MacFarlane and Norton Physiological ecology of Oncorhynchus tshawytscha 



247 



of 1500x. Counts representing daily increments of 

 gi-owth were made from the otolith margin (dorsal 

 edge) to a posterior primordium. Each otolith was 

 counted at least two times by the same reader Data 

 are presented as age, in days, between the hatching 

 check and the otolith margin. 



Stomach contents were identified and quantified ac- 

 cording to methods of Hobson and Chess ( 1976). Prey 

 were identified to the lowest possible taxa, enumer- 

 ated, and their relative volume and size distribution 

 recorded. The derived parameter, index of relative im- 

 portance (IRI), was used to determine the importance 

 of specific taxa to juvenile salmon at each sampling 

 location. The IRI was computed by the equation 



IRI = (N + V)FO. 



where A^, V, and FO 



the percent number, volume, 

 and frequency of occurrence, 

 respectively, of a taxon in the 

 stomach contents (Pinkas et 

 al.,1971). 



Size, age, condition, lipids, and protein data were ana- 

 lyzed for variability among locations by means of the 

 general linear model of analysis of variance. Differ- 

 ences among specific locations were determined with 

 Tukey's studentized range test, set at a = 0.05, which 

 controls for MEEK (maximum experimentwise error 

 rate) under complete or partial null hypotheses. All 

 statistical procedures were performed with SAS soft- 

 ware (SAS Institute, Inc., 1994). 



Results 



We collected and evaluated 310 subyearling chinook 

 salmon from the 1996-97 year class. The catch con- 

 sisted of naturally spawned and hatchery-produced 

 fish of unknown proportions because less than 37r were 

 marked (by adipose fin clips and coded wire tags) and there 

 were no morphological features to distinguish hatchery 

 from naturally produced salmon. Mean fork length (FL) 

 was 89 mm and ranged from 68 to 113 mm. Total body 

 weights ranged from 3.59 to 14.62 g, with a mean of 7.58 g. 

 A power function was fitted to the relationship between FL 

 and weight (Fig. 2A). Ages were determined for 156 juve- 

 niles and ranged from 112 to 209 days after hatching. The 

 relationship of FL or weight to age was not as well fitted as 

 that for weight on FL (Fig. 2, B and C); however, length and 

 weight were positively correlated to age (P<0.0001 ). 



Each sampling location within the estuary was visited 

 more than once on successive surveys through the 30 

 April-15 July period of juvenile emigration. There were 

 no statistically significant trends in size, age, lipid, and 

 protein variables by sampling date at any location: there- 

 fore we combined data for each location from both surveys 

 through the estuary 



While in the estuary, juvenile chinook salmon grew lit- 

 tle in length or weight; but in coastal waters, they grew 



16 



^ 12 



I 8 



5 4 







120 

 ? 

 i- 100 



X 80 



o 



^ 60 



A 



y = 0.0003 x2 2'7 

 r2 = 0.8523 







.•••i!» 



l!.|:|:l'5' 



IS : • 



60 70 80 90 100 



Fork length (mm) 



B 



y = 60.72 + 0.162X 

 r2 = 0.2390 



110 



120 



12 



100 120 140 160 



Age (d) 



c 



y = 2.368 + 029x 

 r2 =0.1654 . .• ". 



180 200 



220 



5 4 





•tjt:'.''. 



100 120 140 160 



Age (d) 



180 



200 



220 



Figure 2 



Fork lengths, weights, and ages of all juvenile chinook salmon 

 collected from the San Francisco Estuary and Gulf of the Faral- 

 lones: (A) fork length-weight relationship; (Bl age-fork length 

 relationship; (Cl age-weight relationship. 



rapidly (Fig. 3). Juveniles entering the estuary at km 68 

 had a mean (±SE) FL of 82.8 ±0.7 mm and weight of 6.36 

 ±0.21 g. At the exit of the estuary (km 3), mean FL and 

 weight of cohorts were 89.5 ±1.1 mm and 7.23 ±0.30 g, 

 representing mean gains of about 7 mm and 0.9 g. Size 

 changes within the estuary were not statistically signif- 

 icant (P>0.05). Year-class cohorts in the coastal waters 

 of the Gulf of the Farallones were significantly longer 

 and heavier than those from the estuary (FL, P<0.0001; 

 weight, P<0.001). Ocean-caught juveniles were 8.2 mm 

 longer and weighed 6.5 g more than fish collected near the 

 estuary exit. 



Juvenile salmon spent about 40 d migrating along the 

 65-km length of the estuary, according to otolith increment 

 counts (Fig. 3C). Mean age at the entry to the estuary was 

 136 ±2 d; at the exit it was 176 ±3 d. Juveniles aged from 

 the Gulf of the Farallones were 156 ±5 days old, indicating 

 a shorter freshwater or estuarine residence than that for 

 those captured from the Golden Gate area. 



Daily growth rates in the estuary can be calculated from 

 differences in mean size over the 40-d estimated time of 



