OCS Study 

•

MMS 2006-052 

Do Platforms Off California Reduce 

Recruitment of Bocaccio (Sebastes 

paucispinis) to Natural Habitat? An 

Analysis Based on Trajectories 

Derived from High-Frequency Radar 

Final Technical Summary 

Final Study Report 

U.S. Department of the Interior 

Minerals Management Service

Pacific OCS Region 

OCS Study 

MMS 2006-052 

Do Platforms Off California Reduce 

Recruitment of Bocaccio (Sebastes 

paucispinis) to Natural Habitat? An 

Analysis Based on Trajectories 

Derived from High-Frequency Radar 

Final Technical Summary 

Final Study Report 

Author 

Libe Washburn 

Principal Investigator 

Prepared under MMS Cooperative 

Agreement No. 14-35-0001-31063 

by 

Coastal Marine Institute 

Marine Science Institute 

University of California 

Santa Barbara, CA 93106-6150 

U.S. Department of the Interior 

Minerals Management Service Camarillo

Pacific OCS Region November 2006
Disclaimer 

This report has been reviewed by the Pacific Outer Continental Shelf Region, Minerals Management 

Service, U.S. Department of the Interior and approved for publication. The opinions, findings, 

conclusions, or recommendations in this report are those of the authors, and do not necessarily reflect 

the views and policies of the Minerals Management Service. Mention of trade names or commercial 

products does not constitute an endorsement or recommendation for use. This report has not been 

edited for conformity with Minerals Management Service editorial standards. 

Availability of Report

Extra copies of the report may be obtained from: 

U.S. Dept. of the Interior 

Minerals Management Service 

Pacific OCS Region 

770 Paseo Camarillo 

Camarillo, CA 93010 

Phone: 805-389-7621 

A PDF file of this report is available at: 

http://www.coastalresearchcenter.ucsb.edu/CMI/ 

Suggested Citation 

The suggested citation for this report is: 

Washburn, Libe. Do platforms off California reduce recruitment of bocaccio (Sebastes paucispinis) 

to natural habitat? An analysis based on trajectories derived from high-frequency radar. MMS OCS 

Study 2006-052. Coastal Research Center, Marine Science Institute, University of California, Santa 

Barbara, California. MMS Cooperative Agreement Number 14-35-0001-31063. 34 pages. 
Table of Contents 

FINAL TECHNICAL SUMMARY....................................................................... 1 

 

FINAL STUDY REPORT...................................................................................... 13 

 Summary ....................................................................................................... 13 

 

 1. Background ............................................................................................... 13 

 2. Methods..................................................................................................... 14 

 2.1 Species modeled.......................................................................... 14 

 2.2 Ocean current measurement and trajectory calculation .............. 15 

 3. Results....................................................................................................... 18 

 4. Discussion ................................................................................................. 20 

 5. Conclusion ................................................................................................ 24 

 

 References..................................................................................................... 26 

 
List of Tables 

FINAL STUDY REPORT 

Table 1. Statistics for computed trajectories and SIO drifters ............................... 19 

 

 

List of Figures 

FINAL TECHNICAL SUMMARY 

Figure 1. Study area in the Santa Barbara Channel................................................. 2 

Figure 2. RMS difference between radar and drifter total velocity components .... 4 

Figure 3. Separation distance between actual drifters and “pseudo-drifters” ......... 5 

Figure 4. Surface velocity field ............................................................................... 6 

Figure 5. Time series of recently settled juvenile bocaccio .................................... 7 

Figure 6. Sequence of daily-averaged surface current maps .................................. 8

FINAL STUDY REPORT 

Figure 1. Map of study area near Pt. Conception, CA ............................................ 15 

Figure 2. Coverage boundary for May – August 1999 and histogram.................... 16 

Figure 3. Coverage boundary for May – August 2002 and histogram.................... 17 

Figure 4. Histogram and cumulative frequency ...................................................... 19 

Figure 5. Trajectories of all drifters deployed......................................................... 21 

 

 

 
Do Platforms Off California Reduce Bocaccio Recruitment 

1

FINAL TECHNICAL SUMMARY 

STUDY TITLE: Observations of the surface circulation in the eastern Santa Barbara 

Channel using high frequency radar and Lagrangian drifters. 

REPORT TITLE: Do platforms off California reduce recruitment of bocaccio (Sebastes 

paucispinis) to natural habitat? An analysis based on trajectories derived from high-frequency 

radar. 

CONTRACT NUMBER: 14-35-0001-31063 

SPONSORING OCS REGION: Pacific 

APPLICABLE PLANNING AREA(S): Southern California 

FISCAL YEAR(S) OF PROJECT FUNDING: FY 02, FY 03, FY 04, FY 05 

COMPLETION DATE OF REPORT: October 2006 

COSTS: FY 02-04 – $135,210; FY 05 – no cost extension 

CUMULATIVE PROJECT COST: $135,210 

PROJECT MANAGER: Russell J. Schmitt

AFFILIATION: Coastal Research Center, Marine Science Institute, University of 

California, Santa Barbara, CA 93106 

ADDRESS: University of California, Santa Barbara, CA 93106 

PRINCIPAL INVESTIGATOR: Libe Washburn 

ADDRESS: Department of Geography, University of California, Santa Barbara, CA 

KEY WORDS: surface circulation, Lagrangian drifter, Santa Barbara Channel, radar array, 

radar sites and hardware 

BACKGROUND: 

Since 1998, Principal Investigator Washburn’s research group has been operating an array of HF 

radars in the Santa Barbara Channel. During the period of this study (25 September 2002 – 30 

September 2004; no-cost extension until 30 September 2005), a total of 2-4 radar sites was 

maintained along the mainland coast of the Channel. The configuration of the array as of 13 

June 2006 is shown in Figure 1. Real time data is displayed and archived at 

http://www.icess.ucsb.edu/iog/realtime/index.php. 
Final Technical Summary – Washburn 

2

Figure 1. Study area in the Santa Barbara Channel. Black arrows show surface current vectors scaled according to 

the scale in the upper right-hand corner. Observations are a 25-hr average centerd on 0900 GMT on 13 June 2006. 

Vectors are spatial averages over circles with radii of 3 km as shown by the circle to the right. HF radar sites are 

shown with yellow triangles. Oil platforms are squares. 

An important new site installed toward the end of this project is at the Summerland Sanitary 

District plant in Summerland, CA. The site at the Mandalay Generating Station (Mandalay in 

Figure 1) was also installed during the project, but has not consistently obtained data due to 

electrical power problems at the site. These power problems have recently been corrected by 

Reliant Energy, the owner of the site. In April 2006 a new and dedicated circuit was installed to 

power our equipment. Radar hardware was obtained through grants from the W.M. Keck 

Foundation and the University of California Marine Council. Two systems were borrowed from 

NOAA’s Environment Technology laboratory in Boulder, Colorado, and a third from Stanford 

University. Operational support for the radar array has come from a number of agencies 

including: Minerals Management Service, Partnership for Interdisciplinary Studies of Coastal 

Oceans (PISCO), University of California Marine Council, and NASA. 

Throughout most of this study, each radar system produced time series of radial current 

components (hereafter referred to as “radials”) out to a maximum range of 40-80 km off shore 

depending on environmental conditions and software version. As discussed by (Emery et al., 

2004), the actual range and overall coverage area changes in time. For the 12-13 MHz systems 

used in this study, radials are obtained as spatial averages over sectors of the sea surface 

measuring 5 degrees in azimuth by 1.5 km in range. Radials in each sector are computed over 10 

minute intervals. Total current velocity vectors are estimated by averaging all radials each hour 

over circular areas of the sea surface with 3 km radii. The circles are centered on points on a 

square grid with two km spacing between points. The least square technique described by 

(Gurgel, 1994) is used to combine radials within the circular areas to estimate the eastward and 

northward current components at each grid point. 
Do Platforms Off California Reduce Bocaccio Recruitment 

3

OBJECTIVES: 

The original objectives of this project were not fully achieved during the project period because 

of difficulties encountered in gaining access to suitable HF radar sites in the eastern Santa 

Barbara Channel. MMS was kept informed of this situation during the project through routine 

quarterly reporting of progress on the project. 

Toward the end of the project, sites were obtained near Summerland, California. A private home 

owner allowed the short-term installation of a site on his property from 1 May – 1 September 

2004. This site supported a MMS-sponsored experiment to examine oceanographic factors 

leading to settlement of fish larvae on two oil production platforms. This project is an on-going 

collaboration with the research group of Dr. Milton Love of the Marine Science Institute at 

UCSB. Later, the Summerland Sanitary District allowed installation of a site on a more 

permanent basis nearby (about 1 km east); that site began operation in April 2005. Below we 

describe results of observations obtained with the other radars which addressed important new 

aspects of the circulation in the Santa Barbara Channel (SBC). Results also address a new 

method to assess the performance of HF radars for measuring ocean surface currents. 

DESCRIPTION: 

Evaluation of HF radar performance using Lagrangian drifters 

In collaboration with Dr. Carter Ohlmann and his research group, comparisons were made 

between velocity estimates obtained by HF radar and those obtained by arrays of drifters in the 

SBC. These experiments were moved from the originally-proposed location in the eastern SBC 

to the central channel when we were unable to secure eastern HF radar sites. Previous 

comparison (e.g. (Shay et al., 1998; Emery et al., 2004)) focused on comparisons with moored 

current meters. Because current meters are effectively point measurements in the ocean, they 

are fundamentally different from current measurements obtained by HF radar. This is due to the 

fact that radar measurements are inherently averaged in space and time. The hypothesis driving 

this analysis was that the velocity differences observed between current meters and HF radars 

result from the different ways the two methods observe the ocean, point measurements versus 

spatial averages. If spatial averaging is accounted using drifters spread over areas comparable to 

those over which the radars measure velocity, then differences should be reduced. An 

experiment was conducted to test this hypothesis. 

As part of the test, dense arrays of surface drifters were used to quantify the flow field on time 

and space scales over which the HF radar observations are measured. Up to 13 drifters were 

repetitively deployed off Coal Oil Point (COP) in the SBC on 5 days during an 18 month period. 

The deployments were within coverage of the radars at COP and Refugio (RFG). During each 

deployment day, a group of drifters was released on a regularly spaced grid within a 2 km x 2 km 

square. To reduce geometric pointing errors, the deployment square was located such that radial 

vectors to the COP and RFG radars were nearly perpendicular. As drifters moved from the 

square they were retrieved and replaced to maintain a broad spatial distribution of observations 

within the sampling area during the day. This sampling scheme resulted in up to 56 velocity 

observations distributed over the time (1 hr) and space (1 to 4 km-2) scales from which surface 
Final Technical Summary – Washburn 

4

current maps from HF radar are computed. As shown in Figure 2, root-mean-square (rms) 

differences between HF radar radial velocities and average drifter velocities are mostly 3 to 5 cm 

s

-1, much less than the 9-20 cm s-1 typically reported in the literature (e.g. (Shay et al., 1998; 

Emery et al., 2004)). The reduction over past validation studies employing current meters is due 

to improved resolution by the drifters of small scale velocity structures that are included in the 

time- and space-averaged HF radar measurements. Roughly 5 cm s-1 of the differences can be 

attributed to sampling on the differing time and space scales observed by drifters and current 

meters. 

Figure 2. RMS difference between radar and drifter total velocity components for area of Coal Oil Point near Santa 

Barbara, CA versus number of drifter observations used to compute the drifter mean. HF radar totals are hourly 

averages within a 2 km square. Drifter velocities are corresponding time and space averages with randomly subsampled sets within the square. The u component (*) is east-west and mostly along-shore in the study region. The v 

component (o) is north-south and mostly across-shore. Error bars show standard error. 

Despite generally good agreement, differences can increase strongly with time. In one instance 

from a HF radar array in San Diego, the difference increased from near zero to more than 20 cm 

s

-1 within 2 hr. The rms difference and bias (mean absolute radial speed difference) for that day 

exceeded 7 and 12 cm s-1, respectively. Overall, however, these observations show that HF 

radars measure surface currents more accurately than has been inferred based comparisons with 

current meters. Results of this work are summarized by Ohlmann et al.(submitted to the J. 

Oceanic and Oceanographic Technology). Results were also presented at the 2006 Ocean 

Sciences meeting in Honolulu. 

Currently, Ohlmann’s and Washburn’s research groups are examining the capability of the HF 

radars for predicting current trajectories. The observational approach is to deploy drifters within 

the observational field of the HF radars over time periods of up to a few days (i.e. before the 

drifters exit the SBC). As shown in Figure 3, separation distances between actual drifters and 

pseudo-drifters derived from HF radar observations increase through time. Separation rates are 

about 0.2 km hr-1 in the along shore (x) direction and 0.3-0.4 km hr-1 in the cross-shore (y) 
Do Platforms Off California Reduce Bocaccio Recruitment 

5

direction. Data of Figure 3 were obtained from the COP and RFG radars for a location in the 

central SBC over a time scale of just over 4 hr. Data from other experiments are being examined 

to evaluate trajectory predictions on scales of about 1 day. A principal goal of this research is to 

quantify trajectory differences for use in models combining HF radar observations with 

stochastic models representing unresolved small scale velocity structure. A collaborator on this 

project, Dr. Arthur Mariano of the Rosentiel School of Marine Sciences at the University of 

Miami, is an expert on these modeling approaches. 

Figure 3. Separation distance between actual drifters and “pseudo-drifters” computed from HF radar time series as 

a function of time. At time t = 0 the separation distance was zero. The x-separation distance (*) is east-west and 

mostly along-shore in the study region. The y-separation distance (o) is north-south and mostly across-shore. Error 

bars show standard error. 

Small coastal eddies in the Santa Barbara Channel 

During this project, small coastal eddies were discovered along the mainland coast of the Santa 

Barbara Channel using observations from the HF network described above. The eddies 

frequently occur along the mainland coast of the SBC between Santa Barbara and Pt. 

Conception. They are 4-15 km in diameter and typically last about 2 days, although some last up 

to 6 days. Most eddies within the radar coverage area are clockwise with rapid rotation rates; the 

rates typically equal to about half the earth’s rotation rate at the latitude of the SBC. Moored 

observations over the inner shelf (12 - 15 m water depths) showed that these eddies can 

occasionally move previously upwelled nutrients on shore. We speculate that these eddies are an 

important transport mechanism for nutrients and biogenic particles to inner shelf regions and 

nearshore kelp forests. They would also advect oil and other pollutants toward shore. Figure 4 

shows an example a small anti-cyclonic eddy on 30 August 2002 lying offshore, just west of 

Refugio. This eddy was centered over the 100 m isobath with strong clockwise rotation (blue 

area). The counter-clockwise pattern southeast of this eddy (red area), only partly resolved by 

the radar coverage in this figure, is a persistent feature of the SBC circulation (Harms and 

Winant, 1998; Nishimoto and Washburn 2002; Beckenbach and Washburn, 2004; Dever, 2004). 

The more frequent occurrence of clockwise eddies suggests that interaction with the bottom is 

partly responsible for their formation. Further discussion of these eddies is given by (Bassin et 
Final Technical Summary – Washburn 

6

al., 2005). This work was also the focus of the Masters Thesis of Ms. Corinne J. Bassin, a 

graduate student in the UCSB Interdepartmental Program in Marine Science. 

Figure 4. Surface velocity field on 30 August 2002 at 0000 GMT showing small anti-cyclonic eddies (clockwise 

arrows) south of Coal Oil Point (COP) and southwest of Refugio (RFG). Color contours show rotation rate (i.e. 

relative vorticity) normalized by the earth’s rotation rate f. Red regions show strong counter-clockwise rotation and 

blue areas show strong clockwise rotation. Yellow-filled black squares are HF radar sites at Point Conception 

(PTC), RFG, and COP. Arrows show velocities based on scale to right. Red arrows are velocities measured at 

nearshore moorings and black arrows are velocities measured from HF radar. 

Delivery of larval bocaccio to oil production platforms in the eastern Santa Barbara 

Channel: 

Another important research activity during the project period has been a collaboration with the 

research group of Dr. Milton Love of the Marine Science Institute at UCSB. This MMS-funded 

study is aimed at 1) exploring links between ocean current patterns and the timing of delivery of 

juvenile rockfishes settling on offshore platforms in the eastern Santa Barbara Channel; and 2) 

estimating the proportion of these juvenile fishes that, if the platforms did not exist, would have 

been transported to natural reef habitat. Surface currents in the eastern Santa Barbara Channel 

were mapped using radar for the first time during this study, and the data are being used to 

pursue both research objectives. 
Do Platforms Off California Reduce Bocaccio Recruitment 

7

Figure 5. Time series of (upper panel) the number of the number of recently settled juvenile bocaccio and (b) the 

subsurface salinity structure at 26 m depth at platform Gail. 

The field observations of this combined biological and oceanographic study were conducted at 

two platforms, Gail and Gilda, in the eastern Santa Barbara Channel from May through August 

2004. This period corresponded with the season of much of the rockfish recruitment in this area. 

These two platforms, Gail (in about 230 m depth) and Gilda (about 65 m depth) are in a dynamic 

area where ocean currents are variable over a scale of several days and where fronts and eddies 

are observed. Fish settlement was quantified every 3-4 days using SCUBA surveys. Ocean 

surface currents were measured by the HF radar array of Figure 1 while other oceanographic 

monitoring was conducted at the platforms. This monitoring included moored acoustic Doppler 

current profilers (ADCP’s) and temperature-salinity (T-S) recording at both platforms. 

Measurements at platform Gail were made near 30 m depth (yellow line, Figure 5). Vertical 

profiles of water properties were paired with every SCUBA survey. 
Final Technical Summary – Washburn 

8

Figure 6. Sequence of daily-averaged surface current maps with hand-rendered flow features (curved orange 

arrows) and abundance of juvenile bocaccio at Platforms Gail and Gilda (gray squares circled in red) during two 

consecutive fish surveys on 28 June and 1 July, 2004. A recruitment pulse of juvenile bocaccio at the two platforms 

is associated with the northwestward flow of surface waters from the Southern California Bight through the eastern 

entrance of the Santa Barbara Channel. 

An analysis of the seasonality of recruitment based on the in situ physical oceanographic data 

has been completed. Multiple lines of oceanographic evidence clearly show that the onset of the 

Gai

Gild

July 1 (Day 183) 

(Fish survey: 200 juvenile 

bocaccio observed at Gilda, 

110 juvenile bocaccio at Gail. 

June 29 (Day 181) 

(No fish surveys on 

days 181 and 182) 

June 28 (Day 180) 

(Fish survey: No juvenile 

bocaccio sighted)
Do Platforms Off California Reduce Bocaccio Recruitment 

9

recruitment season of bocaccio rockfish (Sebastes paucispinis) in the eastern Santa Barbara 

Channel was abrupt and coincided with a change of water masses in June 2004. Bocaccio, 

presently declared over-fished by the National Marine Fisheries Service, was the most abundant 

juvenile rockfish observed in this study. Figure 5 shows that a mid-water tongue of low salinity 

water (highlighted in green) broadened and was centered around the depth where the juvenile 

bocaccio settled on Platform Gail during the recruitment season. The same water mass 

phenomenon associated with bocaccio recruitment at Platform Gilda was also evident (data not 

shown). The temperature-salinity properties observed at the platforms near the depth where the 

juvenile bocaccio settled were compared with reference samples of water mass sources in the 

Southern California Current region collected in June 2004 by the California Cooperative 

Fisheries Investigations (CalCOFI). This comparison showed that waters bringing the larval 

bocaccio originated east and south of the channel in the Southern California Bight. 

Figure 6 shows HF radar current patterns before and during the bocaccio recruitment event to the 

platforms around day 183 of Figure 5. The significant finding is that the currents delivering 

juvenile bocaccio recruits to the eastern Santa Barbara Channel platforms were from the 

Southern California Bight and not from within the channel or north of Point Conception. 

Presently, we are working toward objectively analyzing the radar data to explain how the spatial 

and temporal patterns of fish recruitment in the Santa Barbara Channel is linked to current 

patterns. We will be using the radar data to estimate where recruiting fish would be transported 

if the platforms were not present by simulating drifter trajectories from the radar data—these 

results likely will be referenced for platform decommissioning considerations. Preliminary 

results were presented by Mary Nishimoto at the Ocean Science Meeting in Honolulu, HI in 

February 2006. 

SIGNIFICANT CONCLUSIONS: 

1. Validation of HF radar current measurements is significantly improved if comparisons are 

made with observations on similar time and space scales as used by the radars. Velocity 

differences between the HF radars and arrays of drifters are 0.03-0.05 m s-1, less than half the 

typical differences obtained in comparisons with point measurements such as current meters. 

2. Surface trajectories can be predicted by HF radars, but predictions depart from actual 

trajectories measured by drifting buoys. Based on comparisons with surface drifters, differences 

between predicted and measured trajectories near the coast separate at 0.2-0.4 km hr-1. 

Separation increased more rapidly in the cross-shore direction compared with the along-shore 

direction. Improvements in trajectory prediction may result if models of unresolved scales of 

motion are incorporated. 

3. Small eddies often occur near the mainland coast of the Santa Barbara Channel. The eddies 

are 4-15 km in diameter and typically last about 2 days, although some last up to 6 days. Most 

eddies within the radar coverage area rotate clockwise. It is likely that the clockwise rotation 

results from bottom friction near shore. 
Final Technical Summary – Washburn 

10

4. The nearshore eddies are a new transport mechanism for moving nutrients and biogenic 

particles to inner shelf ecosystems of the Southern California Bight. It is likely that they can 

rapidly move buoyant pollutants such as oil and stormwater runoff toward shore. 

5. Larval bocaccio, an important rockfish species in California waters, can recruit strongly to oil 

production platforms. Rapid onset of delivery of the larvae onto two platforms in the eastern 

Santa Barbara Channel (Gail and Gilda) coincided with the arrival of low salinity waters from 

the Southern California Bight. This suggests that monitoring water masses may be useful in 

predicting delivery of rockfish larvae. 

STUDY PRODUCTS: 

Publications: 

Bassin, C.J., L. Washburn, M.A. Brzezinski, and E.E. McPhee-Shaw. 2005. Sub-mesoscale 

coastal eddies observed by high frequency radar: A new mechanism for delivering nutrients 

to kelp forests in the Southern California Bight, Geophysical Research Letters, 32, 

doi:10.1029/2005GL023017 

Beckenbach, E.H., and L. Washburn. 2004. Low frequency waves in the Santa Barbara Channel 

observed by high frequency radar, Journal of Geophysical Research, 109, 

doi:10.1029/2003JC001999,2004 

Cudaback, C., L. Washburn, and E.P. Dever. 2005. Sub-tidal inner-shelf circulation near Pt. 

Conception, California, Journal of Geophysical Research and Oceans, 110, C10007, 

doi:10.1029/2004JC002608 

Emery, B.M., L. Washburn, and J.A. Harlan. 2004. Evaluating radial current measurements 

from CODAR High Frequency radars with moored current meters, Journal of Atmospheric 

and Oceanic Technology 21(8): 1259-1271. 

Emery, B.M., L. Washburn, M. Love, M.M. Nishimoto, and J. C. Ohlmann. 2006. Do oil and 

gas platforms off California reduce recruitment of bocaccio (Sebastes paucispinis) to natural 

habitat? An analysis based on trajectories derived from high frequency radar. Fisheries 

Bulletin, 104: 391-400. 

Ohlmann, C., P. White, L. Washburn, E. Terrill, B.M. Emery, and M. Otero. 2006. 

Interpretation of coastal HF radar derived surface currents with high resolution drifter data, 

Submitted; Journal of Atmospheric and Oceanic Technology. 

Research Presentations:

Anderson, C.R., M.A. Brzezinski, D.A. Siegel, L. Washburn, and N. Guillocheau. 2004. Are 

harmful algal blooms responsible for the variability in phytoplankton species composition 

during spring blooms in the Santa Barbara Channel? ASLO/TOS Ocean Science Research 

Conference, Honolulu, HI, 15-20 February.Bassin, C.J., L. Washburn, and E. McPhee-Shaw. 

2003. Sub-mesoscale eddies along the northern Santa Barbara Channel: A possible 
Do Platforms Off California Reduce Bocaccio Recruitment 

11

mechanism for nutrient delivery to the inner shelf”, Eastern Pacific Ocean Conference, 

Wrigley Marine Science Center, Catalina Island, CA, 24-27 Sept. 

Bassin, C.J., L. Washburn, E.E., and McPhee-Shaw. 2004. Sub-mesoscale eddies along the 

northern Santa Barbara Channel: A possible transport mechanism for particle transport across 

the inner shelf. ASLO/TOS Ocean Science Research Conference, Honolulu, HI, 15-20 

February. 

Beckenbach, E.H. and L. Washburn. 2003. Vorticity, divergence, and flow states in the western 

Santa Barbara Channel. Eastern Pacific Ocean Conference, Wrigley Marine Science Center, 

Catalina Island, CA, 24-27 Sept. 

Emery, B.M., M.M. Nishimoto, L. Washburn, and M. Love. 2003. Alternative fate estimation of 

oil rig bocaccio recruits using HF radar. Eastern Pacific Ocean Conference, Wrigley Marine 

Science Center, Catalina Island, CA, 24-27 Sept. 

Emery, B., M. Nishimoto, L. Washburn, and M. Love. 2004. Do offshore platforms affect the 

fate of recruitment boccacio? An analysis based on HF radar derived surface trajectories. 

Poster presented at the Thirteenth Western Groundfish Conference, Victoria, British 

Columbia, Canada, 9-13 February. 

Emery, B., M. Nishimoto, L. Washburn, and M. Love. 2004. Do offshore platforms affect the 

fate of recruitment boccacio? An analysis based on HF radar derived surface trajectories. 

Fourth International Radiowave Oceanography Workshop, Townesville, Australia, 21-23 

April. 

McPhee-Shaw, E., L. Washburn, and D. Siegel. 2003. Low-frequency dynamics and nutrient flux 

to the inner shelf of the Santa Barbara Channel. Eastern Pacific Ocean Conference, Wrigley 

Marine Science Center, Catalina Island, CA, 24-27 Sept. 

McPhee-Shaw, E.E., D.A. Siegel, L. Washburn, D. Reed, and M. Brzezinski. 2004. Spring 

Upwelling in the Southern California Bight - Three Seasons of Observation. AGU Ocean 

Science Meeting, Portland, OR, 26-30 January. 

Nishimoto, M. M., L. Washburn, M. Love, D. Schroeder, and B. Emery. 2005. Is the delivery of 

juvenile fishes settling on offshore platforms linked to transport by ocean currents? 

American Fisheries Society Annual Meeting, Anchorage, AK; September 11-15, 2005. 

Nishimoto, M. M., L. Washburn, M. Love, D. Schroeder, and B. Emery. 2005. Is the delivery of 

juvenile fishes settling on offshore platforms linked to transport by ocean currents? 8th 

International Conference on Artificial Reefs and Related Aquatic Habitats (CARAH). April 

10-14. Biloxi, MS 

Nishimoto, M M, L. Washburn, M. Love, B. Emery, and D. Schroeder. 2006. Is the Timing of 

Juvenile Reef Fish Settlement Linked to Local and Regional Ocean Current Patterns? 

AGU/ASLO Ocean Sciences Meeting, 20-24 February, Honolulu, HI. 
Final Technical Summary – Washburn 

12

Ohlmann, C., P. White, L. Washburn, E. Terrill. 2006. Interpretation of HF Radar Derived 

Surface Currents with Arrays of GPS-Located, Reusable Drifters. AGU/ASLO Ocean 

Sciences Meeting, 20-24 February, Honolulu, HI. 

Palomino, E., M. McManus, J.L. Largier, L. Washburn, S. Morgan, K. Stolzenbach, B. Sanders, 

M. Stacey. 2003. Adding Value to NEOCO, the Network for Environmental Observations of 

the Coastal Ocean. Eastern Pacific Ocean Conference, Wrigley Marine Science Center, 

Catalina Island, CA, 24-27 Sept. 

Washburn, L. and M. Lavin. 2003. Inverse estuarine circulation in the northern Gulf of 

California. UC-MEXUS Workshop, Ensenada, Baja California, Mexico; 28 November. 

Washburn, L., E.H. Beckenbach, B.M. Emery, C.J. Bassin, C.N. Cudaback, and J.C. Ohlmann. 

2004. Recent results from an array of HF radars on the South-central California Coast. Fourth 

International Radiowave Oceanography Workshop, 21-23 April, Townesville, Australia, 21-

23 April. 

Washburn, L., C. Blanchette, C.N. Cudaback, B.M. Emery, and C. Gotschalk. 2004. Poleward 

flow events around Pt. Conception, California: An analysis based on HF radar and moored 

time series. Fall AGU Meeting, San Francisco, CA, 13-17 December. 

Washburn, L., J.C. Ohlmann, M.M. Nishimoto, C.A. Blanchette, B. M. Emery. 2005. Some 

applications of current-measuring, high frequency radars on the South-Central California 

Coast. 5th Radiowave Oceanography Workshop, Costanoa Conference Center, Pescadero, 

California, 3-6 May. 
Do Platforms Off California Reduce Bocaccio Recruitment 

13

FINAL STUDY REPORT 

Do platforms off California reduce recruitment of bocaccio (Sebastes 

paucispinis) to natural habitat? An analysis based on trajectories derived 

from high-frequency radar 

 

Brian M. Emery, Libe Washburn, Milton S. Love, Mary M. Nishimoto, 

and J. Carter Ohlmann 

Summary 

To investigate the possibility that oil platforms may reduce recruitment of rockfishes (Sebastes

spp.) to natural habitat, we simulated drift pathways (termed “trajectories” in our model) from an 

existing platform to nearshore habitat using current measurements from high-frequency (HF) 

radars. The trajectories originated at Platform Irene, located west of Point Conception, 

California, during two recruiting seasons for bocaccio (Sebastes paucispinis): May through 

August, 1999 and 2002. Given that pelagic juvenile bocaccio dwell near the surface, the 

trajectories estimate transport to habitat. We assume appropriate shallow water juvenile habitat 

exists inshore of the 50 m isobath. Results from 1999 indicate that 10% of the trajectories 

represent transport to habitat, while 76% represent transport across the offshore boundary. For 

2002, 24% represent transport to habitat, and 69% represent transport across the offshore 

boundary. Remaining trajectories (14% and 7% for 1999 and 2002 respectively) exit the 

coverage area either northward or southward along isobaths. Deployments of actual drifters 

(with 1-m drogues) from a previous multiyear study provided measurements originating near 

Platform Irene from May through August. All but a few of the drifters moved offshore, as was 

also shown with the HF radar-derived trajectories. These results indicate that most juvenile 

bocaccio settling on the platform would otherwise have been transported offshore and perished 

in the absence of a platform. However, these results do not account for the swimming behavior 

of juvenile bocaccio, about which little is known.

1. Background 

The 27 oil and gas platforms off southern and central California have limited life spans. Many of 

these structures have been in place for over 20 years (Love et al., 2003), and it is expected that 

some of these platforms will be decommissioned in the near future. Because decommissioning 

may entail full removal of the platform, agency personnel tasked with determining the best 

course of action in regard to the platforms would likely benefit from an understanding of the role 

that platforms play as fish habitat (Schroeder and Love, 2004). 

The platforms harbor high densities of many species of fishes, although species compositions 

vary with platform bottom depth (Love et al., 1999a; 1999b; Love et al., 2003). About 35 species 

of rockfishes (genus Sebastes) dominate the three distinct assemblages found around many 

platforms in the Santa Barbara Channel and off central California: the bottom, shell mound, and 
Final Study Report – Washburn 

14

midwater assemblages. Fishes around platform bottoms tend to be adult and subadult 

individuals. Those on the shell mounds are usually adults of dwarf species or juveniles of larger 

taxa. The midwater assemblages are composed almost entirely of juvenile fishes. Some of these 

juvenile fishes are one and two-year old individuals, but most are young-of-the-year (YOY) 

rockfishes. Densities of YOY rockfishes around platforms are usually far higher than those at 

nearby natural reefs (Love et al., 2003). These observations have raised a concern (e.g., Krop1). 

that platforms may reduce recruitment to natural reefs by functioning as catchments for pelagic 

juvenile rockfishes.

To investigate the possibility that an oil platform may reduce recruitment of rockfishes to natural 

habitat, we simulated drift pathways (hereafter referred to as “trajectories”) from an existing 

platform to nearshore habitat using current measurements from high-frequency (HF) radars. 

2. Methods

2.1 Species modeled 

Because trajectories derived from HF radar approximate transport pathways of near surface 

water parcels, we chose to model the movements of pelagic juvenile bocaccio (Sebastes 

paucispinis) that dwell near the surface during their time in the plankton. This historically 

important recreational and commercial fishing target in central and southern California (Love et 

al., 2002), is also among the shallowest dwelling juvenile fishes (Lenarz et al., 1991; Ross and 

Larson, 2003). 

Off central and northern California, parturition for bocaccio occurs from January to May and 

peaks in February (Love et al., 2002). Off southern California, the species has a reproductive 

season that spans all year, but most larvae are released from October to July, although January is 

the peak month. Juvenile bocaccio recruit to inshore waters from February to August off central 

California, although May through July is the peak season (Love et al., 2002). The trajectory 

simulation period from May through August was chosen to span this principal recruitment 

season. 

Bocaccio range from the Alaska Peninsula to central Baja California, and adults are usually 

found over high relief boulder fields and rocks in 50−250 m of water (Love et al., 2002). The 

fish most often settle in rocky habitat covered with various algae or in sandy zones with eelgrass. 

Juvenile bocaccio are commonly found in drifting kelp (Mitchell and Hunter, 1970; Boehlert, 

1977) indicating that the fish recruit to natural habitat encountered in offshore surface waters. 

For this analysis, we assumed that waters from the shallow subtidal to the 50-m isobath 

represented suitable habitat for juvenile recruits. This choice reflects the lack of information 

about suitable habitat locations in our study area and likely results in overestimates of the 

abundance of such habitat. 

 

1

 Krop, L. 1997. Environmental user group representative, disposition panel. In Proceedings: Public workshop, 

decommissioning and removal of oil and gas facilities offshore California: recent experiences and future deepwater challenges,

September 1997 (F. Manago, and B. Williamson, eds.), p 172. Mineral Management Service OCS Study 98-0023. Coastal 

Research Center, Marine Science Institute, Univ. California, Santa Barbara, California, 93106. MMS Cooperative Agreement 

Number 14-35-0001-30761. 
Do Platforms Off California Reduce Bocaccio Recruitment 

15

a b

Annual scuba surveys and submersible surveys (1995−2001) in the Santa Barbara Channel and 

Santa Maria Basin regions showed that YOY bocaccio inhabit the upper 35 m around one or 

more platforms for each year surveyed. Platform Irene (34o

36.62 ́N, 120 o43.40W; bottom depth 

73 m) was selected for analysis because fish recruited to it each year from 1995−2001 (Love et 

al., 2001) and it was the site of the highest density of YOY bocaccio observed from submersible 

surveys during these years (Love et al., 2003). Moreover, from May through August, 1999 and 

2002, Platform Irene was also in a region of good HF radar coverage, which allowed 

computation of extensive trajectory ensembles. 

2.2 Ocean current measurement and trajectory calculation 

Near-surface ocean currents were measured hourly by using an array of three HF radars 

(SeaSondes, manufactured by CODAR Ocean Sensors, Ltd. of Los Altos, CA) operating at 

12−13 MHz. At these frequencies, the measurement is an average over the upper 1 m of the 

water column (Stewart and Joy, 1974). The radars were located at Pt. Sal, Pt. Arguello, and Pt. 

Conception (Figure 1a). 

Figure 1. a) Map of study area near Pt. Conception, California, showing trajectories derived from HF radar from 1 

May - 31 Aug 2002. Triangles show HF radar locations and the white square shows Platform Irene. Gray curve 

superimposed on trajectories is the coverage boundary used for 2002. Labeled thin black lines are bathymetric 

contours. b) Solid black lines are 25 sample trajectories, which intersect the coverage boundary. Gray curve is the 

same as in panel a. The trajectories in panels a and b were created from velocity time series that were interpolated 

with EOFs. 

HF radars measure components of surface currents by means of a Doppler technique, at spatial 

increments of 1.5 km in range and 5o in azimuth from the radar location. Surface current vectors 

in an east-north coordinate system were computed on a 2-km grid by using the least square 

technique described by Gurgel (1994). With this technique, all current components obtained 

within a 3-km radius around each grid point were combined to estimate the surface current every 

hour. The 3-km radius limits the spatial resolution of the near-surface current fields. Emery et 

al. (2004) have described the processing of the HF radar data in more detail. Further discussion 

on the use of HF radars for measuring near surface currents is given by Paduan and Rosenfeld 

(1996) and Graber et al. (1997). 
Final Study Report – Washburn 

16

Emery et al. (2004) assessed performance of the three HF radars by comparing them with in situ

current meters at 5 m depth. They found that root-mean-square differences in radial speed 

measurements between HF radars and current meters ranged from 0.07 to 0.19 m/s. Recent 

observations comparing surface currents from HF radars and drifters have indicated that 

differences are substantially reduced if spatial variability in current fields is accounted for 

(Ohlmann, 2005). 

The areas used for computing trajectories were offshore of Pt. Conception and Pt. Arguello as 

shown in Figure 2a for 1999 and Figure 3a for 2002. These areas were selected to maximize the 

spatial coverage, and to minimize the inclusion of grid points with low temporal coverage. 

Variable coverage from individual radars results in differences in coverage between years. 

Boundaries of nominal coverage areas were oriented along and perpendicular to isobaths. 

Platform Irene is about 2 km from the inshore boundaries, which lie along the 50-m isobath 

(Figs. 2a and 3a). At times actual radar coverage exceeded nominal coverage boundaries as may 

be seen by comparing sample computed trajectories (black lines, Figure 1a) with the 2002 

boundary (gray closed curve, Figure 1a). Coverage in 2000 and 2001 for May through August 

was inadequate for producing trajectory ensembles around Platform Irene. 

A new trajectory was begun at the location of Platform Irene every 4 hours from 1 May through 

31 August for 1999 and 2002. Positions along the trajectories were determined by integrating 

current vectors forward in time using a fourth order Runge-Kutta algorithm. Trajectories ended 

where they encountered spatial gaps or where they reached the edge of radar coverage. 

Figure 2. a) Coverage boundary for 1 May – 31 August, 1999. Numbers and tick marks around boundary identify 

bins corresponding to x-axis in panel b. b) Histogram (gray bars, left-hand scale) and cumulative frequency (bold 

line, right-hand scale) show fraction of trajectories (in percent) intersecting bins around coverage boundary. Panel a 

shows bin locations. Distance around the coverage boundary (Figure 2a) in kilometers is also shown (bottom scale). 

Arrow above histogram shows boundary location nearest Platform Irene. 

a) b)
Do Platforms Off California Reduce Bocaccio Recruitment 

17

The number of trajectories reaching the coverage boundaries defined in Figures 2a and 3a were 

reduced by gaps in spatial and temporal radar coverage. For example, of the 670 possible 

trajectories in 2002, 541 (81%) ended within the radar coverage area and 129 (19%) intersected 

the coverage boundary. Changes in spatial coverage on diurnal and longer time scales resulted 

from several factors, such as broadcast interference, and are a characteristic of HF radars 

(Paduan and Rosenfeld, 1996). Gaps in the velocity time series were also caused by outages of 

individual radars. The average durations of these gaps were 4.4 ±22.3 h and 5.9 ±7.9 h in 1999 

and 2002, respectively. Outages of individual radars also produced a few long gaps in the 

velocity time series for each year across the entire coverage area. In 1999 two long gaps 

occurred: one from 1800 coordinated universal time (UTC) 28 June through 2000 UTC 22 July, 

and a second from 2300 UTC 24 July through 2200 UTC 13 August. In 2002 a single long gap 

occurred from 1700 UTC 16 May through 0100 UTC 21 May. These longer gaps were not 

filled. 

Figure 3. a) Coverage boundary for 1 May – 31 August, 2002. Numbers and tick marks around boundary identify 

bins corresponding to x-axis in panel b. b) Histogram (gray bars, left-hand scale) and cumulative frequency (bold 

line, right-hand scale) show fraction of trajectories (in percent) intersecting bins around coverage boundary. Panel a 

shows bin locations. Distance around the coverage boundary (Figure 2a) in kilometers is also shown (bottom scale). 

Arrow above histogram shows boundary location nearest Platform Irene. 

Shorter gaps were filled by interpolation by using empirical orthogonal functions (EOFs; (Emery 

and Thomson, 1998)). EOFs incorporate the underlying spatial structure of all velocities 

recorded at all locations where data existed at a given time. Any velocity component, u say, at 

grid point j may be expressed as 

() () ij j

N

i 1

u j t = ∑ai t φ + u =

 , (1) 

a) b)
Final Study Report – Washburn 

18

where t = time; 

 j u = the time average at location j (computed from available data at location j); 

 ai = the time-varying amplitude function; 

 φij = the ith spatial EOF mode at location j; and 

 N = the number of modes. 

The first seven modes (i.e., N=7) were used for interpolation and explained 64% (1999) and 56% 

(2002) of the variance. EOF interpolation increases the number of trajectories reaching the 

coverage boundary to 99%. As a test, gaps were also filled with linear, spline, and moving 

average interpolation, but EOF interpolation resulted in the most trajectories reaching the 

coverage boundaries. Otherwise, results did not depend strongly on the interpolation method. 

The fraction of filled data with EOF interpolation compared with the total possible data was 4% 

in 1999, 12% in 2002, and 14% with the 2002 data for the 1999 coverage boundary. Here the 

total possible data were the number of grid points within the coverage boundary for either 1999 

(45 grid points) or 2002 (291 grid points) multiplied by the number of hours between 1 May and 

31 August minus the long gaps discussed above (2952 hours – 1057 hours in 1999, 2952 hours – 

104 hours in 2002). Examples of 25 EOF-filled trajectories that started every 120 hours and 

intersected the 2002 coverage boundary are shown in Figure 1b. 

The principal quantity used in our study to estimate how Platform Irene might affect transport to 

nearshore habitat was the histogram of points where trajectories crossed the boundaries of the 

coverage areas. To determine this quantity, coverage boundaries were divided into 4 km-long 

segments, or bins. The first bin of each histogram was less than 4 km because distances around 

the coverage boundaries were not exactly divisible by 4. Bin numbers increased counterclockwise around the boundaries starting from 1 in the southeastern corner (Figs. 2a and 3a). 

The smallest numbers identified bins lying along the 50-m isobath. 

3. Results 

Trajectories originating at Platform Irene were sufficiently dense to fill in much of the 

surrounding area. In 2002 for example, EOF-filled trajectories spread over an area of about 20 

km in the cross-shore direction by 60 km in the alongshore direction (Figure 1a). North of Pt. 

Arguello, several trajectories crossed the 50-m isobath and some ended very near shore. South of 

Pt. Arguello, only a few trajectories approached the 50-m isobath. Instead, most turned 

southward or southwestward and moved offshore. A tendency for trajectories to align parallel to 

isobaths was evident in the northern end of the ensemble, although in other areas, such as the 

southeast, many trajectories lay across isobaths. 

A histogram of points where trajectories crossed the coverage boundary for May−August 1999 

exhibited a peak in bin 11 on the offshore side along the 500-m isobath (Figure 2b, left-hand 

axis). Table 1 and the cumulative histogram (Figure 2b, right-hand axis) showed that 76% of the 

trajectories crossed the offshore side corresponding to bin numbers 9−13. A second peak 

occurred in bin 3 and about 10% of trajectories crossed the inshore boundary on the 50-m isobath 

(bins 1−5). The remaining trajectories crossed either the northern (9%) or southern (5%) sides of 

the coverage boundary. 
Do Platforms Off California Reduce Bocaccio Recruitment 

19

Table 1. Statistics for computed trajectories and SIO drifters. 

In 2002 the radars covered a substantially larger area (Figure 3a), including about 50 km of the 

50-m isobath, 70 km of the 500-m isobath along the offshore boundary, and a portion of the 

western entrance of the Santa Barbara Channel. The histogram of coverage boundary crossings 

for May-August 2002 also exhibited two peaks, one between bins 9−11 along the 50-m isobath 

and a second in bin 31 along the offshore side of the coverage boundary (Fig 3b). In 2002, 24% 

of trajectories crossed the 50-m isobath (bins 1−13), 69% crossed the offshore side (bins 22−39), 

and the remainder crossed the northern side (6%) or the southern side (1%). 

Figure 4. Histogram (gray bars, left-hand scale) and cumulative frequency (bold line, right hand scale) showing 

percent of 2002 trajectories intersecting bins from 1999 coverage boundary. Bottom scale shows distance around the 

coverage boundary in kilometers. Arrow above histogram shows boundary location nearest Platform Irene. 

To compare results more directly between years, a histogram of crossings was generated from 

the 2002 trajectories, using the 1999 coverage boundary. The 1999 coverage boundary was 

completely contained within the 2002 coverage boundary. With the 2002 trajectories, a peak in 
Final Study Report – Washburn 

20

the histogram again occurred along the offshore boundary (Figure 4, left-hand scale), this time at 

bin 12 compared with bin 11 when the 1999 trajectories were used. A second, but much smaller 

peak occurred along the 50-m isobath at bin 3, consistent with the small peak along the 50-m 

isobath of Figure 2b. Table 1 and the cumulative histogram (Figure 4, right-hand scale) showed 

that 18% of trajectories crossed the 50-m isobath, 66% crossed the offshore side of the coverage 

boundary, and the remainder crossed either the northern (7%) or southern (8%) sides. 

The time required for trajectories to cross the coverage boundary, defined in our study as the 

residence time, varied between years and mainly depended on the size of the coverage area. In 

1999 the mean and standard deviation for the residence time was 19 ±12 hours, and the 

maximum was 86 hours (Table 1). In 2002 they were 47 ±34 hours and the maximum was 163 

hours. When the 2002 trajectories were computed over the 1999 coverage boundary, residence 

times were comparable to the 1999 values: 22 ±18 hours and a maximum of 116 hours. 

4. Discussion 

Because of limitations in spatial coverage, the HF radar-derived trajectories could not be used to 

examine the full range of length and time scales over which actual trajectories may extend. We 

used a trajectory data set resulting from the release of Argos drifters in the region to examine 

these scales. Drifters were deployed in the Santa Barbara Channel and Santa Maria Basin at 

irregular intervals from October 1992 through December 1999 as part of a circulation study 

conducted by the Scripps Institution of Oceanography (SIO; see Dever [1998] and Winant et al. 

[2003] for a description of the drifter data set). Drifter positions were obtained up to six times per 

day, typically for 40 days, and had a spatial accuracy of about 1 km. Several trajectories ended 

earlier when the drifters beached. 
Do Platforms Off California Reduce Bocaccio Recruitment 

21

Figure 5. Trajectories of all drifters deployed by the Scripps Institution of Oceanography at various times from 

October 1992 through December 1999 after passing within 10 km of Platform Irene (gray dots). Also shown are all 

drifters after passing within 10 km of Platform Irene for only the time period 1 May to 31 August for all years (black 

dots). White circle is centered on Platform Irene with and has a radius of 10 km.

No drifters were released at Platform Irene although many approached the platform after release 

elsewhere (130 drifters were deployed to the north of Platform Irene, and 440 were deployed to 

the south). To approximate trajectories originating at Platform Irene, all drifters released during 

all seasons for all years, and approaching within 10 km of the platform, were identified. This 

distance is a compromise between proximity to the platform and ensemble size; 93 trajectories 

approached within 10 km of Platform Irene (white circle in Figure 5 is 10 km in radius and is 

centered on Platform Irene). Of these, 34 were released north and 59 south of Platform Irene. 

The ensemble of trajectories beginning within 10 km of Platform Irene (gray and black dots, 

Figure 5) mainly followed the trend of southward advection by the California Current System, 

although a smaller number extended northward from the platform and a few reached Monterey 

Bay. Four trajectories entered the Santa Barbara Channel. 

 
Final Study Report – Washburn 

22

A further sorting of the ensemble of 93 trajectories approaching within 10 km of Platform 

Irene to include only those during 1 May−31 August of all years produced a subset of 21 

trajectories (black dots, Figure 5). Of these, 17 crossed the 2002 coverage boundary: 3 on the 

50-m isobath, 12 on the offshore boundary, 2 on the northern boundary, and 0 on the southern 

boundary. Although the ensemble was small, the fraction of drifters crossing the inshore 

boundary of the 2002 coverage area (18%) was comparable to the fraction of HF radar-derived 

trajectories that did so (24%), as shown in Table 1. Most trajectories crossing the offshore 

boundary continued offshore and southward, consistent with advection by the California Current 

System. Others crossing the offshore boundary extended north of Platform Irene before turning 

southward or offshore. The two trajectories crossing the northern boundary remained near shore 

and crossed the 50-m isobath north of the platform. None of these 17 trajectories entered the 

Santa Barbara Channel. Except for three drifters that beached, all the drifters remained offshore 

for the duration of Argos data logging. 

Trajectories crossing the 50-m isobath tended to do so north of platform Irene, as shown by most 

of the computed trajectories and three of the SIO drifters (Figs. 2b and 3b). This movement 

indicates that transport from the platform to shallow water habitat along the mainland coast 

mostly occurred during times of northward, or poleward, currents. Poleward flow in the region 

north of Pt. Conception results from weakening or reversal of the prevailing upwelling favorable 

winds, the so-called “relaxation” flow state described by Dever (2004), Harms and Winant 

(1998), and Winant et al. (2003). They also described two other flow states, “upwelling” and 

“convergent,” which produce offshore and equatorward transport near Platform Irene. Together 

these flow states have a 69% probability of occurring during May−Aug (36% for upwelling and 

33% for convergent), whereas the relaxation state has a 23% probability of occurring (Winant et 

al. [2003], their Table 3). For comparison, 19−30% of trajectories (HF radar-derived plus actual 

drifters) crossed the inshore and northern boundaries, consistent with the relaxation probability 

and 70−81% crossed the offshore and southern boundaries, consistent with the upwelling plus 

convergent probability. 

The trajectories can also be used to estimate recruit survival, based on the time required for 

transport to habitat. Recruit survival is estimated from a simple exponential decay model: 

P(t) = P0e

-mt, (3) 

where m = mortality (0.02 or 0.06/day, Ralston2

); 

 P = population at time t; and 

 P0 = the initial population. 

Here, P0 represents the population of juvenile bocaccio that recruited at Platform Irene, and 

survival estimates are used to predict their survival in the absence of the platform. In 1999 and 

2002, trajectories to habitat from Platform Irene crossed the 50-m isobath within 19−47 hours 

(Table 1), indicating a high percentage of survival for bocaccio (96−98%). In contrast, offshore 

and southward drifter trajectories from the SIO drifter data indicated much lower survival. 

Pelagic juvenile bocaccio transported by these flows would be carried southward by the 

 

2

 Ralston, S., 2004. Personal commun. NOAA National Marine Fisheries Service, 110 Shaffer Road, Santa Cruz, 

CA 95060 
Do Platforms Off California Reduce Bocaccio Recruitment 

23

California Current and remain far from the mainland and the Northern Channel Islands (Figure 

5) for at least 40 days, the nominal time the drifters were tracked. Survival after 40 days along 

these trajectories ranged from 9−45%. 

It is possible that juvenile bocaccio spend time away from near surface waters during their 

planktonic larval phase and therefore their trajectories may depend on deeper currents. Previous 

observations in the study region show strong correlation between near surface and deeper flows, 

indicating that inferences from surface trajectories apply to deeper trajectories. Dever (2004), 

Harms and Winant (1998), and Winant et al. (2003) generally found high correlation between 

moored currents at 5 and 45 m depth with flow speed decreasing with depth. They also found 

that poleward flow at 45 m occurs during the convergent and relaxation states, and equatorward 

flow occurs during the upwelling state. Therefore, during relaxation and upwelling states, 

currents at 45 m likely have similar directions, but lower speeds compared with surface currents. 

During the convergent state, currents at 45 m are opposite surface currents, indicating that 

surface currents represent deeper currents only to some shallower depth. 

We assumed in this trajectory analysis that juvenile bocaccio effectively behave like passive 

particles. We made this assumption for two reasons: 1) this assumption allowed us to focus on 

the lower bound of their range of possible swimming behaviors; and 2) such an assumption also 

eliminated the need to account for their actual swimming behavior in the open ocean, about 

which little is known. We do not assume that juvenile bocaccio do not swim; rather, we assume 

that they swim randomly such that their effective transport is similar to that of passive particles. 

The other behavioral limit of rapid, consistently directional swimming behavior would likely 

alter the fraction of bocaccio encountering shallow habitats versus the fraction being advected 

offshore. Flume experiments with visual cues for directional orientation demonstrate that coral 

reef fish in the late pelagic stage can swim up to ~100 km in 8 days (Stobutzki and Bellwood, 

1997); therefore behavioral modification of trajectories could be very important. Larval and 

pelagic juvenile fish may possess swimming and sensory abilities to overcome passive drift in 

currents; however, some kind of external reference is necessary for fish to detect and respond to 

the direction of a current. In a review of the behavior of larval and juvenile fish in the pelagic 

environment, Leis and McCormick (2002) pointed out that it is yet to be demonstrated that these 

early-stage fish in offshore “blue water” can effectively modify current-driven trajectories by 

orienting to cues from settlement habitat located at a scale greater than several kilometers away. 

A variety of near-field stimuli, such as light and temperature gradients, sound, and visible prey 

affect swimming behavior. Clearly more research is needed to evaluate the effects of swimming 

behavior of temperate reef fishes, such as bocaccio, in order to model their dispersal. We 

speculate, however, that the assumption of passive dispersal will remain an important lower 

bound on constraining effects of swimming behavior. 

Smoothing and interpolation in the processing of the HF radar velocity data limit the spatial 

resolution of current fluctuations to scales of ~6 km, the diameter of circles used to compute 

velocity vectors. Velocity structures smaller than this scale are not resolved but may be 

important in determining trajectories. For example, Helbig and Pepin (2002) found that errors in 

modeling the spatial distributions of fish eggs in an embayment increased as spatial resolution of 

a circulation model decreased. Assuming effects of unresolved velocity structures on smaller 

scales act as a diffusive process, we speculate that incorporating diffusion would cause spreading 
Final Study Report – Washburn 

24

of where trajectories intersect coverage boundaries. In this case, peaks in the histograms (such 

as in Figures 2b, 3b, and 4) would decrease as diffusion spreads boundary intersections to 

adjacent bins. Velocity statistics at scales of order of a few km and smaller in our study area, 

however, are not available for incorporating the effects of diffusion into the trajectories. Results 

from actual drifters, which do contain velocity structure unresolved by the HF radars, are not 

very different from HF-radar-derived trajectory results (Table 1), indicating that effects of 

unresolved variance are not large. For predicting settlement to habitat, trajectory improvements 

gained through the incorporation of smaller scale flow features may be offset by assumptions of 

swimming behavior and habitat location. 

Questions and issues have arisen in the decommissioning process about the regional importance 

of platform fish assemblages (Schroeder and Love, 2004). For example, does removal of a 

platform impact the bocaccio population? Based on our annual research submersible surveys 

(detailed in Love et al., 2003) conducted in 1997, 1998, 1999, and 2001, estimates of YOY 

bocaccio at Platform Irene ranged from 61 (2001) to 41,000 (1999) (Lenarz3

). YOY bocaccio 

abundances can be even higher than those observed at Platform Irene. We have recently 

estimated that, during 2003, Platform Grace, located in the Santa Barbara Channel, harbored 

over 300,000 YOY bocaccio (Lenarz3

). Under even the most conservative parameters, this would 

translate into many thousands of adults (MacCall4

). In addition, there is evidence that some of 

the bocaccio that recruit to platforms as YOYs migrate, and thus seed, natural reefs. Fish tagged 

at Platform A, located off Summerland, CA, in the Santa Barbara Channel, were later recovered 

over natural reefs over 100 km to the north and south of that platform (Hartmann, 1987). In 

another study, recruiting bocaccio became resident on a deep-water platform and formed the 

highest density of adult fish observed in the Southern California Bight (Love et al., 2003). Thus, 

bocaccio that recruit as YOYs to a platform may benefit natural reefs either through emigration 

to these reefs or through increased larval production. 

5. Conclusion 

Observations of evolving surface current patterns obtained by HF radar are used to estimate 

dispersal pathways for juvenile bocaccio in the vicinity of Platform Irene, an oil production 

platform off the central California coast. Results indicate that most of YOY bocaccio settling 

around Platform Irene would not survive in the absence of the platform. Instead, prevailing 

currents would likely advect them offshore where they would have a very low probability of 

survival. Although it is possible that some individuals would encounter acceptable nursery 

habitat on offshore banks or islands, it is likely that most would perish. Thus, the presence of 

Platform Irene almost certainly increases the survival of young bocaccio in the Point 

Conception–Point Arguello region. 

 

3

 Lenarz, W. 2004. Personal commun. P.O. Box 251, Kentfield, CA, 94914-0251 

4

 MacCall, A. 2004. Personal commun. NOAA National Marine Fisheries Service, 110 Shaffer Road, Santa Cruz, 

CA 95060 
Do Platforms Off California Reduce Bocaccio Recruitment 

25

These results indicate that knowledge of regional ocean circulation patterns is essential for 

evaluating the effects of oil production platforms, or other artificial habitats, on dispersal 

pathways of juvenile fishes. Platform location, local current patterns, and natural habitat 

distribution determine the balance between settlement at a specific platform and settlement on 

natural habitat. The approach used in this study, an analysis of trajectories derived from HF radar 

current measurements, can provide insights into this balance. Additional research on small-scale 

circulations features unresolved by the radars and on swimming behavior of juvenile rockfishes 

will sharpen these insights. 
26

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29

The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has responsibility for most 

of our nationally owned public lands and natural resources. This includes fostering sound use of our 

land and water resources; protecting our fish, wildlife, and biological diversity; preserving the 

environmental and cultural values of our national parks and historical places; and providing for the 

enjoyment of life through outdoor recreation. The Department assesses our energy and mineral 

resources and works to ensure that their development is in the best interests of all our people by 

encouraging stewardship and citizen participation in their care. The Department also has a major 

responsibility for American Indian reservation communities and for people who live in island territories 

under U.S. administration. 

The Minerals Management Service Mission 

As a bureau of the Department of the Interior, the Minerals Management Service's (MMS) primary 

responsibilities are to manage the mineral resources located on the Nation's Outer Continental Shelf 

(OCS), collect revenue from the Federal OCS and onshore Federal and Indian lands, and distribute 

those revenues. 

Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program

administers the OCS competitive leasing program and oversees the safe and environmentally sound 

exploration and production of our Nation's offshore natural gas, oil and other mineral resources. The 

MMS Royalty Management Program meets its responsibilities by ensuring the efficient, timely and 

accurate collection and disbursement of revenue from mineral leasing and production due to Indian 

tribes and allottees, States and the U.S. Treasury. 

The MMS strives to fulfill its responsibilities through the general guiding principles of: (1) being 

responsive to the public's concerns and interests by maintaining a dialogue with all potentially affected 

parties and (2) carrying out its programs with an emphasis on working to enhance the quality of life for 

all Americans by lending MMS assistance and expertise to economic development and environmental 

protection.