FISHERY BULLETIN: VOL. 71, NO. 3 



the points among fish weighing more than 

 about 65 kg is borne out by the following 

 data on white muscle of eight yellowfin tuna 

 taken by a single vessel during one day's 

 fishing in the Gulf of Guinea: 



The large variation in mercury levels among 

 tunas of the same size may be due to a dif- 

 ferential growth rate between sexes. For example, 

 Forrester et al. (1972) noted that for any 

 given length above 65 cm, the mercury content 

 was higher among male spiny dogfish than 

 among females. They pointed out that since 

 mercury has been shown to be cumulative 

 with age in some species (Johnels et al., 1967) 

 the difference in mercury levels between sexes 

 may reflect differences in growth rates since 

 it is probable that males grow slower than 

 females beyond a certain age. Moreover, large 



1.00 



0.25 



25 



50 

 YELLOWFIN 



75 

 WEIGHT (KG) 



100 



Figure L — Relation between mercury (ppm) and fish 

 weight (kg) for 88 yellowfin tuna captured in the Gulf 

 of Guinea between July 1971 and July 1972 (from Ivory 

 Coast Fisheries Service, 1972, see footnote 4). 



differences in growth rates among individuals 

 of the same sex may further complicate the 

 relationship. 



In a study of 22 yellowfin tuna captured 

 off Hawaii (size range 31-98 kg; mercury 

 level 0.24-1.32 ppm) a relationship was found 

 between fish weight and total mercury level 

 (r = 0.47; P = 0.05) (Rivers et al, 1972). 



Also, in a study of 26 Hawaiian skipjack 

 tuna it was found that mercury levels in the 

 white muscle of the smaller specimens (18 fish), 

 weighing from 0.6 to about 1.4 kg, were at a 

 nearly constant level of about 0.15 ppm, whereas 

 mercury in the larger fish (8 specimens), weigh- 

 ing about 1.5 to 1.8 kg, ranged from approxi- 

 mately 0.15 to 0.35 ppm (Lasker and Leong, 

 see footnote 3). 



GEOGRAPHIC VARIATION IN 

 MERCURY CONTENT OF FISHES 



Geographic variations in the mercury levels 

 of fishes have been demonstrated in many cases. 

 Some of these variations have been related to 

 mercury pollution by man such as that observed 

 in some Swedish lakes, Minamata Bay in Japan, 

 and some of the Great Lakes and connecting 

 waters between the United States and Canada. 



Other cases of geographic variation have 

 been observed, but it is not known whether 

 they are related to human activities. For 

 example, an investigation of north Pacific 

 halibut has shown that there is a south-north 

 cline in mercury levels, i.e., halibut from the 

 southern areas (off Oregon and Washington 

 States) have higher levels than those from 

 the more northern areas. Also it was found 

 that there are localized "hot spots" in the 

 halibut fishery where the fish have significantly 

 higher mercury levels on the average than 

 halibut in the surrounding area (Bernard E. 

 Skud, pers. commun.). Also Childs and Gaffke 

 (1973) found significant geographic variation 

 in mercury levels of some groundfish (rex sole, 

 Glyptocephalus zachh'us; dover sole, Micro- 

 stomas pacificus; sand sole, Psettichthys melano- 

 stictus; starry flounder, Platichthys stellatus; 

 lingcod, Ophiodoii eloiigatus; and sablefish, 

 Anoplopoma fimbria) from three areas off the 

 Oregon coast. However it is interesting to note 



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