Ahrenholz et al. : Otolith ageing of Brevoortia tyrannus 



215 



The otolith sections of the laboratory-reared ma- 

 terial, which includes the OTC fish following mark- 

 ing, generally had less contrast between alternating 

 bands than did field material. Warlen (1988) noted 

 similar results for gulf menhaden. This was not the 

 case for the ALC fish, where contrast more closely 

 resembled field material. OTC and known-age speci- 

 mens were raised indoors in relatively small ( 100 L) 

 containers, as compared with the larger (2,100 L) 

 outdoor containers used for the ALC fish. (All groups 

 were reared in ambient sea water.) Problems in vali- 

 dating otoliths with laboratory-reared fish have been 

 noted for other species (Campana and Moksness, 

 1991; Toole et al., 1993). Pannella ( 1980) notes that 

 the transition between increments are unclear with 

 respect to chemical or structural changes in some 

 laboratory-reared material. It may be that otoliths 

 from laboratory-reared specimens are less typical 

 because of confounding effects from container size, 

 growth rates, and other aspects of the rearing condi- 

 tions. The poorer contrast may result in lower accu- 

 racy and precision in increment counting, and the 

 slower growth may result in more variable increment 

 counts for a given time period. 



We obtained detectable OTC and ALC marks in 

 viewing otoliths with the dosages used for immers- 

 ing larvae. Because of the color contrast of the or- 

 ange-against-blue background, ALC was visibly 

 easier to detect under blue light fluoresence than was 

 OTC. ALC has been used for marking eggs and hard 

 parts in fish; it leaves a brilliant mark, does not ad- 

 versely affect growth at low dosages, and does not 

 require dilution procedures as does OTC (Tsukamoto, 

 1988; Kishiro and Nakazono, 1991). 



Although we obtained similar results using either 

 oblique-transverse or transverse sections for those 

 periods when increment formation is on the average 

 one per day, one orientation or the other may be pre- 

 ferred for various reasons. The oblique-transverse 

 method of sectioning may be easier to complete in 

 polishing because the primordium and focus can be 

 detected from a greater distance (thickness) when 

 the material is viewed. This reduces processing time 

 and minimizes the number of overground, unusable 

 preparations. On transverse sections of larger or older 

 individuals, or both, the focus is located more by the 

 outline shape than by early optical discovery. However, 

 some investigators using increment measurements for 

 size back calculation or discriminant analysis may pre- 

 fer the transverse section for ease in keeping the same 

 plane of measurement from otolith to otolith. The two 

 section orientations were useful for cross comparisons 

 and interpretation of certain growth zones. Therefore 

 choice of orientation should depend upon the material 

 being examined and the questions being addressed. 



Acknowledgments 



The oxytetracycline experiment was conducted by 

 Robert M. Lewis and James F. Guthrie. The known- 

 age fish, obtained from a spawning conducted by 

 William F. Hettler, were kindly provided by Stanley 

 M. Warlen, and were cared for by Sheryan P. Epperly 

 and Ronald M. Clayton. Valerie Comparetta cared 

 for the menhaden from the alizarin complexone rear- 

 ing trial. Many of the marked otoliths were sectioned 

 and polished by Theresa V. Henley and Robin T. 

 Cheshire. Douglas S. Vaughan assisted us with the 

 statistical power computations. This study was par- 

 tially supported by Grant NA16RG0492 from the 

 Coastal Ocean Program, South Atlantic Bight Recruit- 

 ment Experiment (SABRE), of the National Oceanic 

 and Atmospheric Administration to the North Caro- 

 lina Sea Grant College program. 



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