Bottlenose dolphins like local lady Slightwin are members of a keystone species, so-called because they make it relatively easy to track the health of a marine environment, like our estuary.
Dolphin watching is always mesmerizing. But it is not always fun, especially when you get that creepy feeling that you might be documenting the slow demolition of their community. I know that sounds awful. But the data are not painting the prettiest picture at the moment.
All animal populations ebb and flow naturally. An essential part of field biology and conservation science is documenting everything about an animal community so you get to know their norm. Only when you know their norm thoroughly can you recognize changes and propose educated guesses about whether any particular change is of little concern - because it is just an unusual part of the norm - or is of considerable concern because it is significantly different from the norm.
Capt. John Heidemann and I have been studying the local dolphins for 10 years now. With over a thousand surveys to draw on, I can say with certainty that if you were to take our survey route, you would encounter an average of 12 dolphins.
But that is only on average, the most representative number of a large number of numbers. The actual number of dolphins we encounter across surveys varies greatly, from zero to 56. The professional dolphin literature contains one reason why: Virtually every bottlenose dolphin community that has been studied exhibits a mini-migration.
Individuals move between different waters to spend the summers and winters. So I checked to see if John’s Pass dolphins did too. If so, seasonal changes explain some of the variability in dolphin sightings.
Yes, John’s Pass dolphins go on mini-migrations like other bottlenose dolphin communities. In our case, they travel back and forth between the Intracoastal Waterway (estuary) for the summers and mighty Gulf of Mexico for the winters.
Thus, for many years, the norm was that local dolphins were the most abundant in the estuary in the summer, least abundant in the winter and equally abundant in the spring and fall. These encounter rates looked like a hockey stick on graph paper. The stick’s elbow (between handle and business end) was at the top of the graph like a mountain peak, representing summer’s abundance. The handle angled down to the lower right-hand corner, representing winter’s ebb.
Alas. Did you notice I used past tense? Norm has changed. The long-term seasonal patterns that looked like an inverted hockey stick on a graph have changed for the last three years.
The change in dolphin numbers was first evident in 2011. The number of dolphins dropped 33 percent. The inverted hockey stick changed into a crooked horizontal line. There was no longer any difference between the number of spring and of summer dolphins. Nor was there any difference between the number of fall and of winter dolphins. That meant that the dolphins came back in the spring as normal but did not continue to appear and swell their ranks until they hit their summer highs in July and August.
Did they fail to return? Or did they return, find inadequate conditions and leave? The change also meant that the few who remained did not slowly fade away as the waters cooled from Labor Day to Halloween, Thanksgiving and Christmas boat parades. Why did they leave early?
I eagerly inspected the dolphin numbers for 2012, hoping to see that inverted hockey stick on my graph again, whom I had taken to calling Norm. There was good and bad news.
The bad news was that Norm was really messed up. The crooked line from 2011 surfaced again in 2012. But alarmingly, this time there was no difference in the number of dolphins across all four seasons. That is frightening. It takes a substantial environmental disruption to change firmly established seasonal movement patterns. If it is permanent, which we have no way of knowing, humanity cannot fix it.
The good news was that the average had bounced back from the dismal lows of 2011 to the former average of a dozen dolphins across the four seasons.
But ripple effects are inevitable. What becomes of those few retiring individuals who winter in the quiet estuary? Would it disturb dolphins like good old Tanks, who at her age perhaps preferred her familiar estuary to the rigors of the open Gulf? Teens like Oyster and VC winter in the estuary where they are relatively free of harassment from bigger bulls there. Where could they go if not the estuary?
I waited impatiently for the end of 2013 to see if its dolphin data heralded a return to my Norm, the inverted hockey stick picture on a graph. Tauntingly, the graph bears some resemblance to Norm. Unhappily, it is not real. There was still no difference between the numbers of dolphins we see across all four seasons. Norm is still messed up.
What kind of environmental disruption could literally change seasonal abundance patterns? The devastating winter of 2009-2010 - the coldest in Floridas history - followed by an exceptionally warm summer, seems likely.
Fishermen still see the effects of that brutal winter. Its devastation is hard to measure and to imagine, such as the projection of 20 years for some populations, like snook, to recover. The best we can do is hope that this is the natural ebb and flow of animal populations and continue to document everything to get to know Norm all over again.
Dr. Weaver studies wild dolphins under federal permit 16299, National Oceanic and Atmospheric Administration. Send her an email at firstname.lastname@example.org or visit her website www.goodnaturedstatistics.com. NOAA advises anyone who sees a stranded dolphin in the Gulf of Mexico to call 877-942-5343 or 877-433-8299.