Environmental Engineering Reference
In-Depth Information
et al., 2008a; Wiltschko and Wiltschko, 1995). Four species of Pacific salmon were
found to have crystals of magnetite within them, and it is believed that these crys-
tals serve as a compass that orients to the Earth's magnetic field (Mann et al., 1988;
Walker et al., 1988). Because some aquatic species use the Earth's magnetic field to
navigate or orient themselves in space, there is a potential for the magnetic fields
created by the numerous electrical cables associated with offshore power projects to
disrupt these movements.
Gill et al. (2005) placed magnetosensitive organisms into two categories: (1) those
able to detect the iE field caused by movement through a natural or anthropogenic
magnetic field, and (2) those with detection systems based on ferromagnetic minerals
(i.e., magnetite or greigite). Johnsen and Lohmann (2005, 2008) add a third possible
mechanism for magnetosensitivity—chemical reactions involving proteins known
as crytochromes (i.e., a class of flavoproteins that are sensitive to blue light and are
involved in circadian rhythm entrainment in plants, insects, and mammals). Those
species using the iE mode may do it either passively (i.e., the animal estimates its
drift from the electric fields produced by the interaction between tidal/wind-driven
currents and the vertical component of the Earth's magnetic field) or actively (i.e.,
the animal derives its magnetic compass heading from its own interaction with the
horizontal component of the Earth's magnetic field). For example, Kalmijn (1982)
suggested that the electric fields that elasmobranchs induce by swimming through
the Earth's magnetic field may allow them to detect their magnetic compass head-
ings; the resulting voltage gradients may range from 0.05 to 0.5 µV/cm. Detection
of a magnetic field based on internal deposits of magnetite occurs in a wide range
of animals, including birds, insects, fish, sea turtles, and cetaceans (Bochert and
Zettler, 2006; Gould, 1984). There is no evidence to suggest that seals are sensitive
to magnetic fields (Gill et al., 2005).
Westerberg and Begout-Aranas (2000) studied the effects of a B field generated by
a HVDC power cable on eels (
Anguilla anguilla
). The B field was on the same order
of magnitude as the Earth's geomagnetic field and, coming from a DC cable, was also
a static field. Approximately 60% of the 25 eels tracked crossed the cable, and the
authors concluded that the cable did not appear to act as a barrier to the eel migration.
In another behavioral study, Meyer et al. (2004) showed that conditioned sandbar and
scalloped hammerhead sharks readily respond to localized magnetic fields of 25 to
100 µT, a range of values that encompasses the strength of the Earth's magnetic field.
Some sea turtles (see Sidebar 7.1) undergo transoceanic migrations before return-
ing to nest on or near the same beaches where they were hatched. Lohmann and
Lohmann (1996) showed that sea turtles have the sensory abilities necessary to
approximate their global position of a magnetic map. This would allow them to
exploit unique combinations of magnetic field intensity and field line inclination in
the ocean environmental to determine direction and/or position during their long-
distance migrations. Irwin and Lohmann (1996) found that magnetic orientation
in loggerhead sea turtles (
Caretta caretta
) can be disrupted at least temporarily by
strong magnetic pulses (i.e., five brief pulses of 40,000 µT with a 4-ms rise time).
The impact of a changed magnetic environment would depend on the role of mag-
netic information in the hierarchy of cues used to orient and navigate (Wiltschko
and Wiltschko, 1995). Juvenile loggerheads deprived of either magnetic or visual
