feeding aggregation site

Joanna L Harris et al. Ecol Evol. .

Abstract

Globally, reef manta rays (Mobula alfredi) are in decline and are particularly vulnerable to exploitation and disturbance at aggregation sites. Here, passive acoustic telemetry and a suite of advanced oceanographic technologies were used for the first time to investigate the fine-scale (5-min) influence of oceanographic drivers on the visitation patterns of 19 tagged M. alfredi to a feeding aggregation site at Egmont Atoll in the Chagos Archipelago. Boosted regression trees indicate that tag detection probability increased with the intrusion of cold-water bores propagating up the atoll slope through the narrow lagoon inlet during flood tide, potentially transporting zooplankton from the thermocline. Tag detection probability also increased with warmer near-surface temperature close to low tide, with near-surface currents flowing offshore, and with high levels of backscatter (a proxy of zooplankton biomass). These combinations of processes support the proposition that zooplankton carried from the thermocline into the lagoon during the flood may be pumped back out through the narrow inlet during an ebb tide. These conditions provide temporally limited feeding opportunities for M. alfredi, which are tied on the tides. Results also provide some evidence of the presence of Langmuir Circulation, which transports and concentrates zooplankton, and may partly explain why M. alfredi occasionally remained at the feeding location for longer than that two hours. Identification of these correlations provides unique insight into the dynamic synthesis of fine-scale oceanographic processes which are likely to influence the foraging ecology of M. alfredi at Egmont Atoll, and elsewhere throughout their range.

Keywords: Langmuir Circulation; acoustic telemetry; boosted regression trees; cold‐water bores; foraging ecology; internal waves.

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Conflict of interest statement

There are no competing financial, professional, or personal interests that might have influenced the performance or presentation of the work described in this manuscript. All authors have no conflict of interest to declare.

Figures

FIGURE 1
FIGURE 1
The Central Indian Ocean with Chagos Archipelago; British Indian Ocean Territory indicated within the red box (left inset). The Chagos Archipelago with Egmont Atoll indicated within the red box (left). Egmont Atoll and the location of the oceanographic and acoustic receiver mooring in Manta Alley (red and yellow dots) and four acoustic receivers (green dots) (top right). Bathymetric view of Manta Alley obtained via multibeam survey (E. Robinson, P. Hosegood, A. Bolton, unpublished data) showing the location of the moorings (bottom right). Bottom right legend showing instrument configurations of the long thermistor string (red dot/pin) and subsurface ADCP moorings (yellow dot/pin) deployed 182 m apart anchored at a depth of 66 m. Z is the height above the seabed
FIGURE 2
FIGURE 2
Reef manta rays (Mobula alfredi) engaged in feeding activities at the Manta Alley feeding aggregation site in north Egmont Atoll. Photo by Simon Hilbourne, Manta Trust
FIGURE 3
FIGURE 3
Original current u and v components (yellow dashed lines) and after clockwise rotation 117° relative to north (white lines). Arrows on the white lines show the direction of longshore u (LS U −ve and LS U +ve) and cross‐shore v (CS V −ve and CS V +ve). Showing mooring locations: long thermistor string (red dot) and subsurface ADCP moorings (yellow dot)
FIGURE 4
FIGURE 4
Percentage of detections at each site
FIGURE 5
FIGURE 5
Percentage distribution of detections by hour of the day at Egmont Atoll for all tagged M. alfredi (left), adults only (middle), and juveniles only (right)
FIGURE 6
FIGURE 6
Resident events at each site showing location (by color) and time at the location (by size)
FIGURE 7
FIGURE 7
Partial dependency plots showing the effect of each predictor variable: temperature 2 m above the bed (Temp 2 m), depth–mean backscatter intensity linearly (Backscatter), temperature 50 m above the bed (Temp 50 m), upward (+ve) and downward (−ve ) current flow (Vertical velocity), time relative to high tide in steps of 5‐min (0.083 hr) with high tide zero, negative values before (flood) and positive values after (ebb) (Time to high tide), near‐surface current 48.5 m above the bed (depth 17.6 m) flowing 117° (−ve ) and 297° (+ve) relative to N (Longshore (u) 48.5 m), near‐bed current 8.5 m above the bed (depth 57.6 m) flowing 117° (−ve ) and 297° (+ve) relative to N (Longshore (u) 8.5 m), near‐surface current 48.5 m above the bed (depth 17.6 m) flowing 27° (−ve ) and 207° (+ve) relative to N (Cross‐shore (v) 48.5 m), near‐bed current 8.5 m above the bed (depth 57.6 m) flowing 27° (−ve ) and 207° (+ve) relative to N (Cross‐shore (v) 8.5 m), on the occurrence of tagged M. alfredi at Manta Alley while keeping all other variables at their mean. The green line shows smoothed partial decency. Rugs display the distribution of the data for presence (top, blue), and absence (red, bottom)
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