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Fishery Bulletin 98(1) 



spine sections or otolith sections made by two 

 readers were contrasted. In addition, for each 

 reader, ages estimated from fin spine sections 

 were compared with ages estimated from oto- 

 hth sections from the same fish. 



Validation of annulus formation 



The periodicity of annulus formation in Atlantic 

 sturgeon fin spines and otoliths was studied by 

 using measurements and chemical microanal- 

 ysis of marginal increments, and observation 

 of annuli in juvenile hatchery-reared sturgeon 

 marked with oxytetracycline (OTC) at a known 

 age. Efforts were concentrated on fin-spine age 

 validation because annuli were difficult to con- 

 sistently interpret in otolith sections. 



Marginal increment analysis measures the 

 opaque zone deposited after the last identifi- 

 able translucent zone at the margin of a struc- 

 ture used for age estimation (Kalish, 1995). 

 Seasonal growth of that opaque zone is used 

 to determine the timing of annulus formation 

 (Cailliet et al., 1986; Beamish and McFarlane, 

 1987; Brennan and Cailliet, 1989). In our anal- 

 ysis, marginal increments were measured with 

 image analysis software (Optimas, Inc., 1994) 

 to the nearest 0.001 mm. Marginal increment 

 ratio (MIR) was calculated as 



MIR = MIx 1/A, 



where MI = the width of the outermost opaque 

 zone (marginal increment); and 

 A = the mean width of the three 

 annuli deposited previous to the 

 marginal increment. 



Mean MIR was computed for each month sepa- 

 rately, with ages and sexes combined. Because 

 very few fish were collected for several months, 

 monthly samples were pooled for four three- 

 month seasons. The winter season (December- 

 February) was eliminated owing to very low 

 sample size. 



Microchemical analysis of calcified struc- 

 tures can verify the periodicity of annuli (Jones and 

 Geen, 1977; Casselman, 1983; Radtke and Targett, 

 1984; Cailliet and Radtke, 1987). Calcium concen- 

 tration is correlated with optically defined growth 

 zones in the hard part (e.g. calcium is increased in 

 the opaque zone and decreased in the translucent 

 zone; Cailliet and Radtke, 1987; Lai et al., 1996). For 

 instance, the most recently formed material in fin 



o 

 O 



14 19 24 29 34 



Fin spine age estimated by Reader 1 (years) 



5 10 15 20 25 30 35 



Otolith age estimated by Reader 1 (years) 



24 



16 



- 



10 20 30 40 



Age estimated by fin spine section (years) 



Figure 2 



Bias m age estimates for Hudson River Atlantic sturgeon are eval- 

 uated by age-bias plots that compare estimates made by two read- 

 ers interpretmg the same (Al fin-spine sections and (Bl otolith 

 sections; and (Cl made by one reader interpreting fin-spine and 

 otolith sections taken from the same fish. Dashed lines are "equiv- 

 alence lines" indicating the hypothetical case where both estimates 

 are identical. Error bars represent the 95'7f confidence interval 

 about the mean age (reader 2) for all fish assigned a given age by 

 reader 1. 



spines might contain high or low concentrations of cal- 

 cium in fish collected during summer (rapid growth) 

 or winter (slower growth), respectively (Cailliet and 

 Radtke, 1987). In this instance, fin-spine microanaly- 

 sis of calcium across annuli might show nadirs in cal- 

 cium associated with translucent zones. 



Calcium concentration in fin-spine sections was 

 measured using a JEOL JXA-840A wavelength-dis- 



