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Fishery Bulletin 107(1) 
presence in laboratory conditions, squid have occasion- 
ally been observed swimming with their tentacles ex- 
tended, perhaps to appear bigger and to deter attacks. 
Maximum length (L Max ) therefore could be used as an 
upper limit of predator-perceived squid size. Conversely, 
in studies where squid is considered the predator rather 
than the prey, maximum length may be useful to inves- 
tigators interested in estimating the reach or striking 
distance of a squid. 
All of the morphological relationships measured in 
this study were found to be highly accurate predictors 
of body size. The proposed models to back-calculate 
original size from the lower rostral length of squid 
beaks provide coefficients of determination similar to 
those found in previous studies where fish bones and 
eye lenses were used to calculate original prey size 
(Scharf et al., 1997, 1998; Wood, 2005). Although it 
appears that cephalopod beaks are less susceptible to 
digestion than fish bones, it is still possible that ero- 
sion may lead to some measurement bias (Tollit et al., 
1997; Santos et ah, 2001). The rostrum and shoulder 
are the sections of the beak formed earliest in develop- 
ment; they are most resistant to digestion and erosion 
and therefore ideal structures for reconstructing body 
lengths (Clarke, 1980). As is true with all allometric 
relationships, the techniques presented here are species 
specific and may not be reliable estimators of squid body 
size if applied to lengths beyond the ranges used to de- 
velop the predictive equations. Both longfin and short- 
fin squids attain body sizes larger than were included 
in the present study; however, our analyses include 
the most commonly observed lengths of squid found 
in predator diets and should be adequate for most diet 
studies. For example, <5% of longfin and shortfin squid 
prey sizes reported in Kohler (1987), Gannon et al., 
(1997), Williams (1999), Chase (2002), and Staudinger 
(2006) exceeded the largest mantle lengths measured 
here. It should also be noted that specimens collected 
for the present study were from a portion of the total 
distributional range of each species and were collected 
on a limited temporal scale (Macy and Brodziak, 2001; 
Hendrickson, 2004). If there is variation in squid al- 
lometry due to seasonal, interannual, and regional dif- 
ferences in population structure, our sampling regime 
may not have fully encapsulated these deviations. 
Knowledge of size-selective feeding behaviors is fun- 
damental to assessing trophic relationships and defin- 
ing ecological niches (Bax, 1998). Some of the greatest 
consumers of cephalopods are large apex predators such 
as pelagic sharks, tunas, swordfish, and marine mam- 
mals; however, the majority of diet data collected on 
these and many other teuthophagous predators has 
been qualitative (Smale, 1996). Further, although it 
has been well established that marine predators are 
size-selective when feeding on piscine prey (Juanes and 
Conover, 1994, 1995; Juanes et al., 2001), evaluation of 
size-based predation on cephalopods has not been well 
explored. Perhaps the reason for this paucity of infor- 
mation, especially in the Northwest Atlantic Ocean, 
is the lack of available tools and techniques (Scharf 
et al., 1998). In previous studies where squid size has 
been taken into consideration, mantle length was used 
to calculate relative prey size and to evaluate trophic 
niche breadths (MacLeod et al., 2006; Menard et al., 
2006), or total length was estimated from anatomical 
drawings (Chancollon et al., 2006). These approaches 
are not recommended because they either considerably 
underestimate total squid size or fail to capture varia- 
tion in size with growth, thereby introducing error into 
subsequent calculations. 
To gain a complete understanding of the energetic de- 
mands of marine predators, it will be necessary for this 
key prey group to be accurately assessed. Discerning 
the squid sizes that are most important to supporting 
predator growth will improve evaluations of age- and 
size-based consumption rates of squid predators, natu- 
ral mortality rates of squid populations, competition 
among species, and resource sharing between the com- 
mercial fishing industry and marine predators. 
Acknowledgments 
Funding for this study was provided by the Woods Hole 
Oceanographic Institute Sea Grant and the University 
of Massachusetts School of Marine Sciences. We thank 
the scientists at the National Marine Fisheries Service 
(Northeast Fisheries Science Center) and the staff of the 
Marine Resources Center (Marine Biological Labora- 
tory), in Woods Hole Massachusetts for their help col- 
lecting squid specimens. Special thanks to N. Jacobson 
and C. McGarigal for helping to measure squid, and M. 
Clarke for providing expertise and advise during the 
analysis of cephalopod beak structures. 
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