Reproductive Ecology and Body Burden of Resident ... - The Love Lab

Discussion
Results from univariate two-factor analysis of variance and multivariate canonical discriminant analysis
indicate that the elemental composition of otoliths differ between the two habitat types—the sampled
platforms and natural sites—and geographic areas, although there were significant interaction effects (habitat
x area) on single element abundances in otoliths. Kelp rockfish collected from platforms tended to incorporate
higher concentrations of Mg and Ba and lower concentration of Sr in otoliths compared to fish
from natural habitat. In comparison, Pacific sanddab collected from platforms tended to have lower concentrations
of Ba and Fe in otoliths compared to fish from natural habitat. Sr Ca -1 in otoliths of kelp rockfish
tended to decrease eastward along the coastal shelf from Point Conception through the east Channel. Mn
Ca -1 in otoliths of sanddab from both habitat types was lower in the three westernmost geographic areas
(Point Conception, west Santa Barbara, and east Santa Barbara) than in the eastern and southern areas (east
channel mainland shelf, offshore east channel, and Southern California Bight). Mn Ca -1 was not detectable
in kelp rockfish otoliths. Variability in otolith element concentrations between habitat types and among
areas differed between the two species.
The success rate of correctly assigning fish to habitat type (platform or natural habitat) using canonical
discriminant functions based on otolith element signatures differed between species. Cross-validation of
the discriminant function showed a very high proportion, 94%, of kelp rockfish individuals were correctly
assigned to habitat type based on the otolith signature of Mg Ca -1 , Ba Ca -1 , and Sr Ca -1 . A lower proportion,
64%, of Pacific sanddab correctly grouped into habitat type based on the otolith signature of Mg Ca -1 , Sr Ca -1 ,
Mn Ca -1 , Cu Ca -1 , and Ba Ca -1 —otolith element concentrations that individually varied between sanddab
habitat types and/or areas.
The usefulness of otolith chemical tags relies on detectable differences between habitats of interest,
such as platforms and natural areas that are spatially and temporally consistent (Campana et al. 2000). We
observed adequate spatial consistency of the kelp rockfish otolith multi-element signatures to discriminate
platforms from natural sites across areas characterized by different water masses and currents (Nishimoto
and Washburn 2002, Nishimoto 2009). Uncertainty in the spatial consistency of platform and natural habitat
differences beyond our sampling of five platforms and five natural areas could be reduced by examining
the otolith microchemistry of fish from a broader sampling of the 23 platforms in the Pacific Outer Continental
Shelf region where kelp rockfish reside as juveniles, for example, and natural areas that fish might
utilize later during their life history.
In comparison with our findings, a “platform fingerprint” in otoliths from the Gulf of Mexico was
identified by Nowling et al. (2005). They investigated the concentration of 15 isotopes of the heavy metals,
V, Co, Ni, Zn, Cu, Ag, Cd, Pb, and U, in whole otoliths of the red snapper (Lutjanus campechanus) from platforms
and artificial reefs off Louisiana and artificial reefs off Alabama. The isotopes were identified as products
of historic drilling activities and natural weathering processes of the platform structure and typically
are not analyzed in otolith microchemistry. Ba was not included in the assay panel. Isotopes of V, Zn, Ag,
U, and Pb were found to be good discriminators between platforms and reefs. Concentrations of 51 V, 107 Ag,
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Ag, 206 Pb, 207 Pb, and 208 Pb were higher and 64 Zn and 66 Zn were lower in fish otoliths from platforms than
from Louisiana or Alabama artificial reefs. U was higher at platforms and artificial reefs in Louisiana than
artificial reefs in Alabama. The elements assayed using solution-based ICP-MS in the Nowling et al (2005)
study were either below detection limits or undetectable using LA ICP-MS in this study.
Different combinations of the elements in otoliths that we examined were useful discriminators of
habitat type and geographic areas in other studies of rockfishes of the California Current region. Warner et
al. (2005) found that the concentrations of Sr and Ba from the outermost growth layer of adult kelp rockfish
differed and Mg concentrations did not differ among mainland and island sites in the Santa Barbara
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