NOTE Secor et al.: Validation of otolith-based ageing in Morone saxatilis 



187 



vironment, Health, and Natural Resources; and the 

 U.S. Fish and Wildlife Service. Between 6 and 8 Feb- 

 ruary 1993, migratory fish were captured off the 

 North Carolina coast by the crew of the NOAA ves- 

 sel Chapman by means of a 90-ft, 2-seam fish trawl 

 that was towed for a maximum of 30 minutes (Laney 

 and Cole, 1993). A second group of 13 CWT fish was 

 obtained from DNR commercial drift gillnet and 

 poundnet surveys between 5 October 1992 and 23 

 February 1993. Pound nets and drift nets were lo- 

 cated in shoal areas of the upper Chesapeake Bay, 

 off Kent Island, MD. 



Fish in the study group had been tagged with bi- 

 nary coded wire tags as age-0+ juveniles. Tags were 

 removed from recaptured fish and decoded by per- 

 sonnel at the U.S. Fish and Wildlife Service labora- 

 tory in Annapolis, MD, to determine their ages. 



Otoliths were extracted, soaked in 10% sodium 

 hypochlorite solution, rinsed with deionized water, 

 and embedded within a Spurr epoxy (Secor et al., 

 1991). Transverse sections, approximately 1 mm 

 thick, were then cut through the otolith cores with a 

 Buehler Isomet saw. The sections were mounted on 

 glass slides, polished on 600-grain sandpaper, and 

 polished again on a slurry of 0.3-fum alumina until 

 their surfaces were free of pits and abrasions. Pol- 

 ished sections were viewed under a light microscope, 

 and otolith annuli were counted by two independent 

 readers. Annuli comprised a narrow opaque zone and 

 a wide translucent zone under transmitted light mi- 

 croscopy (magnification at 60 or 150x). Annuli were 

 counted along the sulcal ridge in transverse sections. 

 The otolith ages were compared with each other and 

 with the known age of the fish. 



Results 



Known-age study 



Striped bass collected in pound nets and drift gill 

 nets in Upper Chesapeake Bay were 3- to 7-year-old 

 males and females. The migratory hatchery striped 

 bass from offshore samples tended to be older, rang- 

 ing in age from 4 to 7 years old (Fig. 1). 



Agreement between known and estimated age for 

 resident striped bass was 100% for both otolith read- 

 ers (rc=13). Exact agreement between estimated and 

 known age for migratory fish (n=24) was 79% and 

 87% for reader 1 and reader 2, respectively. All mi- 

 gratory fish were estimated to be within one year of 

 their true age. The mean age difference between read- 

 ers for migratory fishes was not significantly differ- 

 ent from (paired £-test: rc=37; P=0.66). The mean 

 absolute difference between ages estimated by reader 



1 and known-age, a measure of precision, was esti- 

 mated at 0.13 years. Precision estimated for reader 



2 was 0.08 years. Error in age estimates was not re- 

 lated to fish length or age. 



Scale vs. otolith study 



Age estimates from scales and otolith sections were 

 not significantly different for fish with otolith-esti- 

 mated ages of 5 to 11 years (Fig. 2; paired £-test: n-30; 

 P=0.41). However, fish with otolith-estimated ages 

 of 22 to 31 years had scale-estimated ages which 

 were, on average, 9 years less than otolith age esti- 

 mates (Fig. 2; paired t-test: n=30; P<0.0001). 



Scale vs. otolith study 



Scales and otoliths were sampled from recreational 

 landings of striped bass (>91 cm TL) during the May 

 1992 Maryland "Trophy Striped Bass Fishery." These 

 fish were assumed to have spawned recently in up- 

 per Chesapeake Bay tributaries. Five additional fish, 

 large females (>100 cm) collected in 1991 and 1992 

 from the Patuxent and Nanticoke Rivers by DNR for 

 hatchery propagation purposes, were included in the 

 comparisons. Scales and otoliths were aged indepen- 

 dently by a reader at Chesapeake Biological Labora- 

 tory (otoliths) and at DNR (scales). Otoliths were 

 prepared and aged as described above. Scale samples 

 for ageing had been removed from the left side of the 

 fish above the lateral line and below the first dorsal 

 fin. Age was determined from either direct interpre- 

 tation of the scale's annuli or from acetate impres- 

 sions of the scales. Otoliths and scales were coded so 

 that fish length was unknown to readers. 



3 4 5 6 7 



Age (yr) 



Figure 1 



Age-frequency distribution of known-age striped 

 bass from Upper Chesapeake Bay, Maryland (MD) 

 and coastal North Carolina (NC). 



