台灣白鰱之攝食與蛋白質消化率研究

Abstract

1．竹北施肥魚池屬富養池，魚類所須之各類營養元，藻類等，不虞缺乏。 2．白鰱喜食藍綠藻，且能消化之；對矽藻則不甚能消化。 3．白鰱攝食時間在淩晨4：00?5：00時；屬白天活動性魚類。 4．消化管之內含物平均約須10小時後，可以使之排除出去。 5．白鰱消化管中之PH值為6.8?8.3。 6．白鰱之消化管長度（Y）與體長（X）呈正相關。其直??歸方程式為Y＝9.449 × －102.098 r＝0.959 7．葉綠素在白鰱消化管內不會被消化，因此可以作為指示劑。以代替三氧化二鉻，測定蛋白質之消化率。 8．白鰱蛋白質之消化率為72.33％?92.70％。 9．白鰱消化管中蛋白質之消化在前段最大，中段次之，後段再次之。 10．白鰱雌、雄魚間蛋白質之消化率並無差異。 11．白鰱體重大小不同，其蛋白質之消化率亦不同。體重在200?400克間之魚消化率最大，20?45克間之魚次之，500?900克間之魚再次之。

For controlling the feeding of fish, it is important to estimate the digestibility of protein on phytoplanktophagics, Lately, an indierect method that is utilizing inert substance as a reference for the digestibility has been widely used in the studies on the diet of fishes, because of its accuracy and usefulness (T. Nose 1960). However, in the present study another method was employed for determine the digestibility of phytoplankton in silver carp, and will be discribed as follows. Silver carp Hypophthalmichthys molitrix (Cuier and Valenciennes), a herbivorous stomachless fish, was selected as experimental material. Some of the carps were cohabited in Chupei fertilizer pond, The others were raised in the aquaria (100 cm × 110 cm × 72 cm) with ground water, The experimental diet-chlorella which contained 3-5% of the reference substance, Cr2O3 was fed to those fish, In each experimental period ten to fifteen fish were collected as the sample. The digestive tract was evenly divided into nine segments. In order to obtain more accurate results about digestion and absorption the determinations were made on contents in each segment of the digestive tract. The analysis of total nitrogen per microgram of chlorophyll was carefully made on the contents of each segment of the tract. The digestibility was calculated by using the formula as follows: Digestibility = 100 - 100 (%indicator in feed × % nutrient in feces/% indicator in feces × % nutrient in feed) The % degradation of protein in each segment was calculated by using the protein content in the diet as 100 %. These fish have no ability to select food, but through its large or medium opening of mouth and the spongy like complex network numerous gillrakers, the planktonic algae and microorganic particles which can be filtered. The digestive tract of silver carp is an elongated, relatively undifferentiated tube and the stomach is slightly extented toward the anterior part, and with copious supply of juice. The length of body (BL) and of digestive tract (DL) have the regression relationship that can be expressed as follows: DL = 9.449 × BL - 102.098. The pH value of the digestive tract is in the range of 6.8 - 8.3 when the intestine is full of food. There is no significant difference amoung fore-, mid-, and hind-gut in pH value. The state of nourishment of the fish is expressed as the ratio of the dry weight of the contents in the digestive tract to the body weight of the fish. It has been measured as 0.41% in the present investigation. The feeding periodicity was defined by determining the mean weight of the contents in the digestive tract per gram of body weight at a interval of 6 hours in the 24-hour cycle. The feeding curve shows that the peak is around 9:00 a.m., thus the silver carp is diurnal fish. The actual feeding time is then, in the range from 4:00 to 5:00 a.m.. There is no other feeding time can be observed from the curve. It takes more than 10 hours for the food organism from mouth through the digestive tract to the anus. Silver carp digested about 72.33-92.70% of the protein of the diet which contained 11.9% Chlorophyceae, 10.6% Bacillariophyceae, 62.3% Cyanophyceae, and 2.3% unidentified substance. There is no difference in digestibility between sexes, but is affected by the different size of bodies. Anatomically the mouth structure of the fish is not able to perform food selection, however, the analysis of the food contents of digestive tract shows that the trends of food selectivity is present in this fish. They are in favor of the cyanophyceous algae.