Return of the king

Researchers track giant sea bass populations in Channel Islands National Marine Sanctuary

By Larisa Bennett

December 2018

Translucent, aqua-tinted water surrounds scuba divers as they weave through trailing kelp fronds at Channel Islands National Marine Sanctuary. Above them the surface of the Pacific Ocean is in constant motion. Below the ocean surface, towering stands of giant kelp grow in the cool, nutrient-rich upwelling currents of the national marine sanctuary. While wandering through this seemingly endless underwater forest, lucky divers in Channel Islands National Marine Sanctuary just may find themselves face to face with the elusive king of this watery realm – the giant sea bass.

Giant sea bass are one of the larger visitors to Channel Islands National Marine Sanctuary.

Giant sea bass are at the top of the food web in the kelp forest ecosystem. They are massive animals: the biggest of them can weigh 800 pounds and measure over seven feet in length. Giant sea bass will eat just about anything they can fit in their gaping mouths, including lobsters, rays, and small sharks.

For such a significant species, relatively little is known about these kings of the kelp dominion. The species population has been very low in the past several decades due to a long history of overfishing. The fish is listed as “critically endangered” by the International Union for the Conservation of Nature (IUCN). The rarity of seeing one in the wild has made the giant sea bass a difficult subject to study.

At Channel Islands National Marine Sanctuary there have been recent consistent sightings of the enormous fish. A new multi-year research project led by Ryan Freedman and other sanctuary researchers will study this population using a telemetry array and satellite tags. A local collaboration of researchers plan to look at all aspects of this animal’s life in order to better understand its role in the marine environment, as well as how humans affect it.

giant sea bass
Giant sea bass can be distinguished by the black spots that adorn their bodies. Like a fingerprint, each spot pattern is unique to the individual. Researchers can use these spot patterns to track particular giant sea bass as they move throughout the ocean. Photo: Robert Schwemmer/NOAA
diver in kelp forest
Curious giant sea bass are often unafraid of getting close to divers. These large fish have been seen recently among the kelp fronds at Channel Islands National Marine Sanctuary. Photo: Robert Schwemmer/NOAA

Giant sea bass were once common along the Southern California coast, but in the 1970s the population tanked. Francis Joyce, a researcher studying these fish at University of California Santa Barbara, explains that they typically “aggregate at consistent sites and are docile, which makes [them] easy to hunt in spearfishing.” Their popularity among commercial and recreational fishers led to the species being fished aggressively in Mexico and Southern California in the last century. Biology was not in favor of the fish either. Giant sea bass are slow-growing and do not reach sexual maturity until relatively late in life.

Legislation in California that only allows one incidental take of giant sea bass per commercial fishing trip has reduced fishing pressure on the species since the 1980s. The legislation completely bans any recreational take of the fish, but this can be hard to enforce. Lately, divers have spotted giant sea bass in the sanctuary more frequently than in the past. If these anecdotal reports signal the return of large numbers of giant sea bass to the kelp forest, it will likely significantly affect the ecosystem structure. These huge fish eat smaller predators like rays and mid-sized fishes, which in turn eat smaller fish and invertebrates. More giant sea bass means mid-level predators and animals lower on the food chain can diversify.

sheephead fish in kelp forest
The kelp forest is a diverse ecosystem. In addition to giant sea bass, many other creatures – including sheephead like this one – roam these waters. Photo: Robert Schwemmer/NOAA

The research team at Channel Islands National Marine Sanctuary is taking advantage of the assumed recent increase in giant sea bass population to study the animal’s biology, communication, and habitat use. The telemetry system the team has set up will use receivers positioned around Santa Barbara Island and other eastern islands in the sanctuary to keep track of the tagged giant sea bass as they move around sanctuary waters. Currently there are nine receivers in the water programmed to record signals from five tagged giant sea bass. The sanctuary hopes to eventually increase this to 17 receivers tracking 25 tagged fish.

Along with the telemetry system, the team is also using hydrophones to record the surrounding soundscape. Giant sea bass make a low frequency series of grunts when other large animals are around, including divers. Researchers, such as Dr. Lindsey Peavey Reeves, another sanctuary researcher working on the project, want to understand if giant sea bass use this vocalization as a form of communication and, if so, when and why. Hydrophones can record giant sea bass as they move about in the sanctuary and interact with their environment. By studying the situations surrounding vocalization, researchers hope to piece together the purpose behind the sounds these fish make.

Plus, much of the vessel traffic heading in and out of the ports of Los Angeles and Long Beach passes by Channel Islands National Marine Sanctuary, and these ships emit low frequency underwater noise as they travel. This noise can have several impacts on marine life, including preventing animals from hearing each other and other sounds in their environment. It’s called “masking,” and it’s much like how airplane noise overhead can cause people to pause their conversations, or, if the noise is persistent, move to quieter areas.

By recording the sounds made by wind, waves, storms, animals, and human actions, the hydrophone information will provide an overall portrait of sound throughout the sanctuary. This will help researchers better understand how much noise human activities like shipping are contributing to sanctuary waters. It will also help them understand how that noise might be affecting sanctuary animals, including giant sea bass, that are using low frequencies to communicate and sense their surroundings.

map of channel islands national marine sanctuary
Channel Islands National Marine Sanctuary surrounds several of the Channel Islands. Santa Barbara, San Miguel, Santa Rosa, Santa Cruz, and Anacapa Islands all benefit from the sanctuary’s protections. Click the image above to see a larger version of this map. Image: NOAA

Much is left to be learned about these mysterious kings of the kelp forest. Freedman hopes the project will help researchers understand “the behavior and potential risk to an ecologically important predator that is in the process of recovering.” In case the population is indeed recovering, Freedman adds, “The sanctuary is doing its part with relevant partners to ensure the species can continue its recovery trajectory and sanctuary users can enjoy having this species as part of the ecosystem again.” Using modern technology, researchers will have front row seats to watch the giant sea bass returning to the kelp fronds. Long may he reign!

Larisa Bennett is an outreach intern for NOAA's Office of National Marine Sanctuaries and a graduate student at UNC Chapel Hill.