Studying the Behavior and Habitat Preferences of Giant Sea Bass (Stereolepis gigas)
- Principal Investigator:
- Michael Domeier, Ph.D., Pfleger Institute of Environmental Research
- Timeline:
- Initated May 2000; on-going.
 Since 2000, scientists at the PIER have been performing an involved study of the movements of the giant sea bass, Stereolepis gigas, in the California Channel Islands. This is the first intensive study of the behavior, movements and habitat usage of the giant sea bass. This species has been protected for the past 20 years, after decades of over-fishing; yet the giant sea bass population remains below levels appropriate for re-opening the fishery. The overall goal of this project is to learn more about this species in its natural environment by identifying activity patterns and trends. Ultimately we will propose seasonal and/or area closures, striving to minimize incidental giant sea bass mortalities.
An example of an activity pattern that would lend itself to a successful protective area and/or time closure would be the formation of a spawning aggregation. A spawning aggregation would bring a large number of giant sea bass together during a short period of time, making the fish particularly vulnerable to exploitation at that time and place. Data regarding large and small scale movement near and within the Channel Islands could play an important role in the ongoing process of creating and maintaining effective marine reserves in these islands. Without this kind of data, regulators will continue to use the claim of “insufficient data” to postpone actions critical to maintaining sustainable living marine resources.
 Anacapa Island was the initial location for conducting a detailed behavioral study. At Anacapa, an array of 20 receivers was successfully installed in June 2000 and a deeper ring of an additional 24 receivers was added in 2002. The study area was expanded in 2003 to include Catalina Island, where there are currently 13 receiver stations. Giant sea bass are also seasonally detected at the receivers on Santa Cruz and Santa Rosa Islands. To read more about PIER’s Channel Islands Acoustic Receiver array and how it works, click here.
PIER researchers placed acoustic tags in a significant number of giant sea bass after the acoustic receiver array was assembled around Anacapa. From the summer of 2000 through the summer of 2001, 23 Giant sea bass were tagged and released.
 The tags that were placed in the Giant sea bass are estimated to emit a signal for five years. The receivers surrounding the islands will detect and log the presence of a tagged fish 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.
Now we have data from the Giant sea bass for five years (some tags still continue to emit a signal as of June 2005!). Thus far, we have observed some very interesting patterns. We have found that about one third of the tagged fish leave Anacapa Island during the winter months and return in May or June while the other two-thirds remain at the island during the winter months. We have noticed that, for the fish that stayed at Anacapa 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 decreased and the depths at which the fish were detected became deeper. Soon, the data from the other islands will be interpreted and a more comprehensive picture of movement behavior will be available.
The data collected from this monitoring project has begun 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 off of California.
All of this is crucial information for the protection of this threatened species. We are continuing this study and will persist in collecting behavioral data on these fish for the life of the tags. Because the outer array of acoustic receivers at Anacapa are located in deeper water, we hope to learn more about the movements of tagged fish during the winter months when they move away from the island and into deeper water.
PIER gratefully acknowledges the support of the George T. Pfleger Foundation.
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