Cal. Labor Code § 131

Each witness who appears by order of the appeals board or any of its members, or a workers’ compensation judge, shall receive, if demanded, for his or her attendance the same fees and mileage allowed by law to a witness in civil cases, paid by the party at whose request the witness is subpoenaed, unless otherwise ordered by the appeals board. When any witness who has not been required to attend at the request of any party is subpoenaed by the appeals board, his or her fees and mileage may be paid from the funds appropriated for the use of the appeals board in the same manner as other expenses of the appeals board are paid. Any witness subpoenaed, except one whose fees and mileage are paid from the funds of the appeals board, may, at the time of service, demand the fee to which he or she is entitled for travel to and from the place at which he or she is required to appear, and one day’s attendance. If a witness demands his or her fees at the time of service, and they are not at that time paid or tendered, he or she shall not be required to attend as directed in the subpoena. All fees and mileage to which any witness is entitled under this section may be collected by action therefor instituted by the person to whom the fees are payable.