https://en.wikipedia.org/wiki/Viral_hemorrhagic_septicemia
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Transmission
VHSV can be spread from fish to fish through water transfer, as well as through contaminated eggs,[3] and bait fish from infected waters.[41] The emerald shiner is a particularly popular bait fish in the Great Lakes region, and is among the species afflicted.[42]
Survivors of the disease can become lifelong carriers of the virus, contaminating water with urine, sperm, and ovarian fluids.[1] The virus has been shown to survive two freeze/thaw cycles in a conventional freezer, suggesting both live and frozen bait could be a transmission vector. In Europe, the gray heron has spread the virus, but it does so mechanically; the virus is apparently inactive in the digestive tract of birds.[2] "
They may be trying to prevent the spread of this? If the virus can survive 2 freeze thaw cycles in a conventional freezer? In pictures here of Lake Erie fish die offs there always seems to be Emerald Shiners in the pics, better not to use fish or parts for chum?