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Validation of back-calculation equations for 

 juvenile bluefish {Pomatomus saltatrix) 

 with the use of tetracycline-marked otoliths 



Marja E. Roemer 



Department of Biology 



University of Massacfiusetts Dartmouth 



Nortfi Dartmouth, Massachusetts 02747 



Kenneth Oliveira (contact author) 



Department of Biology 



University of Massachusetts Dartmouth 



285 Old Westport Road 



North Dartmouth, Massachusetts 02747 



E-mail address for K Oliveira: Kolivelra@umassd.edu 



In recent years, a decrease in the 

 abundance of bluefish (Pomatomus 

 saltatrix) has been observed (Fahay et 

 al., 1999; Munch and Conover, 2000) 

 that has led to increased interest in a 

 better understanding the life history 

 of the species. Estimates of several 

 young-of-the-year (YOY) life history 

 characteristics, including the impor- 

 tance and use of estuaries as nursery 

 habitat (Kendall and Walford, 1979) 

 and size-dependant mortality (Hare 

 and Cowen, 1997), are reliant upon 

 the accuracy of growth determina- 

 tion. By using otoliths, it is possible 

 to use back-calculation formulae 

 (BCFs) to estimate the length at 

 certain ages and stages of develop- 

 ment for many species of fishes. Use 

 of otoliths to estimate growth in this 

 way can provide the same information 

 as long-term laboratory experiments 

 and tagging studies without the time 

 and expense of rearing or recapturing 

 fish. The difficulty in using otoliths 

 in this way lies in validating that 1) 

 there is constancy in the periodicity of 

 the increment formation, and 2 ) there 

 is no uncoupling of the relationship 

 between somatic and otolith growth. 

 To date there are no validation 

 studies demonstrating the relation- 

 ship between otolith growth and 

 somatic growth for bluefish. Daily 

 increment formation in otoliths has 

 been documented for larval (Hare 

 and Cowen, 1994) and juvenile blue- 

 fish (Nyman and Conover. 1988). 



Hare and Cowen (1995) found age- 

 independent variability in the ratio 

 of otolith size to body length in early 

 age bluefish, although these differ- 

 ences varied between ontogenetic 

 stages. Furthermore, there have 

 been no studies where an evaluation 

 of back-calculation methods has been 

 combined with a validation of otolith- 

 derived lengths for juvenile bluefish. 

 This study uses tetracycline- 

 marked YOY bluefish otoliths to 

 achieve two objectives: 1) to validate 

 the relationship between somatic and 

 otolith growth for juvenile bluefish, 

 and 2) to compare the effectiveness 

 of the Dahl-Lea equation, Fraser-Lee 

 equation, scale proportional hypoth- 

 esis (SPHi, and body proportional 

 hypothesis (BPH) length back-calcu- 

 lation formulae. 



Materials and methods 



Young-of-the-year bluefish were col- 

 lected by beach seine (15 mx 1.2 m 

 x6mm mesh) from Clarks Cove in 

 Buzzards Bay, Massachusetts, in 

 July and August 2005. The fish were 

 anesthetized with eugenol (clove 

 oil), measured for fork length (FL), 

 weighed, injected with tetracycline 

 hydrochloride (75 mg/kg body weight), 

 and individually marked with visible 

 implant elastomer (VIE) tags (North- 

 west Marine Technology, Shaw Island, 

 WA). The fish were maintained in a 



970 liter flow-through seawater tank 

 for 1 month at ambient water tem- 

 perature and fed with chopped squid 

 and fish once daily to satiation. After 

 32 days, the fish were remeasured, 

 reweighed, and their sagittal oto- 

 liths were removed for analysis. In 

 addition, 417 YOY juvenile bluefish 

 between 62 and 182 mm FL were 

 collected from southeastern Massa- 

 chusetts during the years 2004 and 

 2005 and were used to develop models 

 of the relationship between otolith 

 radius and fork length. ANCOVA 

 was used to compare the slopes of 

 the otolith-size-body-size relationship 

 between wild fish and experimental 

 fish to determine if the experimental 

 conditions caused uncoupling of the 

 relationship. 



Otoliths were prepared for analysis 

 as in Oliveira (1996) with two modi- 

 fications; a 0.48-mm section was cut 

 through the nucleus and visibility of 

 the microstructure was enhanced by 

 soaking the section in trypsin solu- 

 tion. Cross-sectioned bluefish oto- 

 liths have an occluded area in the 

 center which obscures observation 

 of the microstructure of the otolith. 

 To clear this region, a procedure 

 was developed in which the sections 

 were placed in 2% trypsin solution 

 for approximately 48 to 72 hours to 

 remove excess protein. This proce- 

 dure cleared the masked area and 

 enhanced visibility of the rings. 



Otoliths were observed on a com- 

 puter monitor at lOOx magnifica- 

 tion so that the entire otolith from 

 primordia to ventral edge could be 

 viewed. Measurements were taken by 

 using the Image-Pro Plus 5.0 image 

 analysis software (Media Cybernet- 

 ics, Silver Spring, MD). Two radial 

 measurements were made on each 

 otolith. The first measurement, R^, 

 was defined as the distance from the 

 primordium to the ventral edge of 

 the otolith (Fig. 1) and represents the 

 radius of the otolith at the end of the 

 experiment. Because bluefish otoliths 

 develop a curvature that increases 



Manuscript submitted 8 May 2006 

 to the Scientific Editor's Office. 



Manuscript approved for publication 

 13 October 2006 by the Scientific Editor. 



Fish. Bull. 105:305-309(2007). 



