Fish ladders ore series of pools 

 that lead the migrating fish over 

 high dams 



What Man Must Know 



The flat-bodied halibut spends 

 most of its life buried in the sand 

 on the ocean floor 



The brilliantly colored tilefish 

 lives in northern waters although 

 it belongs to a tropical family 



Salmon on their nest 



Men have been fishing for thousands of years but knoiv 

 comparatively little about fish and what affects their abun- 

 dance and movements^ "Sea culture" is a new science and 

 the restlessness of the ocean complicates our efforts to 

 unlock its secrets. 



The supply of fish in the ocean is not inexhaustible and 

 Man must practice conservation in the sea just as he is 

 beginning to practice conservation on the land. 



When, where, and how many .--A. most important fact 

 the fishery conservationist must learn is the "maximum 

 sustainable yield" of each species - -that is, the greatest 

 number that can be harvested each year and leave enough 

 for harvesting the next year and the next. The conserva- 

 tionist must also predict when and where the fish are 

 available; and know that they are available to the fisher - 

 nnan. The fishermen can then prepare for big harvests or 

 small, as conditions warrant. "Predicting abundance" is 

 a step in conservation because it permits a regulated 

 harvesting and ensures protection of the available fish. 



How, how, and how.- -But knowing "when, where, and 

 how many" is only part of the work of the fishery con- 

 servationist. He must be able to tell the fishing industry 

 how to make the best possible catch without damage to the 

 resource; how to get the product to the family table with 

 minimunn loss of fish or quality; and how to make the best 

 industrial use of fish not presently used for human food. 



Intriguing mysteries , - -Solving these mysteries include 

 finding the reason why a species such as the tilefish 

 suddenly disappears from its haunts and is missing for 

 years to reappear suddenly in great numbers; the reason 

 for heavy mortality of a species when known conditions 

 appear normal, the "spark" which causes the "red tide" 

 organism to suddenly increase in numbers, killing millions 

 of fish. 



Some recognized dangers.- -PTedators, such as squaw- 

 fish awaiting young salmon at a river nnouth; starfish and 

 drills ruining an oyster harvest; green crabs killing 

 clams; sea lannprey attacking Great Lakes trout fisheries, 

 must be controlled. Obstructions, as dams blocking fish 

 runs; irrigation outlets and power turbine intakes divert- 

 ing fish from their course, nnust be bypassed. Silt that 

 covers spawning grounds and shellfish beds or ruins 

 growth of aquatic plants by making the water too murky 

 for sunlight to penetrate; water fluctuations that drown 

 out or dry up spawning areas; domestic and industrial 

 pollution; careless use of pesticides, all pose problems 

 of great concern to the conservationist. 



