300 
Fishery Bulletin 107(3) 
to acclimate to the presence of a new visual stimu- 
lus. At the start of the second hour the string of lead 
fishing weights was quietly removed and immediately 
replaced with either a string of three electropositive 
metal bars, or the string of lead fishing weights was 
placed back into the tank. This choice was randomized. 
At the start of the third hour, the string of electroposi- 
tive metal bars or lead fishing weights was removed 
and replaced with the other. Only the video records 
from the second and third hours (i.e., one hour in the 
presence of electropositive metal bars and one hour in 
the presence of lead fishing weights) were subsequently 
analyzed. 
The three electropositive metal bars (~2 cmx2 cmxlO 
cm) comprised neodymium (76%), praseodymium (23%), 
Figure II 
Positions of a juvenile sandbar shark ( Carcharhinus 
plumbeus) at 1-sec intervals obtained with Lolitrack 
automated video analysis software (Loligo Systems, 
Tjele, Denmark). Three lead fishing weights (A) or three 
electropositive metal bars (B) were suspended in the 
tank using monofilament fishing line at the position 
indicated by the triangles. The video record was acquired 
with a digital video camera mounted directly above the 
center of a vinyl circular tank (3.6 m diameter, 0.67 
m water depth). Small portions of the tank at the 12 
o’clock and 6 o’clock positions were out of frame because 
of the maximum available height of the laboratory ceil- 
ing where the video camera was positioned. 
and minor amounts (<0.04%) of cerium, lanthanum, 
samarium, and yttrium (Hefa Rare Earth, Vancouver, 
Canada). The three lead fishing weights had similar 
dimensions to those of the electropositive metal bars. 
The strings electropositive metal bars and lead fish- 
ing weights were constructed by using single pieces of 
nylon monofilament fishing line and were suspended in 
the tank at a position approximately 35 cm from the 
tank sidewall (Fig. 1). This lateral position was chosen 
because preliminary observations had shown that juve- 
nile sandbar sharks swam predominately in a circular 
pattern near the tank wall. There was sufficient space, 
however, for the fish to pass easily between the nylon 
line (holding the electropositive metal bars or lead fish- 
ing weights) and the tank wall. Individual electroposi- 
tive metal bars and lead fishing weights were attached 
to the nylon fishing line so as to be at approximately 
16, 32, and 48 cm below the surface when suspended 
in the tank. 
A digital monochrome video camera (IDS Imaging 
Development Systems Inc., Cambridge, MA) equipped 
with a wide angle lens was used to acquire a continu- 
ous record (on a laptop computer) of the swimming 
patterns of each shark. The camera was mounted on 
the laboratory ceiling, over the center of the tank, 
approximately 1.5 m above the water surface. This al- 
lowed an almost complete view of the tank, although 
small areas at the 12 and 6 o’clock positions remained 
out of frame because of the maximum height of the 
digital video camera imposed by the laboratory ceiling. 
The locations of the sharks were subsequently digitized 
(x, y coordinate system) at one-second intervals from 
the video record by using Lolitrack automated video 
analysis software (Loligo Systems, Tjele, Denmark). 
The software generally digitized the broadest area of 
the shark from the dorsal view (i.e., the area between 
the pectoral fins and first dorsal fin). 
Shark positions were translated into quantifiable 
behaviors by calculating the distances between the 
sharks and the electropositive metal or lead weights 
from the one-second interval location records. These 
data were summarized by compiling frequency dis- 
tributions with 5-cm bins. Fractional values for each 
distance bin were calculated from the total number of 
position estimates for each animal when the electro- 
positive metal bars or lead fishing weights were present 
in the tank. The fractional bins were averaged across 
all fish. A two-way (treatmentxdistance bin) repeated 
measures analysis of variance (ANOVA) procedure was 
used to test for differences in the frequency distribu- 
tions (with the use of arcsine transformed percentage 
data), with post hoc tests for significant differences 
between individual bins (Sigma Stat, vera. 3.0.1, Systat 
Software, Inc., San Jose, CA). The significance level for 
all tests was P < 0.05. 
The digital position records were also used to calcu- 
late swimming speeds, which were subsequently segre- 
gated into swimming speeds recorded when the fish was 
within 100 cm of the electropositive metal bars or lead 
fishing weights, and into swimming speeds recorded 
