402 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
CONCLUSION. 
A summary lias now been given of the present character of the oyster industry in 
Europe. The intention has been, first, to give a glimpse of the actual processes of 
oyster-culture, and, second, to show in a general way the influence exerted upon the 
industry by governmental concessions or restrictions. Together with these a number 
of questions relating to the living conditions of the oyster have been discussed, and 
as the interest connected with them may, in several instances, be more than theoretical, 
they may be briefly summarized. It would appear, for example — 
1. That the degree of density of the water is one of the most important factors 
influencing the spawning and fattening of the oyster. This degree of salinity, which 
represents the optimum of spawning conditions, has been found in all countries to be 
delicately poised. The density of the water recorded in the best spawning-grounds 
of the French coast is practically that of the spawning-grounds of Italy and of north- 
ern Europe. It may be stated that the specific gravity of the water in regions of 
maximum production throughout Europe appears to be uniform at about 1.023* in the 
case of the “flat” oyster (Ostreaedulis), and at about 1.021 in the case of the Portuguese 
species (Ostrea angulata). The influence of warmth is not to be underestimated in 
regard to the time and degree of spawning; it is meant, however, that in one zone or 
locality, i. e., bay or estuary, where the water density is favorable, there is a more 
decided tendency to a fall of spat than in a locality of corresponding temperature 
immediately neighboring, where density may differ by only a few thousandths ( e . <j. 0.002) 
of a degree. 
2. That the amount of spat occurring annually in a region appears to be directly 
in proportion to the number of spawning oysters in that region. This is by no means 
a novel suggestion ; it is one, however, that has been repeatedly impressed upon the 
writer. The older idea, it will be remembered, is that banks can never be exhausted, 
on the ground that the few oysters left by the dredgers will, by the annual spawning 
of several millions of young, cause a very rapid regeneration. That the banks regen- 
erate is true,! but the process is shown to be slow, and beset with many difficulties. 
The older proposition is, in brief, untenable, because, as Mobius pointed out,f the per- 
centage of loss of oyster fry is apparently a normal factor (under natural conditions). 
It may be seen, as an illustration, that the immense annual spawning product of one 
oyster may in time give rise to no more than two adults; and from a practical stand- 
point, accordingly, the large number of eggs produced by a single oyster would make 
little difference in the growth of the natural bed. It should be noted that only in 
those places in Europe where the natural bulk of spawning oysters is actually main- 
tained does a great quantity of spat occur regularly; also that where the number of 
spawning oysters is equal the percentage of spat will be notably greater if the spawn- 
ing oysters are little disturbed. § 
* The observations were made at nearly corresponding seasons, 
t Mobius, loo. c it., p. 77. t Mobius, Joe. cit., p. 79. § Cf. Hoek., Joe. cit., p. 481. 
