There’s Always a Bigger Fish

A Rapidly Changing Fish Market

Like poultry, pork, and beef, seafood is a source of protein.  But, unlike the other three categories of animal flesh that are consumed as food, there is far more variety among fish than chickens, pigs, or cattle.  This is largely because many fish are still wild-caught from the oceans.  But that’s not as true as it used to be; the majority of fish on the market are now produced by aquaculture (i.e. farm-raised), and the percentage of aquaculture seafood on the market is expected to increase as declining stocks of wild-caught fish are replaced with those that are farm-raised.  Major species raised by aquaculture include catfish, crawfish, mussels, salmon, scallops, shrimp, tilapia, and trout (FAO, 2016).   But, even though there are many varieties of seafood raised as aquaculture, there still are many more different varieties that are wild caught.

For the purposes of a discussion of the pluses and minuses of consuming fish while pregnant, there are two main characteristics of fish that are of interest.  The first is methylmercury.  Even though some methylmercury can be found in just about all seafood, the amount varies tremendously.  Since it bioaccumulates, higher methylmercury levels tend to occur in predatory fish that are higher up in the food chain.   Among different individual animals of the same species, larger older fish tend to have higher methylmercury levels than those that are younger.  However, because wild-caught fish supplies are dwindling, the supply of high mercury fish is too.  Since the aquaculture fish that are replacing them are lower down in the food chain and are harvested when they are young, they tend to have uniformly low methylmercury levels.  However, they can have higher levels of other contaminants like antibiotics and dioxins.

In addition to mercury, fish can be an important source of many nutrients.  Besides protein, the most noted of these are omega-3 fatty acids.  There have many studies on potential nutritional effects of isolated omega-3 fatty acids (i.e. fish-oil capsules), and most of the results are negative.  Perhaps the best evidence comes from epidemiological studies of fish consumptions where modest levels of fish consumption appear to slightly reduce the incidence of stroke, cardiovascular disease, and perhaps other outcomes like neurobehavioral development (FDA, 2014).  So, omega-3 fatty acids, or possibly some other constituent in fish, probably do not correct a common nutritional deficiency, but it does appear that they may address  a nutritional deficiency that is only found in people who consume little or no fish.

A Survey of Fish Species Sold in the United States

The FDA has been collecting samples from the U.S. market since the 1990.  While early effects focused only on shark and swordfish, the current data base has over 60 categories of fish.  The table below shows estimated average methylmercury for 51 categories, which as of 2012 included over 99% of all species sold in the United States.  However, the FDA database hasn’t been updated since 2010, and the fish on the market are constantly changing, so this table is not exactly current.  But, it will have to do for now.

Also included in the table are omega-3 concentrations.  Most of these are taken from the USDA nutrient database.  While the FDA methylmercury concentrations are reported with ranges (and raw data if you like), the USDA only reports average values.  In order to emphasize the nutrient value of each category, they are sorted by the ratio omega-3 concentrations to methylmercury concentrations.  If you consume fish on a regular basis anyway, then the omega-3s are probably not really an issue, so you may be more interested in just methylmercury concentrations.  They are sorted that way on the FDA web summary and in the FDA risk-benefit report (FDA, 2014).

Fish Category	Hg (µg per g)	Total Ω-3 g/100g	µg Hg per g Ω-3
Sardines	0.02	1.19	1.7
Salmon	0.02	1.18	2.0
Oysters and Mussels	0.02	0.70	2.1
Anchovies, Herring, and Shad	0.05	2.02	2.5
Shrimp	0.01	0.35	3.1
Trout, Freshwater	0.03	0.93	3.4
Scallops	0.01	0.19	3.7
Mackerel, Atlantic and Atka	0.05	1.20	4.1
Mackerel, Chub	0.09	1.25	7.0
Pollock	0.04	0.53	7.0
Smelt	0.07	0.89	7.5
Catfish	0.02	0.22	7.6
Butterfish	0.06	0.73	8.0
Whitefish	0.10	0.91	11.0
Clams	0.02	0.20	11.6
Tilefish, Atlantic	0.11	0.91	12.2
Squid	0.07	0.54	12.9
Tilapia	0.01	0.09	14.3
Crabs	0.06	0.38	16.6
Sablefish	0.37	1.81	20.4
Crawfish	0.03	0.16	20.7
Lobsters, Spiny	0.11	0.48	22.9
Flatfish	0.08	0.30	25.3
Bass, Saltwater	0.25	0.97	25.9
Mackerel, Spanish	0.37	1.25	29.7
Halibut	0.22	0.71	31.4
Bluefish	0.35	0.99	35.4
Carp and Buffalofish	0.17	0.45	37.7
Croaker, Atlantic	0.08	0.20	38.6
Tuna, Albacore Canned, Water	0.35	0.86	40.6
Haddock, Hake, and Monkfish	0.07	0.16	41.9
Bass, Freshwater	0.32	0.76	41.9
Trout, Saltwater	0.26	0.62	42.0
Tuna, Light Canned, Water	0.12	0.27	44.4
Skate	0.14	0.30	45.7
Perch, Ocean and Mullet	0.16	0.32	49.4
Perch, Freshwater	0.15	0.29	50.9
Pike	0.14	0.27	52.3
Cod	0.09	0.16	55.6
Lobsters, American	0.11	0.20	56.4
Tuna, Fresh	0.39	0.65	59.9
Snapper, Porgy, and Sheepshead	0.16	0.26	62.6
Marlin	0.49	0.50	98.0
Croaker, Pacific	0.30	0.30	100.0
Lingcod and Scorpionfish	0.29	0.26	108.7
Swordfish	1.00	0.90	111.2
Shark	0.98	0.69	142.2
Tilefish, Gulf	1.45	0.80	181.3
Mackerel, King	0.73	0.40	182.0
Grouper	0.46	0.25	185.5
Orange Roughy	0.57	0.03	1838.7

Software

One of the problems with public health advice is that it often doesn’t take into account other things that individual consumers want.  Maybe you would rather eat more fish, or less.  Maybe you prefer tuna to salmon.  Maybe fish is too expensive.  So, instead of handing out advice I’m going to concentrate on giving out information.  Then you can figure out whether or not it’s worth changing your fish eating habits for yourself.  The first step is figuring out how your fish consumption choices affect how much methylmercury and omega-3 are in your diet.

Personal_Seafood_and_MeHg_Intake_Estimator.xls

References

Food and Agriculture Organization (2016).  Aquaculture Fact Sheets.

U.S. Food and Drug Administration (2014). Quantitative Assessment of the Net Effects on Fetal Neurodevelopment from Eating Commercial Fish (As Measured by IQ and also by Early Age Verbal Development in Children).  

Official Post Soundtrack

Murphy, Peter (1989).  Deep Ocean Vast Sea.  In: Deep, Track 1.

Post  Notes

Thesis Post #56.  The is the first of what I think will be a five part series that describes a individualized version of the risk-benefit model I created for the FDA [part two was the last post, so part three will be next].  I'm putting it out in parts largely to provide documentation of the model -- the code is all open.  In the end, I will combine them all into a single program.