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We tested the hypothesis that strontium:calcium (Sr:Ca)
in otoliths are reflective of environmental salinity
experienced by two estuarine fishes during early life. Laboratory and field
experiments were performed to examine the effects of salinity and
temperature on Sr:Ca in otoliths of black drum (Pogonias cromis) and red
drum (Sciaenops ocellatus). Otolith Sr:Ca of juveniles reared at four
salinities (5‰, 15‰, 25‰, 35‰) differed significantly for P. cromis while no
salinity effect was observed for S. ocellatus. Otolith Sr:Ca of both species
were not affected by temperature (238C and 308C), suggesting that
partitioning of Sr in otoliths of these taxa is constant over the
temperature range examined. A field verification trial was conducted for P.
cromis and a positive relationship between otolith Sr:Ca and ambient
salinity was observed, even though the percent variability explained was
modest. A series of Sr:Ca point measurements were taken from the core to the
edge of the otoliths of wild P. cromis and S. ocellatus, and otolith Sr:Ca
chronologies of both species showed conspicuous declines during the first
few months of life. While Sr:Ca chronologies of both species suggest that
ingress is associated with a reduction in otolith Sr:Ca, inconsistencies in
laboratory and field experiments intimate that Sr uptake in the otolith may
be insensitive to salinity and regulated by other factors (aqueous
chemistry, ontogenetic shifts in habitat, or physiology). Results from early
life history transects of otolith Sr:Ca conform to expected patterns of
estuarine ingress-egress during early life and indicate that the approach
may be useful for detecting large-scale habitat transitions (marine to
estuarine habitats). |
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There has been increased use of otolith composition
data to track eel Anguilla spp. migrations in coastal and estuarine
environments. Numerous studies have used strontium (measured as Sr:Ca) to
infer salinity related habitat use, yet the method remains largely
unverified. It is no known whether otolith Sr:Ca is primarily related to
ambient salinity, or whether this relationship is confounded by temperature
or growth. We manipulated experimental rearing environments of young
American eels Anguilla rostrata to determine the amount of
variability in otolith Sr:Ca related to salinity, temperature, and growth;
and estimate the lag time in response of otolith chemistry to changes in
salinity. Our results suggest that otolith Sr:Ca in eels can be used to
discriminate broad scale life history periods of fresh, brackish, and ocean
habitat occupation, but finer scale interpretations cannot be supported. In
addition to salinity, temperature, and growth effects, further study is
needed concerning the influence of dietary sources of Sr on eel otolith
composition. |