Joshua Niland has spent enough years working in the kitchens of fine dining restaurants to know the extent of potential food being wasted in the service industry.

"It's a shame, you spend all this money on all this beautiful fish," the 29-year-old head chef and restaurant owner said.

"If you work off the mantra of 40 per cent yield from a fish, then what's the 60 per cent?"

Tackling with the issues of waste and fish sustainability, he opened up his latest culinary offering, Fish Butchery, in the eastern Sydney suburb of Paddington.

Mackerel hung to dry-age inside a cool room at Fish Butchery. ( ABC Radio Sydney: Luke Wong )

The retail store's large cool room and preparation area also serves his established restaurant just a few doors down, Saint Peter, which offers seafood as its main protein.

"Fishmongers have always provided the service of selling fish; we're merging the butchery world into the fish world," he said.

Curiosity driving experimentation

During his early years honing skills under seasoned chefs, Mr Niland began adding up how much seafood by-product could have been put to good use.

Customers can view an open work area where kitchen staff debone whole fish on request. ( ABC Radio Sydney: Luke Wong )

"We got a lot of fish in all the time and I was curious to know what was inside all of them because we were getting so much of a particular species," he said.

"I kept seeing liver, kept seeing roe, kept seeing eyeballs, and I continued to weigh them and put a price tag to them."

It inspired him to experiment with the parts that were normally discarded.

"We started developing recipes that were delicious for a Western palate, and from that I just became fascinated in the price point and what we were actually putting in the bin."

Mr Niland offers up what he calls underutilised fish such as leatherjacket, garfish, bream and small whiting. ( ABC Radio Sydney: Luke Wong )

Changing consumer habits

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Mr Niland actively hooks onto fishers who use sustainable practices and opts for a combination of varieties — either line or net-caught — sourced locally and interstate.

He also promotes the seasonal use of certain types and is championing diverse and underutilised fish such as leatherjackets, tommy ruff, garfish, bream and small whiting.

"Just so it takes the pressure off some of the more frequently used species," he said.

"We hope that we can encourage people, with the right advice, and all the little tedious bones removed, then they'll enter our pans and our homes again."

He also sees the potential his online presence can have in influencing consumer tastes further afield.

"If we take a few photos of blue mackerel and that reaches the 35,000 [Instagram followers] that I have the privilege to post to, and then see it on a few restaurant menus ... that just filters out and trickles down."

Serving up sustainability

Sajad Akhlaghi is also passionate about offering sustainable fish varieties on the menu at his restaurant, Fish & Co, in Forest Lodge.

He is driven by his concerns about ocean pollution and overfishing, particularly in fisheries where stock numbers and catch methods are poorly regulated.

Sajad Akhlaghi is comfortable with serving customers a limited variety of fish to meet his criteria of sustainability. ( ABC Radio Sydney: Luke Wong )

"I just hope there is fish for the future generations," Mr Akhlaghi said.

"The way it is going with these massive boats absolutely clearing the ocean of the fish, the future is looking bleak."

To meet his criteria of sourcing sustainably caught wild fish, he serves species including hoki and hake from New Zealand and salmon from Alaska.

But he is also a fan of local varieties such as leatherjacket caught on the north Sydney coast.

"You want to use the type of fish that is plentiful and you only use a certain amount of it," he said.

"It has to be caught in the right numbers in the right part of the ocean at the right time of the year; there's a lot involved."

He urged people to ask restaurants and shops where their fish was sourced from, which in turn forced service providers to be more informed about their offerings.