Friday, September 24, 2010

New Research - no Bull!

First things first!
Juerg has published the sat tagging paper!

I say, finally! And very well done!
But then, having been very much involved in the Fiji portion, I’m of course fatally biased!

Lemme tell ’ya: it has been challenging!
Talk about the new shiny hammer failing spectacularly! Not that he really had a choice, but Juerg took the decision not to take the easy way and to try and catch the Sharks, drag them to a boat and then drill holes into them. Instead, Gary Adkison armed himself with pole spears and spear guns and proceeded to try and attach the tags on the fly at the end of the Shark feeds – and that was the easy part!

Big problem number one were the animals themselves.
No, not because they are pumped with testosterone (see below) and aggressive as a consequence: but boy, are those Bull Sharks tough to pierce – and not for lack of trying!
Suffice to say that the success ratio was dismal, and at the cost of a rapidly diminishing supply of supposedly unbreakable stainless steel shafts, anchors and leaders. What should have been a walk in the park quickly morphed into a logistical and also, motivational nightmare requiring that the team take several trips to Fiji instead of the planned single one.

And then, the gizmos!
All detached prematurely and happily floated away prior to uplinking.
The most spectacular fail sent an e-mail from no less than north of Cairns, prompting a premature Heureka! that quickly morphed into abject depression once Juerg started looking at the depth data that displayed an endless and disheartening series of zeros.
With all that in mind, the results are truly spectacular and a testimony to what somebody with a brain can do even when he basically gets fatally skunked.

My take on the adventure: nothing comes easy and above all, beware of those hi tech gizmos and the panache that goes with them!
Like I said in another context, those things require some major retro engineering, especially when it comes to the attachment! And before you ask: the SPOT tags are at least as unreliable, if for different reasons linked to the problems of establishing adequate connectivity.
But having said that: we may have an idea – keep watching this space!

So, without further ado, here are excerpts from Juerg’s paper.


Adult bull sharks Carcharhinus leucas were monitored with electronic tags to investigate horizontal and vertical movements in the Atlantic and Pacific Oceans.

In both locations, C. leucas showed some fidelity to specific coastal areas with only limited horizontal movements away from the tagging sites after tag attachment. Fish tagged in the Bahamas were detected mostly in the upper 2
0 m of the water column in water 25–26° C, whereas C. leucas tagged in Fiji spent most of their time below 20 m in water usually >26° C. The results highlight the importance of coastal inshore habitats for this species.


Ninety per cent of the 20 deployed PSATs uplinked to the Argos satellite system resulting in a cumulative total of 235 days on C. leucas depth and ambient water temperature occupancy and some limited information on movements away from the tagging sites.
These first such data available for large C. leucas expand the knowledge about the ecological niche of this species. In both the Atlantic and Pacific Oceans, C. leucas showed some fidelity to specific coastal areas and did not move away from these shallow habitats at large scale during the time of tag attachment. Fish in both locations used the water column from the surface down to c. 200 m, but in the Bahamas the species spent most of the time in shallow waters while in bathymetrically less constraining Fijian coastal habitats C. leucas were usually found deeper than 30 m. The results highlight the importance of coastal inshore habitats for this species. Reliable satellite tag performance still faces many challenges (Hays et al., 2007), with premature pop-off being one of the prime reasons for obtaining shorter than planned tracks from marine vertebrates (Chapman et al., 2007; Arrizabalaga et al., 2008). In most cases, it is difficult to identify reliably the reasons for premature detachment (Hays et al., 2007). The tag attachment method chosen in this study to attach pop-off tags to C. leucas from a platform (Bahamas) or by scuba (Fiji) might have resulted in insufficient tag anchoring and consequently a higher rate of premature tag loss. For example, environmental factors such as currents or the fact that the fish to be tagged could move around freely in the water column pose difficulties to the diver when approaching the animal or might affect the accuracy of tag placement.
Nevertheless, data sets obtained via satellite telemetry that span days rather than months or years can still provide meaningful insight into horizontal and vertical movement patterns and behaviour of sharks (Dewar et al., 2004; Chapman et al., 2007; Pade et al., 2009).


Previous research has shown that large-scale movements tend to be comparatively limited in C. leucas with pronounced site fidelity in coastal and continental shelf waters (Kohler et al., 1998; Kohler & Turner, 2001; Tremain et al., 2004; Curtis, 2008).

Resightings and the admittedly limited horizontal movement data obtained from this study support this proposition.
It remains, however, largely unknown why C. leucas show fidelity to certain coastal inshore habitats. A plausible factor that might explain a certain degree of site fidelity in this study is that at both tagging sites, Walker’s Cay and in the Shark Reef Marine Reserve, C. leucas have been attracted with food for the purpose of sightings by to
urists. This might have affected the behaviour of the tagged C. leucas in that, for example, fish would not leave the tagging sites on large-scale movements, but remain in the area. The majority of the tagged C. leucas in this study were resighted at the tagging sites indicating indeed some level of site fidelity. None of the C. leucas tracked for >17 days (B1, B4, F4, F6, F9, F11 and F12) with the exception of F11, however, was resighted more than twice (mean = 0・8) in the weeks following tagging, despite the fact that the means of attracting the C. leucas to the site continued. Carcharhinus leucas F11 was resighted most during tag attachment (Table III), but all seven appearances at the tagging site occurred in a relatively narrow time frame (18 October to 3 November 2004) of its entire 53 days track. This, together with results from other studies (Kohler et al., 1998; Kohler & Turner, 2001; Tremain et al., 2004; Curtis, 2008), indicates that tagged C. leucas in this study showed specific horizontal movements.

The finding of tag B2, which was attached to a female C. leucas, floating on the surface in the Stuart Inlet, Florida, at the entrance to the Indian and St Lucie Rivers is noteworthy.
The Indian River Lagoon system is considered a nursery area for different shark species including C. leucas (Snelson & Williams, 1981; Snelson et al., 1984; Curtis, 2008). The light-based longitude estimates indicate that fish B2 moved away from the tagging site in the Bahamas in a westward direction and the tag popped off in waters off the east coast of southern Florida (Fig. 4). Given the lack of accurate geolocation and the fact that this tag was floating on the surface for several days before starting to transmit its position, it is unknown how close to the coast this tag detached from the animal. Despite this finding, it therefore remains unknown whether or not female C. leucas from the Bahamas move to nursery grounds in Florida.
The possibility of such a migratory link to reproductive areas in Florida could be tested conclusively in the future using, for example, acoustic telemetry methods.


Clear diel vertical migration patterns known from other shark species (Weng
& Block, 2004; Sims et al., 2005; Weng et al., 2007; Andrews et al., 2009) were not identified in C. leucas in this study. Although diel patterns were found in some C. leucas from the Bahamas and Fiji, the relatively small differences between day and night mean depths are unlikely to be of biological significance.

Fig. 3. Histograms of mean + s.d. per cent time spent at (a) depth and (b) temperature for Atlantic ( ) and South Pacific ( ) Carcharhinus leucas tagged with pop-off satellite archival tags (PSAT).

The general notion that C. leucas spend the majority of their time in water shallower than 30m (Compagno, 1984) was confirmed for individuals tagged in the shallow waters of the Bahamas, but not for C. leucas tagged in Fiji, where they were recorded below 30 m for most of the time.
Most depth data collected to date come from studies looking at the behaviour of immature C. leucas in estuarine habitats (Simpfendorfer et al., 2005; Curtis, 2008); data on depth preferences of adult C. leucas away from shallow habitats are limited.

The data from this study provide the first evidence that adult C. leucas preferably stay between 30 and 100 m in bathymetrically non-constraining habitats with frequent visits to the surface and only occasionally dive to below 100 m. Dives below 200 m would have been possible in both locations. Defining the northern edge of the Little Bahama Bank, Walker’s Cay is part of the Abacos and the northernmost island of the Bahamas. The shallow waters of the Bahama Banks surrounding the island descend into deep Atlantic Ocean waters a few km north-east of Walker’s Cay. This is contrary to the situation on the southern coast of Viti Levu where the ocean-facing sides of the fringing reefs immediately drop to well below c. 250 m deep waters. Depth and location records obtained from PSATs and direct observation indicate that C. leucas tagged in the Bahamas stayed in shallow waters close to the tagging site, but showed increased diving activity in deeper waters off the Bahama Banks. Such behaviour is similar to what was found for C. leucas tagged on Shark Reef in Fiji where fish spend the majority of their time in the deeper waters just off the fringing reefs on the southern coast of Viti Levu.


The fact that adult C. leucas show a certain degree of site fidelity punctuated by limited migratory movements up and down coastlines makes this apex predator an important species of coastal ecosystems and underpins the need for local and possibly national or even international co-operation.
Estuarine habitats have been determined to be habitats of great concern for coastal sharks, in that these areas continue to suffer from dramatic anthropogenic environmental interactions and destruction. Identifying the movement corridors between or along coasts will therefore help when devising conservation strategies for C. leucas.

And now, to the aforementioned testosterone.
While I was, and still am battling issues of connectivity, Patric has issued a challenge that was accepted and, I believe, brilliantly met by Christie Wilcox of Observations of a Nerd.
Having heard (and believed!) it a thousand times, I consider this an absolute must read! Very, and I mean: VERY well done!

And indeed Patric: thanks Nigel and Discovery for having propagated this stupidity to millions of viewers!

And talking of Bull.
The myth goes one step further, that is, to claim that because of the testosterone, Bull Sharks are particularly aggressive. Certainly not in my experience: among the species I’ve dived with, I consider them among the most circumspect and timid, orders of magnitude less assertive than, say, Silvertips and Galapagoses!
Apparently and contrary to popular myth, the postulated link does not even exist in humans and may even be actually quite the opposite! Like always, it’s probably way more complicated and direct causality can be safely ruled out.

Plus: why, exactly, do bodybuilders dope themselves with testosterone?
Is it because they want to be more aggressive? Just look at how Bull Sharks are built – would it not have been much more plausible to assume that testosterone in Bulls is linked to their spectacular muscle mass?

Anyway, I’m digressing as usual.
All I wanted to say is Bravo Juerg and Bravo Christie!

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