Acoustic Telemetry Analysis of California Gamefish Reveals the Functional Performance of the Wheeler North Artificial Reef

By Chelsea Black, SRC MS Student

Submerged structures such as ships, steel frames, or boulders placed on the seafloor deliberately to mimic attributes of a natural habitat are known as artificial reefs (ARs). Since the development of the National Fishing Enhancement Act of 1984, most AR construction in the United States has been focused on enhancing fishery resources and opportunities (Logan & Lowe 2018). These ARs provide new habitat for both fish and benthic organisms to colonize, which increases food resources and overall fish biomass. However, to establish reef productivity for mobile species it is pertinent to know when and for how long species are resident to a reef, which has historically been overlooked by previous AR studies.

The largest constructed rock reef in the US, the Wheeler North Artificial Reef (WNAR) off the coast of San Clemente in the Southern California Bight, was built by the Southern California Edison Company (SCE) as mitigation for the loss of the San Onofre Kelp Bed after discharged cooling water from a nuclear generating station resulted in restricted giant kelp growth and subsequently the loss of species diversity (Figure 1). For SCE to receive mitigation credit, WNAR must either meet or exceed the abiotic and biotic performance standards relative to community performance from two nearby natural reefs, the San Mateo Kelp Bed (SMK) and the Barn Kelp Bed (BK).

Figure 1. Construction of the WNAR [Source:].

In a recent study, Logan & Lowe (2018) examined the movement patterns of three economically and ecologically important fishes, the kelp bass (Paralabrax clathratus), the barred sand bass (P. nebulifer) and the California sheephead (Semicossyphus pulcher) within WNAR. 195 fish were captured, tagged with acoustic transmitters, released, and monitored by underwater receiver stations for a study period of two years to determine residency indices (Figure 2). When examining these indices, the authors concluded kelp canopy surface area was the single environmental parameter that explained the most variation in residency. This makes sense considering previous research demonstrates that artificial structures with high vertical relief provide important habitat conditions, including refuge from predators, for many species (Martin & Lowe 2010).

Figure 2. Locations of acoustic receivers and neighboring natural reefs [Logan & Lowe 2018].

Figure 3. From top to bottom: California sheephead, kelp bass, and barred sand bass [Source: Wikimedia Commons].














After analyzing movement patterns of the tagged fish across WNAR, SMK and BK, this study suggests that during favorable conditions fish were highly resident to WNAR and demonstrate that it is currently functioning similar to the surrounding natural reef habitats, effectively reaching the set performance standards. In addition, the results reveal that future AR management plans in California would be most successful when implementing structures that are ideal for giant kelp growth to aid in increased species diversity in abundance by providing an ideal habitat for important species.

Works Cited

Logan, R. K., & Lowe, C. G. (2018). Residency and inter-reef connectivity of three gamefishes between natural reefs and a large mitigation artificial reef. Marine Ecology Progress Series, 593, 111-126.

Martin CJ, Lowe CG (2010) Assemblage structure of fish at offshore petroleum platforms on the San Pedro Shelf of southern California. Mar Coast Fish 2:180−194.

Food abundance, prey morphology and diet specialization influence individual sea otter tool use

By Molly Rickles, SRC Intern

In this study, Fuji looked into how ecological factors impact sea otter’s tool use. Sea otters are known to be near shore foragers that carry food on the surface, and have been observed to use tools to eat their prey. In many other species, tool use has been found to be a response to a lack of food, since the tools allow the animal to eat less preferential species easier. This means that individuals that are found at sites with limited food will have a more specialized diet. Researchers wanted to see if this would be the same with sea otters.

In this study, sea otters were tagged from 2000-2014 in five study areas across California. The individuals were tagged using a plastic cattle tag on their hind flippers as well as with an implanted VHF radio transmitter. The animals were observed as foraging for food when they were found to be repeatedly diving underwater and returning to the surface. The results of these foraging dives provided the data, which included the dive outcome, prey identification, time spent handling prey and if tools were used. Once data was collected, groups were classified based on the most prevalent prey item, which included: Abalone, crab, mussel, clam, urchin and snail. In addition, AIC statistics were made for each model, and then compared to select the best-supported model based on the data.

Figure 1

This figure shows the various sites where sea otters were tagged and researched. Even though many of the sites are close together, each site had different prey species present which presented different scenarios for the sea otters. (Source: Fujii, J. A., Ralls, K., & Tinker, M. T. (2017). Food abundance, prey morphology, and diet specialization influence individual sea otter tool use. Behavioral Ecology. doi:10.1093/beheco/arx011)

It was found that crab was the most common prey item. In addition, in sites where food was limited, sea otters were found to have more diversity in prey. Results showed that adult females were the most common tool-users, and that snails were the most common prey eaten with tools. Sea otters who most often ate snails were found to be more likely to use tools to eat other types of prey, since they were accustomed to using tools to eat snails. This also went the other way, since species that ate prey that did not require tools were less likely to use tools with more difficult to eat species.

Figure 2

This graph shows the predicted probabilities of tool use among the different food groups consumed by sea otters. Sea otters that eat snails have the greatest chance of using tools to consume snails as well as other prey. (Source: Fujii, J. A., Ralls, K., & Tinker, M. T. (2017). Food abundance, prey morphology, and diet specialization influence individual sea otter tool use. Behavioral Ecology. doi:10.1093/beheco/arx011)

These results show that dietary differences among the individual sea otters played a role in tool use, especially when food resources declined. Researchers observed that older sea otters used tools more frequently, showing that tool use requires time and energy spent learning to be successful. In addition, pups raised by females who used tools were commonly seen using the same tools and eating the same prey, suggesting that mothers teach it, which could be an area for future research. This study proved both the necessity hypothesis and the opportunity hypothesis, showing that tool use increases when prey is limited and that tool use improves prey capture, making it a beneficial skill.

Work Cited

Fujii, J. A., Ralls, K., & Tinker, M. T. (2017). Food abundance, prey morphology, and diet specialization influence individual sea otter tool use. Behavioral Ecology. doi:10.1093/beheco/arx011