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STUDYING THE BEHAVIOR AND HABITAT PREFERENCES OF GIANT SEABASS
The summer of 2000 was the beginning of a very exciting, high-tech research project specifically aimed at learning more about the behavior of giant seabass. Using new acoustic tagging equipment, scientists at PIER have been able to track the movements of individual giant seabass within at least 1 km of Anacapa Island for over 2 ½ years! This was done by first selecting a good study site where giant seabass can still be found in relatively high numbers. Only a few such areas still exist, and among these, Anacapa Island proved to be the best study site. Anacapa Island is very accessible, situated only 10 miles off the coast of Ventura, California. Anacapa has long been know to be the home of giant sea bass, in fact, the International Game Fish Association's recognized all-tackle world record of 563 pounds 8 ounces was caught off this island in 1968. In recent years, a team of sport divers led by Kathy deWet-Oleson, have kept track of the numbers of giant seabass present at one of the fishes' favorite spots around the island. It seems this spot is the gathering place for spawning giant sea bass during the summer months. Kathy's team have seen as many as 35 giant sea bass at one time!
All of these things made Anacapa Island our first choice for conducting a detailed behavioral study. Our goal was to use acoustic tags on a significant number of giant sea bass, and then place a continuous array of acoustic receivers around the island. An array of 20 receivers was successfully installed in June 2000, and a deeper ring of an additional 24 receivers was added in 2002. Five giant seabass were successfully captured and tagged in the summer of 2000 and 12 in the summer of 2001, giving us the ability to track a total of 17 tagged fish. Each fish had an acoustic tag surgically implanted in its abdominal cavity. Internal placement of the tag is best for long-term studies, since the tag will not bother the fish once the incision is healed. The receivers can detect the tags for up to 5 years, after which the tag's batteries will expire. Each receiver will detect and log the presence of a tagged fish, as long as it is within 500 meters of the receiver. The tag not only tells the receiver that it is in the area, it also tells the receiver how deep the fish is swimming. Each receiver can log data from the tagged fish for six months, then the receiver needs a battery change before it is good for another six months.
The data collected from this monitoring project has started to give us an idea of the seasonal patterns, spatial distribution, feeding habitats and potential spawning aggregations for the protected giant sea bass. We have noted patterns in spatial and temporal distribution that may be indicative of a spawning aggregation, but that are more complicated that we originally expected. In a classic spawning aggregation the fish would aggregate at a given location and then disperse. In this case not all fish left the island during the off-season, although there were several fish that were noted to leave the island during the winter months. Those that did not leave moved further away from the island and into deeper water. One possible explanation for this behavior is that the fish may have been moving out to deeper water to feed on squid, which spawn during fall and winter in this region.
We have begun a collaboration with Jenn Caselle, a researcher at UC Santa Barbara working through PISCO, who has tagged both sheephead and cabezon at Anacapa Island this past summer and is currently tracking them using our system. Our plan is to share our information with the public and in particular the Channel Islands National Marine Sanctuary in an attempt to provide data for establishing and monitoring marine reserves in the area. This has been a very successful and exciting project and we look forward to continuing data collection and adding it to what we have already learned. This study has been funded through a generous grant from the General Services Foundation and the George T. Pfleger Foundation.
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We have now been retrieving data from our array for
4 years, and have observed some very interesting patterns. We have had 1/3 of the tagged fish leave the island during the winter months and return in May or June, and the other 2/3 remain at the island during the winter months. We have noticed that for the fish that stayed at the island for the majority of the year, there was a distinct change in activity patterns. During the winter months the number of detections at the array went down while the depths that the fish were recorded at went up.
All of this is crucial information for the protection of this threatened species of fish. We are continuing this study and will persist in collecting behavioral data on these fish for the life of the tags (approximately 5 years). We have added a second array of acoustic receivers in deeper water in an attempt to detect the movements of tagged fish during the winter months when they move away from the island and into deeper water. Our acoustic array has also generated excitement from other researchers.