In the past, researchers who wanted to study fish movement could choose from a suite of conventional tags (spaghetti tags, Peterson disc, T-tags, various dart tags, internal anchor tags, dyes etc.), that all resulted in the same data: a location of tagging and a location of recovery. Although interesting, these data can be misleading. In many cases it gave us more information about where people were fishing than where the fish were actually spending time. For example, a species may have a wide ranging seasonal pattern of movement that passes through a popular fishing area, resulting in the tag to be recovered very near the spot it was originally tagged a year earlier, but giving no indication of the (probably) vast migration in between. Sampling strategies can be developed to avoid such potential bias.
Many aggregating fishes are known or believed to migrate to spawning aggregations. These migrations can be a daily event (typically for resident aggregations) or may occur only during a limited portion of the year (for transient aggregations). Often biologists want to know from what area does a known aggregation draw its members, and until recent times it was usually necessary to tag fish and hope for their recovery at a later date. This question can be approached in two ways. Fish can be tagged when they are dispersed, prior to an aggregation, with possible capture at the aggregation. Otherwise, they can be caught and tagged at an aggregation, then be captured later after they have dispersed. Ideally both strategies could be used, however the particular circumstances may favor one or the other. This passive tagging requires some way to physically tag the fish and often involves capture of the fish. Physical tags can be of many types, such as dart tags with a streamer or beads exposed externally, or other markings such as freeze branding by liquid nitrogen or acrylic paints. Internally fish can be tagged with sonic tags or micro-chip tags.
Among studies of spawning aggregations, Myrberg et al. (1988) tagged Acanthurus nigrofuscus by first herding the fish into large monofilament nets, then captured by hand net. They were tagged with "textile tags". The tags had no effect on the behavior of the fish, some tagged fish spawning the same day after capture and tagging. Myrberg et al. (1988) included studies on migration, surveying fish swimming to spawning sites from the point along the shallow reef where they normally left the reef to proceed over sand to the spawning site. The site selected as the place to count the temporal occurrence of fish was critical and provided much interesting data in this study.
External tags can also sometimes be placed without capturing the fish. They can be emplaced by spear whenever the fish can be approached closely enough to accurately place the tag. Colin (1992) used this technique to tag dispersed Nassau grouper, while C. Koenig and others have used similar methods on goliath groupers (jewfish) in the Atlantic. Difficulties include getting accurate placement of the tags, and the limited time that can be spent underwater finding the fish and placing the tags. When the fish can be easily captured by hook and line or traps that do not negatively affect their survival, they can be tagged quickly and released with high quality placement.
Freeze branding is another marking technique that has been applied to the study of reef fish. Zeller and Russ (1997) freeze-branded adult P. leopardus using liquid nitrogen cooled numerals, tagged specimens remaining healthy with brands evident during visual censuses for a period of at least 2-3 months. This might be a useful technique to tag fish captured from an aggregation site and immediately released after branding. Their continued presence at the aggregation site would indicate the number of days an individual fish remains at the aggregation.
In general, tagging while dispersed is the most feasible way to determine the "capture area" of an aggregation. There are several reasons for this. If fish are tagged while dispersed, then the most concentrated fishing effort and observational effort can be focused on the aggregation itself, hence a greater chance of finding tagged fish when they are concentrated and often being caught. Also a very high reward can be offered to fishermen for capture of any tagged fish and this provides a modest economic incentive to cooperate with researchers. It also offers an opportunity to let a fishermen, who captures a tagged fish, know exactly where the fish was tagged (as long as the information is readily accessible to the field workers), something which again may pique their interest in the research. Also if a tagged fish is seen in an aggregation and the tag is externally identifiable (say by color coded beads), the fish can be searched for regularly over a period of days, providing extra information on duration of individuals at aggregations, etc.
If fish are tagged during an aggregation, there are several problems. First, the tagging operation, either through capture or underwater tagging, may disrupt the aggregation. Fish captured, say by fish traps, tagged and released quickly at the aggregation site may not behave normally and observations of such fish may distort ideas of normal behavior, etc. On the other hand, tagging of aggregated fish does offer the chance of doing tag-recapture studies to assess total populations, etc.
Johannes et al. (1999) tagged groupers caught by hook and line from aggregation sites in Palau. After capture, fish were held in a tub of seawater, then measured, sexed (if possible), excess gas removed from the swimbladder using a hypodermic needle, and then tagged. Ten-12 cm Hallprint dart tags were used, inserted so the barb was anchored beneath vertebral spines or pterygiophores. Although results from this study were mixed, with reluctance of fishermen to return tags having a large influence on success, recaptures showed that at least some individuals returned to the same aggregation sites from one year to the next.
Migration routes can be determined in a number of ways. Where there are large numbers of fish undertaking a daily migration, the route can be marked by dropping painted stones as the fish are moving and later the route can be surveyed by compass and tape (Myrberg et al. 1988). Alternatively, if the migration path is in shallow water a snorkeler can swim the route of the migration above the fishes, and either have a GPS unit in a water proof housing, writing down the coordinates at regular intervals, or have a boat follow close by that has an observer recording GPS readings. In the latter case, it is more feasible to have the swimmer going in the direction opposite to the migrating fishes, in this way the person meets the migrating fishes head on and does not potentially disturb their choice of pathways by swimming along with them. This method was used successfully in tracking scarids in Palau that were migrating along a shallow reef towards a channel in the reef for a daily spawning (see Fig. 10). The migration path determined by GPS coordinates can be applied to other GPS determined maps of the spawning area, such as bathymetry, habitat maps and aerial photography.
Telemetry to Document Fish Movements, Behavior and Habitat Use
The last decade has seen the development of new technologies that have proven to be valuable tools in the study of fish movements and would have considerable application to spawning aggregations. Archival tags, acoustic tags and satellite pop-up tags have all revolutionized the study of fish movement, behavior and habitat usage. A data logger coupled to a pressure sensor, thermistor and a light receptor allows for the nearly continuous collection of data that permits us to determine the temperature, depth and geographic position (calculated from light levels) of the fish while carrying the data logger. An archival tag is simply a data logger that is placed in, or on, the fish and later recovered if the fish is recaptured. Pop-up satellite archival tags (PSATs) collect the same information as an archival tag, but the fish need not be recaptured because the tag detaches from the fish, floats to the surface and transmits the information to a satellite. Because of the relatively slow baud rate of the data transfer to the satellite, much of the data must be compressed into histograms. Because of this data compression, PSATs do not provide nearly as much resolution as an archival tag, but the rate of data return is usually much higher because the majority of archival tags are never recovered.
Figure 47. Handling of this large black sea bass is similar to that which any fish being implanted with an acoustic tag should undergo, if properly handled. In the left photo the tag is being inserted intramuscularly into the dorsal musculature. On the right, the fish is having its gills irrigated with fresh water from a hose and the eye is covered which calms the fish.
Archival tags have not yet been applied to the study of reef fish aggregations, but PSATs have been deployed on goliath grouper in Florida. One difficulty with both of these tag types is that the geo-location data that result from the light data are not very precise (resolution of 1 degree of latitude and longitude), so the fish must move a great distance to provide a useful track. Longitude is more accurately estimated than latitude from light data. In some cases where the coastline is running east/west, a combination of longitude and depth (if the behavior of the species allows for confident determination of bottom depth) can be used for a fairly good position. For reef species that spend much of their time near the bottom this technique holds promise. Additionally, the location of the tag when it surfaces is pinpointed by the satellite with a great deal of accuracy, providing an end point from which to interpret earlier position data. Acoustic tags are different in that they do not usually collect data (the exception is the very large CHAT tag [Vemco] that is not presently not suitable for reef fishes), but instead continuously transmit a pulse at a specific frequency that allows for very fine scale movement studies. Some have an integrated pressure sensor that allows for determination of the depth, or a thermistor that can transmit internal and/or external body temperature. They can be externally affixed via a leader and dart (can be done on a vessel, at the side of the vessel or in situ with a modified spear gun) or surgically implanted in the peritoneal cavity or intra-muscularly (Fig. 47). These tags can be monitored through either manual tracking or remote sensing. Manual tracking allows for extremely precise movement data but is usually limited to 24-48 hours due to the requirement that the researcher must follow the tagged fish with a boat and listening device. Eventually the researcher becomes exhausted and the boat needs fuel. For species that do not move much, individuals can be relocated and tracked many times over the life of the tag. Short term tracks are always suspect due to the fact that the trauma of capture and surgery can have an effect on behavior. Specimens that are followed over a period of weeks to years (by relocating the fish) result in more valuable data since the tracks are not influenced by the immediate trauma of capture and tagging.
One excellent example of manual tracking of individuals moving to and from a spawning aggregation (Plectropomus leopardus) was conducted by Zeller (1997). The Nassau grouper has also been the subject of studies using acoustic tags. Bolden (2000) tagged Nassau grouper with surgically implanted acoustic tags, held the fish for several days before release and successfully monitored their presence for several weeks thereafter. One fish was captured months later at a spawning aggregation, and the tag discovered when the fish was cleaned. This fish had migrated over 200 km from the tagging site to the aggregation site. Carter et al (1994) acoustically tagged two Nassau groupers prior to the spawning aggregation, and one was recovered two years later 150 miles north of the tagging location. They also tracked a number of non-aggregating fish to determine home ranges and daily activity patterns. One fish was captured months later at a spawning aggregation, and the tag discovered when the fish was cleaned. This fish had migrated over 200 km from the tagging site to the aggregation site.
Remote tracking can be done by mooring data logging hydrophones in the study area. Multiple hydrophones can be spaced to create a continuous array that gives complete coverage of the study area. If the tagged individual remains in the study area a continuous record of its movements is obtained. When depth coded tags are used the researcher gets both movement and depth data. Multiple specimens can be tracked simultaneously. When tags with long battery life are used (up to 5 years), a tremendous dataset can result. This technology has been applied to the study of spawning aggregations. One example exists at Anacapa Island, one of the southern California Channel Islands, where an array completely encircles the island. This array has been used to study giant sea bass, thought to form a spawning aggregation at the island, for almost 2 years. Individual fish have been documented to return to the island during the spawning season after an absence of 11 months. Other individuals have been shown to reside at the island year- round.
The technologies described here hold enormous potential for the study of spawning aggregations in the future. As the technology for PSATs evolves the tags will become smaller and therefore applicable to a wide array of species. At this time PSATs can only be used on the largest species (we would not recommend tagging an individual weighing less than 25 kg). Acoustic tags and archival tags can be purchased in a variety of sizes and can be used to study most species of concern.