The Plight of Dolphin Carrying PSATs
Any time a prey species alters its basic movement patterns or appearance, it will be noticed by a predator. Most times a large dolphin can either out-swim or out-maneuver its predators, like sharks, marlin, or toothed whales, but there are always some times when they don’t. Large predators have to eat, too.
Unfortunately, you cannot place a pop-off satellite archival tag (PSAT) on a dolphin without altering its appearance and possibly its swimming behavior. Changes in either of these will draw the immediate attention of predators. You might as well place a bull’s-eye on the fish. After tagging, it will be up to the fish to avoid all of the additional predator-attention the tag has brought its way, if we are to acquire any long-term data.
Nature being what it is, one of the dolphin tagged June 3, 2013, with a PSAT as part of a long-term tracking study ended quickly. Just 36.75 hours into a planned six-month deployment, the unit popped to the surface at 1:30AM EDT and began to drift with the prevailing surface currents of the ocean. While we cannot absolutely say what caused its separation from the fish, but having been the one who attached the tag, I say an outside force had to sever the200-lb. mono tether to which it was attached. Since the tag surfaced from a depth of two meters, this suggests that it was attacked by a critter with sharp teeth, such as possibly a mako shark, which has a known fondness for dolphin.
All is not lost, because in its brief liberty the bull dolphin did provide a look at a new behavior we have not recorded before. Long-term tags such as the ones in this study only record data on the quarter-hour, so that leaves large gaps of unrecorded behavior. With that said, this fish did not like the surface. It was observed in the top 10 meters (m) of the water column only on 12 percent of the records with most occurring near the end of the track.
It stayed at an average depth of 29.5 m or 97 feet during its brief tracking period. The maximum depth observed for this fish was 69.9 m or 229 feet. The fish was observed to occupy waters between 65 and 115 feet 51 percent of the time. So, if you don’t already pull a deep line when fishing in blue water, this may be a good reason to start.
The fish was observed to move between temperatures of 69.6 to 80.4 o F and occupied waters having an average temperature of 76 o F. It did show a preference for 74 oF water, spending 35 percent of its time there. The hottest water was encountered when it was at the surface near the end of its life. The coolest water was noted during a dive to 172 feet a few hours after it was released.
This fish remained at depths of around 100 feet for the longest period observed for any of the 20 dolphinfish tracked with PSAT’s by the DRP. Most of the other dolphin tracked off South Carolina, southeast Florida and the Caribbean were observed to dive to these depths but remained there for only brief periods. While this could be a unique behavior for just this one fish, it does open the possibility of a new behavior that some mature bull dolphin could spend long periods well below the surface of the water.
While this was an abbreviated tracking effort, as I write this article, there is one tagged fish still out there that has not been heard from for 140 days. This leaves 40 days in the Sat tag’s planned monitoring period to achieve the six-month monitoring goal. These fish represent the start of a long-term study made possible by a grant from the Guy Harvey Ocean Foundation. Three more pop-off satellite archival tags will be deployed in 2015 to complete this initial segment of the ongoing study.
Capt Jimbo Thomas and his brother Rick, who operate the Miami, Florida, based charter boat Thomas Flyer have become the first crew to tag 2,000 dolphinfish under the DRP. Rick Thomas tagged their two-thousandth fish on August 1, 2014, off Miami.
Capt. Thomas joined the DRP tagging effort in 2004. Since that time he and his brother Rick have averaged tagging 204 dolphin each year. Their best year was in 2007 when they tagged 367 dolphin. They have earned the DRP top charter boat award on all but one year since they have been participating.
Thanks to their efforts in tagging dolphin, we have been able to track fish from Palm Beach, Florida, all the way to New Jersey. Their fish have even been recovered as far away as the Dominican Republic.
Hot Fall Dolphin Fishing
The northeastern Caribbean from the Lesser Antilles through the Greater Antilles experienced some of their worst dolphin fishing during the 2013-14 season. In the words of one charter captain, there was no grass and no fish, nothing. Well, the ocean made a 180-degree turnaround starting in late August 2014. Large lines of Sargassum that stretched for miles began showing up with large dense mats. With this grass came dolphin, and an unusually large number of small 15- to 18-inch fish. These reports have come in from participating anglers in the Virgin Islands, Antigua, the southern and northwestern coasts of Puerto Rico as well as the southeastern corner of the Dominican Republic.
It appears that this large influx of Sargassum, an oceanic algae, could be a repeat of the 2011 event when vast amounts of the weed washed ashore on Caribbean beaches. While fishermen look upon this oceanic weed as a blessing to their fishing, it can create serious problems for the islands. During these events tons of the weed can blanket the white-sand beaches, threatening the vital tourism of the islands.
While the grass may pose a threat to island beaches, it may offer a great opportunity for tracking the movements of dolphinfish around the Caribbean. During September 2014 60 fish were tagged in the Caribbean Sea and adjacent waters. This is more than the average total number of fish tagged each year in this region. The surge in Caribbean tagging also pushed the total number of fish tagged for the month of September to a new record high of 148.
The September tagging activity was a great start but it slowed way down in October. Now we will have to wait to see if the abundance of dolphin picks back up during the fall and winter fishing season. If tagging rebounds, this could be a great opportunity to advance our knowledge of the movements of dolphin in the Caribbean Sea.
Donations are Needed
It is not too late to provide financial support to the Dolphinfish Research Program. Donations to the 2014 program have been the lowest in the research program’s nine-year history. Please consider making a contribution to keep this important research effort alive.
Donations are fully tax deductible. Checks should be made out to the Reef Foundation/Dolphin Study and sent to the address shown below.
For More Information, Contact
Dolphinfish Research Program
Cooperative Science Services, LLC
961 Anchor Rd., Charleston, SC 29412
Telephone – FAX (843) 795-7524
Web site www.dolphintagging.com