Hobbs et al : Modification of the biological intercept model to account for ontogenetic effects in Hypomesus iranspacificus 



31 



relationship into the back-calculation model (for details 

 see Vigliola et al. [2000]). Therefore, although these 

 models can compensate for growth effects when age 

 and ontogenetic effects are linked, they cannot provide 

 accurate estimates when ontogenetic influences act in- 

 dependently of effects caused by growth and age. One 

 means of compensating for ontogenetic shifts in the 

 OS-FS relationship is to use the BI model separately 

 for specific life stages. This model can be useful for 

 providing accurate back-calculations across stage-spe- 

 cific transitions, such as metamorphosis (Jenkins, 1987; 

 Campana, 1990; Otterlei et al., 2002). 



We first assess whether the number of ring incre- 

 ments in otoliths accurately reflects absolute age in 

 larvae and juvenile cultured (known-age) delta smelt 

 (Hypojyjesi/s transpacificus). Second, we evaluate the de- 

 gree to which age-independent variability occurs in the 

 OS-FS relationship, as well as whether growth rate and 

 ontogenetic effects modify the proportionality of otolith 

 growth to fish growth. Finally, we modify the BI model 

 to account for ontogenetic stage-specific transitions of 

 the OS-FS relationship and compare these results with 

 those from the TVG and MF models to a to assess the 

 performance of our model for size-at-age back-calcula- 

 tions for delta smelt. 



The delta smelt is a small osmerid fish endemic to the 

 San Francisco Bay estuary (McAllister, 1963; Moyle, 

 2002). Historically one of the most common fishes, it 

 was listed as "threatened" under the Federal and State 

 Endangered Species Acts in 1993 (Moyle et al., 1992; 

 USFWS, 1995; Sweetnam, 1999). Recent precipitous 

 declines in the populations have led to a major inves- 

 tigation regarding the cause of such declines. A few of 

 the potential sources include contaminants and exotic 

 species (Bennett and Moyle, 1996; Bennett et al., 2005), 

 both of which may result in reduced growth rates and 

 poor recruitment. Examination of these hypotheses 

 requires accurate estimates of back-calculated size-at- 

 age for a growth-history assessment of the delta smelt 

 (Fujiwara et al., 2005). 



Materials and methods 



Larval rearing and otolith examination 



All known-age delta smelt larvae were reared at the 

 University of California Davis's delta smelt culturing 

 facility located at the U.S. Bureau of Reclamation John 

 F. Skinner fish facility in Byron, California. Newly 

 hatched larvae were raised in 75-L black tanks at 20 

 larvae/L. Each tank maintained a flow-through system 

 with filtered water from the Sacramento-San Joaquin 

 delta. Ambient water temperatures during the rearing 

 period ranged from 17° to 20°C and salinities from to 

 1 ppt. The tanks were held in refrigerated cargo con- 

 tainers under 40-W florescent lights for a 12 h light: 12 

 h dark cycle. Larvae up to 30 days after hatching (dah) 

 were fed Isochyrisis spp. enriched rotifers. Nanochlorop- 

 sis spp. (green micro-algaej paste was added to create 



"green" conditions necessary for larval feeding (Basker- 

 ville-Bridges^; Baskerville-Bridges et al., 2004). Larvae 

 more than 30 dah were fed Artemia sp. (brine shrimp) 

 nauplii enriched in Super Selco(fi) (Inve Aquaculture 

 NV, Belgium). Ten larvae up to 10 dah were sacrificed 

 daily with an overdose of MS222 (FINQUEL MS-222 

 Argent Chemical Labratories, Inc., Redmond, WA) and 

 fixed in 95% ethanol. Fish from 10 dah to 60 dah were 

 sampled every other day and fish from 60 dah to 100 

 dah were sampled every 5-7 days (Table 1). Samples 

 were collected from bins representing two spawning 

 pairs of fish. 



Sagittal otoliths were dissected from larvae and juve- 

 niles with fine forceps and dissecting needles. For lar- 

 vae, care was taken to photograph and analyze only the 

 sagittal otoliths, which were identified by orientation to 

 the notochord. In delta smelt, the sagittal otoliths are 

 located near the notochord, whereas lapilli are located 

 distally and posteriorly in relation to the sagittae. Oto- 

 liths were mounted in Crystal Bond thermoplastic glue 

 (Crystalbond™ 509, Ted Pella Inc. Redding, CA) with 

 the sulcus side down. Otoliths from fish 92 dah and 

 older were sanded with 1200-grit wet and dry sandpa- 

 per and polished with 0.3-micron alumina and a polish- 

 ing cloth. Sagittal otolith radii were measured with a 

 light microscope at 400-100 x magnification. All otoliths 

 were analyzed with Image Pro 4.0(S) (Media Cybernet- 

 ics, Silver Spring, MD). The first increment was deter- 

 mined as the first ring after the nucleus core. Otolith 

 radius was measured with the aid of image analysis, 

 from the core to the dorsal edge, because this direction 

 was consistently the clearest trajectory. All references 

 to otolith size are radial measurements from the core 

 to the dorsal edge. Sagittae were read by two readers 

 without knowledge of fish age. Two readings were made 

 by each reader and reread if age differences exceeded 

 10%. If age estimates continued to vary, the otolith was 

 removed from the data set. 



Statistical analyses 



To determine if increment formation occurred on a daily 

 basis, a Student's t-test was used to determine the sta- 

 tistical significance of the slope from a linear regres- 

 sion of increment counts on known age. Under the null 

 hypothesis, the slope of increment counts on known age 

 was equal to one. We rejected the null hypothesis if the 

 slope significantly differed from one. 



Statistical analyses, similar to those of Hare and 

 Cowen (1995), were used to evaluate age-independent 

 variability in the OS-FS relationship, as well as growth 

 rate and ontogenetic effects. First, regression models 

 (linear and polynomial) of standard length and otolith 



1 Baskerville-Bridges, B. 2001. Progress and development 



of delta smelt culture: year-end report 2000. Interagency 

 Ecological Program news letter 14:24-29. Department of 

 Water Resources 827 7"' St., Room 301, Sacramento, CA 

 95814. 



