556 



Fishery Bulletin 92(3). 1994 



Although recent studies have shown that for many 

 species the scale method underages older fish at the 

 point where fish growth becomes asymptotic 

 (Beamish and Chilton, 1981; Beamish and McFar- 

 lane, 1983; Barnes and Power, 1984), there has been 

 little evaluation of other weakfish hardparts. 

 Merriner (1973) compared weakfish scales to whole 

 vertebrae and otoliths, and Villoso ( 1989) compared 

 scales to whole otoliths. Both concluded that scales 

 were best. However, Merriner's study was conducted 

 before thin-sectioning of otoliths (Williams and 

 Bedford, 1974; Beamish, 1979; Beamish and Chilton, 

 1981) and other hardparts became common and 

 Villoso ( 1989) did not consider thin-sectioning. 



A decline in weakfish landings since 1980, coupled 

 with greater competition between fisheries, caused 

 the Atlantic States Marine Fisheries Commission 

 (ASMFC) to develop a weakfish management plan 

 in 1985 (Mercer, 1985). Since then the ASMFC has 

 issued an updated stock assessment 1 and suggested 

 a 25% reduction in coast-wide exploitation rates 

 (Amendment No. 1 of the Weakfish Fishery Manage- 

 ment Plan of the ASMFC). However, it is essential 

 to proper weakfish management that a validated 

 ageing technique be developed and used, as improper 

 ageing can lead to faulty estimates of model param- 

 eters such as age at maturity, growth, longevity and 

 mortality (Beamish and McFarlane, 1983). 



The objectives of this study were 1) to compare 

 otolith, dorsal-fin spine, and pectoral-fin ray sections 

 with scales in terms of legibility and interpretation 

 of potential annual marks, ease of collection and pro- 

 cessing, and precision, 2) to validate the hardpart 

 demonstrating the greatest clarity by mar- 

 ginal increment analysis for each age 

 group found in the Chesapeake Bay area, 

 and 3) to conduct a more in-depth compari- 

 son of the validated hardpart with scales 

 in terms of precision and accuracy, time of 

 annulus formation, growth estimates, and 

 use in back calculation of body length. 



Methods 



Preliminary comparison of 

 hardparts 



Four hundred weakfish were collected ev- 

 ery other week during April-October in 

 1989 from three Chesapeake Bay commer- 

 cial pound nets. On each collection day, one 



1 Vaughan, D. S., R. J. Seagraves, and K. West. 1991. 

 An assessment of the Atlantic weakfish stock, 

 1982-1988. Atl. States Mar. Fish. Comm. Spec. 

 Rep. 21. Wash. DC, 29 p. 



anterior 



22.7 kg (50 lb) box of each available grade of weak- 

 fish — small, medium, or large — was bought and all 

 fish within it processed. Fish were measured for to- 

 tal length (TL ±1.0 mm), sexed, and both sagittal 

 otoliths were removed and stored dry. Scales were 

 removed from an area just posterior to the tip of the 

 left pectoral fin, below the lateral line. The left pecto- 

 ral fin and the entire dorsal fin were removed by cut- 

 ting below the base of the rays. Scales and fins were 

 stored in paper envelopes and kept frozen until prepa- 

 ration for ageing. 



A total of 45 fish, 15 from each grade, were ran- 

 domly selected from the fish collected in 1989 for a 

 preliminary comparison of hardparts. These fish ranged 

 from 244 to 615 mm TL and each of their four hardparts 

 was prepared for reading as described below. 



The right otolith from each fish was transversely 

 sectioned through the nucleus with a Buehler low- 

 speed Isomet saw. Sections 350-500 urn thick were 

 mounted on glass slides with Flo-Texx clear mount- 

 ing medium and viewed under a dissecting micro- 

 scope at 24x magnification by using transmitted light 

 and bright field, with the exception of samples from 

 the period April-May, when sections were also read 

 with reflected light and dark field to help identify 

 the last annulus. Thin opaque bands, presumed to 

 represent annual marks, were counted along the 

 otolith sulcal groove (Fig. 1). Because opaque bands 

 inhibit light passage, they appeared dark in trans- 

 mitted light (Fig. 2A) and light in reflected light. 



Scales from each fish were soaked in water until soft, 

 after which they were washed gently with a soft-bristled 

 tooth brush. Three or four clean, unregenerated scales 



•]■ ".,-.i! 



posterior 



proximal 



distal 



ventral arm of Ihe sulcal groove 

 thin opaque bands 



Figure 1 



Schematic representation of a transverse section taken through 

 the right sagittal otolith. The ventral arm of the sulcal groove, 

 along which otoliths were measured, is indicated. The whole otolith 

 is positioned as it would be in a weakfish, Cynoscion regulis. 



