Fish have long been a darling of the diet world. Praised for being a filling yet low-fat source of protein, the likes of salmon, cod and sea bass are recommended by nutritionists across the globe.

The NHS advises we eat at least two portions per week to reach our quota of omega-3 fats, which are beneficial for heart and brain health.

But are some types better than others? Yes, according to dieticians.

Speaking to the Daily Mail, experts have revealed that some kinds are much lower in fat, and therefore better for those with pounds to lose. Others, meanwhile, could even be harmful when eaten regularly.

Some species, such as mackerel for instance, contain far higher quantities of vital nutrients many of us are lacking, such as vitamin B12, according to dietician Sophie Medlin, a consultant dietician and founder of City Dieticians. So which should you choose for your fish supper? 

Per typical fillet: 214 calories, 15.7g protein, 16.8g fat, 4.4g sat fat, 0.15g salt

Mackerel is a fast-swimming oily fish that can be found across the UK and European coastlines, and belongs to the same Scombridae family as the tuna. Experts say it's among the healthiest fish to eat.

Its star quality is the abundance of omega-3 fatty acids – more than any other fish and almost twice as much as you'll find in an average salmon fillet. 

A comprehensive Harvard study published in the early Noughties found that eating two mackerel fillets per week could reduce the risk of heart attack death by 36 per cent. 

Ms Medlin points out that, like salmon, mackerel is high in fat. And unlike other fish, a significant proportion – around a third – is saturated fat, the type that can damage arteries.   

However, this is still less than a quarter of a woman's daily limit and a fifth of a man's, meaning, for most people, the fat is worth the benefits. 

Verdict: 4.5/5 

Per typical fillet: 112 calories, 25.1g protein, 1.3g fat, 0.35g salt

A bottom-dwelling fish with white, flaky meat that’s often a favourite to use for fish and chips, it’s often imported from overseas – mainly from China, Iceland and Norway.

Experts say cod – baked or roasted, rather than deep fried – is a great choice for those on a low-calorie diet, looking to lose weight. 

'Cod is an excellent lean protein source,' says Ms Medlin. There's roughly twice the filling protein in cod, compared to mackerel, meaning it's low risk for after-dinner hunger pangs.

Ms Medlin adds it also contains uniquely high levels of iodine, which is essential for fuelling the hormones that control metabolism and regulate heart rate. 

It is also critical for proper brain and nervous system development during pregnancy and early childhood. 

'Cod's main disadvantage is that it contains much less omega- 3,' says Ms Medlin. 'And most people aren't getting enough omega-3 in their diet.' It contains roughly ten times less than the equivalent portion of salmon, according to the University of California Davis Department of Nutrition. 

Verdict: 4/5 

Per typical fillet: 149 calories, 17.9g protein, 8.4g fat (almost all unsaturated), 0.11g salt

Seabass are native to the North East Atlantic Ocean, Mediterranean Sea, and the Black Sea – although they can sometimes be found in freshwater rivers.

If you prefer white fish to mackerel, seabass is a great option due to its high protein count and hefty omega-3 content, says Ms Medlin. 

'You generally get more omega-3 here than you do with other white fish,' she adds.

However, there could be risks involved with eating seabass several times a week. 'Seabass often contains more mercury than other fish such as salmon and cod,' says Ms Medlin. 

Mercury enters water from sources such as coal burning, mining, industrial processes and waste, and it is then absorbed by small, underwater organisms.

Fish eat these organisms, causing mercury to build up in their tissues. Certain fish – usually those that are larger and higher up the food chain – contain higher traces than others. 

The chemical is known to be toxic to humans at certain levels, causing problems with vision, coordination and memory. 

'Mercury levels in seabass are usually low enough that you could eat it, say, once every week,' says Ms Medlin.   

Verdict: 3/5

Per typical fillet: 197 cals, 23.8g protein, 11.3g fat, 0.12g salt

As an oily fish, salmon is high in omega-3 fatty acids, which are associated with lower risk of chronic disease and protecting brain cells from age-related damage. 

Interestingly, studies have shown that the benefits of these fats are only delivered when they're delivered via food, rather than supplements.

According to Ms Medlin 'salmon is an excellent all-rounder'.

'It is high in omega-3, protein, vitamin D, B12, and selenium.'  It's estimated that up to a third of older Britons are lacking in B12 – which is essential for healthy blood flow and energy levels. 

Vitamin D deficiency is also common. The nutrient is vital for helping our bones absorb strengthening calcium. 

Salmon does, however, contain roughly ten times the fat of fresh tuna – although 80 per cent of it is the unsaturated, healthier kind.

This makes it higher in calories than low-fat options like sea bass and cod. 

'But given the additional nutrients it supplies, it's worth the extra calories, says Ms Medlin. For instance, the protein in the fish contains all nine essential amino acids, meaning our body can easily use it for energy and muscle building.

Verdict: 5/5 

Per typical 'steak' fillet: 144 calories, 31.9g protein, 1.6g fat, 0.09g salt 

Large, fast-swimming saltwater fish that can be found in the Indian, Pacific and Atlantic Oceans, tuna are members of the mackerel family.

Its meat is firm and dense, and it has a mild, savoury flavour when cooked – or a rich, buttery taste when eaten raw.

Ms Medlin rates it lower than other fish, explaining that, 'although a lean protein sauce that's rich in selenium and B12, mercury contamination is a concern.' 

'Larger tuna species like yellowfin and Atlantic bluefin contain particularly high levels,  like so we need to be conscious of how much we consume regularly.'

 However studies show tinned tuna contains far lower quantities of mercury, compared to fresh. 

Ms Medlin adds: 'While fresh tuna contains some omega-3, it's far, far less than mackerel and salmon, although slightly more than you'll find in cod.'

Verdict: 3/5 

Per serving of king prawns: 51 calories, 11 protein, 0.3g fat, 1.19g salt 

Not technically fish, but instead crustaceans, prawns can be found in freshwater and seawater, depending on their species.

Unlike most fish, they’re also more likely to trigger an allergic reaction – with between 0.5 per cent to 2.5 per cent of the population believed to have a shellfish allergy.

While they're a very low calorie, low fat choice, they're the highest in salt of all the options analysed.

Although a small portion makes up only a sixth of your daily salt intake, this could quickly add up as you often have to eat large amounts of prawns to feel full, experts say. 

'But they do contain some omega-3, although much less than oily fish,' says Ms Medlin.

'The problem with prawns is they're often served breaded or fried, which counters some of the health benefits, so make sure they're steamed.'

Verdict: 3/5 

 *Nutrition information taken from tesco.com