Emery el al : Do oil and gas platforms reduce recruitment of Sebastes paucispinis to natural fiabitat' 



397 



within 10 km of Platform Irene (gray and black dots, 

 Fig. 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. 



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 pro- 

 duced a subset of 21 trajectories (black dots. Fig. 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 on the southern bound- 

 ary. 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. Oth- 

 ers crossing the offshore boundary extended north of 

 Platform Irene before turning southward or offshore. 

 The two trajectories crossing the northern boundary re- 

 mained 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 trans- 

 port 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 weaken- 

 ing or reversal of the prevailing upwelling favorable 

 winds, the so-called "relaxation" flow state described 

 by Dever (2004), Harms and Winant (1998), and Wi- 

 nant et al. (2003). They also described two other flow 

 states, "upwelling" and "convergent," which produce off- 

 shore 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. [2003J, 

 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: 



38N- 



36- 



34- 



32- 



30- 



28- 



iV 







r-'^ 





-124 



-122 



-120 



~~l 



-new 



Figure 5 



Trajectories of all drifters deployed by the Scripps Insti- 

 tution of Oceanography at various times from October 

 1992 through December 1999 after passing within 10 

 km of Platform Irene I gray dots). Also shown are all 

 drifters after passing within 10 km of Platform Irene 

 for only the time period 1 May-Si August for all years 

 (black dots). White circle is centered on Platform Irene 

 and has a radius of 10 km. 



P = population at time t\ and 

 Pq = the initial population. 



Here, Pq 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 sur- 

 vival 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 California Current and remain far from the 

 mainland and the Northern Channel Islands (Fig. 5) 



P(t) = P^e-'", (3) 



where m = mortality (0.02 or 0.06/day, Ralston^); 



■■^Ralston, S. 2004. Personal conimun. NOAA National 

 Marine Fisheries Service, 110 Shaffer Road, Santa Cruz, 

 CA 95060. 



