Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. They caught the fish on the Sabbath. [al-Ḥasan, Muqātil]
2. They placed traps for the fish before the Sabbath but collected them the following day. This means that a man would dig a pit and make a channel to the sea. When the Sabbath came, he would open the channel, even though Allah had forbidden work on the Sabbath. The tide would bring the fish into the pit, and when the fish tried to escape, they could not. He would then collect the fish on Sunday. [al-Suddī]

How did they transgress in the matter of the Sabbath?