Listeria would have contaminated the product during handling. Although frozen which would prevent growth during frozen storage, the directions of "Defrost before use and keep refrigerated for up to 30 days", would have provided opportunity for growth. This product would then be consumed as a ready-to-eat product.
https://www.fda.gov/safety/recalls-market-withdrawals-safety-alerts/bi-overseas-trading-recalls-product-due-potential-contamination-listeria-monocytogenes
B&I Overseas Trading Recalls Product Due to Potential Contamination with Listeria Monocytogenes
Summary
Company Announcement Date: December 23, 2020
FDA Publish Date: December 24, 2020
Product Type: Food & Beverages Fish
Reason for Announcement: Listeria Monocytogenes
Company Name: B&I Overseas Trading, Inc
Brand Name: Veladis
Product Description: Herring in oil
https://www.fda.gov/safety/recalls-market-withdrawals-safety-alerts/bi-overseas-trading-recalls-product-due-potential-contamination-listeria-monocytogenes
B&I Overseas Trading Recalls Product Due to Potential Contamination with Listeria Monocytogenes
Summary
Company Announcement Date: December 23, 2020
FDA Publish Date: December 24, 2020
Product Type: Food & Beverages Fish
Reason for Announcement: Listeria Monocytogenes
Company Name: B&I Overseas Trading, Inc
Brand Name: Veladis
Product Description: Herring in oil