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Dolphin Watch
Bridge goes up; dolphin numbers go down
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The compelling culture of intelligent dolphins outside your doorstep is even more precious for failing to come with a guarantee of continued existence.
My first evaluation of the John’s Pass Causeway construction as a potential man-made or anthropogenic threat to the local community of bottlenose dolphins is done.

Conservation projects like this are profoundly expensive in money and time: It takes eight years of data to see if the number of animals using an area stays the same, goes up, or goes down. This extensive investment in tracking population trends is because most populations fluctuate naturally, and you need several years of information to tell if any particular oscillation is something other than natural variation.

We now have eight years of data. Construction was associated with a definitive decline in dolphin presence in the study area, which showed a steady downward trend with one exceptional “blip.”

“But we still see dolphins!” you exclaim. Yes, but this is what’s diabolical about environmental degradation.

In our case, the actual number of dolphins using the area did not change but their behavior did. They used the area the most before construction started, used it progressively less as construction was underway and are using it even less now that construction is done. Their decline is statistically significant (not a matter of chance), linear and reflected by both resident and non-resident dolphins.

How does one begin? If Capt. John Heidemann and I see 12 dolphins on each of 12 surveys, we’ve seen somewhere between 12 and 144 actual dolphins. I derived the actual number of dolphins we saw for each season. These numbers didn’t change before, during and after construction. It means that the same core group of dolphins continues to use our local waters.

But the next two measures show that they use our local waters less often now.

Part of the previous measure takes into account how often we see individual dolphins. Such sighting probabilities are partly a function of how often we survey for dolphins, but luckily, we’ve managed a steady effort of about a dozen surveys per month for the last 90 months.

Say we go out on four surveys, and if on all four surveys we see the local colorful character we call DD2 in all his bullish glory, his sighting probability is 100 percent, or in statistical parlance, 1.00. If we see him on three of the four surveys, his sighting probability is .75 (75 percent); if we see him on two, his sighting probability is .50, and so on.

For all the dolphins that we’ve known since before construction began, I calculated each dolphin’s average sighting probability before, during and after construction. Their individual sighting probabilities have declined significantly. That is, we still see DD2 but not as often as we did before construction.

A different way to look at dolphin behavior is to establish how many dolphins there are and how often you find them at sea. We’ve identified 300 dolphins using the study area, but they don’t all use it at the same time.

To see if the number of dolphins changed before, during and after construction, I looked at how many dolphins we see on each survey of the study area, which is our frequency of encounter rate. A comparison of the average frequency of encounter rates before, during and after construction showed a 40 percent drop from before to after construction.

As bleak as these numbers appear, coastal construction may not have been the main or only culprit. These data show that the dolphin decline was concurrent with construction, but further analysis is needed to establish that whether the decline was contingent upon construction.

I mentioned a “blip” in the steady linear decline, which may be cause for optimism. The blip was 2009, the only year where the dolphin numbers returned to their pre-construction baseline levels. It occurred during year four of the five years of construction. It may reflect something like “old habits die hard.” Perhaps the dolphins eventually adjusted to construction and were on their way back to pre-construction behavior.

Unfortunately, 2009 was followed by the terrible winter of 2010, which was responsible for substantial fish kill across the study area and may account for some of the dolphin decline. However, when I statistically removed the effect of water temperatures on dolphin encounter rates, the decline in encounters before, during and after construction were still statistically (non-chance) present.

In addition, could the icy winter of 2010 have put strain on dolphin food supplies, causing them to slowly vacate our local waters, but not affect the fishing of the last two years?

Finally, there is a confound that I would like to resolve but do not wish on the dolphins or other sea creatures. That is Red Tide. The substantial fish kill of the 2005 Red Tide may have been wholly or partly responsible for the dolphins’ exodus; perhaps the fish populations recovered four years later in 2009.

Still, the blip is cause for optimism because it suggests that whatever combination of influences accounted for change in dolphin numbers did not have a permanent effect. We watch for another “blip” like 2009 in 2013 or 2014.

The story isn’t over because there’s another angle. There are two ways to look at the dolphin decline: Either they don’t like this area or they like another area better.

Dr. Weaver studies wild dolphins under federal permit 16299, National Oceanic and Atmospheric Administration. Send her an email at or visit her website NOAA advises anyone who sees a stranded dolphin in the Gulf of Mexico to call 877-942-5343 or 877-433-8299.
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