A 200-year archaeozoological record of Pacific cod (Gadus macrocephalus) life history as revealed through ion microprobe oxygen isotope ratios in otoliths

Abstract

We measured δ18O values in modern and archaeological Pacific cod (Gadus macrocephalus) otoliths recovered from Aialik Bay on the Pacific coast of the Kenai Peninsula, Alaska, using a high precision ion microprobe. Values of δ18O were measured in as many as sixty 10-μm spots along 2–3mm transects from the otolith core to its margin with high spot-to-spot analytical precision (δ18O±0.3‰). We obtained sample densities along a linear transect that were at least 2 to 3 times greater than micromilling/conventional mass spectrometry techniques. From modern Pacific cod otoliths (using in situ temperatures from electronic archive tags) we calibrated an empirical fractionation equation of aragonite δ18O to sea water temperature (r2=0.75, p<0.001, δ18OA=2.13–0.25T°C) and from which we predicted the thermography of fish life history and historic nearshore water temperature in the Gulf of Alaska (GOA). Sinuous variability of δ18O values along core-to-margin transects likely reflect seasonal temperature changes and suggest similar longevity between modern and archaeological cod. Generally increasing δ18O values from the otolith core region to the margin revealed an ontogenetic migration from warmer nearshore habitat during the first year of life to cooler deeper waters at later ages, a behavior that has not changed over the past 200years. A decline in the average δ18O of otolith cores from archaeological (~200+, ~100+ years before present, 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 GOA over the last 200years. Implications of indigenous subsistence resource use and settlement patterns are discussed in light of major shifts in GOA water temperatures and biological regimes.