freshwater fishes contain saturated fatty acids ( safas ) , monounsaturated fatty acids ( mufas ) , and long - chain polyunsaturated fatty acids ( pufas ) that have significant role in human health .
polyunsaturated fatty acids ( pufas ) are particularly important due to their ability to prevent cardiovascular disease , psychiatric disorders , and some other illnesses such as atherosclerosis , thrombogenesis , high blood pressure , cancer , and skin diseases .
pufas are commonly categorized into two main groups omega 3 ( 3 ) and omega 6 ( 6 ) depending on the position of the first double bond from the methyl end group of the fatty acid .
the main 3 pufas playing important role in human health include -linolenic acid ( ala ) , docosahexaenoic acid ( dha ) , eicosapentaenoic acid ( epa ) , and docosapentaenoic acid ( dpa ) and 6 pufas include linoleic acid ( la ) and arachidonic acid ( ara ) .
these pufas are not synthesized in the human body and therefore inclusion in human diets is a necessity . ultimately , it is important to take a proactive approach to ensure sustained access and uptake of pufas for proper maintenance of our health .
the variation in fa of fish is due to diet consumed , reproductive cycle , temperature , season , and geographical location [ 47 ] .
nile perch lates niloticus , nile tilapia oreochromis niloticus , tilapia zillii , and rastrineobola argentea fishes constitute great resources for communities living around lake victoria .
the fishes are widely distributed throughout the lake representing the most fished species for human consumption .
they have significant commercial and ecological roles in the lake ecosystem   and aquaculture potential particularly for oreochromis niloticus and tilapia zillii [ 10 , 11 ] .
the nutritional quality of these species in the area is important for rural communities since they are crucial diets and provide livelihoods subsequently influencing community health .
previous studies on fas of fish species in lake victoria have concentrated on growth stages and general proximate analysis particularly in kenyan and ugandan waters of lake victoria .
kizza et al .   analyzed the lipid content and fa profiles of lates species whereas turon et al .  
the fa compositions of muscle and heart tissue of nile perch , lates niloticus , and nile tilapia , oreochromis niloticus , in lakes victoria and kioga have been investigated by kwetegyeka et al . .
mwanja et al .   characterized the fish oils of mukene ( rastrineobola argentea ) of nile basin waters , lake victoria , lake kyoga , and the nile river .
established the fa profiles of the eggs and juvenile muscle of nile perch ( lates niloticus ) . the use of fa profile of the polar fraction as a taxonomic marker for nile perch lates niloticus , nile tilapia oreochromis niloticus , marbled lungfish protopterus aethiopicus , bagrus docmak , and african catfish clarias gariepinus was compared by kwetegyeka et al . .
the effects of heavy metal pollution on 3 pufas levels in tilapia fish from winam gulf of lake victoria were explored by muinde et al . .
the proximate composition of rastrineobola argentea ( dagaa ) of lake victoria - kenya has been analyzed by ogonda et al . .
although these studies have been conducted in lake victoria , none was carried out in the tanzanian waters .
moreover , there is lack of information on the composition and levels of pufas of freshwater fish species at different trophic levels in lake victoria .
limited literature is available on the fa profile of tilapia zillii although it is among the most consumed fish around lake victoria .
the only data available in this species is that from olagunju et al .   in nigeria who analyzed the nutrient composition of tilapia zillii .
the present study investigated the types and levels of 3 pufas in four different species , nile perch ( lates niloticus ) , nile tilapia ( oreochromis niloticus ) , tilapia zillii , and dagaa ( rastrineobola argentea ) in mwanza gulf of lake victoria .
a total of 48 individual species of lates niloticus , oreochromis niloticus , tilapia zillii , and rastrineobola argentea were collected from mwanza gulf and morphologically identified using keys given by eccles   and skelton .
after collection each sample was stored in plastic bags and preserved by using dry ice while still in the boat and later frozen at 20c .
frozen samples were shifted by airplane to the zoology laboratory at the university of dar es salaam for analysis . in the laboratory , the fish samples were thawed to remove the ice for easy cutting of tissues .
each individual fish was cut , starting from the upper part below the dorsal fin down to the abdomen .
sample tissues were washed in water to remove blood and dried with a tissue paper to remove excess water . for each sample
extraction of lipid was done by using methanol and chloroform at a ratio of 2  :  1 and mixed by a vortex for 2 minutes .
the samples were then stored in a refrigerator for two days to speed up extraction of lipid .
addition of extra 1  :  1 methanol to chloroform was done to extract the remaining lipids from the tissues .
the two layers formed by fish tissues ( lipids and aqueous solution ) were separated by using a separating funnel to obtain lipids followed by addition of sodium sulphate to remove traces of water from the lipids .
evaporation to remove chloroform was done locally in air conditioned room at 16c for 24 hours .
five ( 5 ) mg of lipid was suspended in 1  ml toluene prior to derivatization .
then , 2  mls of methanoic sulphuric acids ( 1% v / v ) was added to each sample in vials and sealed .
the samples were heated in a stopper tube at a temperature of 50c overnight for 16 hours to speed up the reaction .
this was followed by addition of 2  mls of water containing sodium bicarbonate ( 2% : w / v ) to each sample to neutralize the acids .
extractions of product were done by addition of hexane / diethyl ester ( 1  :  1 , by vol ; 2  5  m ) .
evaporation to remove acids was done locally in air conditioned room at 16c for 48 hours .
the determination of types and levels of omega 3 pufas was done by using a gas chromatograph mass spectrometer ( gc - ms - qp2010 ultra ) , which was equipped with flame ionization detector ( fid ) .
1  l of fame in hexane was injected into the gc - ms in a split ratio 1.0 .
helium was used as the carrier gas at a flow rate of 2  ml / min .
temperature was programmed as follows : column oven was set at 90c , held for 2 minutes , and then increased to 260c and held for 5 minutes and total time was 41 minutes .
the 3 pufas ( ala , epa , and dpa ) were identified by comparing their retention time with those of commercial standards .
the data is presented as mean  standard error and was tested for homogeneity of variances using levene 's test
. then the data was analyzed by using one way analysis of variance ( anova ) to compare the levels of 3 pufas .
tukey 's hsd multiple comparisons test was done to evaluate specific differences in the levels of the selected 3 pufas in the four fish species
all analyses were done using statistical package for social science ( spss ) version 20 for windows .
a total of 36 fas were identified in the four commercial species of fish from mwanza gulf of lake victoria ( table 1 ) .
the unsaturated fas were relatively many ( 27 ) compared to safas ( 9 ) .
the most dominant safas were palmitic acid , pentadecanoic acid , stearic ( octadecanoic ) acid , tetracosanoic acid , and heptadecanoic acid .
twenty ( 20 ) of the 27 unsaturated fas were pufas and 7 were mufas . among the 20 types of pufas , the 3 pufas were relatively more abundant ( 8) , followed by 6 pufas ( 7 ) .
the dominant 3 pufas were docosatrienoic , docosapentaenoic , docosahexaenoic , eicosapentaenoic , and eicosatetraenoic acids .
the 9 pufas and mufas were dominated by eicosadienoic acid and oleic acid , respectively . 

o. niloticus had comparatively the highest number of fas particularly in mufas , 3 pufas , and 6 pufas and other unsaturated fas compared to l. niloticus , t. zillii , and r. argentea ( figure 1 ) .
some of the types of fas that were found only in o. niloticus included hexadecadienoic acid , alpha linolenic acid , 9-tetradecenoic acid , 11-tetradecenoate , and 11 , 13-eicosadienoic acid ( table 1 ) .
the dominant 3 pufas in all four sampled commercial fish species were docosatrienoic acid , docosahexaenoic acid , docosapentaenoic acid , eicosapentaenoic acid , and eicosatetraenoic acid ( table 2 ) .
similar to types of fas , o. niloticus recorded more types of 3 pufas ( 8 equivalent to 31% ) than l. niloticus ( 7 ) , t. zillii ( 7 ) , and r. argentea ( 4 ) ( figure 2 ) .
the results showed a significant difference in epa levels among the four commercial fish species ( f = 6.19 , p = 0.001 ) .
tukey 's hsd multiple comparisons test showed significant higher levels of epa in r. argentea than o. niloticus ( p = 0.001 ) , t. zillii ( p = 0.036 ) , and l. niloticus ( p = 0.009 ) .
the dpa levels were not significantly different among the four commercial species ( f = 0.652 , p = 0.583 ) .
this study found 36 types of fas with different saturation levels in the four commercial fish species .
these results are reasonably similar to those obtained by mohamed and al - sabahi . in their study
out of the 36 fas , the saturated safas were 9 ( 25% ) and unsaturated ones were 27 ( 75% ) which included 20 pufas and 7 mufas .
the dominant unsaturated fas were octadecanoic acid , arachidonic acid , docosapentaenoic acid , eicosapentaenoic and docosahexaenoic acid , and oleic acid .
the existence of more unsaturated fas than saturated fas in the fish samples is similar to mwanja et al .   who obtained more categories of unsaturated fas ( 53.91% ) than saturated fas ( 46.24% ) in r. argentea .
the dominant unsaturated fas in this study are similar to those obtained by zenebe et al .
mohamed and al - sabahi , osibona , grgn and akpinar , effiong and fakunle , and muinde et al . .
the more unsaturated fas than saturated fas obtained in this study are probably due to their natural ubiquitous occurrence .
freshwater species are known to contain appreciable amount of unsaturated fas   and sometimes more than saturated ones [ 15 , 24 ] .
the dominant 3 pufas found in all four commercial fish species were of epa , dpa , and dha .
this finding is similar to zenebe et al .   and grgn and akpinar   who described that the most abundant fas in freshwater species are epa and dha .
the domination of these 3 pufas might be attributed to the feeding habit of the four species .
the three species ( r. argentea , o. niloticus , and tilapia zillii ) feed lower in the food chain mainly on microalgae ( diatoms and dinoflagellates ) which are excellent sources of epa , dpa , and dha . for example , a study by mfilinge et al .   and meziane et al .   reported that diatoms and dinoflagellates contain higher concentrations of epa and dha , respectively , and have been used as markers of diatoms and dinoflagellates in the aquatic food web .
o. niloticus and r. argentea are the major prey of l. niloticus in lake victoria [ 33 , 34 ]
. equally , t. zillii which has relative similar size and shape is also consumed by l. niloticus .
based on this feeding chain , it is more likely that the epa , dpa , and dha contained in the herbivorous fish species were transferred to the carnivorous ones via the food chain .
the present results showed that o. niloticus had relatively more types of 3 pufas than the other species , t. zillii , l. niloticus , and r. argentea .
the more types of 3 pufas in o. niloticus might be attributed to the diverse food items consumed by the fish .
o. niloticus is an omnivorous fish consuming diverse species of phytoplankton , insects , and juveniles fish .
a study by rumisha and nehemia   reported that o. niloticus feeds primarily on cyanophyta , diatoms , dinoflagellates , desmids , and green algae .
it has also been found to expand its diet from plant materials to include insects and fish .
the fas composition reflects the composition of the diet , because  you are what you eat  .
thus , expansion of diet and diversity of microalgae species contribute to o. niloticus having more types of 3 pufas which are beneficial for the health of consumers as well as fish .
the other three species , l. niloticus , r. argentea , and t. zillii , have specialized feeding as they increase in size .
l. niloticus is a predator that feeds on fish ( including its own species ) , crustaceans , and insects .
r. argentea is zooplanktivorous feeding on zooplankton , surface insects , chironomids , and the prawns ( caridina nilotica ) .
t. zillii adults feed on phytoplankton , detritus , and macrophytes . due to their specialized feeding , they limit the diversity of food and therefore pufas compared to the omnivorous o. niloticus . in addition , more types of 3 pufas in o. niloticus could be as a result of desaturation and elongation of fas .
the ability to elongate and desaturate fas is not the same in all species of fish .
o. niloticus have the ability to bioconvert stearic acid , oleic acid , and other fas , which belong to group c:18 fas , to highly unsaturated fas .
for example , arachidonic fa ( 20:4n-6 ) is a product of an elongation and desaturation of metabolic precursor of linoleic acid ( 18:2n-6 ) , whereas epa and dha their metabolic precursor are alpha linolenic acid .
stearic acid , oleic acid , and other c:18 groups were found to be dominant fas , contributing to higher composition of fas in o. niloticus . by virtue of this capability ,
classified o. niloticus as an excellent source of 3 pufas and being ideal for production of 3 supplements .
the present study indicated that r. argentea have relatively higher levels of both epa and dpa .
this finding is contrary to mwanja et al .   who found low levels of epa in r. argentea .
in this study the higher levels of epa and dpa in r. argentea could be attributed to its feeding behavior .
adult r. argentea explore the bottom zone during daytime which is the habitat for zooplankton and macrobenthic invertebrates .
furthermore , the relatively higher levels of epa and dpa in r. argentea may also be attributed to the swimming mode and pattern .
r. argentea is a slow swimmer commonly exhibiting vertical movements for avoidance of predators in search for food . on the contrary , o. niloticus ,
t. zillii , and l. niloticus are fast swimmers utilizing more energy for movements against current .
for this reason it is more likely for r. argentea to conserve more epa and dpa , which account for higher levels of these fas than those in the other species .
thus fish consumers should eat more r. argentea due to its high nutritional value and low selling price .
the current study identified 36 fas in four commercial species of fish from mwanza gulf of lake victoria .
o. niloticus was found to contain more types of fas and 3 pufas than l. niloticus , t. zillii , and r. argentea .
moreover , r. argentea has significantly and relatively higher levels of epa and dpa than the other three commercial fish species from mwanza gulf of lake victoria .
thus fish consumers should eat more o. niloticus to get a variety of fas types and r. argentea to obtain high levels of epa and dpa .
 fatty acids ( fas ) particularly 3 and 6 polyunsaturated fatty acids ( pufas ) play important role in human health . 
 this study aimed to investigate the composition and levels of selected 3 pufas in four commercial fish species , nile perch ( lates niloticus ) , nile tilapia ( oreochromis niloticus ) , tilapia zillii , and dagaa ( rastrineobola argentea ) from mwanza gulf in lake victoria . 
 the results indicated that 36 types of fas with different saturation levels were detected . 
 these fas were dominated by docosahexaenoic ( dha ) , eicosapentaenoic ( epa ) , docosapentaenoic ( dpa ) , and eicosatetraenoic acids . 
 o. niloticus had the highest composition of fas ( 34 ) compared to l. niloticus ( 27 ) , t. zillii ( 26 ) , and r. argentea ( 21 ) . the levels of epa differed significantly among the four commercial fish species ( f = 6.19 ,  p = 0.001 ) . 
 the highest epa levels were found in r. argentea followed by l. niloticus and o. niloticus and the lowest in t. zillii . 
 the dpa levels showed no significant difference among the four fish species studied ( f = 0.652 ,  p = 0.583 ) . 
 the study concluded that all four commercial species collected from mwanza gulf are good for human health , but r. argentea is the best for consumption because it contains higher levels of 3 fas , mainly epa . 