Fish - from the Perspective of Health

High in Omega-3 and Low in Mercury

Fish	Omega-3	Mercury
Salmon Atlantic	2.147	0.015
Salmon King (Chinook)	1.737
Salmon Pink	1.288
Salmon Coho	1.279
Salmon Sockeye	1.23
Salmon Chum (Keta)	1.175
Trout	1.154	0.025
Herring	2.149	0.048
Sardine	0.982	0.010
Anchovy	2.055	0.014
Shad (River Herring)	2.407	0.039
Caviar/Fish Roe	6.541
Mussel	0.782
Oyster	1.376	0.012

The fishes actually belong to only two groups.
1) The Salmonidae family includes salmon, trout, char, and whitefish.
2) The Clupeiformes order includes herring, sardine, shads, menhaden, and anchovy.

It is unfortunate that "shad" can refer to Alosa (river herring), a low mercury fish, or Pomatomus, a high mercury fish. 

High in Mercury

Fish	Mercury
Shark	0.979
Swordfish	0.870
Tuna (except Light)	0.560
Marlin	0.390
Mackerel King	0.730
Mackerel Spanish	0.454
Orange Roughy	0.562
Grouper	0.448
Tilefish (Golden Snapper, White Snapper)	1.450
Chilean sea bass	0.303
Bluefish	0.305
Sablefish (black cod)	0.265
CROAKER WHITE (Pacific)	0.280

references
FDA
//www.fda.gov/food/foodborneillnesscontaminants/metals/ucm115644.htm

//www.health.gov/dietaryguidelines/dga2005/report/html/table_g2_adda2.htm

EWG
//www.ewg.org/research/us-gives-seafood-eaters-flawed-advice-on-mercury-contamination-healthy-omega-3s
//www.pirg.org/toxics/reports/brainfood/brainfoodreport.pdf
Consumer Reports