Seahorses, stalked jellyfish, dolphins and seagrass meadows are among the marine wildlife gaining better protection with the announcement of 23 new marine conservation zones (MCZ) by the government on Sunday.

However, a leading expert criticised the MCZs as useless “paper parks” that offer no real protection from the dredging and trawling that has devastated large areas of England’s seas for decades.

The 23 new zones stretch from the coast of Northumberland down to Land’s End and include Europe’s longest chalk reef off Cromer in Norfolk. But, with the 27 MCZs designated in 2013, the total of 50 is far below the 127 sites proposed by an earlier £8m government consultation. The 50 MCZs, along with other types of protected areas, now cover 20% of all English waters, almost 8,000 sq miles (20,700 sq km).

“As an island nation, the UK is surrounded by some of the richest and most diverse sea life in the world – from the bright pink sea-fan coral colonies off the south-west coast, to the great chalk reef stretches in the east,” said marine environment minister George Eustice. “It’s vital we protect our marine environment to ensure our seas remain healthy, our fishing industry remains prosperous and future generations can enjoy our beautiful beaches, coastline and waters.”

Professor Callum Roberts, at the University of York and one of the UK’s leading marine conservation experts, welcomed the new MCZs but said: “We need more because the network we have is far from complete. Despite the [50] MCZs, the UK’s rich marine life has very little protection. That may sound paradoxical, but six years after the Marine Act was passed, MCZs are still paper parks. They have no management at all, so life within them remains unprotected. They will be worse than useless, giving the illusion of protection where none is present.”

The government said it was working to ensure management measures are put in place within two years, but Roberts said: “I am deeply skeptical of what it will achieve.” He said, for example, there are already moves to open up a “special area of conservation” in Cardigan Bay to scallop dredging: “It is one of the most destructive fishing methods in the world, turning habitats into rubble and leaving trails of dead and dying creatures in its wake.”

Bloody Henry starfish at the new Holderness Inshore MCZ. Photograph: George Stoyle/Wildlife Trusts

He said 65 “reference areas” of complete protection from fishing, proposed on top of the original 127 MCZs had been abandoned: “The one bit of the network that was really critical was dropped.” Prof Roberts led 86 marine scientists in condemning the government in 2013 for reneging on the recommended 127 MCZs.

Among the newly created MCZs are Mounts Bay, covering St Michael’s Mount and the Marazion where seagrass, stalked jellyfish and crayfish live, and Greater Haig Fras, the only substantial area of rocky reef in the Celtic Sea.

Joan Edwards, head of Living Seas at The Wildlife Trusts, said: “We are pleased by this government’s commitment to addressing the decimation of our seabed over the past century. This second step towards the completion of a ‘blue belt’ in UK seas is crucial but there’s still work to be done.”

She said the 50 MCZs must be properly managed and a third and final tranche of MCZs must be delivered. The government will open a consultation on further MCZs later in January.

Melissa Moore, the Marine Conservation Society’s head of policy, said: “We’re recommending that the final tranche in 2017 includes South Celtic Deep – a site that supports short-beaked common dolphin; Norris to Ryde, which is rich in seagrass meadows; Mud Hole off the north west coast - 35 metres deep and home to rare sea pens - and Compass Rose off the Yorkshire coast, which is an important spawning and nursery ground for herring and lemon sole.”

The government is also set to consult on new special areas of conservation for harbour porpoise and special protection areas to protect feeding and bathing areas used by birds, such as spoonbills in Poole Harbour and puffins on the Northumberland coast.

But Prof Roberts said the low level of protection the government was giving to UK waters was in sharp contrast to its leadership on the international stage: “The UK is giving full protection from fishing to huge areas of our overseas territories in the Atlantic (Ascension Island), Indian (Chagos) and Pacific Oceans (Pitcairn). We urgently need the same high levels of protection in our home waters.”

Kerry McCarthy, Labour’s shadow environment secretary, said: “It is now six years since the last Labour government’s Marine and Coastal Access Act and during that time the government has delayed and prevaricated on delivering a much-needed ecologically coherent network of marine protected areas.

White Beaked Dolphins at the Coquet to St Mary’s MCZ. Photograph: Martin Kitching/Wildlife Trusts

The 23 new Marine Conservation Zones

North Sea

1. Coquet to St Mary’s

2. Farnes East

3. Fulmar

4. Runswick Bay

5. Holderness Inshore

6. Cromer Shoal Chalk Beds

South East

7. The Swale Estuary

8. Dover to Deal

9. Dover to Folkestone

10. Offshore Brighton

11. Offshore Overfalls

12. Utopia

13. The Needles

South West

14. Western Channel

15. Mounts Bay

16. Lands End

17. North-west of Jones Bank

18. Greater Haig Fras

19. Newquay and the Gannel

20. Hartland Point to Tintagel

21. Bideford to Foreland Point

Irish Sea

22. West of Walney

23. Allonby Bay