The top five largest aquaculture companies in the world, based on annual revenue, are:

1. Nutreco – Nutreco is a Dutch-based company that specializes in animal nutrition and aquafeed. In 2019, the company reported revenue of €3.2 billion.
2. Mowi – Mowi is a Norwegian company that is the world’s largest producer of farmed Atlantic salmon. In 2020, the company reported revenue of €4.8 billion.
3. Cermaq – Cermaq is another Norwegian company that specializes in the farming of Atlantic salmon and rainbow trout. In 2020, the company reported revenue of €1.4 billion.
4. Grieg Seafood – Grieg Seafood is a Norwegian company that specializes in the farming of Atlantic salmon and rainbow trout. In 2020, the company reported revenue of €1.2 billion.
5. Tassal – Tassal is an Australian company that specializes in the farming of Atlantic salmon. In 2020, the company reported revenue of AUD 335 million.

It is worth noting that the aquaculture industry is highly globalized, with many companies operating in multiple countries and regions. Additionally, the industry is constantly evolving, so the ranking of these companies may change in the future.