Brodeur et al.: Distribution, growth, condition, origin, and associations of |uvenile salmonids 



27 



during the first cruise, prevented us from sampling all 

 the stations along each transect. At each station, a Nordic 

 264 rope trawl built by Nor'Eastern Trawl Systems, Inc. 

 (Bainbridge Island, WA) was towed in surface waters by 

 a chartered fishing vessel (FV Sea Eagle) at a speed of 6 

 km/h. This rope trawl has a maximum mouth opening of 

 approximately 30 m x 18 m. Mesh sizes ranged from 162.6 

 cm in the throat of the trawl near the jib lines to 8.9 cm in 

 the codend. To maintain catches of small fish and squid, a 

 6.1-m long, 0.8-cm mesh knotless liner was sewn into the 

 codend. All tows were 30 minutes in duration. All fish and 

 squid caught were counted and measured at sea. After fork 

 length (FL) was measured to the nearest mm, all juvenile 

 salmon were immediately frozen for later determinations 

 of growth, condition, food habits, genetic analysis, and as- 

 sessment of pathological condition. 



The physical and biological environment was monitored 

 and sampled at each station immediately prior to setting 

 the trawl. A CTD (conductivity, temperature, and depth) 

 cast was made with a Sea-Bird SBE 19 Seacat profiler to 

 100 m at deep stations or within 10 m of the bottom at 

 shallow stations. Chlorophyll and nutrient samples were 

 collected from 3 m depth with a Niskin water sampler. A 

 neuston tow with a 1-m 2 mouth containing 333-,(im mesh 

 net was towed for 5 minutes out of the wake of the vessel 

 at each station. General Oceanics flow meters were placed 

 inside the net to measure the amount of water sampled. 

 Additional details on the analysis of these neuston trawls 

 are available in Reese et al. 2 



Condition and growth analysis 



Each salmonid was remeasured (FL to the nearest mm) 

 and weighed (to the nearest 0.1 g) in the laboratory. A por- 

 tion of hepatic and muscle tissue was excised, placed in 

 individual capsules, frozen in liquid nitrogen, and stored 

 at -80°C until analyzed. The bioenergetic health of juve- 

 nile salmon was evaluated by assessing changes in water 

 content (as a surrogate measure of fat accumulation) of 

 liver and muscle to estimate dry tissue weight. The water 

 content was determined by drying tissue samples to a con- 

 stant weight at 105°C. The accumulation of energy reserves 

 during the growth season ( energy reserves of salmon in 

 August in relation to salmon collected in June) that would 

 enhance survival of juveniles during the winter when food 

 availability is lower was also measured. The condition of 

 juvenile salmon was assessed by examining weight residu- 

 als (by using either the wet weight or dry weight) derived 

 from the allometric relationship between length and weight 

 of individual juvenile salmon after logarithmic transforma- 

 tion (Jakob et al., 1996) of salmon captured in June and 

 August. Wet-weight residuals are representative of the 

 traditional condition index of animals and are a reflection 



2 Reese, D.C., T.W.Miller, and R.D. Brodeur. 2003. Community 

 structure of neustonic zooplankton in the northern California 

 Current in relation to oceanographic conditions. 22 p. Unpubl. 

 manuscript. Northwest Fisheries Science Center, NMFS. 2030 

 S. Marine Science Drive, Newport, OR 97365. 



of somatic tissue growth. Dry-weight residuals are respon- 

 sive to accumulation of fat stores and are a reflection of the 

 bioenergetic health of the individual animal (Sutton et al., 

 2000; Post and Parkinson, 2001). 



To contrast growth characteristics during 2000 in differ- 

 ent latitudinal ranges of the California Current, we com- 

 pared ocean growth rates of juvenile coho salmon south 

 and north of Cape Blanco in the GLOBEC study area, 

 and in the area from Newport, Oregon, north to northern 

 Washington. The physical and biological characteristics of 

 these three regions of the coastal ocean differ greatly (U.S. 

 GLOBEC, 1994), and these differences may impact the dis- 

 tribution and abundance of prey of juvenile salmonids and 

 therefore may also affect salmonid growth. Data north of 

 Newport, Oregon, were collected during a separate study of 

 the Columbia River plume and the adjacent coastal ocean 

 (hereafter called the "plume study") using the same trawl 

 and a similar sampling strategy as in the GLOBEC study 

 (see Emmett and Brodeur [2000] and Teel et al. [2003] for 

 details). 



Scales were examined from 45 juvenile coho salmon 

 caught during the June and August 2000 GLOBEC 

 cruises and 252 juvenile coho salmon caught during the 

 2000 plume cruises. The scales were mounted on gummed 

 cards from which acetate impressions were made. Using 

 a video camera attached to a compound microscope and 

 Optimas® imaging software (vers. 5.1, Optimas Inc., Se- 

 attle, WA) we measured the distance (scale radius) along 

 the anterior-posterior axis of each scale from the focus 

 (F) to the ocean entry mark (OE) and to the scale margin 

 (Fig. 1). The fork-length of each fish at the time of ocean 

 entry (FL 0E ) was estimated from the scale radius (SR 0E ) 

 at ocean entry using the Fraser and Lee back-calculation 

 method (Ricker, 1992): 



FL„ 



(FL- 36.07) 

 SR 



xSR of . +36.07, 



where FL = length at capture; 



SR = scale radius at capture; and 

 36.07 = the intercept from a regression of SR on FL 

 for juvenile coho salmon caught in the ocean 

 (Fig. 2A). 



In an analogous fashion, fish weight at time of ocean entry 

 (Wr 0£ ) was back-calculated f 

 length at ocean entry (FL 0E ): 



(Wt 0E ) was back-calculated from the estimated fish fork 



\ni Wt 0E ) = 



(ln(Wr 1 + 12.633) 

 ln(FL) 



xln(FL r , F 1-12.633, 



where Wt = weight at capture; and 

 -12.633 = the intercept from a linear regression of 

 ln(Wr) on ln(FL) for juvenile coho salmon 

 caught in the ocean (Fig. 2B). 



The growth rate in FL, 



(FL-FL 0E )lAd, 



