Fishing authorities say they have been surprised to learn of reports of a rare mud crab being caught in the Swan River.

Mud crabs are commonly found 2,000 kilometres away in Western Australia's north where a small commercial fishery operates.

Daniel Curulli said he was stunned when his family caught a lone mud crab this week in the Perth river.

Daniel Curulli was very surprised to catch a mud crab in southern WA. ( Supplied: Daniel Curulli )

"Mum starts pulling the net up and says it's really heavy, and dad's saying it's probably just a jellyfish," he said.

"As it gets up closer, mum sees the big mud crab in there and she sort of freaked out a bit holding it in the water.

"We're all screaming to hurry up and get it in the boat, and there it was, a nice big mud crab."

Mud crabs are commonly found in the Pilbara, Kimberley and NT. ( ABC Radio Darwin: Jesse Thompson )

Crab made the journey alone

Mr Curulli's encounter was backed up by the WA fishing authority's senior crab research scientist, Danielle Johnston, who said while mud crab sightings in the Swan River were uncommon, they had been reported in the past.

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"All the fisherman who have ever reported mud crabs in the Swan are quite shocked because it is a rare occurrence," Ms Johnston told Di Darmody on ABC Radio Perth.

"Last year, the water temperatures in the metro area were quite cool and it was a late start to summer, so I'm actually quite surprised by this sighting.

"Generally, they occur only in warmer water years where there's a strong Leeuwin current that pushes them down the coast from the Kimberley and the Pilbara region."

Ms Johnston said it was likely the mud crab made the long journey alone.

"There might be more but it's unlikely — they certainly wouldn't be reproducing in the Swan.

"This would just be a lone mud crab that has made its way down the coast."

Bustling with marine life

While the Swan is an inner-city river sandwiched between a busy port at its mouth and constant marine traffic upstream, Mr Curulli said he had no problem eating from it.

"We're lucky to have what we've got in the Swan," he said.

"There's whiting, tailor, herring, flathead; there's blue manna crabs, prawns, bream, mulloway.

"At the end of the day, it is a clean river — it may look dirty but it's a good river to eat from."

Hundreds of fish species live in the 72-kilometre Swan River. ( ABC Radio Perth: Gian De Poloni )

Dolphins are commonly spotted in the Swan, while bull sharks have also been seen on occasion.

"We haven't seen bull sharks, but we have heard of people who catch bull sharks in the Swan," Mr Curulli said.

"Friends of mine have been diving in the Swan for prawns and have caught some nice, big king prawns.

"We've seen squid and I've even heard of people catching Spanish mackerel."

And for the mud crab?

"It was delicious. I highly recommend mud crab, it's beautiful," Mr Curulli said.

"That's why it was a bit of a bonus to catch one in the Swan — we didn't have to travel up to Karratha or Kununurra or Exmouth to try and catch some."