666 U. S. BUREAU OF FISHERIES 
Not only were the herring affected by the pollution, but presumably also the 
pickerel, perch, suckers, carp, and all other species of fish that grow in Saginaw 
Bay ; and the total damage done to these species involved much greater financial 
losses than those of the herring, for they yield the bulk of the commercial 
eatches of the bay and possess an average value greater than that of the 
herring. The significance of a pollution such as described here is seen in the 
fact that the herring industry was still affected in 1923—five years after the 
pollution ceased. 
Finally, this study shows that from the point of view of the commercial 
fisheries, it is not profitable to allow the herring, at their present rate of 
growth, to become much older than four years, as they increase slowly in 
weight after the third year. Further, relatively few third-year fish should be 
taken. This will permit most of the herring to spawn once, and some twice, 
as virtually all the individuals of a year class reach sexual maturity in their 
third or fourth year of life. 
If present conditions remain constant, the herring fishery does not appear to 
be in any immediate danger of extermination, in spite of the fact that no 
closed season exists for this species. This, no doubt is due partly to the fact 
that the intensive fishing is carried on during the spawning run when mature 
fish only are taken. Many of the immature second, third, and fourth year fish 
are thus unmolested. Further, many of the mature third-year fish escape the 
nets and therefore have a chance to spawn twice. But the handwriting is 
appearing on the wall. That the herring fishing is very intense is attested by 
the paucity of old individuals in the catch and by the shifting in the age compo- 
sition. Relatively very few fourth-year herring escape the nets to comprise, a 
year later, the fifth-year group. The data show that commercial fishing is so 
intense that a year class is practically wiped out during its year of dominance— 
the fourth. Briefly stated, the history of the majority of individuals of a year 
class seems to be as follows: They are spawned in the fall, hatched in the 
spring, grow as immature fish for two or three years, attain sexual maturity in 
the third or fourth year, and are captured by the fishermen before or during 
their fifth year. Hach year class is depleted rapidly in the year of its dominance. 
Captain Watuace. With regard to the pollution of the Great 
Lakes, I wonder if any account is being taken of the ashes that are 
dumped overboard by the steamers that have used the Great Lakes 
for a number of years. I have heard fisherman say that the quantity 
of ashes dumped overboard in Lake Erie and Lake Michigan must 
have some effect on the vegetable growth at the bottom. 
Doctor Van Oosren. There is no effect, as far as I know. 
Captain Watuace. I was wondering if it had been taken into 
account with regard to pollution ? 
Doctor Van Oosten. Yes; it has been considered in regard to 
pollution. 
Doctor Fisn. For the most part vessels follow more or less definite 
routes, and the accumulation of ashes would be more or less along 
these routes. I should think that the bottom accumulation would 
be in the greater depths of water, where the only plant or animal life 
to be found would be the diatoms gotten in the open waters. 
Doctor Van Oosrren. The places where the growth of plants is 
thickest are near the waters edge, and this water probably would not 
be affected by the ashes. The places where these ashes are dumped 
may not be the vital places of plant production. 
Mr. Rapcrirre. Is it not true that these cinders, etc., do shift 
around on the bottom on the spawning areas and are actually destruc- 
tive of eggs and young fish? 
Doctor Van Oosten. There is no question but what anything that 
sinks to the bottom becomes deleterious to spawning. There are a 
great many species that never come off of the bottom at all—they get 
their food and all there. 
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