In most cases attempts to track the movement of eggs and larvae involve some type of tagging of the water mass containing the propagules, usually by dye, drift cards/bottles or "current-following" drifters. The "tagging" of eggs is generally unfeasible, unless there is some special biochemical tag or genetic marker that might be used (e.g., Jones et al., 1999).
Dye has uses in some studies where a very "clean" start to the tracking is desired. Hensley et al. (1994) and Appeldoorn et al. (1994) used Rhodamine dye to track the transport of eggs after spawning. A saturated solution of dye (presumably in sea water) was placed in thick walled rubber balloons, then attached to anchored lines at a height above the bottom at selected spawning sites. When it was desired to release the dyed water, the balloon was burst from a distance using a long spear. Carter et al (1994) attempted to track the spawn of Nassau grouper using florescent dye, but unless the dye is tracked by sampling water using a fluorometer, it quickly becomes too diffuse to be visible to the human eye.
The drift card or bottle is another way to approach the question of where eggs go after spawning. Long a tool of physical oceanography, they can be used to address questions of where recruits resulting from a spawning aggregation might settle. One of us (Domeier, in prep) successfully used drift bottles to investigate the downstream dispersal from a known mutton snapper aggregation site (Fig. 51). A large number of scintillation vials ballasted with BB’s (copper coated lead pellets) so that they were barely buoyant were released at the aggregation site at the presumed time of spawning to model potential recruitment pathways. A label placed inside instructed anyone finding the vial to contact the researcher. This technique is inexpensive, provides statistically meaningful sample sizes and is appropriate for use in populated regions (i.e., where returns are likely). The method may not be suitable or useful where there are few persons to find drifters, an inability to return information, or vast areas of open ocean, since relatively few drifters may eventually be grounded and found.
At their simplest level, current-following drifters are nothing more than an in-water object (sea anchor, vane or drogue) with a high resistance to lateral movement and held at a certain depth by a line and floats tethered to some type of marking device to allow it to be followed. For short-term use, up to a day or so, simple vane drifters with a pole marker ("high- flyer") can be used (Fig. 52). The drifter is tracked from a small boat, and its position determined by coming alongside it at intervals with a GPS receiver. The position data are then plotted on a map of the area and a recorded track of the drifter derived from that.
Current-following drifters work relatively well for the initial few days of the existence of a fish egg and larvae in the plankton. Eggs are buoyant and take about 24 hours or less to hatch into yolk sac larvae. This initial larval stage does not swim much, but may be able to control its depth to some extent. After larvae begin feeding (3-4 days after hatching), they may well move into water depths below the depth of the drogue, and consequently be transported differently from the drifter. Each day increases the chance that drifter tracks do not reflect the movement of larvae. As larvae approach settlement, they may well swim actively towards reefs, perhaps in response to sound or olfactory clues, rendering drifter tracks almost meaningless at that time. Drifter data should always be interpreted cautiously, that level of caution increasing as time after spawning increases.
Figure 51. (Left) Drift bottle made from standard scintillation vial ballasted with BB's to the point of being only marginally positively buoyant. (Right) Locations of recoveries of drift bottles along the south Florida coast released at Riley's Hump (a spawning area) at the time of presumed spawning.
There is a need for low-cost current-following drifters that exceed the capabilities of the manually tracked drifter described above. Potentially basic GPS units could be integrated into the simple systems logging the track of the drifter at regular intervals. These stored data could be
Figure 52. Simple current-following drifter. A. Underwater view of the drogue or vane is to the right, 1 m by 1m extending downward to 50 cm depth, attached by a line to the pole and float. B. Surface view of the marker pole, with colored flag to aid in locating from a boat.
accessed after several days by either recovering the drifter or by some sort of electronic transmission. It is likely that some sort of locating means would be needed to make such a system work, such as VHF radio beacons or something similar.
For longer duration tracking, satellite-tracked drifters are the only viable option. They are similar to the manually tracked drifter, with a drogue set at a depth of interest, but the surface float has a satellite transmitter and battery pack (Fig. 53). Position information is acquired several times per day by satellite, and can be accessed by the user via modem. The data are near real time (most recent positions are often only a few hours old), but like other data, the track of the drifter between points must be estimated.