A millennium of trophic stability in Atlantic cod (Gadus morhua): transition to a lower and converging trophic niche in modern times

Abstract

Stable isotope analyses of zooarchaeological material can be used to examine ecological variability in exploited species at centennial to millennial scales. Climate change is a notable driver of marine ecosystem change, although historical fishing is also likely to have impacted past marine systems. Fishing removes the oldest and largest individuals and may thereby result in shorter trophic pathways and reduced niche width of predatory fish species. In the current study we examine the trophic niche of Atlantic cod, haddock and Atlantic wolffish, in the last millennium using δ¹³C and δ¹⁵N values of bone collagen. We report a lower trophic level of Atlantic cod and haddock but higher level of wolffish in present times, following centuries at consistent and higher trophic levels of Atlantic cod. This results in a concurrent converging trophic niche of the demersal fish. We suggest that the current data set provides a valuable historical baseline facilitating interpretation of current variability in the trophic ecology of northern demersal fish.

Figure 1

Map showing the excavation sites of Breiðavík (BRV), Kollsvík (KOV) and Siglunes (SGN) as well as the approximate position of the fishing grounds of the present-day comparison samples (ISA) indicated as a shaded area.

Figure 2

δ¹³C and δ¹⁵N values from all of the samples used dated from 970 CE and until present-day. The box plots show the stable isotope values in each of the temporal groups, specifically, the median (horizontal line), first (box) and third (whiskers) quartile. Values falling out with the third quartile are presented as circles.

Figure 3

Generalized additive model (GAM) estimates of non-linear pattern of change in δ¹³C (A) and δ¹⁵N (B) values over time for Atlantic cod, wolffish and haddock. The bottom panel (C) shows the smoothed winter and summer sea surface temperature, as well as temperature estimates (grey dots), based on diatom proxies north of Iceland.

Figure 4

SEA_B of demersal fish for each century were a sample size of five or more stable isotope values were available. The boxplots show the distribution of SEA_B estimates. Specifically, the median (horizontal black line). First (box) and third (whiskers) quartile. Values falling out with the third quartile are presented as circles.

Figure 5

δ¹³C and δ¹⁵N values and standard ellipses of demersal fish for the centuries were five or more samples of two or more species were available. Atlantic cod and wolffish in the four temporal groups were samples for both species were available. Black ellipses show Atlantic cod, red ellipses are wolffish and green ellipses are haddock. The insert text shows the likelihood of each species being observed in the isotope niche of the others as estimated by nicheROVER (see “Materials and methods”) with the direction of the effect indicated by an arrow.