386 
Fishery Bulletin 1 1 1 (4) 
Table 3 
Habitat and size of the most abundant prey species found in stomachs of Sowerby’s beaked whales ( Mesoplodon bidens) 
taken in the pelagic drift gillnet fishery for Swordfish ( Xiphias gladius) in the western North Atlantic between August 1989 
and July 1996. Habitat/depths are taken from Fishbase (http://www.fishbase.org/search.php, accessed June 2013). Standard 
length was used to measure fish lengths. i? 2 =coefficient of multiple determination; NA=not available 
Prey species 
Habitat 
Diurnal 
migrant 
Depth 
range 
(m) 
Number 
measured 
Mean 
otolith 
length 
(cm) 
Otolith 
length 
range 
(cm) 
Otolith length 
(OL) - fish 
length regression 
Mean length 
of prey (cm) 
with range 
Laemonema 
barbatulum 
Benthopelagic 
No 
50-1620 
136 
0.4 
0.2-0. 6 
NA 
NA 
Lobianchia 
gemellarii 
Mesopelagic 
Yes 
200-800 
140 
0.7 
0.6-0. 8 
Fish length = 
0.06430L + 1.0482 
R 2 = 0.9799 
9.7 (8-11) 
Nezumia 
bairdii 
Benthopelagic 
No 
90-700 
198 
0.8 
0 
1 
o 
Fish length = 
0.0350L- 0.0961 
i? 2 =0.9348 
22 (13-28) 
Lampanyctus 
spp. 
Mesopelagic 
Yes 
40-1000 
127 
0.3 
0.2-0. 4 
NA 
NA 
Phycis chesteri 
Benthopelagic 
No 
90-1400 
74 
1 
0.6-1. 3 
NA 
NA 
Helicolenus 
daetylopterus 
Bathydemersal 
No 
50-1100 
40 
0.3 
0.2-0. 6 
NA 
NA 
Scopelogadus 
beanii 
Meso- to 
bathypelagic 
Yes 
400-1000 
38 
0.3 
0.2-0. 4 
Fish length = 
0.01690L + 1.6267 
i? 2 =0.5816 
8.1 (5-11) 
Arctozenus 
risso 
Mesopelagic 
No 
200-1000 
43 
0.4 
0.3-0. 4 
Fish length = 
0.00860L + 1.6552 
R 2 = 0.8359 
21.5 (10-27) 
Ceratoseopelus 
Mesopelagic 
Yes 
330-600 
24 
0.3 
0.2-0. 3 
Fish length = 
5.4 (4-6) 
maderensis 0.05110L - 0.1393 
fl 2 =0.9454 
fish we recovered (4. 0-6. 3 cm) and their low numbers. 
This fish species is generally abundant and found in 
schools in the deep scattering layer (DSL) along the 
shelf break on the southern side of Georges Bank 
(Backus et al., 1968) where the Sowerby’s beaked 
whales that we examined were taken. 
The mechanisms by which the Sowerby’s beaked 
whale locates and captures prey are largely unknown. 
All whales in the genus Mesoplodon have relatively 
small mouths and few teeth, and they are believed to 
employ suction while feeding (Mead et al. 1982; Heyn- 
ing and Mead, 1996). The Sowerby’s beaked whale 
has 2 teeth that erupt only in sexually mature males 
(Mead, 1989; Heyning and Mead, 1996). The relatively 
small mouth and 2 teeth of this species may explain 
why Sowerby’s beaked whales typically are found to 
have only small prey items in their stomachs. 
Studies that employed digital acoustic tags on other 
beaked whales in this genus have provided brief but 
exceptionally rich glimpses into the foraging behav- 
ior of these animals. For example, tagged Blainville’s 
beaked whales in the Canary Islands have been report- 
ed to feed on prey in the lower part of the DSL and 
within the benthopelagic zone (Arranz et al., 2011). 
Almost half of the attempts at prey capture made by 
these whales in the Canary Islands occurred in the 
benthic boundary layer, reinforcing the importance of 
benthopelagic prey for them. In addition, these Blain- 
ville’s beaked whales appeared to focus on the oxygen 
minimum layer just below the DSL in areas of steep 
topography. Johnson et al. (2008) described the behav- 
ior of a tagged Blainville’s beaked whale in the Baha- 
mas that appeared to provoke a schooling reaction in 
mesopelagic prey that resulted in a school of prey up 
to 4 m in diameter and created an opportunity for the 
whale to more easily capture those prey. Until a tag is 
deployed for the deep-diving Sowerby’s beaked whale, 
we can only speculate about the foraging behavior of 
