Migration pathways, behavioural thermoregulation and overwintering grounds of blue sharks in the Northwest Atlantic - PubMed (original) (raw)

Migration pathways, behavioural thermoregulation and overwintering grounds of blue sharks in the Northwest Atlantic

Steven E Campana et al. PLoS One. 2011.

Abstract

The blue shark Prionace glauca is the most abundant large pelagic shark in the Atlantic Ocean. Although recaptures of tagged sharks have shown that the species is highly migratory, migration pathways towards the overwintering grounds remain poorly understood. We used archival satellite pop-up tags to track 23 blue sharks over a mean period of 88 days as they departed the coastal waters of North America in the autumn. Within 1-2 days of entering the Gulf Stream (median date of 21 Oct), all sharks initiated a striking diel vertical migration, taking them from a mean nighttime depth of 74 m to a mean depth of 412 m during the day as they appeared to pursue vertically migrating squid and fish prey. Although functionally blind at depth, calculations suggest that there would be a ~2.5-fold thermoregulatory advantage to swimming and feeding in the markedly cooler deep waters, even if there was any reduced foraging success associated with the extreme depth. Noting that the Gulf Stream current speeds are reduced at depth, we used a detailed circulation model of the North Atlantic to examine the influence of the diving behaviour on the advection experienced by the sharks. However, there was no indication that the shark diving resulted in a significant modification of their net migratory pathway. The relative abundance of deep-diving sharks, swordfish, and sperm whales in the Gulf Stream and adjacent waters suggests that it may serve as a key winter feeding ground for large pelagic predators in the North Atlantic.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1

Figure 1. Blue shark PAT tag and pop-up locations.

Map shows tagging (∗) and pop-up (•) locations for 23 blue sharks tagged off the eastern coast of Canada. Pop-up symbols are coloured to match the corresponding tagging symbol. Month of pop-up indicated by number.

Figure 2

Figure 2. Blue shark migration pathways by year.

Reconstructed migration pathways (one colour-coded solid line per shark) of blue sharks tagged with PATs, overlaid on the SST satellite imagery on the date corresponding to their presence. Sharks not entering the Gulf Stream within 2 weeks of the date corresponding to the satellite imagery are not shown. Imagery date is shown in the lower left corner of each panel, and tag pop-up month is indicated at the end of each track. Tracks of tags 56390 and 56395 (2005), 66390 and 66391 (2006), and 34517 (2007) have been truncated by 0–2 degrees at the eastern edge of the SST imagery.

Figure 3

Figure 3. Maximum daily depth of blue sharks across months.

Maximum depths varied with the month, but were much greater after entry into warm Gulf Stream waters (red) than prior to entry (green). Symbols show mean ± 1 SE.

Figure 4

Figure 4. Mean depth and temperature of blue sharks across months.

Symbols show mean ± 1 SE depth (A) and temperature (B) while in (red) or out of (green) warm Gulf Stream waters.

Figure 5

Figure 5. Frequency distribution of depths occupied by blue sharks by time of day.

Panels show depth frequencies by 6-hr interval while in (red) or out of (green) warm Gulf Stream waters. Sharks only show extensive diel vertical migration while in the Gulf Stream, and tend to be deepest during daylight hours.

Figure 6

Figure 6. Diel changes in vertical distribution of blue sharks.

Mean ±95% CI depth (A) and temperature (B) of blue sharks by 6-hr interval while in (red) or out of (green) warm Gulf Stream waters. Sharks only show extensive diel vertical migration while in Gulf Stream.

Figure 7

Figure 7. Examples of blue shark dive profiles overlaid on the temperature field.

Time-weighted depths of individual blue sharks (solid black lines) from 2006 (left panels) and 2007 (right panels) at 6-hr intervals, overlaid on the colour-coded water temperature field as recorded by the PAT. Note the initiation of daily deep diving behaviour shortly after encountering the warm surface waters of the Gulf Stream.

Figure 8

Figure 8. Blue shark depth at midnight versus phase of the moon.

Sharks in the Gulf Stream moved to significantly greater depths as the moon became fuller. A loess curve has been fit to the data.

Figure 9

Figure 9. Net influence of diel diving behaviour on blue shark displacements.

Ocean currents averaged over 4 years at depths of 35 m (A, thin vectors) and 600 m (B). The longest vector corresponds to a speed of 45 cm/s. Displacement vectors associated with the diving behavior are shown at the pop-up locations as bold vectors in A. Note that these displacement vectors are averaged over the full lifetime of the PATs and are normalized to give the change in displacement over a 200 day period. The inset in B shows the zonal velocities through the section indicated by the bold dashed line; solid contours indicate eastward flow while the dashed contour line indicates westward flow.

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