During the week of October 17 – 23, 2010, a group of scientists, guides, and anglers worked together on Abaco to tag and track the movements of bonefish. The overall goal of the tagging effort was to identify bonefish movement patterns to determine the amount of area required by bonefish for survival, and whether Abaco bonefish undertake long distance spawning migrations. This information will be applicable to designing appropriate fisheries policies and habitat protections to ensure healthy bonefish populations. While previous tag and release results suggested that bonefish on Abaco occupy relatively small home ranges, these findings may have greatly underestimated the amount of space actually needed by bonefish in order to complete spawning migrations. We were particularly interested in learning whether bonefish from the Marls on the western side of Abaco migrate the considerable distance to the southern end of the island in order to reproduce. If this type of long distance spawning migration was identified, it would suggest that additional habitat protection along the migration corridor and at the spawning location may be needed for bonefish conservation on Abaco.
Recent research on Eleuthera found that bonefish migrate long distances to specific locations, and then move offshore to spawn at night during a period spanning the new and full moons between October and May. Prior to spawning, the bonefish collect in large schools in shallow bays near deep water, and move offshore as a group at dusk. Because of the extremely wide, shallow shelf on the west side of Abaco, bonefish in the Marls would have to swim a considerable distance to the west or south in order to reach deep offshore spawning waters. Recent observations by Abaco guides suggested that bonefish may group to spawn in an area near Cross Harbour. Large schools of fish were seen in a shallow bay adjacent to deep water during peak spawning season, mirroring the observations made on Eleuthera.
We used acoustic (sonic) tags to determine whether bonefish from the Marls traveled to Cross Harbour. While standard passive tags (“spaghetti tags”) require the recapture of a tagged fish in order to identify movements, a combination of high tech acoustic tags and underwater receivers allows movements to be detected and recorded without having to recapture the fish. The acoustic tags send out sonic pings (much like a submarine), which are detected by the underwater receivers at distances up to one thousand feet. Each tag has a unique code, allowing each fish to be individually identified. During the week of October 17 – 23, the group surgically implanted acoustic tags in 25 bonefish in the southern Marls and Cross Harbour. Prior to tagging, 22 underwater receivers were placed in and near Cross Harbour to detect bonefish movements.
A bonefish is stitched up after an acoustic tag is placed in its abdomen. After surgery bonefish were given time to recover in a holding pen before being released.
The expectation was that if bonefish from the Marls move to Cross Harbour to spawn, the receivers in Cross Harbour would detect one or more of the bonefish tagged in the Marls. We also expected that if bonefish spawn near Cross Harbour, we would see patterns whereby bonefish would be detected by the receivers (which were placed in shallow habitats) during quarter moon periods, but would be absent during periods around the full or new moons.
In May 2011, the underwater receivers were retrieved taken back to the lab to download the data. Twelve of the 25 bonefish implanted with acoustic tags were detected by the receivers in and near Cross Harbour between October 2010 and May 2011.
One of the bonefish tagged in the Marls on October 17 (tag number 33353, a female, 18” fork length) was detected by receivers north of Cross Harbour on December 1, on the southern side of Cross Harbour on December 2, and in the bay where guides observed a possible pre-spawning school of bonefish on December 3. This fish moved at least 37 miles (60 km) from where it was tagged. The fish then moved out of range of the receivers. The new moon was on December 5, so the movements of this fish coincide with expectations for a fish migrating from the Marls to a spawning location. The fact that a bonefish tagged in the Marls was detected at Cross Harbour is a significant result of the study.
Of the bonefish that were tagged in Cross Harbour, two fish exhibited behaviors that also suggested movement away from the shallow flats to spawn. These two bonefish (tags 33363 (a 17” male) and 33369 (a 17” female)) were detected by receivers in shallow water almost every day during the course of the study. However, both fish disappeared from the shallows for several days around the new moons of December 2010 and January 2011. Both fish disappeared December 4 – 7 (new moon was December 5), and then returned to the shallows. In January, fish 33363 disappeared January 6 – 8, and fish 33369 disappeared January 5 – 9 (new moon was January 4) and then returned to the shallows.
Although we did not observe bonefish spawning during this time, and their exact locations during their periods of absence from shallow water are unknown, their movements mirror those reported by the researchers at the Cape Eleuthera Institute. In addition, the migration of a bonefish from the Marls was perfectly timed to correspond with the new moon in December, which suggests that the Cross Harbour area may be an important spawning location for Abaco bonefish. Similar long-distance movements in association with spawning have been documented on north Andros and Eleuthera. Even if this is not the case, this research has documented the long distance movement of a bonefish on Abaco, and shown a connection between the Marls and Cross Harbour, underscoring the need for large scale conservation of bonefish habitats. This connectivity suggests that impacts to migrating and spawning bonefish outside of the Marls could negatively affect the world class fishery found within the Marls.
The goal now is to obtain additional funding to conduct a follow-up study, with intensive tagging of bonefish in the Marls with acoustic tags, and a redistribution of the receivers at Cross Harbour based on movements detected during this first study. This follow up study will better define the connection between the Marls and Cross Harbour, and identify the location used by the pre-spawning schools of bonefish.
