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

ture and salinity on otolith elemental uptake rates, Elsdon and Gillanders (2002) and Martin and Thorrold
(2005) suggested that uncontrolled variation of Mn in experimental treatments may have confounded
results. Martin and Thorrold (2005) suggested that bacterial growth in the treatments may have served as
microbial catalysts forming manganese oxides that were scavenged onto particles lowering dissolved Mn
concentrations in ambient waters. It is possible that the particle scavenging rate of Mn by bacteria in the
bottom waters of the westernmost areas of our study area may have been higher than the other areas.
In contrast, under anoxic aquatic conditions, the availability of dissolved Mn Ca -1 for uptake by fish
can increase. Dorval et al. (2007) found that Mn Ca -1 in otoliths of spotted seatrout (Cynoscion nebulosus)
was positively correlated with Mn Ca -1 in Chesapeake Bay waters but the relationship was not significantly
linear. Dorval et al. associated the field observations of concentrations of biologically available Mn (Mn +2 )
in ambient waters peaking in particular locations due to the reduction of oxidized Mn during summer when
anoxic conditions dominate bottom water and surface sediment. Another explanation for our observations
of otolith Mn Ca -1 differences among areas may be that the Mn reduction rate may be higher in bottom
waters of the eastern Channel and Southern California Bight if waters are relatively anoxic compared to the
western areas of our study.
Forrester and Swearer (2002) found no relationship between Mn concentration in otoliths and sediments
within bays of the Southern California Bight; however, we cannot rule out differences in the Mn load
in sediments in our study that may have contributed to the differences otolith chemistry among areas that
we observed.
In summary, our results showed that the elemental concentrations of some elements in the otoliths of
kelp rockfish and Pacific sanddab differed between platforms and natural sites sampled in the Santa Barbara
Channel and Southern California Bight; however, the differences were not always consistent among
geographic areas. 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. Mn Ca -1 , in otoliths of Pacific sanddab, was the only element in otoliths
of either species that showed a well-defined pattern in its distribution among collection areas. 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 usefulness of otolith chemical tags relies on the spatial and temporal consistency of the otolith elemental
signatures that discriminate between habitats of interest. We observed adequate spatial consistency
of the kelp rockfish otolith multi-element signatures to discriminate platforms from natural sites across five
geographic areas. There was less certainty in identifying the type of habitat where Sanddab were captured
using otolith signatures.
Uncertainty in the spatial consistency of otolith signatures that discriminate platform and natural
habitat beyond our sampling sites 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 and natural areas that
fish might utilize later during their life history. Future study should include a multiyear sampling effort to
examine the temporal stability of a “platform fingerprint.”
Results from this study imply that platform habitat “fingerprints,” unique microchemical markers, may
be identifiable along the time-keeping, growth trajectory of the otolith to identify adult fish that have moved
between platforms and natural areas during the life history of the individuals. Such reconstructions would
benefit from more study on the spatial variability of platform elemental signatures from all existing platforms
and the stability of platform markers in populations over time. Otolith microchemistry, along with
genetics and acoustic tag telemetry, might be used to estimate the degree of connectivity between platforms
and local or regional populations.
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