Two satellite tags were deployed
on February 1, 2006 just north and west of the western end of Grand Bahama
Island in the Bahama Islands, onto fish of approximately 45 lbs. and 50 lbs.
Both fish were observed to swim down and away from the vessel upon release, in
apparently good condition. On-deck time was less than one minute for each of the
two tagged fish.
The
first of these tags released prematurely on March 27, 2006, after 54 days at
liberty. Satellite transmissions from the freed tag indicated that the position
of this tag (and presumably the fish it was attached to) when it popped-off was
approximately 100 miles east of Savannah, Georgia. This corresponds to a net
movement of approximately 365 miles, as well as at least one crossing of the
Gulf Stream. Unfortunately, the remaining position data from the tag was poor
and we were not able to determine any intermediate positions. Depth and
temperature data for this tag revealed that the fish spent most of its time
between 30 and 60 feet and between 65° F and 80° F, however its average
nighttime depth was deeper than its average daytime depth. It also made brief,
deep trips to near 600 feet every night and visited the surface at least once
during all but one of the 54 days for which data was recorded.
The other tag deployed on February 1, 2006 remained attached for the full 120
days and then released as scheduled, in the vicinity of Fort Pierce, Florida.
This corresponds to a net movement of approximately 65 miles and at least one
crossing of the Gulf Stream. Position data from this tag was better than for the
first tag, indicating that this fish traveled north almost to Cape Hatteras,
North Carolina, then turned south and traveled back to the area of initial tag
and release. This suggests that the total distance traveled and the route taken
were quite different than what would have been concluded using only the
capture/release and pop-off locations. The depth profile for this fish was very
similar to that of the other fish, spending most of its time between 30 and 60
feet, making daily visits to the surface, and nightly trips to near 600 feet.
The temperature profile for this fish was also very similar to the first fish.
Two additional tags were deployed on October 27, 2006 onto wahoo of
approximately 45 lbs. and 53 lbs., just off Palm Beach County, Florida. Both
fish were observed to swim down and away from the vessel upon release, in
apparently good condition. On-deck time was less than one minute for each of
these two tagged fish.
The first of these tags released prematurely on November 26, 2006 at 1:46 a.m.
in the Gulf Stream approximately 150 miles east of the mouth of the Chesapeake
Bay. The data from the tag indicates that prior to this premature release, the
wahoo was exhibiting behavior consistent with the two previously tagged fish,
spending most of its time between 30 and 60 feet of depth, and making brief but
daily visits to the surface as well as to depths approaching 600 feet. It rarely
went deeper than 600 feet. It had been meandering slowly north from the point of
tag and release, along the western edge of the Gulf Stream. Then, on November
26, 2006, the tag (and the fish it was presumably attached to) quickly reached a
depth of 3000 feet and a temperature of 33°F., where it remained for
approximately 3 hours. This depth was reached in approximately 30 minutes, which
is equivalent to a descent rate of 1.5 feet/sec. The tag's position at the time
of this event (based on transmitted tag data) places it in an area where the
bottom depth is approximately 3000 feet. After approximately 3 hours at this
depth, the tag returned to the surface, ascending in approximately 30 minutes.
This rate of ascent is consistent with a freed, buoyant tag floating up to the
surface. Based on the data from this tag, we propose that the fish perished
quickly at or very near the surface at approximately 1:46 am (local time, EST)
on November 26, 2006, possibly as a result of predation. The deceased fish
either sank very quickly to the bottom (most likely) or was taken there by a
predator (unlikely but not impossible). The descent from the surface to a depth
of about 3000 feet occurred at a rate of approximately 1.5 feet/second. We
performed a simple test using a fully intact 20 lb. wahoo carcass, and found
that a dead wahoo could in fact sink at this rate, so we believe this as the
most likely explanation. However, it is not impossible that a predator carried
the captured wahoo to depth to feed in relative safety from competitors. Several
species of shark common to the area, including the Mako shark (Isurus oxyrinchus),
have demonstrated this behavior. In either case, the tag, which was anchored
just below the second dorsal fin, apparently continued to record data throughout
this period and therefore seems to have been undamaged by the events.
Approximately three hours after reaching 3000 feet, we believe the tag became
free from the fish (or whatever was left of the fish) and floated to the
surface. The tag remained at the surface, and as it was programmed to do began
transmitting its stored data after it
registered no change in depth for a period
of 5 days. The depth and temperature readings from the tag are in agreement
throughout the deployment period, including during the deep descent and
subsequent ascent. The location of the descent event on November 26, 2006 (37N,
73W) and the GPS location recorded 5 days later on December 2, 2006 upon
transmitter activation (38N, 64W) are consistent both in terms of speed and
direction with the freed tag drifting in the Gulf Stream for those five days
following release.
The other tag deployed on October 27, 2006 was scheduled to pop-off on February
28, 2007. This tag has not reported and is presumed lost. Thus, data has been
recovered from 3 of the 4 tags deployed.
The
results of these PSAT deployments indicate that wahoo over 45 lbs are excellent
candidates for the application of PSAT technology. The depth and temperature
profiles exhibited remarkable consistency between the tagged fish. All three
fish spent the most of their time between 30 and 60 feet of depth, however
nighttime depths were uniformly deeper for all three fish and all made regular
nightly trips to near 600. Also, all three fish made daily visits to the
surface. Temperature profiles were also similar for all three fish, with the
majority of time spent between 65°F and 80°F. The quality of the position data
was only average, but it was good enough to show that all three fish traveled
north from the area of the Florida Straits. This is contrary to the prevalent
opinion among recreational and commercial fishers that wahoo in the western
North Atlantic travel south in the Fall and Winter, and north in the late Spring
and early Summer.
Clearly, there is a lot of valuable information that can be gathered using
PSAT's that can not be gathered by any other means.