748 



Fishery Bulletin 91(4). 1993 



200 400 600 



DISTANCE (MICRONS) 



800 



Figure 21 



Relationship between Sr/Ca ratios and distance from boundary 

 of enclosure towards edge in a 172.2-mm juvenile Dover sole. 

 Microstomia pacificus, collected on 11 September 1989. The speci- 

 men had four post-settlement annuli and was prepared in trans- 

 verse section. Solid line represents five-term running averages 

 of Sr/Ca; dashed lines represent 2-a measurement error, calcu- 

 lated as in Toole and Nielsen (1992); shaded areas represent 

 opaque zones and unshaded areas represent translucent zones 

 in the otoliths; and numbers above peaks represent post-settle- 

 ment annuli. 



There were no dramatic changes in otolith structure 

 to mark the transition from Stage 3 to Stage 4. 



Postmetamorphic juveniles Formation of the first 

 post-settlement annulus occurred during the fall, sev- 

 eral months after the transition from larval Stage 4 to 

 juvenile Stage 5 that defines the end of metamorpho- 

 sis (Markle et al., 1992). While the first and subse- 

 quent post-settlement annuli are useful for establish- 

 ing the number of years since an individual settled 

 and completed metamorphosis, they are not useful for 

 determining either duration of the metamorphic pe- 

 riod or the date associated with the end of metamor- 

 phosis. No other significant structural landmarks were 

 associated with this stage. 



Relation of otolith microchemistry to 

 metamorphic stages 



Premetamorphic larvae Within the clear central area 

 formed during Stage 1, Dover sole otoliths exhibited 

 high Sr/Ca levels relative to levels in adjacent opaque 

 areas. Translucent zones of otoliths are rich in arago- 

 nite and have a poorly developed protein matrix, while 

 opaque areas have higher protein concentrations 

 (Dannevig, 1955; Williams and Bedford, 1974; Morales- 

 Nin, 1987). Because organics are oxidized during beam 



exposure and 1:1 concentrations of C0 2 relative to 

 cations are assumed in microprobe calculations 

 (Toole and Nielsen, 1992), differences in concentra- 

 tion of organic materials could differentially affect 

 accuracy of Ca and Sr measurements in translucent 

 and opaque areas. However, Sr and Ca do not frac- 

 tionate from one another in response to beam dam- 

 age, so Sr/Ca ratios should not be affected by con- 

 centration of organics (Toole and Nielsen, 1992). 

 Protein-poor areas of otoliths are generally associ- 

 ated with rapid growth (Dannevig, 1955; Williams 

 and Bedford, 1974; Morales-Nin, 1987). However, 

 many northeastern Pacific fishes, including adult 

 Dover sole, form translucent zones (annuli) on 

 otoliths during slow winter growth periods (Chilton 

 and Beamish, 1982). 



Sr/Ca ratios reflect the substitution of Sr for Ca 

 in otolith aragonite, possibly owing to reduced abil- 

 ity to physiologically discriminate between Sr and 

 Ca during biological precipitation (Radtke et al., 

 1990). The effect appears to be mediated through 

 Sr concentration in the saccular endolymph, which 

 in turn may be related to plasma protein levels 

 (Kalish, 1989). Sr/Ca ratios have been described as 

 inversely proportional to environmental tempera- 

 ture (Radtke, 1984, 1989; Townsend et al, 1989; 

 Radtke et al., 1990) and growth rate (Sadovy and 

 Severin, 1992), and directly proportional to age 

 (Kalish, 1989), "stress" (Townsend et al, 1989), Sr con- 

 centration in ambient water (Kalish, 1990), and salin- 

 ity of ambient water (Radtke et al., 1988). The latter 

 two effects have only been documented under extreme 

 conditions of seawater vs. freshwater residence. The 

 effects of age described by Kalish (1989) were linked 

 primarily to adults and may not apply to larvae and 

 juveniles in this study. 



Elevated Sr/Ca ratios in premetamorphic Dover sole 

 otoliths could be related to a combination of low tem- 

 perature, slow growth rate, and high "stress" (all of 

 which may be expected to covary) if correlations ob- 

 served in other studies apply to this species. However, 

 confirmation of these effects in Stage- 1 Dover sole is 

 impractical, given available information. Temperatures 

 experienced by premetamorphic larvae depend upon 

 their distance offshore and depth, both of which are 

 variable during this stage (Pearcy et al., 1977; Markle 

 et al, 1992). Because periodicity of increment forma- 

 tion prior to AP formation has not been validated, 

 growth rate cannot be determined with certainty. While 

 it is possible that "stress" may be associated with de- 

 velopmental changes, the strong Sr/Ca spike approxi- 

 mately 48 urn from the central primordium appeared 

 to occur subsequent to events such as hatching (at a 

 length of =6 mm; Ahlstrom and Moser, 1975) and ini- 

 tiation of eye migration, caudal fin flexion, and growth 



