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I'm sure you remember this post - or not? :)
There we speculated that an analysis of Bull Shark teeth could not only indicate whether they have entered freshwater habitats but could even indicate the specific river nursery they have visited.
Alas, it appears, we were wrong.
The results clearly confirm that those Sharks whose teeth were sampled between January and March did not sojourn in freshwater, at least not for an extended period of time.
- We did collect the teeth on the reef without being able to allocate them to any individual Shark, and therefore don't know whether any of the Sharks was a returning female that had given birth in the river nurseries. Maybe those 42 teeth we collected were all from Sharks that had not migrated to the rivers at all - improbable but by no means impossible!
- If memory serves me right, Bull Sharks have six or seven rows of teeth, with the first, outer "active" row being erect whereas the last row is not fully developed and largely covered by gum tissue.
Even assuming that tooth turnover is rapid, one would have to assume that it takes quite a while, likely weeks (?), for a tooth to migrate to the first row and then fall out. Unfortunately the paper does not expound how long it takes for a tooth to fully form whilst adding trace elements - but in view of the above, I have to assume that the entire tooth formation is a rather lengthy process.
If so, it would mean that for a tooth to show a composition that is highly indicative of freshwater exposure, the Shark would have to reside in a freshwater habitat for an extended period of time.
- Our observations of pregnant females generally show that depending on ENSO, they disappear for approx. 4-5 weeks sometimes in late October to late November before coming back to the SRMR.
As the Bulls are not resident within the SRMR, those 4-5 weeks are however not at all representative for the time the females spend in the river. Instead, they are likely to comprise a) a period spent in other marine habitats before departing to the river, b) the time spent traveling to the river nursery, c) the time spent in the river which is the only time where the teeth would pick up a freshwater signature, d) the time traveling back from the river and finally, e) the time spent elsewhere in the ocean before visiting us, and being recorded in the SRMR.
It is thus only fair to assume that the time spent in the river proper is rather short, maybe even just enough to swim in, give birth and swim back out.
- Furthermore, the Fijian Bull Sharks are not known to establish proper riverine/freshwater populations like e.g. those in the Brisbane River, the Breede or Lake Nicaragua but are instead quasi exclusively marine
- With all of that in mind, chances to find robust freshwater signatures in the teeth of the Fijian Bull Sharks were very slim to start with!
- But OF COURSE they DO go and give birth in the rivers! Big female Bulls are being seen and even caught there, and that's where we do find the juveniles!
Long story short?
Nice hypothesis, great evidence collection, really interesting and cutting-edge analysis - but alas, a rather total bust!
And so it goes - can't win them all!