filter/select by hierarchal value

I am looking for ideas or recommendations on how to filter or select data based on a ranked hierarchy. I've attached a very small portion of a larger file. I am trying to filter/isolate/select CAS Numbers with the lowest (minimum) LD50 based on a ranked hierarchy -- Salmonid, Fathead Minnow, and than "other". If salmonid present, its lowest value is used. If salmonid data is not present than Fathead Minnow lowest value is used. If neither salmonid or fathead is present than its the lowest of "other" is used. I'm open to general solutions but would rather code since the data will be periodically downloaded and may change.