The number of illegal fishing offences reported on the Great Barrier Reef has more than tripled since 2011 and authorities say rapid growth in boat ownership across the state is largely to blame.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 4 minutes 59 seconds 4 m 59 s Great Barrier Reef Marine Park Authority acting general manager Richard Quincey discusses illegal fishing Download 9.2 MB

During the last financial year, 634 illegal fishing offences were reported on the reef, many of them in the protected "green zones".

The vast majority of offences were by recreational fishers, who were reported 538 times, while commercial fishers were reported for 96 offences.

Acting general manager of the Great Barrier Reef Marine Park Authority (GBRMPA), Richard Quincey, said better surveillance was another reason for the higher numbers, but there was no doubt illegal fishing was happening more often.

"People are intentionally breaking the law and intentionally going into the [green] zones and fishing; both commercial and recreational fishers," he said.

"One of the reasons for that is they know there are more fish in there.

"There can be two or more times higher [fish numbers] as a minimum in protected, closed zones and therefore it becomes an attractive proposition."

One of the key roles of green zones is to allow fish stocks to grow in those areas and then spill over into legal fishing zones.

Mr Quincey said by damaging fish stocks in the green zones, illegal fishers were preventing the full spill-over benefit occurring legal fishing zones, which disadvantages fishers who obey the laws.

While the number of offences seems high, Mr Quincey said they must be taken in context; the Great Barrier Reef covers an area the size of Italy and millions of people fish on it every year.

On the other hand, he admitted that if 634 offences had been reported, it was difficult to know how many other fishers were getting away with illegal fishing.

"Our information from analysing some of the detections over the past years is that about 50 per cent of people [caught fishing illegally] either know or should know what they were doing," he said.

"In offshore locations with large vessels, with people navigating by GPS to get there, there is no excuse and most people know what they are doing.

"Mum and dad and the kids fishing inshore might be a different story and that is where we pick up a lack of awareness."

Fishing is allowed in about 70 per cent of the marine park area, while the remainder is covered by protected zones.

For commercial fishers, Mr Quincey said fishing in the green zones was a lucrative proposition, as they can catch double the fish in the same period.

He said the impact of illegal fishing on fish stocks could be "quite significant."

"You are really quickly removing those larger breeding fish and that can have a much longer-term impact."

Illegal fishing on the Great Barrier Reef comes in many forms, but the most common was line-fishing, wither with rod and reel, or handline.

Recreational fishing groups and conservationists have argued penalties need to be tougher for fishers caught breaking the law on the Reef, but Mr Quncey said the penalties handed down by courts were "substantial".

"Our concern has been that [those penalties] still don't provide a disincentive for some people and they are repeat offenders," he said.

"There have been two cases recently where we have used other sanctions for repeat offenders."

In one of those cases, a commercial fisher was banned from the marine park area for two years.

In the other, a fisher has been banned from stopping or slowing below five knots in the green zones.