FISHERY BULLETIN: VOL. 70, NO. 4 



was collected soon after voided with a fine-tipped 

 eye dropper. It was placed in a Syracuse dish 

 in saline solution (0.6''r CaCU, 2.5';y NaCl) and, 

 under a dissecting microscope, cleaned of all ad- 

 hering extraneous material such as sand grains. 

 Torn or broken strands were rejected. To re- 

 move adherent salts, the fecal material was then 

 washed with 3Sr ammonium formate on preig- 

 nited, washed, and tared glass fiber filters and 

 dried at 65 °C to constant weight. Reproduc- 

 ibility of blank filters washed with two 1-ml 

 aliquots of 3Sf ammonium formate varied from 

 0.01 to — 0.05 mg. No correction was made for 

 the sporadic weight loss of the blanks. Dried 

 samples were frozen until analyzed. Sufficient 

 fecal material was obtained for 2-8 replications 

 of each analysis. Analyses of the fecal pellets 

 produced on the detritus diet used 0.2 to 1 mg 

 samples. The high variability of results with 

 these samples was greatly reduced by increasing 

 sample size. Therefore, subsequent analyses of 

 fecal material employed 1 to 10 mg feces per 

 replication. Errors in samples less than 1 mg 

 in weight were presumed to be caused by in- 

 homogeneity of fecal material and magnification 

 of the absolute errors involved in weighing and 

 chemical analyses. Replications for each anal- 

 ysis were chosen from fecal samples collected 

 at diflferent feeding intervals. There was no in- 

 dication of a change in the assimilation efficien- 

 cy of a given diet with time. 



To avoid undue handling, shrimp were sized 

 and identified to species only after feeding. 

 Therefore, feeding occurred with mixed sizes 

 and species. Three diets — diatom, Chow, and 

 algal mat — were fed to white shrimp only, and 

 two diets — detritus and AF-1 — were fed to white 

 and brown shrimp. In tests in which individual 

 shrimp were fed diff"erences were not found in 

 assimilation efficiency due to size (see for in- 

 stance Table 7) or species. Therefore, the re- 

 sults reported here are assumed to apply equally 

 to both species within the size range used. 



addition, in a fifth test, shrimp were allowed to 

 graze on the algal community coating the base of 

 culms of the oyster grass, Spartina alterniflora, 

 and the assimilation of this food source was mea- 

 sured. 



The four defined diets were: (1) an axenic 

 culture of the periphyton diatom, CyUndrotheca 

 fusiformes; (2) a heavily bacterized culture of 

 the same diatom (termed hereafter "detritus"); 

 (3) AF-1,' an artificial food consisting of rice 

 bran (52.0^/ ), shrimp meal ( 30.5 '/r ), fish meal 

 (S.O^/f), fish solubles (2.0';r), mineral mix 

 (2.0%), soy protein (S.O'/c), and Calgon 

 (0.29r); and (4) Trout Chow,' consisting pri- 

 marily of fish meal, fish solubles, soybean meal, 

 ground wheat, and ground yellow corn, with a 

 guaranteed minimum analysis of 40 ^r crude pro- 

 tein, 2.5 Sr crude fat, and 5.59^ crude fiber. 



The method used rests on the assumption that 

 the food is representative of the ingested mate- 

 rial, that is, that selection does not occur. In 

 order to minimize the possibility that the shrimp 

 could select by sorting of particles, and to pre- 

 vent dispersion and suspension of the defined 

 diets during feeding, all these diets were bound 

 by mixing with a 2% algin solution (Meyers, 

 Butler, and Hastings, 1972). The mixture was 

 extruded through a syringe of 1-mm pore di- 

 ameter into a 0.6 to 1.2% CaCb solution. The 

 calcium ion sequestered the algin, producing 

 either strands or small spheres of bound food 

 which were readily accepted by the shrimp. De- 

 tails of this method are discussed at length in the 

 aforementioned article. 



CHEMICAL ANALYSES 



After drying, samples of food and feces were 

 digested along with the glass fiber filters on 

 which they were isolated. Filter blanks carried 

 through the same analytic procedures indicated 

 no detectable contamination from this source. 



FEEDS 



The digestibility of four defined diets, two ar- 

 tificial and two natural, was investigated. In 



* Prepared by Dr. Samuel P. Meyers and D. Butler, 

 Department of Food Science, Louisiana State University, 

 Baton Rouge, La. 



'' Ralston Purina Company, St. Louis, Mo. Reference 

 to trade names in the publication does not imply endorse- 

 ment of commercial products by the National Marine 

 Fisheries Service. 



1284 



