In this paper, we compare the magnetic field emitted by TBC to the anomalies in the geomagnetic field created by the three large bridges running perpendicular to the migration routes of these species. This knowledge permits researchers to estimate to a first order the magnitude of the fields from the TBC cables in the future just based upon the documented (or alternatively, the assumed) line load, without having to conduct exhaustive gradiometer surveys in the future. A strong concordance was found between the measured and calculated anomalies in the geomagnetic field, as well as between the measured and calculated distance between the anomalies’ maxima and minima. The magnetic field profiles measured along survey lines that ran approximately perpendicular to the cable’s heading at the four locations together with the magnetometer’s height above the sea floor were entered into a regression model to estimate load current, the cable’s depth and the angular rotation of the cable’s two conductors from the horizontal (the ‘twist’ angle). Seventy eight of 167 survey lines at these sites yielded profiles in which an anomaly in the geomagnetic field indicated the cable’s presence. Gradiometer surveys were conducted at four sites: 1) San Francisco-Oakland Bay Bridge (BB), 2) Richmond-San Rafael Bridge (RSR), 3) Benicia-Martinez Bridge (Ben), and 4) San Pablo Bay (SP). The static magnetic field was measured while traversing the cable roughly perpendicular to its path at varying heights above the sea floor. Previously, we described the effect of the load current carried on the TBC cables on the local magnetic field. The inset shows both a picture of the cable and its construction. Magnetic field surveys were conducted over an area with a width of 1 km (pink survey lines) at the following sites: 1) Benicia Bridge, 2) San Pablo Bay, 3) Richmond Bridge, and 4) Bay Bridge. The Trans Bay Cable (dark blue line) conducts electricity from the city of Pittsburg, CA to San Francisco.
10.1371/001 Fig 1 Overview of the Trans Bay Cable route and survey area. Anatomical studies have revealed that the sturgeons have electroreceptors, and hence are capable of using similar fields to provide guidance during migration. Experimental and observational studies have established that species within the salmonid family orient to magnetic fields and may use them to guide their movements during migration. These are Chinook salmon smolts ( Oncorhynchus tshawytscha) that emigrate downstream through the San Francisco Estuary to the Pacific Ocean and adult green sturgeon ( Acipenser medirostris), which immigrate upstream to their spawning habitat in the upper Sacramento River and subsequently, back down to the ocean after spawning occurs. We have studied the effect of the Trans Bay Cable (TBC), an 85-km long, high voltage, direct current transmission line leading from Pittsburg, CA to San Francisco, CA, on two fish species migrating through the San Francisco Estuary ( Fig 1). Concern exists that animals that migrate along the continental shelves might orient to the magnetic fields from the cables, and move either inshore or offshore away from their normal path. The magnetic fields produced by undersea cables that carry electricity from electric power generation sources to shore-based power stations may produce similar local distortions in the earth’s main field. While suggestive, the conclusions that can be drawn from such correlational studies are limited without experimental confirmation. Evidence exists that baleen whales strand where rotation of oceanic plates results in weakly magnetized sections of crust intersecting the western coastline of Great Britain and eastern coastline of North America.
The paths coincide with magnetic maxima (“ridges”) and minima (“valleys”) leading away from the seamount. For example, scalloped hammerhead sharks have been shown to use these magnetic gradients to guide their diurnal migrations from a seamount to and from nighttime feeding grounds. Observational evidence exists that some marine animals perceive and orient to local distortions in the earth’s main geomagnetic field. The latter are referred to as magnetic anomalies in the earth’s field. The earth’s magnetic field has two components, the dipolar main field, produced by the convective movements of molten iron in the earth’s core, and distortions in this field from magnetic particles embedded in the earth’s outer crust.
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