Fish Behavior and Offshore Oil Platforms

How Do Reef Fishes Use Offshore Petroleum Platforms?

Offshore oil platforms create artificial reef habitats in which diverse groups of marine life accumulate. However, a problem arises when once-productive platforms become obsolete. A State Rigs-to-Reef program would introduce several alternatives for dealing with obsolete oil platforms besides full decommissioning, which include topping, partial removal, toppling, or leaving in place (Fig. 1). However, to determine whether these are worthwhile options it is first important to quantify the ecological significance of these structures for marine organisms.

While the Rigs-to-Reefs Program has been applied to hundreds of platforms in the Gulf of Mexico, managers of the California oil platforms are still undecided. Since oil platforms provide new habitats for marine animals, the method in which they are decommissioned may have serious effects on the diversity and survival of established populations. Determining community structure, abundance and behavior of fishes associated with offshore platforms has helped managers determine how different decommissioning options change the importance of the artificial reef habitat. While community structure studies provide researchers with an idea of the marine fish that colonize the platforms, acoustic telemetry studies are essential to determining behavioral patterns of abundant populations (e.g. site fidelity, homing behavior, etc.). Oil platforms in the San Pedro Shelf and Santa Barbara Channel have been studied to address these questions (Fig 2).

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Fig. 1. oil platform removal types — **Fig. 1.** Obsolete oil platforms may be a) left in place, b) “topped,” or removed above the water line, c) partially topped and removed up to 25 m depths, d) “toppled,” or pushed fully into the water, or e) fully removed via explosives. Each method may pose particular threats to the reef habitats that have formed since the oil platform’s insertion.

Fig. 2. California Offshore platform locations — **Fig. 2.** There are 27 offshore oil platforms in California waters, primarily in the Santa Barbara Channel and San Pedro Shelf. Many of these platforms have been in place for over 40 years, which has led to the creation of unique environments with high levels of rugosity and vertical structures. These characteristics make offshore oil platforms artificial habitats for species with a close association to reef substrata. In addition, the platforms off southern California span a range of environmental conditions, meaning that not all platforms may possess the same marine assemblages of organisms and cannot all be considered to perform the same way.

The Santa Barbara Channel – Site Fidelity of Platform Associated Fishes

Lowe et al. (2009) conducted a study in the East Santa Barbara Channel, in which they studied the site fidelity and movement patterns of groundfishes that were associated with offshore petroleum platforms (Fig. 3). Fifteen species were caught and surgically fitted with acoustic transmitters. Acoustic receivers were places on nearby support vessel moorings and within the platform structures to monitor for emigration of tagged individuals. Results showed that the degree of site fidelity varied among platforms and across species (Fig. 4). In addition, there was evidence of fish moving from shallower platforms to deeper platforms over time, indicating possible ontogenetic depth shifts with age.

Fig. 3. Monitoring locations for offshore oil platforms Gild — **Fig. 3.** Both Lowe et al. (2009) and Anthony et al. (2012) studied the behavior of oil platform associated groundfish in the East Santa Barbara Channel at three oil platforms: Grace (91 m depth), Gilda (61 m depth), Gail (225 m depth). While Lowe et al. (2009) found that site fidelity varied with species and location, Anthony et al. (2012) transported individuals from oil platforms to Anacapa Island to address questions of homing ability, resident time, transit times, and temporal movement patterns.

Fig. 4. degree of site fidelity — **Fig. 4.** Of the 15 species tagged by Lowe et al. (2009), lingcod and widow rockfish showed high degrees of site fidelity (60-71%), while cabezon, rosy rockfish, brown rockfish, green rockfish, and striped rockfish showed moderate site fidelity (31-60%), and copper rockfish and vermillion rockfish showed a relatively low site fidelity.

The Santa Barbara Channel – Translocation Potential of Platform Associated Fishes

Anthony et al. (2012) used the resulting data and methods from Lowe et al. (2009) to study translocation, homing behaviors, and habitat utilization of lingcod and rockfishes caught at the same platforms in the Santa Barbara Channel. Lingcod and rockfishes from three oil platform locations were translocated to natural habitat of similar depth at Anacapa Island, in order to determine whether the animals were capable of returning to their home platform. The purpose of this study was determine the potential efficacy of successfully transplanting fish from platforms to be potentially fully decommissioning to natural, overfished habitats, to mitigate loss of fish life during the full decommissioning process. However, the success of such an effort would depend on whether translocated fish would home back to the platform they were captured at or whether they preferred natural habitat over that of platform habitat.

Of those tagged, nearly 25% of individuals exhibiting homing behavior (Fig. 5, 6). Translocated lingcod were found to home back to their platform of capture in as few as 10 hrs! (Fig. 7). Furthermore, lingcod and rockfish showed different movement patterns while homing, illustrating differences in movement patterns and homing across species (Fig. 8, 9).

Fig. 5. homing behavior — **Fig. 5.** Anthony et al. (2012) found that 25% of those that were translocated exhibited homing behavior. On average, it took 14.7 ± 30.6 days for an individual to return to a platform. Homing lingcod left Anacapa Island before the vermillion rockfish and took an average of 1.4 ± 1.22 days to return home. Of those that did not home, 12 took up residency elsewhere and the others left the detection range.

Fig. 6. residence — **Fig. 6.** Anthony et al. (2012) found that lingcod showed much lower residence times than both vermillion and brown rockfish. In other words, the lingcod were the first to return home after being moved to Anacapa Island, and usually did so within 24 hours of translocation. Fish pictures are by Larry Allen.

Fig. 7. transit time — **Fig. 7.** Anthony et al. (2012) also found that lingcod exhibited the shortest transit times once homing behavior began. It took an average of 1.4 ± 1.2 days for lingcod to return home. Alternatively, homing vermillion rockfish took 11.1 ± 19.0 days and brown rockfish took even longer.

Fig. 8. movement probability of lingcod — **Fig. 8.** The movement probabilities of lingcod that were translocated by Anthony et al. (2012). Arrows show the direction of movements and thickness of arrows represent degree of probability of movement.

Fig. 9. movement probability of vermillion rockfish — **Fig. 9.** Vermillion rockfish studied by Anthony et al. (2012) showed seven different patterns of behavior. All individuals translocated to Anacapa Island moved either 1) to Grace, 2) to Gilda, 3) to Gail, or 4) remained at Anacapa. Some individuals then moved from 5) Grace to Anacapa, while others remained at their first destination of 6) Platform Grace or 7) Platform Gilda. Those caught at Grace were most likely to stay there if they returned (64.5%), while translocated fish had a 26.3% probability of being detected at Anacapa Island, and 12.0% were frequently detected at Platform Gilda.

Each of these studies suggests that some species have a relatively high affinity for artificial reef habitats and decommissioning efforts may significantly change habitat structures. Furthermore, although relocation efforts were successful for some individuals, the process is both costly and time consuming.

The San Pedro Shelf

There are 7 active petroleum platforms on the San Pedro Shelf and while limited data exists, the upper reaches of these platforms appear to be dominated by reef fish species such as blacksmith and California sheephead.

Martin and Lowe (2010) evaluated the assemblage structure of fish at different depths for six platforms (2 inshore, 4 offshore) along the San Pedro Shelf (Fig. 2, 10). Fish community structure was influenced by seasonality and location (Fig. 11). Alternatively, neither species composition nor community structure changed from year to year, unless there are dramatic changes in oceanographic conditions, such as water temperature (e.g. La Niña) (Fig. 12-14). Furthermore, inshore and offshore platforms showed interesting correlations between species composition parameters and water temperatures (Fig. 13, 14).

Fig. 10. transects — **Fig. 10.** Following the procedure from Love et al. (2003), Martin and Lowe (2010) conducted bi-monthly transects at six oil platforms in the San Pedro Shelf at three depth levels, if possible. The researchers were unable to obtain level 3 transects from two of the platforms (Elly, Eureka).

Fig. 11. species compositions — **Fig. 11.** Martin and Lowe (2010) found that oil platforms Esther and Eva (inshore) had similar species compositions. However, the compositions of inshore platforms were significantly different than those offshore. Inshore platforms had serranid (34% density, 55% biomass) and surfperch species (“San Diegan Province species”) that have a high association with the ecotone (edge), while blacksmith and other pelagic species (95% density, 23% biomass) (“Oregonian Province species”) dominated offshore platforms. Adult and young of the year rockfish were also present at offshore sites.

Fig. 12. community depth stratification — **Fig. 12.** Martin and Lowe (2010) found that blacksmith composed 92% of total reef fish density at offshore oil platforms, but only contributed to 19% of the biomass. Most of those observed were young of the year and were always associated with structures that provided planktivorous prey and nightly shelter. Blackfish are unlikely to move to deeper waters because of thermal preferences and the risks involved with increased predation and reduced food availability at increasing depths.

Fig. 13a. species diversity as a function of water temperatu — **Fig. 13a.** Martin and Lowe (2010) found that the species compositions of inshore oil platforms were correlated with water temperatures. Both species richness and species diversity significantly increased with increasing water temperatures.

Fig. 13b. fish density as a function of water temperature — **Fig. 13b.** Martin and Lowe (2010) found that the species compositions of inshore oil platforms were correlated with water temperatures. Fish densities also increased with water temperature.

Fig. 14. species density and fish biomass of oil platforms f — **Fig. 14.** According to Martin and Lowe (2010), oil platforms further offshore in the San Pedro Shelf showed increases in species density and richness, but decreases in fish biomass with increasing temperatures.

Mireles (2010) simultaneously conducted a study to measure the site fidelity of four commercially important fishes at two oil platforms in the San Pedro Shelf. Fifteen adults of each of four species (CA sheephead, cabezon, kelp and grass rockfish) were acoustically tagged and monitored for 1.5 years (Fig. 15, 16). Ten of the fifteen tags just detected the presence or absence of the individuals, and the five additional tags had depth sensors. Results showed that each species had a high degree of site fidelity and that most species remained within shallow waters. Each species showed diel behavior, with sheephead traveling the greatest distance between day and night. However, this movement, as well as the depth distribution of cabezon, appears to change depending on the season. Since the animals remain within shallow waters, any decommissioning effort that removes the platform above 55 meters depth would eliminate essential habitats that are occupied by each of the studied species.

Fig. 15. acoustic receiver placement and detection efficienc — **Fig. 15.** Acoustic receivers were placed at various depths at each platform to ensure the highest detection efficiency possible. The data were downloaded on a bi-monthly basis.

Fig. 16. cabezon, grass rockfish, kelp rockfish, and Califor — **Fig. 16.** Mireles (2010) studied the site fidelity of cabezon (*Scorpaenichthys marmoratus*), grass rockfish (*Sebastes rastrelliger*), kelp rockfish (*Sebastes atrovirens*), and California sheephead (*Semicossyphys pulcher*) at two oil platforms in the San Pedro Shelf.

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