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Fishery Bulletin 96(2), 1998 
Year class 
Figure 4 
A 14 C values in the otolith cores of adult black drum in relation to the published A 14 C 
chronology for the atmosphere (data from Nydal [1993]). The data have been scaled 
so as to emphasize the similarities in the rate of increase and decline in the two time 
series. Otolith cores formed in riverine or estuarine waters would be expected to reflect 
more closely the atmospheric pattern of A 14 C than that found in marine carbonates. 
otolith ageing studies reported 
to date, whether in the North 
Atlantic (Fig. 3) or in the South 
Pacific (Kalish, 1993, 1995a, 
1995b; Kalish et al., 1996). How- 
ever, the A 14 C chronology recon- 
structed from the black drum 
otoliths was noticeably differ- 
ent; whereas prebomb 14 C val- 
ues were comparable among all 
taxa, all postbomb year classes 
of black drum were significantly 
higher than those observed in 
any other open-ocean carbonate, 
and appeared to be phase- 
shifted 2-4 yr towards earlier 
years. This difference cannot be 
explained on the basis of age- 
ing error, because the peak A 14 C 
values in the otolith were as 
much as four times higher than 
those recorded in any year of 
any other open-ocean species. 
However, the apparent anomaly 
is fully explicable when the life history of the black 
drum is considered. The black drum is a marine fish 
but spends the first year of its life in rivers or estu- 
aries (Richards, 1973). Thus the elemental and iso- 
topic composition of the otolith core reflects the riv- 
erine or estuarine environment, not that of the open 
ocean. Because estuaries are shallow, well-mixed 
areas with strong riverine input, there is a rapid and 
relatively complete exchange of radiocarbon between 
the atmosphere and the water. As a result, the 14 C 
chronology of an estuary is a much closer reflection 
of the atmospheric chronology than of the marine 
chronology (Erlenkeuser, 1976; Spiker, 1980; Tanaka 
et al., 1986). Thus it is to be expected that the A 14 C 
values of the black drum otolith core would he inter- 
mediate, in magnitude and phase, to those of the at- 
mosphere and the marine series, although closer to 
the former. This is indeed what was observed; thus 
black drum age assignments based on annular counts 
appear to be considerably more accurate than was 
originally thought, and not biased by 2-4 yr as would 
have been indicated by comparison with the marine 
carbonate chronologies. Nonetheless, the black drum 
measurements are the first demonstration of this 
phenomenon in fish and confirm Kalish’s (1995b) 
suggestion that precise age calibrations against bomb 
radiocarbon signals must take into account the na- 
ture of the habitat in which the young fish has lived. 
Whereas the the collection of black drum cores 
provided an unambiguous view of the period of A 14 C 
increase, individual variability within any given year 
class was large and greatly exceeded the analytical 
error of the 14 C assays. Individual ageing errors un- 
doubtedly contributed to this variability because even 
a 1-yr ageing error (in a 30-yr-old fish) would have 
resulted in a A 14 C change of about 40 during the early 
1960’s. A similar argument applies to errors associ- 
ated with the isolation of the otolith core from the 
surrounding, more recent, material. However, we 
believe that environmental heterogeneity and estua- 
rine food sources were the dominant source of vari- 
ability in the year-class-specific measurements. Vari- 
ability in the isotopic composition of estuarine wa- 
ters can be much larger than that of the open ocean: 
Spiker (1980) documented nearly twofold variabil- 
ity in the A 14 C of total dissolved inorganic carbon 
(DIC) among locations in the Chesapeake Bay nurs- 
ery area of black drum. The potential for variability 
due to diet in an estuary is also large. Because ter- 
rigenous carbon sources reflect atmospheric (en- 
riched) A 14 C in their isotopic composition and because 
terrigenous input and A 14 C tend to decrease with 
increasing salinity, the isotopic composition of an 
animal’s diet is bound to be more variable in an es- 
tuary than in a more homogeneous environment like 
the open ocean. Approximately half of the carbon 
incorporated into the shells of estuarine bivalves is 
derived from 14 C-enriched metabolic (dietary) 
sources, as opposed to DIC, with the result that there 
is a high variability among individuals (Erlenkeuser, 
1976; Tanaka et al., 1986). Similar arguments un- 
doubtedly apply to fish otoliths because metabolic 
