1 IOilL.ni UULiLiLillll. »VU. Ol, l*KJ. 1 



rates to field populations is the constancy of deposi- 

 tion rates between these environments. Most labora- 

 tory studies have occurred under constant tempera- 

 ture and salinity and under conditions of artificial 

 food types and densities and low light intensities 

 compared with the field. Often, increments from 

 otoliths of laboratory-grown larvae are much fainter 

 than those from otoliths of field-captured larvae 

 Since field conditions can fluctuate to extents that 

 have been shown to cause increment disruption in 

 laboratory situations, a way to verify daily deposi- 

 tion in the field would be an important contribution. 

 A transitional step between the laboratory and the 

 field has been made by Laurence et al. (1979) and 

 0iestad (1982). Laurence et al. (1979) raised known- 

 age larvae in a flow through enclosure This study 

 was designed to measure the growth and survival 

 of fish larvae exposed to varying prey concentrations 

 in the field. Modifications of this system could be 

 used to study increment deposition in known-age lar- 

 vae exposed to field conditions. 0iestad (1982) pre- 

 sented a review of larval fish studies performed in 

 enclosures. Gjrisaeter and 0iestad (1981) reared 

 known-age larvae in large enclosures and determined 

 increment deposition rates (Table 1). Few inves- 

 tigators have used such enclosures for validation of 

 otolith increment deposition rates for field simulated 

 studies. Enclosures should prove particularly 

 valuable for validation and simulation of suboptimal 

 field conditions on growth and increment deposition. 



Statistical Applications 



Once the veracity of daily increment deposition is 

 established, a wide variety of statistical methods can 

 be used in otolith studies. Statistical methods that 

 have been employed in larval otolith studies have 

 been linear regressions to establish increment 

 deposition rates and curve fitting techniques to es- 

 tablish growth rates from length-at-age data. Linear 

 regression has also been applied regardless of 

 whether it actually fits the data. It is important to 

 check for lack of fit, selection of the appropriate 

 model, and weighting before applying linear regres- 

 sion blindly. It is recommended that, when possible, 

 confidence intervals and standard deviations be in- 

 cluded in the data presentation. 



Investigators are beginning to relate increment 

 widths, as indicators of growth, with environmen- 

 tal conditions (Methot and Kramer 1979; Lough et 

 al. 1982). When increment widths are correlated 

 directly with environmental factors, either no 

 correlations are seen (Neilson and Geen 1982) or 

 correlations may be spurious. Problems exist in 



measuring the physical conditions to which the lar- 

 vae have been exposed, especially since larvae may 

 move from one area to another. In addition, there 

 are questions concerning food availability and its 

 concentration and patchiness. Another consideration 

 in relating growth to environmental conditions is 

 that, as the fish grows, the width of the outer incre- 

 ments decreases proportionately to decreases in 

 length. Better results might be obtained either with 

 covariance analysis or by fitting a growth function 

 to data then using the residuals in correlation tests. 

 Investigations of residuals with exploratory tech- 

 niques such as principal component analysis or 

 canonical correlation might prove fertile 



Comparison of Scanning Electron and 

 Light Microscopy 



Scanning electron microscopy (SEM) has been 

 used to confirm otolith structure (Dunkelberger et 

 al. 1980; Watabe et al. 1982) and to compare incre- 

 ment counts with those obtained by transmitted light 

 microscopy (Radtke and Waiwood 1980; Campana 

 and Neilson 1982; Neilson and Geen 1982; Radtke 

 and Dean 1982; Tsuji and Aoyama 1982; Ralston and 

 Miyamoto 1983). Under optimal conditions, counts 

 using both methods were equivalent except for lar- 

 val cod. Radtke and Waiwood (1980), using SEM, 

 determined that cod produced daily increments from 

 hatch onward, while Gj«teaeter (1981), using a light 

 microscope, did not observe increment formation un- 

 til 4-5 d after hatch. 



Most investigators did not verify deposition seen 

 with the light transmission microscope with SEM 

 studies. Confirmation with SEM is highly desirable 

 when increments are nondaily. However, extensive 

 use of the technique for field surveys is prohibited 

 by the additional cost and preparation time when 

 compared with light microscopy. In cases where 

 suboptimal or abnormal field conditions may result 

 in nondaily increment formation (Jones 1984), SEM, 

 used in conjunction with ancillary techniques, may 

 assist identification of the proportion of larvae for 

 which age is underestimated with light micros- 

 copy. 



CONCLUSIONS 



The report of the otolith workshop held in Bergen, 

 Norway (Anonymous 1982) stated that the ap- 

 pearance of increments in otoliths of larval fish living 

 in diverse habitats and representing many families, 

 argues strongly for the universality of this phenom- 

 enon. Validation that these increments are indeed, 



100 



