Scharf et a I.: Diet analysis of piscivorous fishes 
587 
differences between genera relative to opercles. 
Newsome (1977) and Hansel et al. (1988) revealed 
similar findings, demonstrating the limitations of 
opercles for identifying prey fishes beyond the fam- 
ily level. Moreover, Hansel et al. (1988) found that 
cleithra and dentaries were more resistant to diges- 
tion than opercles or pharyngeal arches, when ex- 
amining several freshwater fishes, and concluded 
that, for young prey fish, the cleithrum may be the 
most reliable bone because of its large size and early 
development in relation to other diagnostic bones. 
None of the three diagnostic bones appear useful 
for differentiating between the two species of the 
genus Alosa examined in this study. Similarly, Hansel 
et al. (1988) found that for certain congeneric fresh- 
water fishes, identification was inconsistent and not 
dependable when opercles, cleithra, dentaries, and 
pharyngeal arches were used. The size range of fishes 
used in this study included juveniles and young 
adults (Table 1) and is typical of size ranges exam- 
ined in previous studies (Trippel and Beamish, 1987; 
Hansel et al., 1988). Therefore, for these life stages, 
opercles, cleithra, and dentaries may be potentially 
useful as tools for identifying prey fishes to the fam- 
ily and, in some cases, the generic taxonomic level, 
but likely are not adequate for distinguishing be- 
tween species of the same genus. 
Our descriptions of the diagnostic bones provide a 
simple means to distinguish prey fishes based on 
general bone structure. Indeed, there have been more 
rigorous efforts directed at identifying the system- 
atic relationships among several of the fishes exam- 
ined in this paper. However, the bone descriptions in 
this paper were provided as a simple guide for field 
identification of prey fishes commonly recovered 
from piscivore stomachs, and thus centered on gen- 
eral overall shape and features that were common 
to all of the fishes. The differences among the taxa 
that are outlined here should be readily observable 
to a large range of workers with varying levels of 
fisheries training. Therefore, we suggest that 
the level of analysis and description presented here 
may compliment existing information and may be 
ideally suited for identification of prey fishes in field 
settings. 
The bone descriptions and regressions presented 
in this paper clearly have the potential to increase 
the amount of dietary information obtainable from 
stomach contents analyses of Northwest Atlantic 
piscivores. Further, the generation of a series of re- 
gression equations relating measurements from both 
internal hard parts and external morphological fea- 
tures to original fish size may provide a means to 
cross reference prey size estimates, thus facilitating 
the identification of erroneous estimates. 
Acknowledgments 
We thank the scientists at the National Marine Fish- 
eries Service (Northeast Fisheries Science Center in 
Woods Hole, MA) for fish collections and allowing us 
to participate in research cruises to collect samples. 
We are especially grateful to Arnie Howe and the 
Massachusetts Division of Marine Fisheries for col- 
lecting numerous samples. We thank Brian Hanra- 
han for providing considerable assistance with fig- 
ure graphics and William Bemis for assistance with 
osteological literature. We are grateful to John 
Boreman, Karsten Hartel, and Rodney Rountree for 
comments that improved the manuscript. This study 
was supported by the Cooperative Marine Education 
and Research Program of the National Oceanic and 
Atmospheric Administration and the Five College 
Coastal and Marine Sciences Program. 
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