Deep sea fish – helpful or harmful? Part 2

Author: Ms. Agnes Lam, Nutritionist at HK BioTek

We talked about the pros of eating deep sea fish in the last issue, but we often avoid eating deep sea fish due to its mercury level. So how should we choose which deep sea fish, and how much should we eat, as to benefit from its advantages without intaking excess intake of mercury? The table above can serve as a reference for every smart consumer when choosing deep sea fish.


The mercury level in fish depends on its species, age, foraging mode and habitat. Those which are older or in higher position of the food chain (fish larger in size) usually contains more mercury in their body.


Table 1: Fish traffic light

Red light Green light
Swordfish Salmon
Shark Sardine
Tilefish Atlantic Mackerel
Marlin Flounder
Big-eye Tuna Pollock
Bluefin Tuna Rainbow Trout
King Mackerel Herring
Source: Hong Kong Centre for Food Safety, US FDA, British Columbia Ministry of Health

Those listed in the red light’ column are fish that contains higher average mercury level (Table 2), and should not be frequently consumed. The maximum safe intake level of mercury, set by the Food and Agriculture Organization of the United Nations and the WHO Joint FAO/WHO Expert Committee on Food Additives (JECFA) in 2003, was set at 1.6 microgram per kg weight per week. For example, if an adult of 60kg body weight intake 3 ounces (85g, around size of a palm) tilefish, the mercury level already exceeds the maximum safe level by 30%. As the impact of mercury is particularly susceptible to pregnant women, women preparing for pregnancy and infants, people falling in these groups should avoid eating fish that might contain high level of mercury.


Fish listed in the green light’ column are those with lower average mercury level (Table 2), and can be consumed regularly and in varieties. The American Heart Association recommends weekly intake of deep sea fish for 2-3 times, 3 ounces each to meet the requirement of recommended intake of omega-3.

Table 2: Average mercury level and omega-3 content of different kinds of fish

Species Average mercury level
Omega-3 content
(gram/3 ounces)
Fish with higher mercury level
Swordfish 970 – 1820 0.97
Shark 540 – 1500 0.83
Tilefish 1450 0.90
Marlin 1100 – 1430
Big-eye Tuna 639 – 740 1.36
Bluefin Tuna 730 1.17 – 1.50
King Mackerel 730 0.36
Fish with lower mercury level
Salmon 10 1.10 – 1.90
Sardine 13 1.30 – 2.00
Atlantic Mackerel 50 1.10 – 1.70
Flounder 50 0.48
Pollock 60 0.45
Rainbow Trout 70 0.84 – 0.98
Herring 80 1.70
Source: Hong Kong Centre for Food Safety, US FDA, American Heart Association