Hurricanes are typically associated with loss of life, loss of property, and economic devastation. Hurricane Katrina, which blew through the gulf coast in summer 2005, brought all those things and more. It also brought lots of baby dolphins. Hurricanes and other major storms tend to be related to increased strandings of marine mammals, so why might a hurricane be associated with more dolphins, rather than fewer?
Starting in December 2004, before the hurricane, and continuing through November 2007, a group of scientists from the University of Southern Mississippi, led by Lance J. Miller (now at San Diego Zoo Global), motored around the gulf looking for Atlantic bottlenose dolphins (Tursiops truncatus), initially in an attempt to understand their social organization.
What they discovered, at first glance, is perhaps a bit surprising. Around two years after the hurricane struck, there was a massive increase in the number of dolphin calves observed. In other words, bottlenose dolphins living in the Mississippi sound experienced a reproductive increase during the two years following the storm. Either, they made more baby dolphins, or more baby dolphins were surviving, or both.
Miller reasoned that there were probably three related phenomena that, combined, could explain the apparent increase in dolphin calf observations.
After a female dolphin loses her calf, she can give birth again much sooner than if her calf had matured to adulthood. "If a large number of calves perished as a result of Hurricane Katrina" the researchers say, "this would allow for a greater percentage of females in the population to become reproductively active the following year." Indeed, the available stranding data indicated that there was a slight increase in juvenile dolphins stranded in the months directly following Hurricane Katrina. The decrease in the number of juvenile dolphins who survived the hurricane is also evidenced by the fact that the researchers observed a decrease in the proportion of juveniles to adults in the fall of 2005, after the hurricane passed through.
By itself, this doesn't seem to adequately explain the increase in dolphin calves. If the females who suffered losses in the hurricane simply replaced their calves, then the increase that the scientists observed would not have been as big as it was. Something else was going on. That something was distinctively human.
When Hurricane Katrina blew through the gulf, the local shrimping, crabbing, and fishing industries were ravaged. In Mississippi, according to one estimate, 87% of commercial fishing vessels were damaged or destroyed, which meant a decrease in the amount of seafood brought into shore by nearly fifteen percent, in 2005 and 2006, compared with 2004. There were also forty to fifty percent fewer commercial and recreational fishing licenses granted by the state of Mississippi in 2005 and 2006. Between 2000 and 2005, there were over one million recreational fishing trips each year in the area, which decreased to around seven hundred fifty thousand in 2006, coinciding with one million fewer pounds of fish landing on Mississippi docks.
What this means it that in the months and years following Katrina, there were fewer boats in the gulf and, consequently, more fish. Both of these are especially good for dolphins.
Despite the common notion that dolphins enjoy playing in the wakes created by boats, there is plenty of evidence that dolphins actually avoid them. When boats are around dolphins spend considerably less time foraging for food. With a reduction in the number of boats in the water, both commercial and recreational, dolphins may have been able to spend a more time eating, and less time traveling or diving in an effort avoid boats.
Finally, according to Miller and his colleagues, the decrease in fishing could have resulted in increased prey availability for dolphins, "similar to the effects of creating a marine reserve." Of the species that the Mississippi dolphins are known to prey upon, seventy-five percent are the targets of commercial fishing, and sixty-three percent are fished for sport.
Not only is more food beneficial to pregnant females, who become more likely to deliver healthy offspring, but it also makes their milk more nutritious and plentiful for nursing calves. And there is more food for the calves themselves to eat, increasing their survival rate over what it would have been if human fishing had continued undisturbed.
While Hurricane Isaac seems not to have impacted gulf fishing industries to the extent Katrina did, it's important to remember that the short- and long-term effects of massive environmental disruptions like hurricanes are extremely complex. This research suggests that destructive hurricanes may actually improve the viability of gulf fisheries, at least in the short term, to the point where the effects of the hurricane resemble the effects of creating a marine reserve! And that's a good thing, if you're a dolphin. There's more to eat.
Lance J. Miller, Angela D. Mackey, Tim Hoffland, Moby Solangi, & Stan A. Kuczaj II (2010). Potential effects of a major hurricane on Atlantic bottlenose dolphin (Tursiops truncatus) reproduction in the Mississippi Sound Marine Mammal Science, 26 (3), 707-715 DOI: 10.1111/j.1748-7692.2010.00371.x