212 FISH HATC'HF.RY MANAGKMENT 



At this point, energy, instead of being funneled into the building of body 

 tissues, is channeled into the formation of eggs and sperm. When sex prod- 

 ucts are released a weight loss as much as 10-I5"/ii occurs. Fish also have 

 high metabolic rates during the spawning season, associated with the 

 spawning activities. Conversely, during winter, resting fish have very low 

 metabolic rates. Fish suffering from starvation have 20% lower metabolic 

 rates than actively feeding fish. Excitement and increased activity elevate 

 the metabolic rates. All these affect the amount of energy which must be 

 supplied by the feed. 



OTHER ENVIRONMENTAL FACTORS 



Factors such as water flow rates, water chemistry, and pollution can put 

 added stresses on fish, and result in increased metabolic rates in relation to 

 the severity of the stress. Water chemistry, oxygen content, and amount of 

 other gases, toxins, and minerals in the water all affect the metabolic rate. 



For many species, darkness decreases activity and energy requirements. 

 These fish grow better if they have "rest periods" of darkness than they do 

 in constant light. 



Crowding, disease, and cultural practices also can have an affect on the 

 metabolism and well being of fish. 



Digestion and Absorption of Nutrients 



Feed in the stomach and intestine is not in the body proper because the 

 lining of these organs is merely an extension of the outer skin. Feed com- 

 ponents, such as simple sugars, can be absorbed as eaten. The more com- 

 plex components such as fats, proteins, and complex carbohydrates, must 

 be reduced to simpler components before they can be absorbed. This 

 breaking- down process is termed digestion. Feeds cannot be utilized by the 

 animal until they are absorbed into the body proper and made available to 

 the cells. 



Absorption of nutrients from the digestive system and movement of the 

 nutrients within the body is a complicated process and not fully under- 

 stood. For nutrients to be available for biochemical reactions in the cell, 

 they must be absorbed from the digestive system into the blood for trans- 

 port to the cells. At the cellular level, they must move from the blood into 

 the cell. 



Fish also are able to obtain some required elements directly from the wa- 

 ter, this being especially true for minerals. 



A brief anatomical review of a fish's digestive tract will illustrate the 

 sites of feed digestion and absorption. 



The mouth is used to capture and take in feeds. Most fish do not chew 



