Direct validation of black drum 

 (Pogonias crom/s) ages determined 

 from scales 



Gary C. Matlock 

 Robert L. Colura 

 Lawrence W. McEachron 



Texas Parks and Wildlife Department 

 4200 Smith School Road 

 Austin. Texas 78744 



In the 1980's increased harvest of 

 black drum (Pogonias cromis) in the 

 Gulf of Mexico raised concerns 

 about the possibility of overfishing 

 (Anonymous 1 ; Murphy and Taylor, 

 1989). Growth information neces- 

 sary to examine the effects of fishing 

 on population size was limited to 

 rate estimates based primarily on 

 temporal changes in length- 

 frequency distributions (Pearson, 

 1929; Simmons and Breuer, 1962) 

 and length changes in recaptured 

 tagged fish (Osburn et al., 1980; 

 Doerzbacher et al., 1988). Age in- 

 formation was limited to scale 

 analysis of fish in Texas and Vir- 

 ginia (Pearson, 1929; Richards, 

 1973). Consequently, scales and 

 otoliths became the focus of study 

 for estimating age and growth rates 

 (Cornelius, 1984; Music and Pafford, 

 1984; Murphy and Taylor, 1989; 

 Beckman et al., 1990; Peters and 

 McMichael, 1990), but, age data 

 were not directly validated. For ex- 

 ample, Beckman et al. (1990) used 

 the intra-year progression of annu- 

 lus formation on otoliths to conclude 

 that one annulus forms per year. 

 While their indirect evidence is com- 

 pelling, age was not directly vali- 

 dated, and no attempt has been 



'Anonymous. 1989. Saltwater finfish research 

 and management in Texas, a report to the 

 Governor and the 71st Legislature. Tex. 

 Parks Wildl. Dep.. PWD R-3400-061-12/88, 

 Austin, TX, 65 p. 



558 



made to validate age by using the 

 scale method. Thus, the reliability 

 of conclusions concerning the effect 

 of fishing on black drum may be 

 suspect. More cost-efficient studies 

 would result from using scales in- 

 stead of otoliths for ageing if reli- 

 able data could be obtained from 

 scales. The objectives of this study 

 were to validate directly the forma- 

 tion of annuli (one growth check per 

 year) on scales, the time of annulus 

 formation, and the effect of tagging 

 on annulus formation. 



Methods 



Black drum were caught in gill nets 

 at randomly selected sites in nine 

 Texas bay systems (Dailey et al, 

 1991) during spring (April-June) 

 and fall (Sept. -Nov.) 1985-1991. 

 Captured fish were measured for to- 

 tal length (TL) to the nearest mm, 

 tagged with internal abdominal tags 

 with external plastic streamers (Os- 

 burn et al, 1980), and released. Prior 

 to release, at least two scales were 

 removed from the area beneath the 

 distal end of the left pectoral fin 

 immediately ventral to the lateral 

 line (Matlock et al, 1987). Scales 

 were removed from a total of 9,088 

 released tagged black drum (195- 

 1,257mm TL) between April 1985 

 and December 1991. Anyone captur- 

 ing a tagged fish was requested 

 through posters and news-media 



advertisements to report date, lo- 

 cation, and TL of each recaptured 

 tagged fish, and to return at least 

 one scale collected from the same 

 area from which scales were re- 

 moved at release. Scales from 22 re- 

 captured tagged fish (354-635 mm 

 TL) were returned to the Texas 

 Parks and Wildlife Department 

 (TPWD) by fishermen between May 

 1985 and December 1991. All re- 

 ported data were assumed accurate. 



Scales were prepared by washing 

 in soapy water and impressing on a 

 cellulose acetate slide with a roller 

 press (Smith, 1954). The impres- 

 sions were examined at 32 diam- 

 eters magnification with a micro- 

 projector by using incandescent 

 light. Annuli, characterized by 

 breaks in circuli and new radii, were 

 identified following Pearson's (1929) 

 description and separately counted 

 by two examiners without collabo- 

 ration. Scales were read blind (i.e., 

 without knowing TL or date each 

 scale was obtained). Agreement be- 

 tween readers was obtained for all 

 scales on the first reading. Scale ra- 

 dii and distances from the focus to 

 successive annuli were measured 

 along a diagonal line (Fig. 1) to the 

 right antero-lateral scale corner 

 (Klima and Tabb, 1959). Magnified 

 (32x) scale measurements are re- 

 ported unless otherwise noted. Six- 

 teen of the 22 fish had both usable 

 scales and TL measurements at 

 both release and recapture; 17 had 

 usable scales at both times but one 

 fish lacked TL data at recapture 

 (Table 1). The usable data from the 

 remaining five fish were used to ac- 

 complish some of the objectives. 



Annulus formation was validated 

 by comparing the number of scale 

 growth checks observed at release 

 to those at recapture (Fig. 2). The 

 difference in number of annuli was 

 compared to the expected difference 

 under the null hypothesis: number 

 of scale growth checks per year * 1. 



Manuscript accepted 24 dune 1993. 

 Fishery Bulletin: 91:558-563 (1993). 



