Wednesday, February 29, 2012

Great Whites - Great Paper!

GW picture by Chip Scarlett

It should come as no surprise that after the Junior fiasco, I'm certainly not a great admirer of the GW researchers within the TOPP program, at least not when it comes to their ethical attributes.

But I must say, this is as good as it gets.
From the paper.

About what the Sharks may be doing in the White Shark Café.

Despite the likely reduction in foraging opportunities in the Subtropical Gyre, white sharks are offshore during a period of time when prey availability in this region may be relatively high.
It is logical that white sharks would time their offshore migrations to coincide with periods of increased prey availability. The reported spawning area of the neon flying squid (Ommastrephes bartrami) generally overlaps temporally and spatially with white sharks in offshore waters as does that of the purpleback flying squid (Sthenoteuthis oualaniensis), suggesting that spawning aggregations of Ommastrephid squid may be an important prey resource in offshore waters. Similar to the neon flying squid, Pacific pomfret (Brama japonica) also make seasonal migrations, foraging in the transition zone or subarctic waters during the summer then moving south to subtropical waters to spawn during winter and spring. Catches of pelagic fishes and sharks by the Japanese long line fishery can be substantial, though not necessarily at their peak, in this region during the time of year that white sharks are present. It has also been noted that the occurrence of white sharks in Hawaii coincides with birthing of humpback whales.
Therefore, it appears that white sharks may be using these offshore habitats when prey availability, though likely still lower than in coastal habitats, is relatively greater than at other times of the year.

Although these offshore habitats may not provide optimal foraging conditions at present, it is important to remember that historical conditions may have been very different.
It is possible that with the decline of large pelagic fishes, sharks and whales, the historical forage base of white sharks in these habitats has diminished. In addition, the decline of the Hawaiian monk seals, may have also removed an important prey item for white sharks. It seems likely that historically there was a more abundant prey field available for white sharks to exploit in pelagic habitats.
Therefore, to some extent white shark migrations may be vestigial and reflect historical conditions.

About why they may be traveling there

One of the fundamental questions regarding white sharks in the NEP is why they undertake these extensive and regular offshore migrations, leaving the highly productive California Current for oligotrophic waters of the central Pacific.
The two primary hypotheses are that these movements are related to foraging or reproduction. While our data are not able to directly address the reproductive hypothesis, they are useful for evaluating the foraging hypothesis. Our results indicate that 1) white sharks do forage in offshore habitats, though at a lower rate, and 2) white sharks may initiate these offshore migrations around the size of first maturity.
These results, in combination with the observation that white sharks returning to central California aggregation sites often are lean, suggest that although white sharks feed offshore, it appears foraging may not be the primary motivation for offshore migration.

If white sharks are not moving offshore to feed, an alternative explanation is that the offshore migrations have a reproductive purpose, possibly playing a role in gestation, parturition or mating.
The particular reproductive function that offshore habitats may play in white shark life history is unclear, but it is possible that use of these habitats is related to gestation and/or mating as parturition is believed to occur in the southern California Bight. It is possible that females use the warm waters of the Subtropical Gyre to aid in gestation, and indeed there is some evidence that females segregate from males in offshore habitats, especially when in the Hawaiian focal area. If female white sharks have an extended gestation period (12 to 18 months), extended use of warm offshore waters may explain the return of some large females to coastal aggregation sites every other year.

Very interesting!
Needless to say, I very much look forward to finding out how this dovetails with Michael Domeier's multi-year tracking results, and his own interpretation of the data!

But look for yourselves.
Thankfully, the paper is open access and you can read it in its entirety right here.


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