570 U. S. BUREAU OF FISHERIES 
our fishes is being maintained only by drawing on new resources. If so, we 
have a disquieting picture. Our substitute species may decline when they are 
subjected to intensive fishing as long as the others have been. 
Among the shellfishes we find similar tendencies. Referring to Figure 11, we 
see that oysters, crabs, and clams have declined moderately; lobsters have 
declined more; while shrimp is the only shellfish that has an upward trend in 
yield. Oysters, clams, and lobsters were fished more extensively in the 
earlier years and they began their decline first. The most intensive fishery for 
crabs began when the dredge was introduced in the early nineteen hundreds. 
This kept the yield up for over a decade, when it too declined. Similarly, the 
otter trawl, introduced into the shrimp fishery after 1908, has been responsible 
for greater yields, which are still continuing to increase. If it were not for the 
greater yield of shrimp, the shellfish fisheries would show a distinct decline 
since 1908. 
Similar charts could be shown to demonstrate trends of the same nature in 
fisheries of other regions of the United States, but it will not be necessary to 
trouble you with the details. Suffice it to say, that while the Pacific Coast 
States show a remarkable upward trend in the total yield of their fisheries, the 



MILLIONS OF POUNDS 


° 1 ° 0 Cc 
3 q D o re) ro} Q at rx 
2 2 © a o ® ® ® 
Fig. 11.—Changes in the yield of the important shellfish on the Atlantic sea- 
board (on a logarithmic scale) 
salmon and halibut, which have been fished the longest time, are barely maintain- 
ing their yields. The great increase is due to the sardine and tuna fisheries 
of comparatively recent origin. As for the Great Lakes, Doctor Koelz, in his 
report, has shown that although they have a relatively constant yield, this 
constant yield has been maintained by the increased use of so-called “rough” 
fish, which to a marked extent, have replaced the fishes that formerly were 
considered more desirable. In the Mississippi River, we have a rise and 
decline during the period, and if we omit the mussel fisheries we find that the 
yield has declined quite markedly. 
In conclusion, I wish to emphasize again that yield statistics have been 
used throughout this discussion. According to these statistics, our yield has 
been maintained and in some regions increased, but not by general increases 
in all species. On the contrary, many of the highly prized species have failed 
to provide yields comparable with those of former years. Their places have 
been taken by species that formerly were not sought after. While this does not 
mean that depletion is proven in every case where decreased yields occurred, 
it does point out the fact that there is need for critical studies of many species. 
If the yield of our fisheries is to be maintained, we must determine first what 
fluctuations in abundance are occurring, then find out whether they are due 
to natural or other causes, and finally determine what steps should be taken to 
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