By: Alyssa Clevenstine
Understanding the movement patterns and habitat associations of fish helps us understand their ecological functions in marine ecosystems. Numerous factors affect their movement such as searching for food resources, better habitat conditions, and breeding areas to improve fertilization success. As a result, many species of fish, particularly large carnivorous species, migrate potentially long distances to form aggregations. Breeding and spawning aggregations represent one of the most critical processes to reproductive success, but they also tend to be heavily fished because they make it easy to target the fish en masse.
Giant sea bass (Stereolepis gigas) are an example of a species that was assumed to aggregate for spawning purposes and were targeted by anglers throughout California and Mexico in the early 20th century. Historically, landings of S. gigas occurred during the summer months (May through September) and included multiple individuals, suggesting that they may form spawning aggregations. Due to the consistency of these events, the giant sea bass population in southern California was drastically depleted during the mid-1900s.
The estimated lifespan of giant sea bass is at least 76 years and the age at sexual maturity is 11-13 years. This long reproductive phase makes this species particularly vulnerable to overfishing. A maximum length over 2 m and weight of 200 kg, in addition to a curious nature and little fear of humans, also made them quite vulnerable to spearfishers.
Commercial landings in California fell from 115 tons in 1932 to only 5 tons in 1980; tonnage in Mexico simultaneously dropped from 363 tons to just 12 tons. Due to the drastic reduction in landings California banned recreational fishing for giant sea bass in 1982, and limited commercial take to allow only the incidental catch and sale of one individual per trip.
In the last decade, the scientific, diving, and fishing communities report an increase in sightings and incidental catch of giant sea bass around the Channel Islands. This suggests the species may be recovering south of Point Conception. However, scientists and managers have very little information on where the species moves and when they aggregate in coastal waters.
I am addressing this question by quantifying the patterns of giant sea bass movements over time (i.e. daily, lunar, tidal) and space to provide high quality resolution of individual movements over the next 18 months. Passive monitoring tags will be used to confirm whether specific locations are aggregation sites, and quantify their movement patterns in relation to space use, home range size, and site fidelity.
As a Wrigley Fellow, I am able to conduct my research at 13 sites across Catalina Island to identify giant sea bass aggregation sites, determine abundance and aggregation size at different aggregation sites, and characterize the movement patterns of individuals between those sites. With the help of scientific divers from Cal State Long Beach and the Aquarium of the Pacific, 17 giant sea bass have been tagged so far this summer. We will tag 13 more in the next two weeks. The fish are tagged with coded acoustic transmitters with a lifespan of approximately two years. We also video-record tagged fish to estimate their size using a parallel laser system.
Along with tagging, we conduct visual surveys at each of our sites monthly to determine fish abundance. Thankfully, water visibility has been improving this summer, making it easier to count and measure. Just this week we saw seven giants at one site where we had previously tagged a dozen individuals, and only one of them had one of our acoustic tags! This is a great sign that fish may be moving sites during spawning season.
With the help of the USC Wrigley Fellowship I have been able to easily access my sites all summer and reduce many of the high costs associated with boating and diving around Catalina Island. The summer has already been very successful and I am looking forward to the final month of my fellowship to finish tagging and collect telemetry data during the height of giant sea bass spawning!