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NCCOS Scientists Seek Ways to Reduce Bottlenose Dolphin Strandings

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Are human activities contributing to dolphin strandings and dolphin mortality generally?

For well over a decade, researchers from the National Ocean Service’s Center for Coastal Environmental Health and Biomolecular Research in Charleston, S.C., have been grappling with that question and the science behind it.

Led by the National Centers for Coastal Ocean Science’s (NCCOS) Wayne E. McFee, the research is the first to identify crab pot fisheries as a significant source of dolphin mortality along the South Carolina coast. That finding has led to reclassification of the fishery and stepped–up oversight. The research into the causes of dolphin entanglements has also led also to recommendations involving buoy line deployment and line length.

It must be a particular treat to work with dolphin research. Can you give us some personal insights into this work?

we want to know something about dolphin behavior to see how much time they spend around the fishery

WM: We all have a passion for dolphins and know the need to protect them. The opportunity to work in the field as well as in the lab makes for a diverse workplace, and to be able to do this important research on marine mammals is very rewarding.

We of course understand that human interactions can cause or contribute to dolphin deaths through incidental fishery interactions such as crab line entanglements; accidents such as boat collisions; deliberate acts such as gunshot wounds; and dolphin exposures to environmental contaminants caused by agricultural runoff, pesticide use, or oil spills. But it’s equally clear that not all premature dolphin deaths are linked to human activities.

We concentrate our efforts on finding out how fisheries, illegal feeding of dolphins, boat traffic, habitat degradation, etc. affect dolphins. But we’re also interested, for instance, in how suggested gear modifications in the fisheries may affect the economics of fisheries.

Besides the human aspects, we are interested in the health of our oceans generally. Dolphins, serving as sentinels, can tell us a lot about infectious and emerging diseases that humans may need to be concerned about. We can only get these answers by conducting necropsies, collecting tissues, and trying to determine causes of death. These data also help us determine the contaminant load these dolphins have, and that may be a concern to humans that consume many of the same prey species that dolphins do.


Bottlenose dolphin found dead stranded in marsh next to Stono River, Charleston, SC
There have been at least a few instances of substantial die–offs or groundings of dolphins. Does your scientific research shed understanding on those?

WM: Bottlenose dolphins died in large numbers in 1987–88—killing, by some accounts, up to half of the migratory population. Between June 1987 and April 1988, 742 dolphin deaths were recorded – 10 times more than average. Including dolphins that died at sea and were not washed ashore, estimates of the actual deaths from this event run as high as 2,500.

Human activities are not directly implicated in these cases. Though there is still some minor debate among researchers, most have concluded that the evidence points to a morbillivirus infection that spread through the coastal population.

There have been other unusual mortality events of bottlenose dolphins such as in Texas and Florida. In these unusual mortality events – we call them UMEs – researchers find themselves driving down the beach and encountering one animal after another over a long stretch of beach—essentially collecting samples and moving on to the next one. It is exhausting work, and it takes a lot of coordination among team members. This initial effort is followed by long days spent packaging and sending samples to researchers for appropriate testing.

For coastal managers in South Carolina, I think the dolphin die off in 1987–88 amounted to the proverbial wake–up call. They knew they needed more information about what was killing the dolphins, how the dolphin population as a whole was being affected, and what historic trend data might tell them.

In South Carolina, the effort led to more organized statewide reporting on dolphin mortality, and creation of a database on dolphin ‘strandings.’ After fifteen years of data collection, the database has information on more than 600 strandings along South Carolina’s coastline. [Editor’s Note: While the term "stranding" for dolphins typically means any animal found on land, either dead or alive, for data collection purposes, researchers expanded its meaning to also include dead and floating animals found at sea.]

You say it amounted to a wake–up call. Can you elaborate on what happened next?


Bottlenose dolphin found dead stranded on Kiawah Island, SC with crab pot rope attached to the tail after pot was removed.

WM: With more and better data on dolphin strandings along South Carolina’s coastline, researchers began to look more deeply into trends and causes. Our research team joined forces with Sally R. Hopkins–Murphy from the South Carolina Department of Natural Resources, DNR, to look at dolphin strandings data from 1992 to 1996. We basically wanted to determine trends and the extent of human involvement. Our studies concluded that there was evidence of human interaction as the cause, or contributing factor, in the deaths of some bottlenose dolphins.

But it wasn’t entirely clear just what kinds of human interactions might have been involved. We wrote at the time that it was still highly speculative as to which fishery in South Carolina may be responsible for the incidence of entanglements associated with rope or line marks. Our subsequent research was able to establish a direct link to South Carolina’s Atlantic blue crab fishery, and that research led to its reclassification for a higher level of monitoring.

We think it’s important to point out that the Atlantic blue crab fishery is one of South Carolina’s largest fisheries in terms of volume and value. It takes place in areas known also to as bottlenose dolphin habitats, and the research pointed to probable interactions between the blue crab fisheries and the dolphins. Our analyses of historical strandings showed that approximately 24 percent of 42 entanglement cases in South Carolina between 1992 and 2003 had resulted from the blue crab fishery. That finding confirmed the appropriateness of the Category II reclassification that has now taken place. And as a result, the fishery is at a higher level of awareness, and the fishermen have more stringent reporting requirements and can be asked to carry an observer aboard their vessels.

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Same dolphin as above showing just the tail with the rope.

Your research colleagues and you emphasize the value of long–term trends in your research. Can you explain the particular importance?

WM: Lori Schwacke of the National Ocean Service, and South Carolina DNR’s Sally R. Hopkins–Murphy, and I published a research study on bottlenose dolphin stranding trends looking at data from 1997 to 2003. Their long–term database enabled us to better recognize seasonal trends in dolphin strandings, reproductive trends in particular. The long–term trends also allowed us to recognize unusual events.

Looking at only one or two years of data will not tell you much about detecting something unusual or managing living populations. You really need a number of years to determine temporal and spatial trends in strandings. We wouldn’t have known, for instance, that the number of dolphin strandings in South Carolina in 2001 was statistically higher than in any other year without the availability of a long–term database.


Underwater video recording system used in the first crab pot study on the movements of line in shallow water.

Are there particular challenges involved in determining whether human activities play a role in these losses?

I think the dolphin die off in 1987–88 amounted to the proverbial wake–up call

WM: Determining human involvement, and in particular fishery involvement, is far from simple. Animal carcasses are found in various stages of decomposition. Some show evidence of drowning, rope abrasions, or other trauma. And the carcasses are rarely found with fishing gear still attached.

Looking only at those cases in which we could detect human involvement of one sort or another, our research team determined that 25 percent of the strandings were caused by humans. And of those, rope or line entanglement was most common. Of the 302 strandings that occurred between 1997 and 2003, we identified 143 as showing firm evidence that there either was or was not human interaction.

Of those 143 strandings, 36, or about one–quarter, showed signs of human interaction. And of those, 44.4 percent involved rope entanglements, including confirmed crab pot interactions. Net entanglements, which are a problem in neighboring North Carolina and Virginia, numbered only five, and two of those coming from a single trammel net set. That’s the case in large part because gill net fisheries, the principal cause of fishery mortality in Virginia and North Carolina, are rare in South Carolina.

Along the South Carolina coast, it’s not unusual to see color–coded buoys marking the underwater location of baited cages or crab pots. Are those pots somehow related to the risks the dolphins are facing?


"Nesting" of the line on the bottom caused from improper deployment of the crab pot as seen in the first crab pot study.

WM: Those buoys float on the surface, attached by a rope to the baited underwater trap sitting on the bottom. For fishers to locate them, the attached lines have to be long enough to reach the surface at high tide. Increasing the length of those ropes, according to many of the fishers our research team interviewed, helps to keep the pots from rolling over in strong currents.

Keep in mind that part of dolphins’ charm and allure for us humans is that they seem playful and curious. They often travel in small groups of fewer than 10, but sometimes more, and beach visitors and vacationers often see them from the shore and watch them trailing fishing boats in hopes of a handout. Sometimes they see the dolphins riding bow waves of large oceangoing vessels. Bottlenose dolphins, with their distinctive gray color and trademark dorsal fin, are the most common species of dolphin in near–shore waters and the most likely to be encountered by people along the coasts. They have a long snout, attain a length of as much as nine feet, and have conical–shaped teeth with anywhere from 20–25 teeth in each row of the jaws.

As an aside, I want to point out that if you are close enough to see the shape of the dolphin’s teeth - it’s unlikely to be good news for the dolphin, more than likely stranded. In that case, proper procedure is to contact authorities and avoid human or domestic pets’ physical contact, with it. Dolphins can transmit diseases to people. For live dolphins found on shore, the public is similarly advised to contact authorities and not to try to push the animal out to sea.

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Dolphins, serving as sentinels, can tell us a lot about infectious and emerging diseases

Are there particular steps that crab pot fishers can take to minimize risks to bottlenose dolphins?

Our research team studied the movements of buoy lines used in the crab pot fishery to determine where and when bottlenose dolphins may be more susceptible to entanglement. Our findings suggest that how the gear is deployed can be as important as the specific type of gear used.


Getting ready to deploy crab pots with DSTmilli data recorders attached to the line during the second crab pot study to determine movements of three different line types of various stiffness.

If the fisher puts the pot over the stern, and then makes an arc around the pot, letting the line out smoothly to the end, the line goes straight to the bottom and lies straight on the bottom next to the pot, which would reduce the risk of entanglement.

But if the fisher instead puts the pot over and picks up the coil of line and just drops the whole thing over, the line has a tendency to ‘nest’ on the bottom with coiled loops of rope that float upward from the bottom. That increases the risk of entanglement. Neatness, in effect, counts for a lot when it comes to sparing the dolphins.

And another important factor is the length of the buoy line relative to the depth of the water, the strength of the current, and the time of deployment relative to high and slack tides. Our studies point to several clear conclusions:


DSTmilli data loggers on coiled line with buoy and pot prior to deployment in the second crab pot study.

  • Little or no arcing occurred in fairly strong currents, water current velocities of more than one–fifth of a meter per second;
  • In depths of 10 feet or less, ropes longer than 50 feet produced waving in the water column and arcing on the bottom; and
  • Slack tide—that interval of relative stillness between an incoming and outgoing tide—was a period of increased risk of entanglement.

Can you briefly describe the specific actions taken as a result of these findings?

WM: Acting on the study results, NOAA’s National Marine Fisheries Service, NMFS, has adopted specific recommendations for crab fishers. They stipulate that line should be deployed in an untangled, straight line to help reduce the risk of line coming off the bottom and arcing. They also limit the line to the minimum length necessary, especially in shallow or slack water.

A particularly interesting aspect of our research is that we determined that the characteristics of the buoy lines themselves, the type of line, also makes a difference. We observed the movements of three different buoy line types with varying degrees of stiffness to determine which type would pose the least threat of entanglement to bottlenose dolphins.


Close–up of DSTmilli data logger on line during second crab pot study.

A similar study involving captive manatees had concluded that the type of line used does make a difference. In the case of our research, we did not use animal subjects but rather did studies attaching sensors to each type of line to record their movements under similar conditions. The study provided some evidence that a stiffened buoy line, as opposed to nylon, for instance, created a better profile. The stiffer line may reduce the risk of entanglement of bottlenose dolphins in the crab pot fishery because it creates less line in the water column and reduces the kind of erratic movement that can lead to arcs in the line.

From the standpoint of the fishers, that kind of deployment issue does not take any additional time, and the stiffened rope is actually less expensive than the most common line used by commercial fishers. For fishers not changing line length when moving their pots between deep and shallow water, making the switch could cost some time initially, he said, but that time also can be minimized.

One positive outcome from this whole project is that fishers are not afraid to contact us when they have an entanglement in their gear. And we reciprocate by giving them back their gear if they are fishing legally.


Bottlenose dolphin swimming past our study crab pots in the second crab pot study. The elongated buoy marks our current velocity meter that sat on the bottom to record current water velocity and direction.

So what comes next in this line of research? What are the plans for further understanding and protecting bottlenose dolphins and the potential harms they might face as a result of human activities?

WM: Now that we’ve looked at the gear, we want to know something about dolphin behavior to see how much time they spend around the fishery and what behaviors they exhibit in close proximity to the buoy lines or pots.

As for future directions for our research, we hope to continue to work on the crab pot issue and also conduct more research on impacts of the shrimp trawl fishery on dolphin mortality. By continuing to work closely with NMFS law enforcement on the issue of illegal feeding of dolphins by humans, we hope the public increasingly will recognize that illegal practices pose risks to the dolphins we all enjoy watching.