in canada , fish contribute to over 50% of the dietary intake of mercury ( hg ) by adults ( dabeka et al . 2003 ) , and fish containing the highest concentrations of hg are predatory fish , such as shark , swordfish and tuna ( health canada 2007 ) . hg , present in fish as both methyl - hg and inorganic hg , is a potent neurotoxicant . on the other hand , fish are one of the best food sources of vitamin d and omega-3 fatty acids , dha and epa , and it is important to weigh the benefits of eating fish against the potential toxicity of hg present in the fish ( health canada 2007 ) . to promote consumption of fish while at the same time providing advice to vulnerable population groups about limiting consumption of predatory fish , and to evaluate the risk - to - benefit ratio of eating fish , it is important to know the levels of hg in predatory fish sold at the retail level in canada . in 2002 , 2004 ) which found that predatory fish , such as shark , swordfish and tuna , contained high concentrations of hg , averaging 1820 ng g for shark , 1430 ng g for marlin and 930 ng g for fresh or frozen tuna . this survey is a supplement to the previous work and provides levels of hg found in retail fish purchased in 2005 . marlin , sea bass , red snapper , orange roughy , fresh water trout , grouper , black cod ( sablefish ) , arctic char , king fish ( king mackerel ) and tilefish are included in this survey . , samples were purchased as cleaned fillets or whole fish at the retail level in toronto , vancouver and montreal in 2005 . the retail stores included four supermarket chains and four specialty fish seafood outlets in each city . each sample was shipped frozen to maxxam analytics inc . for cleaning ( if necessary ) , homogenization and bottling in pre - cleaned polyethylene bottles supplied for total hg and glass jars with teflon cap liners for methyl hg ( i - chem brand , thermo fisher scientific ) . the samples were then frozen and shipped to health canada 's food research division in ottawa for analysis . information provided with each sample included , where available , the species name as provided by the retail outlet , the state of the sample when purchased ( frozen , fresh or previously frozen ) , the city of sample pickup , name of store where purchased , country of origin and date of collection . the countries of origin for the samples were usa ( 7 ) , argentina ( 1 ) , australia ( 5 ) , canada ( 26 ) , chile ( 13 ) , hungary ( 2 ) , mexico ( 1 ) , new zealand ( 3 ) and taiwan ( 1 ) , with the remaining samples being of unknown origin . analyses were performed using the reagents , instrumentation and methodology described in dabeka et al . briefly , after a low - temperature nitric / hydrochloric acid and hydrogen peroxide digestion of roughly 1 g fish tissue and dilution to 50 ml with water , measurements were made using a cetac-6000a dedicated hg analyser equipped with an asx-500 autosampler and adx autodilutor . quality control measures for each analytical batch included three reagent blanks , two reagent blank spikes ( 200 ng hg ) , one sample spike of 400 ng hg ( in duplicate for both the unspiked and spiked 400 ng sample ) and duplicates of three different standard reference materials ( srms ) with certified hg concentrations . two cross - check standards ( from a different manufacturer ) were included in each batch for standard verification during the run . the solution limit of detection ( lod ) was estimated for each analytical batch by multiplying the standard deviation of the three reagent blanks by 3 . sample lods were calculated by multiplying the solution lod by the dilution volume and dividing by the weight of the actual sample taken for analysis . solution lods averaged 0.045 ng ml and sample lods averaged 1.9 ng g. recoveries from spiked blanks and samples averaged 97 and 101% , respectively . one of the sample spike recoveries was high ( 155% in batch 8) , and this was due to the high concentration of hg in the sample selected for spiking in the batch compared with the spike added . agreement with certified levels in the national research council of canada ( nrcc ) dolt-2 and dorm-2 srms and the national institute of standards and technology ( nist ) oyster tissue 1566a srm was generally satisfactory , although results for batches 9 , 10 and 11 were unusually high . blank spaces in the tables denote that the particular test was not included in the batch . note : blank spaces denote that the particular test was not included in the batch . , samples were purchased as cleaned fillets or whole fish at the retail level in toronto , vancouver and montreal in 2005 . the retail stores included four supermarket chains and four specialty fish seafood outlets in each city . each sample was shipped frozen to maxxam analytics inc . for cleaning ( if necessary ) , homogenization and bottling in pre - cleaned polyethylene bottles supplied for total hg and glass jars with teflon cap liners for methyl hg ( i - chem brand , thermo fisher scientific ) . the samples were then frozen and shipped to health canada 's food research division in ottawa for analysis . information provided with each sample included , where available , the species name as provided by the retail outlet , the state of the sample when purchased ( frozen , fresh or previously frozen ) , the city of sample pickup , name of store where purchased , country of origin and date of collection . the countries of origin for the samples were usa ( 7 ) , argentina ( 1 ) , australia ( 5 ) , canada ( 26 ) , chile ( 13 ) , hungary ( 2 ) , mexico ( 1 ) , new zealand ( 3 ) and taiwan ( 1 ) , with the remaining samples being of unknown origin . analyses were performed using the reagents , instrumentation and methodology described in dabeka et al . ( 2002 ) . briefly , after a low - temperature nitric / hydrochloric acid and hydrogen peroxide digestion of roughly 1 g fish tissue and dilution to 50 ml with water , measurements were made using a cetac-6000a dedicated hg analyser equipped with an asx-500 autosampler and adx autodilutor . quality control measures for each analytical batch included three reagent blanks , two reagent blank spikes ( 200 ng hg ) , one sample spike of 400 ng hg ( in duplicate for both the unspiked and spiked 400 ng sample ) and duplicates of three different standard reference materials ( srms ) with certified hg concentrations . two cross - check standards ( from a different manufacturer ) were included in each batch for standard verification during the run . the solution limit of detection ( lod ) was estimated for each analytical batch by multiplying the standard deviation of the three reagent blanks by 3 . sample lods were calculated by multiplying the solution lod by the dilution volume and dividing by the weight of the actual sample taken for analysis . solution lods averaged 0.045 ng ml and sample lods averaged 1.9 ng g. recoveries from spiked blanks and samples averaged 97 and 101% , respectively . one of the sample spike recoveries was high ( 155% in batch 8) , and this was due to the high concentration of hg in the sample selected for spiking in the batch compared with the spike added . agreement with certified levels in the national research council of canada ( nrcc ) dolt-2 and dorm-2 srms and the national institute of standards and technology ( nist ) oyster tissue 1566a srm was generally satisfactory , although results for batches 9 , 10 and 11 were unusually high . blank spaces in the tables denote that the particular test was not included in the batch . note : blank spaces denote that the particular test was not included in the batch . a summary of the results ( table 2 ) found that arctic char , red snapper and fresh water trout contained the lowest concentrations of hg , averaging 37 , 148 and 55 ng g , respectively . average levels in the other fish types varied from 384 ng g for black cod to 854 ng g for marlin . note : designated fish subject to a 1000 ngg regulatory limit , others to a limit of 500 ngg average concentrations of hg in the different species were in general agreement with those found in other canadian and international surveys ( table 3 ) . differences among the surveys can be attributed to the size of the individual fish from which the sample was taken , as fish mercury concentrations vary directly with fish size . additionally , regional differences in hg concentrations may be a function of the food source . for example , tilefish caught in the atlantic ocean contained 144 ng g hg , whereas those sampled in the gulf of mexico averaged 1450 ng g ( fda 2009 ) . it is uncertain whether this difference is due to different food sources or to the situation that the tilefish in the two areas are different species . comparison of hg levels ( ngg ) found in this study with those found previously in canada and other countries . average of three tilefish - two samples purchased in 2008 containing 128 and 3000 ng g , and one purchased in 2005 contained 80 ng g. fda ( 2009 ) ; environmental protection agency ( 2000 ) ; national marine fisheries service ( 1978 ) . the primary reference for the gulf tilefish and sablefish data in the fda report is a national marine fisheries service report ( hall et al . 1978 ) , while that for the kingfish or king mackerel is an epa report ( ache et al . 144 ng g in atlantic tilefish and 1450 ng g in those caught in the gulf of mexico ( fda 2009 ) . the current canadian regulatory limit for total mercury in the edible portion of commercially sold fish is 500 ng g , with the exception of a 1000 ng g limit and accompanying consumption advice ( health canada , 2008 ) for specific species of piscivorous fish : shark , swordfish , marlin , orange roughy , escolar , and fresh and frozen tuna ( health canada , 2010 ) . the proportion of individual samples exceeding the relevant standards was : 13% of black cod , 24% of grouper , 40% of king fish , 28% of marlin , 20% of sea bass and 50% ) of tile fish ( table 2 ) . however , the uncertainty in these percentages is high due to the small numbers of each species tested . the mean and median concentrations of mercury in black cod , grouper , king fish , marlin and sea bass were all below the standards for the respective species . additional data would aid in characterizing the typical mercury concentrations in some types of fish , in particular king fish and tilefish . for example , the us fda has reported a median mercury value in tilefish ( atlantic ) of only 99 ng g ( n = 32 ) ( fda , 2009 ) . it should also be noted that the occasional consumption of infrequently consumed fish containing mercury levels greater than the regulatory limits would not be expected to pose a health risk to consumers . thanks are due to elizabeth elliott and mark feeley of the chemical health hazard assessment division , bureau of chemical safety for review and helpful suggestions .
total mercury was analysed in 188 samples of predatory fish purchased at the retail level in canada in 2005 . 
 the average concentrations ( ng g1 , range ) were : sea bass 329 ( 381367 ) , red snapper 148 ( 36431 ) , orange roughy 543 ( 279974 ) , fresh water trout 55 ( 20430 ) , grouper 360 ( 81060 ) , black cod 284 ( 71651 ) , arctic char 37 ( 2854 ) , king fish 440 ( 42923 ) , tilefish 601 ( 791164 ) and marlin 854 ( 1252346 ) . 
 the canadian standard for maximum total mercury allowed in the edible portions of fish sold at the retail level is 1000 ng g1 for shark , swordfish , marlin , orange roughy , escolar and both fresh and frozen tuna . 
 the standard is 500 ng g1 for all other types of fish . in this study , despite the small number of samples of each species , the 1000 ng g1 maximum was exceeded in five samples of marlin ( 28% ) . 
 the 500 ng g1 maximum was exceeded by six samples of sea bass ( 20% ) , four of tilefish ( 50% ) , five of grouper ( 24% ) , six of king fish ( 40% ) and one of black cod ( 13% ) ,