553 



Preliminary use of oxygen stable isotopes and 

 the 1983 El Nino to assess the accuracy of 

 aging black rockfish (Sebastes melanops) 



Kevin R. Piner 



Southwest Fisheries Science Center 



National Marine Fisheries Service, NOAA 



8604 La Jolla Shores Drive 



La Jolla, California 92037 



E-mail address Kevin. Pinena'noaa. gov 



Melissa A. Haltuch 



School of Aquatic and Fishery Sciences 

 University of Washington, 

 1122 NE Boat Street 

 Seattle. Washington 98105 



John R Wallace 



Northwest Fisheries Science Center 

 National Marine Fisheries Service, 

 2725 Montlake Blvd East 

 Seattle, Washington 98112 



(Campana, 2001). Recently, stable 

 oxygen isotopes from Pacific halibut 

 (Hippoglossus stenolepis) otoliths 

 were used to examine regime shifts 

 in the Northeast Pacific for the iden- 

 tification of changes in bottom wa- 

 ter temperatures (Gao and Beamish, 

 2003). In addition, otolith chemistry 

 may be used to identify environmen- 

 tal events that serve as natural tags 

 for such studies (Campana and Thor- 

 rold, 2001). We used a strong region- 

 al environmental event, the 1983 El 

 Nino, as a time marker to judge the 

 accuracy of age assignment for black 

 rockfish <15 years of age. The 1983 

 El Nino produced anomalously warm 

 oceanic conditions along the coast- 

 lines in the eastern Pacific; therefore 

 the stable oxygen isotope ratio from 

 1983 should reflect this change in 

 oceanic conditions. 



Materials and methods 



Black rockfish (Sebastes melanops) 

 range from California to Alaska and 

 are found in both nearshore and shal- 

 low continental shelf waters (Love et 

 al., 2002). Juveniles and subadults 

 inhabit shallow water, moving deeper 

 as they grow. Generally, adults are 

 found at depths shallower than 55 

 meters and reportedly live up to 50 

 years. The species is currently man- 

 aged by using information from an 

 age-structured stock assessment 

 model (Ralston and Dick, 2003). 



In many studies, ages are assumed 

 to be accurate and there is no effort 

 to validate the accuracy of the ages 

 (Beamish and McFarlane, 1983). Re- 

 cent methods of age validation rely 

 upon environmental events that serve 

 as time markers (Campana, 2001). 

 Bomb radiocarbon released during 

 nuclear bomb testing has been used 

 to validate fish ages (Kalish et al., 

 1996; Campana, 1997; Kalish et al., 

 1997). Unfortunately, bomb radiocar- 

 bon can be used only for fish that 

 lived during the informative period 

 (-1960-70); thus the technique has 

 been used primarily on older ages. 

 For many stock assessments, the 

 validation of younger ages is more 



critical because of their importance 

 in estimating vital rates, such as 

 growth and maturity schedules. 



In this note we apply the well- 

 studied relationship between water 

 temperature and the ratio of oxygen 

 stable isotopes in otoliths to assess 

 the accuracy of young black rockfish 

 ages. Oxygen isotope ratios serve as 

 a record of past water temperatures 

 because the isotope ratio is incorpo- 

 rated into the otolith in near equi- 

 librium with the ratio found in the 

 environment (Patterson et al., 1993; 

 Thorrold et al., 1997) and ambient 

 water temperatures are inversely cor- 

 related with 18 0/ 16 ratios (Gao et 

 al., 2001). Calcified structures have 

 a strong history of being used in en- 

 vironmental reconstructions based on 

 incorporated trace elements and iso- 

 topes (Chivas et al., 1985; Holmden 

 et al., 1997). Otolith microchemistry 

 has been used to successfully recon- 

 struct the environmental history of 

 fish and to answer questions about 

 natal homing (Thorrold et al., 2001) 

 and population mixing (Campana et 

 al.. 1999). Variation in oxygen iso- 

 topes has been used to confirm vi- 

 sually observed growth increments 



We obtained nine pre-aged black rock- 

 fish otoliths collected during 1987-91 

 from recreationally caught fish off 

 Cape Lookout, Oregon (~45.25°N, 

 145°W), from approximately 15-30 

 m water depth. One otolith was aged 

 by Oregon Department of Fish and 

 Wildlife scientists by using the tra- 

 ditional break-and-burn method; the 

 matching otolith was used in the 

 stable isotope analysis. Fish from a 

 range of years and ages (Table 1) were 

 selected to include the 1983 El Nino 

 year. A time series of annual summer 

 bottom water temperatures from the 

 same region and depth where the 

 black rockfish otolith samples were 

 obtained, were provided by the Pacific 

 Hindcast from the Columbia Univer- 

 sity International Research Institute 

 for Climate Prediction, Palisades, 

 New York. 



To estimate the year containing 

 the warmest oceanic conditions, we 

 examined otolith material from all 

 opaque growth zones within each oto- 

 lith for oxygen isotopes ( 18 0/ 16 0) and 



Manuscript submitted 13 February 2004 

 to the Scientific Editor's Office. 



Manuscript approved for publication 



8 February 2005 by the Scientific Editor. 



Fish. Bull. 103:553-558 (2005). 



