April 27 - Dolphins for Dinner?

A bit of follow up on yesterday's post...I got questions about my reference to orcas. On 13 March, we observed two large killer whales passing through the area, in waters where franciscanas had been seen. We know nothing of the diet of killer whales in this area, whether they are strictly fish eaters, or whether they also eat dolphins, porpoises, whales, sea lions, elephant seals, penguins, etc.

April 26 - Where is Marta?

Unfortunately, our daily maps do not include Marta... We originally tagged four Franciscana dolphins, but lost the signal for one fairly early on.

We do not know why we lost Marta’s signal. There were some indications of a possible problem with the electronics of the tag over the last few days of transmission (the tag sends out function status reports periodically when it is working properly, in addition to location data). There are plenty of scenarios that might explain this: the tag could have come off (as it is designed to do if it should become entangled, for example), or sadly the dolphin could have been lost from the population. There are plenty of potential dolphin predators abound in Bahia San Blas. The town’s claim to fame is as a recreational shark fishing site. On our last day, we observed killer whales passing through the area where we had recently received signals from Marta. (I posted the orca picture on the left when we first started the blog.)

While we do not know why we lost the tag, I know the problem was not the battery... The tag indicated good voltage through its last status report. The tag is programmed to provide potentially 100 days of transmissions and we were nowhere near that point.

While it was not a battery problem, the way the tag is affixed to the dolphin is a bit more dicey. The tag is attached to the fin with plastic pins held on with corrosible metal nuts. These are designed to corrode in saltwater. Temperature and salinity will affect how fast this will happen, but it should take several months, at which point the tag will fall off the fin.

So...after all that, we just don't know what happened with Marta...

April 25, 2007

The dolphins are continuing their pattern of the past few days; with Roberto deep in the bay, Lea farther to the east, and Yaana outside the mouth of the bay.

April 24, 2007

From west to east, Roberto, Lea, and Yaana. She continues to spend time near the mouth of the bay.

April 23 - Dolphins Are Different...

Dolphins are different... Different from us and from one another! Here's an example: the tagged Franciscana dolphins have stayed in or VERY close to the mouth of the bay the entire time. I study Atlantic bottlenose dolphins and as a species, they show far more variation in the way they behave depending on whether they live close or far from shore---to the point that some taxonomy folks wonder if there might be subspecies or two different species (but I'll leave that debate to those who specialize in taxonomy).

But with the coastal/estuarine species like the Franciscanas, we do not yet know much about the inshore/offshore distribution. Data from Franciscana dolphins caught in fishing nets suggest that adult males may be found further offshore than mothers with calves. This is consistent with our three years of tagging experience, in which we have caught mostly adult females and juvenile males - no adult males - in the shallow inshore waters.