On Tuesday the captain of a UK-registered vessel was fined £10,000 for unlawfully fishing in Manx waters.
It follows the prosecution at Douglas courthouse last week of a Scottish fisherman who was fined £15,000 for breaching Manx regulations.
Richard Ronan MHK said the "significant fines send out a clear message".
The Deputy High Bailiff ordered both skippers to forfeit their catch of King Scallops worth nearly £5,500 each (about 90 bags).
Both vessels were routinely boarded by officers from the Manx government's fisheries protection vessel Barrule before being escorted back to the island and detained.
Mr Ronan, the Minister for Environment, Food and Agriculture, said: "The Isle of Man is committed to the protection and development of stocks of shellfish within its territorial sea and is vigilant over illegal fishing activity.
"These significant fines must act as a strong deterrent."
He added that the Isle of Man "welcomes responsible skippers" with registered vessels to sustainably fish in the Isle of Man's territorial sea with the appropriate licence.
Manx territorial waters extend out to 12 miles around the island's coastline and contain a number of restricted zones.

Habyarimana Mucebo, a senior member of the Rwandan FDLR rebel group, was captured in Rutshuru, north-eastern DR Congo.
Members of the FDLR fled into DR Congo after the 1994 genocide in Rwanda.
The slaughter of some 800,000 ethnic Tutsis and moderate Hutus sparked years of unrest in the region.
Mr Mucebo, the FDLR intelligence chief, has been taken into custody for interrogation, the army says.
Most senior figures of the group are wanted by Rwanda for their alleged role in the 1994 killings.
Mr Mucebo's arrest is not linked to genocide charges as initially reported.
Congolese officials accuse him of links to atrocities blamed on the FDLR.
For many years, the Hutu rebel group had become a source of instability in DR Congo's North Kivu province.
Rwanda has sent troops and militias into DR Congo on several occasions since the genocide, saying they were going after the FDLR.

The pro-Rouhani reformist group Omid (Hope) won all 21 seats, ousting their conservative rivals who had been in power for 14 years.
Mr Rouhani won Friday's presidential election in the first round.
He promised voters a moderate and outward-looking Iran and criticised the conservative-dominated judiciary.
Council elections were held alongside the presidential vote and the results were announced on Sunday.
The new city council must convene within 45 days when it will elect a new mayor to oversee services for Tehran's nine million residents.
Incumbent Mayor Mohammad Baqer Qalibaf was a candidate in the early stages of the presidential election but dropped out to back hardliner Ebrahim Raisi.
Mr Raisi only received 38.5% of the vote, which was not enough to take the election to a second round.
President Rouhani, 68, said his resounding victory showed that voters rejected extremism and wanted more links with the outside world.
Analysts say he is in a strong position to seek reforms and to revive Iran's ailing economy.
After his re-election there were celebrations in the capital, Tehran, with crowds of young people singing and dancing in the central Vali Asr Square.
In his first speech after the result was announced Mr Rouhani said: "The Iranian nation has chosen the path of interaction with the world, a path which is distant from extremism and violence."
Mr Rouhani supports the landmark deal with world powers to curb Iran's nuclear programme.
The prosecution of a second UK skipper in a week for breaching Manx fishing regulations sends a "clear message to the industry," a politician has said.
[NEXT_CONCEPT]
A Rwandan Hutu rebel commander wanted for atrocities committed on Congolese soil has been arrested in eastern Democratic Republic of Congo.
[NEXT_CONCEPT]
Iran's re-elected moderate President Hassan Rouhani has received a further boost after reformists won key council elections in the capital, Tehran.