Individual movements between and within regions can be very important in defining a population structure. We are currently at the point in our research where this type of data would be very helpful, especially when combined with the genetic data that we have already collected and analyzed.
For this type of investigation, researchers usually deploy some type of wildlife tag, with the hope of eventually recapturing the tagged animal. Assuming that the fish can be caught, tagged, and released in healthy condition, these traditional tag and release methods are good because they are inexpensive, and because sport anglers can usually deploy many of the tags. When a fish is recaptured, one can theoretically determine how much it grew in the amount of time since it was first captured, and also get some idea of how far it traveled. Traditional tags are also small, so they can be attached to fish of almost any size.
Traditional tags are limited, though. One drawback is that although the total straight-line distance between initial capture and recapture locations can be determined, there is no way to know the meandering that may have occurred between those two points. It is also impossible to determine the depths or water temperatures at which the fish traveled, or the time it spent at various locations before moving on. A fish could be caught off Bimini in the fall, travel to Bermuda in the spring and spend its summer there, travel back to the Bahamas the following fall and be recaptured off Freeport. There would be no way of knowing anything about the spring/fall movements. All that would be known is that the fish was initially caught off Bimini and recaptured 1 year later off Freeport.
Archival tags can solve
some of these problems. An archival tag is essentially a small computer which
can be attached to a fish, and which records information such as water depth,
temperature, and position. When (and if) a previously tagged fish is recovered,
the computer is removed and the stored data can be downloaded.
A requirement for both archival tags and "traditional" tags is that the tagged animal be recaptured, and this unfortunately is a potential problem. Recapture rates for tagged fish are quite low, especially for pelagic species. About 15 recaptures out of 1000 tagged fish is considered a good rate. With conventional tags the price per tag is quite low so the low rate of return is not too costly, even if relatively inefficient. But archival tags are expensive, and they must be deployed by a trained biologist, so the low rates of return start to be a concern. Still, since the recapture can be performed by anybody, this is still a reasonable method for certain popular fisheries. These same limitations apply to wahoo, as well, but in addition the wahoo’s solitary habits makes recaptures even less likely than for other species.
Recently, a new type of tag called a pop-up satellite archival tag (PSAT) has
been developed and
is being increasingly
used to track the movements of various species of pelagic fish. PSAT's consist
of an archival tag, a transmitter, and an antenna, all attached to
the
fish via a corrodible link. As the fish moves, data such as lat/lon, water
temperature, pressure (which is converted to water depth) and elapsed time are
recorded just as they are in an archival tag. However, unlike an archival tag,
a satellite tag does not need to be physically recovered. When the link
corrodes through (after some pre-determined time, i.e. 4 months), the tag
becomes free, floats to the surface, and begins to transmit its recorded data to
a satellite. The data can then be downloaded to the researcher's computer for
analysis.
These tags provide much better data, but at a cost. Each such tag costs approximately $3,500.00 (compared to about $5.00 for a conventional tag). This is in addition to the costs associated with actually catching and releasing the fish, which must be done by a properly trained researcher working off of a properly outfitted fishing vessel, and using crew who are particularly experienced in catching wahoo. Despite the cost, the data is so uniquely valuable that the federal government has funded several tagging projects using these satellite tags, primarily for billfishes and tunas.
We
recently deployed four PSAT’s onto wahoo in the western North Atlantic Ocean,
and we have two more ready to go. Three of these tags have reported back,
providing valuable preliminary data concerning the movement patterns and
environmental preference of wahoo in this region. One tag never reported and
was presumably lost due to an unknown cause. This is currently an active part
of our research and we are in the process of raising money for additional tags
and tagging trips.
For more information visit our
Satellite Tagging Results page