Movement patterns of juvenile whale sharks tagged at an aggregation site in the Red Sea

Abstract

Conservation efforts aimed at the whale shark, Rhincodon typus, remain limited by a lack of basic information on most aspects of its ecology, including global population structure, population sizes and movement patterns. Here we report on the movements of 47 Red Sea whale sharks fitted with three types of satellite transmitting tags from 2009-2011. Most of these sharks were tagged at a single aggregation site near Al-Lith, on the central coast of the Saudi Arabian Red Sea. Individuals encountered at this site were all juveniles based on size estimates ranging from 2.5-7 m total length with a sex ratio of approximately 1∶1. All other known aggregation sites for juvenile whale sharks are dominated by males. Results from tagging efforts showed that most individuals remained in the southern Red Sea and that some sharks returned to the same location in subsequent years. Diving data were recorded by 37 tags, revealing frequent deep dives to at least 500 m and as deep as 1360 m. The unique temperature-depth profiles of the Red Sea confirmed that several whale sharks moved out of the Red Sea while tagged. The wide-ranging horizontal movements of these individuals highlight the need for multinational, cooperative efforts to conserve R. typus populations in the Red Sea and Indian Ocean.

Figure 1. Study sites for Rhincodon typus in the Saudi Arabian Red Sea.

(A) Location of the study area within the Red Sea. (B) Locations of 59 satellite tag deployments on juvenile R. typus near Al-Qunfudhah (n = 2) and Al-Lith (n = 57). (C) Detail of tag deployments around Shi’b Habil near Al-Lith (n = 55). Symbol color indicates the year of tag deployment. Basemap sources: ESRI, AND, USGS, TANA.

Figure 2. Size frequency histogram of Rhincodon typus individuals of known sex tagged with satellite tags at an aggregation site in the Saudi Arabian Red Sea.

Bars represent the number of individuals estimated to the nearest 50

Figure 3. Movements of 47 Rhincodon typus tagged with satellite tags in the Saudi Arabian Red Sea.

(A) Most individuals (n = 39) made basin-scale movements within the southern Red Sea. (B) Three R. typus performed excursions into the northern Red Sea as far as Sharm el-Sheikh. (C) Five sharks departed the Red Sea and moved into the Gulf of Aden and northern Indian Ocean. Green and red diamonds indicate tagging and tag pop-off locations, respectively. Track lines were removed from (A) for clarity. Basemap sources: ESRI, AND, USGS, TANA.

Figure 4. Daily depth-temperature plots for four Rhincodon typus tagged near Al-Lith in the Saudi Arabian Red Sea.

Depth is indicated on the y-axis and by the length of the colored data column. Time is indicated on the x-axis with each data column representing a day of reported data. Days without a column indicate that no data were received for that day. Water temperate at a given depth is indicated by the color of the column (temperature scale in °C indicated on the right-hand axis). (A) Platform Terminal Transmission ID 106745, a male shark of unrecorded length tagged in April 2011. (B) PTT ID 106752, a 4.5 m female shark tagged in April 2011. (C) PTT ID 52585, a shark tagged in March 2010. (D) PTT ID 52588, a 3 m shark tagged in April 2010.

Figure 5. Composite time-at-depth histogram for 32 Rhincodon typus tagged with pop-up satellite archival tags in the Saudi Arabian Red Sea in 2010–2011.

(Sharks tagged in 2009 were excluded from this analysis due to low bin resolution in transmitted time-at-depth data.) Data in horizontal bars represent the reported mean time spent in a particular depth range by individuals over the course of tag deployment. Note variable depth intervals on y-axis.

Figure 6. Time-at-depth plots for three Rhincodon typus tagged near Al-Lith in the Saudi Arabian Red Sea that exhibit considerable occupation of deep water.

Depth is indicated on the y-axis and by the length of the colored data column. Time is indicated on the x-axis with each data column representing a day of reported data. Days without a column indicate that no data were received for that day. The percentage of time spent within a given depth range on a given day is indicated by the color of the column (percentage scale indicated on the right-hand axis adjacent to panel C). (A) Platform Terminal Transmission ID 52535, a 3 m shark tagged in April 2010. (B) PTT ID 52571, a 5 m male shark tagged in April 2010. (C) PTT ID 95972, a 4 m female tagged in June 2009. Note variable scale of x-axis.

Figure 7. Habitat utilization distribution (UD) aggregated for all 47 whale sharks tagged with pop-up satellite archival transmitting (PSAT) tags in the Saudi Arabian Red Sea in 2009–2011.

Seasons were defined according to lunar calendar. UD is composed of all track locations based on methods indicated in Table 1. The overall distribution indicates core-use areas (warm colors) near Al-Lith in the spring and further offshore and southward through the remaining seasons. Color terminates at 95% UD (peripheral-use areas).