Scharf et a I.: Diet analysis of piscivorous fishes 
579 
r 2 values typically greater than 0.95 and mean percent 
prediction errors generally less than 10% (Table 7). 
Diagnostic bones 
For each of the bones examined here, clear differences 
in diagnostic features exist among family taxa. Distin- 
guishing bone characteristics were also evident among 
genera within the families Gadidae and Clupeidae. 
Differences between two species from the same genus 
(Alosa ) were, however, difficult to discern from the three 
bones used here. Therefore, bone illustrations are pre- 
sented for only nine species, with alewife, rather than 
blueback herring, representing the genus Alosa. 
Opercles 
The opercle is the largest and most 
dorsal bone in the opercular series, 
which consists of the opercle, the 
subopercle, and the interopercle. 
Together, these three bones pro- 
vide the skeletal support for the 
muscular operculum, or gill cover 
in fishes. The opercle articulates 
with the opercular process of the 
hyomandibula. The articulation 
site is located in the anterodorsal 
region of the opercle, and repre- 
sents a consistent morphological 
feature for orientation of the bone 
during examination. The opercles 
of the fishes examined here can be 
differentiated by two major diag- 
nostic characteristics. First, the 
general shape of the opercle is 
clearly unique to several of the 
families examined. The opercles of 
some families share a general tri- 
angular shape with three well-de- 
fined margins, whereas others are 
not triangular and possess four 
definable margins. A second dis- 
tinctive feature is the presence or 
absence of ridges originating at the 
site of hyomandibular articulation 
and extending ventrally or posteri- 
orly along the margins of the opercle. 
Of the taxa examined here, sil- 
ver hake ( Merluccius bilinearis ) 
and red hake ( Urophycis chuss ) 
have opercles with the most pro- 
nounced triangular shape and 
three clearly defined margins (Fig. 
1, A and B ). The hake opercles are 
further distinguished by the pres- 
ence of two prominent ridges origi- 
nating at the site of hyomandibu- 
lar articulation and extending ven- 
trally and posteriorly along the 
anterior and dorsal margins, re- 
spectively. Hake opercles differ 
