ISOTOPIC RECONSTRUCTIONS OF SWORDFISH DIETS IN THE GULF OF MAINE

Stable isotope tracers can be valuable tools for interpreting the way an ecosystem has functioned over time. Both modern and archaeological swordfish rostra spanning 4,200 years were collected from the Gulf of Maine and off the coast of California for ¹⁵N and ¹³C analysis to understand shifts in swordfish populations and reconstruct their diets through time. In addition, several selected whole rostra were subsectioned and analyzed for bulk carbon and nitrogen isotope analysis in order to evaluate the isotopic variability that can occur along the length of a rostrum.

Preliminary results show notable variability in δ¹³C along individual rostra, with more enriched values closer to the tip of the rostrum. This variability may reflect natural changes in the diet of the swordfish throughout its life, movement patterns with growth, or the increased presence of isotopically depleted lipids in collagen farthest from the rostrum tip. The δ¹⁵N of the archaeological rostrum collagen is relatively constant between 4.2 ka BP and 1.2 ka BP, and then becomes 2‰ depleted in ¹⁵N in modern samples. These data may reflect (1) a decrease in trophic level of the swordfish, (2) a change in nutrients at the base of the food web, or (3) a fundamental shift in swordfish populations in the last 1.2 ka. Swordfish used to be an important resource to human cultures in the nearshore region of the Gulf of Maine; thus, the study of the dietary habits provides useful information on changes in their ecology as well as human exploitation.