A. Mapping Aggregation Sites

Preparation of a reasonably accurate site map is one of the best methods to start work on any aggregation site. Since aggregations have a physical presence, their distribution in time and space needs to be documented. This can be done by a series of observations or measurements displayed on a two or three-dimensional map or bathymetric chart of the area. The accuracy of GPS (global positioning system) plotting and the power of computer displays in two and three dimensions allow us to prepare very useful representations of aggregation structure. It is important to consider that the area outside the aggregation proper is also of interest, for example to compare geomorphology or bathymetry, and the mapping survey ideally should include a much larger area than just the aggregation site proper.

It may not be possible to prepare a map of the area in advance of the aggregation, particularly if its precise location is not known. The mapping process may be disruptive to the aggregation and in such a case the map survey will have to be done after the aggregation. Notes, drawings and survey markers should be used to indicate the extent of the aggregation relative to whatever features (that can be identified in the future mapping effort) may be present at the site.

Uses of Aggregation Site Maps

The benefits of a good map of the area are many:

1. It provides a reference guide for all those doing work at the site, and allows communication between participants about sites within the mapped area.

2. It allows others, who may not have participated in the original surveys, to be able to find and revisit the same sites and do repeat quantitative work on the original site(s).

3. It shows the distribution of depth and habitat relative to the aggregation, can show changes in aggregation location over time, and allows for comparison of different aggregation areas.

Maps can range from simple sketches drawn by divers, showing features of importance relative to one another with approximate distances, to detailed depictions of the bottom, with depth contours, habitat types and particular features. Maps can be prepared with basic tools no more complicated than measuring tapes and a compass, or by using GPS, depth sounders and aerial photographs. Whatever the level of sophistication, a map is intended first to serve as a common reference for observers making initial observations and, just as important, for those who continue work in the future.

Some Examples of Previous Efforts to Map Aggregation Sites

Many workers have prepared maps of their study areas and these are usually included in any publications arising from the work. They are vital for subsequent work, whether for detailed scientific studies or for long-term monitoring. Randall and Randall (1963) provided a map indicating the location of their parrotfish aggregation site, plus a verbal description of the site. These materials were sufficient for Colin (1996) and Colin and Clavijo (1977) to relocate the site many years later.

As part of a study of spawning and dispersal of the eggs of Thalassoma bifasciatum, Hensley et al. (1994) mapped three study reefs with 48 spawning sites on the inshore shelf off La Parguera, Puerto Rico. They tracked the movement of water, represented by dye patches and current drifter, from both spawning and non-spawning sites, plotting the paths on these maps.

Figure 6. Maps of transect routes from Zabala et al. (1997a). A. shows the general area of the study, B. indicates the path swum in general surveys and C. details the path taken in surveys of spawning occurrence. (Reproduced with kind permission of the authors).

This study, done before GPS technology was available, used a microwave trisponder unit that allowed them to construct an x-y grid over their study reefs for plotting positions of drifters and dye patches. This equipment took a lot of effort and expense to put in to the field. We should feel lucky today that we can do the same thing with a simple hand-held GPS that costs less than $US 200!

Maps have been useful in studies of grouper aggregations. Samoilys and Squire (1994) mapped a 1700 m2 area on the reef slope of Scott Reef, Great Barrier Reef, which had been observed to be an aggregation site for Plectropomus leopardus. Their map was used to standardize observations collected during censuses. Zabala et al. (1997a and 1997b) provided detailed maps of their study sites, including bottom communities, the route taken by divers doing underwater visual census and a rough cross sectional view of the reef communities and grouper occurrence (Fig. 6). They designated two different transects, one to estimate annual density changes (200 m long by 10 m wide swum at mid-day) and a second to survey different depths and topographies where groupers occur (100 by 10 m swum at various times of day). These provided consistent, repeatable censuses. Colin (1992) provided a map and cross section view showing the distribution of a Nassau grouper, Epinephelus striatus, aggregation off Long Island, Bahamas that indicated changes in the location of the aggregation between years (Fig. 7). Aguilar-Perera (1994) had a sketch map of a spawning site for another E. striatus aggregation off Mahahual, Yucatan, Mexico, including a vertical view of general reef and aggregation site. While he did not state how this map was prepared and it isn't particularly detailed, this map shows the value of even a simple mapping effort.

How To Do An Aggregation Site Mapping Survey

The value of a good map of the aggregation site is obvious, both as a reference during the study and for future use by others who might continue working at the site. There are a number of ways to produce a site map, and the detail and accuracy of the map can vary greatly. Since working time underwater is usually limited, anything that can be done to construct the map without working underwater at the site is probably useful. Available bathymetric or topographic maps may be a good starting point. The area of interest can be copied or scanned from available charts for use as a base map. If you are using an older map, be careful as GPS determined coordinates may not match up with the latitude and longitude indicated on earlier charts, due to errors in earlier mapping processes. If such is the case, you might be able to figure in a correction factor, based on the charted position of some known object, like a lighthouse, on the older chart.

Any aerial photographs of the area of interest are also of great use. If available, then they may show underwater features, such as large pieces of reef against a sandy bottom, which are identifiable underwater at the spawning site. If the aerial photos are properly scaled, or other known features are visible, a rough map with a scale can be prepared, and then related to bathymetric or topographic maps.

Doing Bathymetric Mapping Surveys

For areas where no bathymetric charts are available, it is possible to prepare a reasonably accurate bathymetric map using a GPS, digital depth sounder with data output, and a laptop computer. GPS coordinates and depth data are logged through the serial port(s) by the computer using a variety of different software programs. These data are then plotted using a software program for contour mapping.

Figure 7. Example maps of Nassau grouper aggregation sites off Long Island, Bahamas (after Colin, 1992); (a) was prepared using available topographic maps and aerial photographs, (b) and (c) were based on underwater observations and other sources.

Combination GPS/depth sounder units have advantages since there is only a single input cable to the computer, whereas separate GPS and depth sounder inputs would require two cables and dual com ports on the computer. Free software provided by Windmill Software, UK, which provides for system set up and logging has provided high quality data logging. This company also has commercial versions with more extensive capabilities, but for most purposes the free software has proven sufficient. Finally the coordinate and depth data are logged on a laptop computer in a watertight plastic box. The serial port cable enters the box through a watertight fitting. If the weather is good, the computer box can be left open to avoid the buildup of heat. If there is spray or rain, the box can be sealed tight after the computer has started logging. However allowance must be made for the heat produced by the computer. Be particularly conscious of not having the computer sitting on something (like a towel) that prevents heat transfer from its lower surface to the air. We would suggest running some tests in the lab to see how much heat builds up from the logging computer inside the protective box to be used.

Once the equipment is set up in a boat on the survey site, the computer can start logging and the boat is driven in a pattern over the survey area to acquire the GPS/depth data. The more sophisticated GPS units have the capability to show the tracks the boat has traversed during the survey, making it simple to efficiently cover the survey area. Care should be taken to verify that the GPS/depth sounder are acquiring good data and that the computer is properly logging information. Some electronic depth sounders use automatic depth ranging, and, if depths changes are extreme and rapid, they sometimes lose their fix on bottom depth. In such cases the depth sounder may provide no depth data, or spit out some totally erroneous figures. Be careful when transitioning between deep and shallow water, or vice versa. When setting up the equipment,

Figure 8. Aerial photograph (top) and contour bathymetric (bottom) map of Ngermekoal (Ulong Channel), Palau, a site for aggregation of groupers of the genera Epinephelus and Plectropomus. (PLC)

make sure the computer clock is set on the correct time (these time data will be logged with the GPS/depth data) and set your wristwatch to the same exact time. When doing the survey have a note pad handy, so that if the depth sounder starts generating obviously erroneous data, you can quickly make note of the time. Later you can go back and delete the bad data from the file.

After returning to shore, the data are transferred to a contour mapping program for plotting. Bad data gathered can be deleted by going into the spreadsheet files based on the times such data were recorded. Once a clean data set is obtained, the data can be used to prepare a bathymetric map. We have used the program "Surfer" by Golden Software for preparing such maps, and the program can deal with 2D and 3D plots with any depth range and contouring available. Some examples of aggregation site maps are shown in Figures 8, 9 and 10. Such maps have the advantage that they are already GPS accurate and additional objects or features can be plotted accurately on the maps using GPS determined coordinates.

Additional information such as the limits of aggregation sites or migration pathways to and from a spawning site can be determined and plotted on the bathymetric map. For example, the migration pathway shown in Figure 10 was determined by having a snorkeler follow migrating fishes, while a boat closely followed him and recorded the GPS coordinates every minute. These were then transferred to the map to show the migration pathway in relation to bathymetry.

Figure 9. Contour bathymetric map in 3D of Ngermekoal (Ulong Channel), Palau, a site for aggregation of groupers of the genera Epinephelus and Plectropomus. Depths in meters (PLC)

Figure 10. Aerial photograph of Lighthouse Reef, Palau with vertical aerial photograph of the reef above and the bathymetric map below with migration pathway shown (PLC).

Underwater Surveys

Surveys "in water" can be undertaken to map out the aggregation site using a measured line and underwater compass. Waterproof survey tapes, usually on a plastic reel, are well suited to underwater surveying and can be strung between identifiable points in the study area. The compass bearing and distance between each pair of points is determined. The survey might be done in a circle around the area of interest, closing the survey circle to the starting point. Or a central point might be chosen and survey lines run out from it to the surrounding areas of interest. Usually the bottom features are sketched on underwater slates relative to the survey lines. Alternatively the area of the survey lines could be photographed looking vertically downward using a wide-angle lens on an underwater camera (such as a Nikonos with 15 mm lens), including the actual lines in place in the photographs (Fig. 11). The photographs are used subsequently to fill in the features of the bottom. A digital camera might be particularly useful for this type of mapping, as the images could be downloaded quickly and would probably provide sufficient resolution for mapping purposes. It is even possible to prepare mosaics of vertical underwater photos using programs such as Photoshop 4.0 and later. Again digital images would be particularly easy to use in this regard.

Figure 11. Vertical underwater photomosaic of area of reef in Ngermakol (Ulong Channel), Palau, a grouper aggregation area (PLC)

A readily available, inexpensive marine hand-bearing compass by Davis Instruments Inc. is excellent for taking underwater bearings. The compass has a hemispherical dome with the compass card and marks for sighting along easily. Interestingly, the liquid-filled clear plastic dome effectively disappears to sight once underwater, so it appears the compass has lost its dome (although this is hopefully not the case!) when using it underwater. Diving compasses can also be used, but be careful that the compass is sighted directly along the line for accurate bearings.

Back on shore, the survey lines are plotted out, using something as simple as a protractor and ruler, or as complicated as a computer mapping program. Then the features are sketched into place, a suitable scale added and other features included. Most underwater maps, if the maker wishes high accuracy, usually require subsequent refinement through more surveying underwater (see Fig. 12). A general, more simple, map, though, might be useable after a single survey.

Figure 12. Detailed underwater survey map of a spawning study site of a small fish species, Koror, Palau. Each square is 1 meter square and shaded areas indicate different substrate features such as coral type. Surveys can be done with tape and compass, plotted and then refined over many hours. This process can be quite time-consuming, however. (George Mitcheson, unpublished)

In some cases, it may be of interest to map out the limits of the fish present in an aggregation area, but such a survey may not be feasible while the aggregation is present. Painted rocks, or other small heavy distinctive objects can be dropped on the bottom while swimming around the limits of the aggregation, and then surveyed, using the line and compass techniques at a later time.

It is often advisable to leave some sort of permanent reference markers on the bottom to aid in relocating a site later and quickly orienting. It is amazing how quickly the memory of a reef site can fade and make it time consuming to get reoriented. If it is aesthetically feasible (the site is not a tourist dive area for example) markers of some type can be left between seasons. Subsurface floats are often useful to mark an area, and various types are available. In general hard plastic floats, such as those used in the fishing industry as net floats, are best. They are pressure proof to considerable depths and have an eye for a line built in. They should be attached via a line to a weight on the bottom. The line should be heavy enough to withstand chafing, surge and motion of the float. Ideally it should be abrasion resistant and, if carefully installed and situated, it will last for years. Soft inflatable floats are convenient, but should be avoided unless the installation is just temporary, a few weeks or less. Perhaps a series of substantial float anchors could be installed and the floats removed at the end of a field season, for reinstallation the next year. Where standard materials are difficult to obtain, blocks of polystyrene foam can be useful as floats for short term marking, but should be removed at the end of the study.

If visually acceptable, concrete blocks can be left in selected areas as permanent markers. They won't float away, their man-made shape is easy to recognize even years later, they are readily available in most countries, and are cheap and non-toxic. They are light enough to be moved underwater by a diver inflating their buoyancy compensator. If located in sandy patches away from reefs they can be very easy to find. Floats can be tied directly to the reef, as is often done, but must be positioned carefully to avoid chafing of the line against the reef and possible damage to coral or other benthic life. Floats attached to concrete blocks are a good system as long as chafing of lines is minimized. This can be done by tying lines through the holes with no slack and using abrasion resistant line.

Whatever system is used, you should try to have markers that are easily found and seen from a distance underwater. Nothing is more frustrating than spending your precious minutes on an aggregation searching for the start or end point to transects or other reference markers. In all cases, remove all temporary markers once the study has been completed.