Status of mercury and other heavy metals in South African marine fish species

Bosch, Adina Cornelia (2015-12)

Thesis (PhD)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Fish is an important food source in South Africa as it is globally, thus the importance of determining and monitoring its safety in terms of metal contaminants and consumer safety. Effective methodology for analysing total metal concentrations and toxic metal components together with representative sampling protocol for sampling individual fish and larger catches are therefore required for accurate assessment of meat safety. Both inductively coupled plasma mass spectrometry (ICP-MS) and high pressure liquid chromatography coupled to ICP-MS was validated as effective methods for accurately determining concentrations of total metals and individual Hg species, respectively. Consequently, it was found that total mercury loads in fish meat consisted mainly of toxic methylmercury (MeHg) components with a minor addition of an inorganic Hg (iHg) component and ethylmercury concentrations being negligible. This proportion of MeHg to iHg varied between muscle types in yellowfin tuna (Thunnus albacares), with higher iHg concentrations in dark muscle than in white muscle, whereas the toxic MeHg concentrations did not vary across the carcass. The MeHg to tHg relationship was caused to vary with variation in fish weight, being described by the following prediction model: 𝑐𝑀𝑒𝐻𝑔=0.073+1.365βˆ™𝑐𝑡𝐻𝑔−0.008βˆ™𝑤; taking into account fish weight as covariate. For the eight other fish species studied [blacktail (Diplodus sargus capensis), hottentot (Pachymetopon blochii), yellowtail (Seriola lalandi), snoek (Thyrsites atun), blue shark (Prionace glauca), shortfin mako (Isurus oxyrinchus), soupfin (Galeorhinus galeus) and smoothhound (Mustelus mustelus)], the relationships between MeHg and tHg were constant even with varying fish sizes as iHg components were considered an insignificant portion of tHg (ctHg = cMeHg). The tHg measurements could therefore be used as accurate indicators of MeHg concentrations without requiring addition speciation analyses. Sampling from the cephalic region of the dorsal white muscle tissue proves representative of the entire edible portions (white muscle) of larger fish (tuna and sharks spp.) for determining both total metal concentrations and toxic Hg components. Where Hg concentrations are positively correlated to fish size (yellowfin tuna, yellowtail and soupfin), subsamples should include individuals representing the entire size range present per catch. A summary of metal concentrations in all eight species studied indicate that Hg is the main metal of concern where a single portion of certain fish (yellowfin tuna, shortfin mako, soupfin and smoothhound shark) consumed per week could exceed regulatory limits for safe Hg intake, whereas other fish species (hottentot) could be consumed daily without concern of Hg toxicity. Information provided by this study will prove useful to both the fishing and processing industry as well as to health authorities providing information for dietary exposure assessments.

AFRIKAANSE OPSOMMING: Vis word gesien as ‘n belangrike voedsel bron in Suid-Afrika sowel as globaal en die bepaling en monitering van visveiligheid in terme van metaal kontaminasie is dus van uiterse belang. Effektiewe metodes vir die analise van totale-metaal konsentrasies en protokol vir verteenwoordigende steekproefneming van enkele visse sowel as van groter vangste is dus belangrik vir die akkurate bepaling van voedselveiligheid in die visbedryf. Beide ICP-MS en HPLC-ICP-MS is onderskeidelik gevalideer as effektiewe metodes vir die akkurate bepaling van totale-metaal konsentrasies en indiwiduele kwik (Hg) spesies. Gevolglik is daar gevind dat totale Hg ladings grootliks bestaan uit metiel-kwik (MeHg) met kleiner bydraes van anorganiese kwik (iHg). Die bydrae van etiel-kwik (EthHg) is egter nietig bevind. Die proporsie MeHg tot iHg het verskil tussen spier-tipes in geelvin tuna (Thunnus albacares) met hoër iHg konsentrasies in die donker spier as in die ligte spier. Die konsentrasies van toksiese MeHg toon egter nie verskille tussen die twee spier tipes of regdeur die viskarkas nie. Die verhouding tussen MeHg en tHg in visse het wel verskille getoon soos die gewig van die visse verskil en kan beskryf word deur die volgende vergelyking: 𝑐𝑀𝑒𝐻𝑔=0.073+1.365βˆ™𝑐𝑡𝐻𝑔−0.008βˆ™𝑤, waarin die gewig van die vis in ag geneem word. Vir al agt ander visspesies (kolstert (Diplodus sargus capensis), hottentot (Pachymetopon blochii), geelstert (Seriola lalandi), snoek (Thyrsites atun), blou haai (Prionace glauca), shortfin mako (Isurus oxyrinchus), vaalhaai (Galeorhinus galeus) en spierhaai (Mustelus mustelus)) is die iHg komponent beskou as onbeduidend en dus is ‘n konstante verhouding gevind tussen MeHg en tHg onafhalklik van visgewig. Die metings van tHg kan gevolglik gebruik word as ‘n akkurate indikasie van die MeHg hoeveelhede teenwoordig sonder aparte metings van individuele Hg spesies. Steekproefneming vanaf die voorste deel van die dorsale witspierweefsel blyk verteenwoordigend te wees van die totale eetbare weefsel (wit spiere) van groter visse (tuna en haai spp.) vir die bepaling van totale-metaal konsentrasies sowel as toksiese Hg komponente. Waar Hg konsentrasies positief gekorreleer is met visgrootte (geelfin tuna, geelstert en vaalhaai), moet visse van verskillende groottes geneem word as steekproefmonsters om verteenwoordigend te wees van alle grootte visse in die vangs. ‘n Opsomming van die metaal konsentrasies wat in al agt visse getoets is, dui daarop dat Hg die metaal is wat die grootste moontlike gevaar inhou in visvleis waar selfs enkele porsies van seker visse (geelfin tuna, shortfin mako, vaalhaai en spierhaai) die weeklikse limiet van Hg inname kan oorskry. Ander visspesies (hottentot) kan weer daagliks geëet word sonder gevaar van toksiese Hg inname. Die huidige studie verskaf waardevolle inligting vir beide die visindustrie en gesondheidsowerhede waar dit kan bydra tot die bepaling van diëet-bloodstellingsperke.

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