for ^13 C, nitrogen-isotope ratios are favored for trophic-level estimation. A well-
worked example has been provided by Olson et al. (2010), an estimate of yellowfin
tuna trophic level in the northern portion of its eastern tropical Pacific population.
They compared the δ^15 N of tuna muscle sampled from commercial catches to δ^15 N of
“omnivorous” copepods sorted from net tows taken across the same area as the tuna.
Copepods were chosen as a “proxy for the base of the food web”, to simplify the
skein of the initial pathways from primary production to tuna, and Olson et al.
assigned them an approximate trophic level of 2.5, which may be as good a guess as
any. Thus, the trophic level (TL) approximation from δ^15 N values of yellowfin tuna
(YFT) and copepods (COP) is:
(^) in which TL
COP = 2.5 and TEF = 2.4‰.
(^) In the comparison of tuna to omnivorous copepods, the δ (^15) N values of copepods
turned out to vary in relatively smooth fashion across the sampling region (Fig. 9.3a),
a pattern explained by the δ^15 N distribution of upper-water-column nitrate in this area.
That in turn is a product of the regional distribution of δ^15 N in nitrate, which is
increased by bacterial denitrification in the shallow suboxic layers, a process that
favors and elevates ^15 N abundance in residual nitrogenous nutrients (Sigman et
al. 2005). Moreover, the δ^15 N of tuna muscle had a similar geography, implying that
the tuna move about over only modest distances within the larger distribution pattern,
at least relative to the tissue turnover time of tuna, which has been measured in
laboratory studies as ∼37 days. Olson et al. fitted the statistical surface shown in Fig.
9.3a to the copepod δ^15 N at the station points, and then plotted the tuna δ^15 N against
the copepod δ^15 N predicted by the surface (Fig. 9.3b). Dividing the differences by
2.4‰ and adding TLCOP = 2.5, the estimated TLYFT varied from 4.1 to 5.7, averaging
4.7.
Fig. 9.3 (a) Contour plot of δ^15 N values (‰) of omnivorous copepods fitted by a
smoothing routine to 68 sampling stations in the Pacific west of Mexico. (b)
Measured δ^15 N values of yellowfin tuna versus δ^15 N values of omnivorous copepods
from the fitted contours in (a). The dashed line is the 1 : 1 relationship.
(^) (After Olson et al. 2010.)