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A 200-Year Archeozoological Record of Pacific Cod Life History as Revealed Through Ion Microprobe Oxygen Isotope Ratios in Otoliths 

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Summer 2014
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Pacific cod is an abundant marine fish species inhabiting the Alaska continental shelf whose importance for food spanned centuries from modern industrial fisheries back to traditional subsistence use by Alutiiq communities. Alutiiq residents of the remote Kenai coast settlements were referred to as Unegkurmiut or “people out that way” by the other more densely settled inhabitants around the Kodiak archipelago. Subsistence artifacts recovered at the Early Contact Village (~1785-1820) and Denton (~1850-1890) Kenai archeological sites consisted of arrowheads and darts, shouldered lance points, adzes, and fish hooks. Archeological recovered fish hooks, museum examples of fishing equipment, and ethnohistoric records indicate that the Alutiiq subsistence effort focused on adult Pacific cod in waters  approximately 30-50 m deep. Large samples of bone and otoliths confirm the importance of Pacific cod in their diet. Intact fossilized Pacific cod otoliths found at archeological sites in the Gulf of Alaska (GOA) provided a unique opportunity to explore the interactions between climate and fish populations on temporal scales not typically available to modern ecologists. Using otoliths recovered from the Early Contact Village and Denton archeological sites dated from 200+ and 100+ years before present (YBP) along with modern collections in Aialik Bay, Alaska (Fig. 1), we analyzed oxygen isotope ratios (δ18O) to reconstruct the near-shore temperature regime and Pacific cod habitat use in the GOA since the Little Ice Age (1350-1850).

map refer to caption
Figure 1. Location of recovered Pacific cod otoliths at two archeological sites in Aialik Bay, Alaska.

Nine Pacific cod otoliths (3 from 200+ YBP sites, 3 from 100+ YBP sites, and 3 from modern caught (2004) fish in Aialik Bay) were thin sectioned, polished, and gold coated in preparation for microsampling using the WiscSIMS (University of Wisconsin, GeoSciences) ion microprobe (Fig. 2). The Ion Microprobe-Secondary Ion Mass Spectrometer (SIMS), funded by the National Science Foundation, was used to explore new applications of in situ analysis of stable isotope geochemistry. The advantage offered by the IMS-1280 ion microprobe coupled with the SIMS over conventional isotope mass spectrometry is the dramatic reduction in analysis spot sizes; from 1 to 10 micrometers. In particular, recent developments in the analytical capabilities of the WiscSIMS ion microprobe, a focused ion beam coupled with a double-focusing mass spectrometer, have allowed in situ analysis of otoliths on sub-annual, even daily, time scales. The ion microprobe can analyze discrete samples (~ 2 ng) that are thousands of times smaller than those required by conventional acid digestion/gas-source mass spectrometry (10-100 μg). The increased spatial resolution (sample diameter = 10 μm with depth of ~1 μm) allows for finer temporal resolution of measurements while maintaining high accuracy and precision.

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Figure 2. Thin sectioned Pacific cod otolith (A) recovered from 200+ year old archeological site in Aialik Bay, Alaska. WiscSIMS ion microprobe spot samples from otolith core to edge (B). Electron micrograph (C) shows high spatial sampling resolution of spot samples.  

Full life-history transects comprising between sixty to eighty 10-micron spot samples from the otolith core (juvenile stage) to edge (adult stage) were sampled with the ion microprobe (Fig. 2) and values δ18O measured from a secondary ion mass spectrometer were plotted as ‰ relative to Vienna PDB standard. Measured δ18O was converted to temperature using a fractionation equation developed from ion microprobe analysis of seven modern Pacific cod otoliths from which in situ bi-hourly temperature and depth records were recorded in electronic archived tags (Fig. 3). Specifically, spot samples of measured δ18O that were sampled near the outer edge of the otolith representing the aragonite material accreted during the period at liberty were regressed with average monthly in situ instrumental temperatures.  The fractionation equation was used to predict the thermography of Pacific cod’s life history from spot samples over the entire transect and to reconstruct near shore Gulf of Alaska temperatures from spot samples taken within the otolith core.

We obtained sample densities along a linear transect that were at least 2 to 3 times greater than micromilling/conventional mass spectrometry techniques with high spot-to-spot analytical precision (δ18O ±0.3‰). Measured values of δ18O were typically lower near core samples (-4.08 to -2.21 ‰ PDB) than spot samples near the otolith edge (0.52 to 1.44 ‰ PDB) (Fig. 4). Rapid rise in of δ18O after the first year of life followed by higher but cyclical δ18O concentrations reflect ontogenetic migratory behavior from warmer near-shore habitat during the first year of life to cooler deeper waters at later ages (Fig. 4). Predicted temperature of Pacific cod habitat use, estimated from the fractionation equation from archive tagged Pacific cod (Fig. 3; r=0.75, p<0.001), show a thermography consistent with what is known about migratory behavior. A decline in the average δ18O of core spot samples from archeological (200+, 100+ YBP) to modern otoliths suggest increasing sea surface temperatures from the late Little Ice Age to present. Temperatures calculated from the δ18O in aragonite suggest a 2°-3°C rise in coastal marine sea surface temperatures in the Gulf of Alaska over the last 200 years (Fig. 5).

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Figure 3. Sequence of ion microprobe spot samples measuring stable oxygen isotopes d18O ( VPDB, 2 S.D.) made at WiscSIMS from a traverse sectioned Pacific cod tagged with an electronic data logger (temperature and depth) and at liberty for 716 days. Spot samples 1-31 were sampled near the outer edge of the otolith and represented the aragonite material accreted during the period at liberty. As expected, relationship between Pacific cod otolith aragonite (d 18O) and bottom temperature showed an inverse, statistically significant linear relationship ( r=0.75, p<0.001).  

High resolution sampling for δ18O, using tools such as the ion microprobe, provides a unique perspective on Pacific cod biogeography and migratory behavior, showing habitat preference for warmer near-shore water during early life stages followed by migration to cooler deeper water. This life history strategy has not appeared to have changed over the past 200 years. Near-shore temperatures in the Gulf of Alaska, inferred through archeological and modern δ18O samples from Pacific cod otoliths, appeared to have increased since the late Little Ice Age. The difference of about 2°-3°C cooler around the decade 1800 A.D. from otolith δ18O is consistent with tree-ring derived estimates of cooler air temperature during the same period. Next steps will be to conduct microprobe sampling on Pacific cod otoliths recovered from even deeper strata at the Kenai archeological sites dating to over 1,000 YBP. Studies of fish otoliths recovered from archeological sites and analyzed with innovated tools such as the ion microprobe, provide a window into the past that opens our understanding of climate change, fish populations, and human resource utilization of the Alaska region over the past thousand years.

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Figure 4. Sequence of ion microprobe spot samples from otolith core to edge measuring stable oxygen isotopes δ18O (‰ VPDB, ±2 S.D.) made at WiscSIMS with predicted temperatures estimated from fractionation equation. Circles show spot samples within the otolith core used to reconstruct near shore temperature change since Little Ice Age.  
 
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Figure 5. Predicted near shore water temperature since the Little Ice Age (200+ YBP) to modern times from 9 Pacific cod otoliths (six of which were recovered from archeological sites and dated to 200+ and 100+ YBP) sampled for stable oxygen isotopes δ18O. Temperature was reconstructed from fractionation equation.

 

By Thomas Helser and Craig Kastelle

 

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