Phytoplankton, especially diatoms, in the gut contents and feces of two plantivorous cyprinids—Silver carp and bighead carp

Abstract

In order to clarify whether the planktix~)rous silver carp and bighead carp can collect phytoplunkton as small as Cyclotella(<20 lun) in Donghu Lake, studies on phytoplankton in their gut contents and feoes v~ere made in 1990. The fish v~ cultured in both net cage in Donghu Lake and aquaria with the lake water. Past the intestine, the average valve diameter of Cyclotella changed little.The average ratio of empty frustule of Cyclotella to total Cyclotella in the fotegut contents of the fishes v~re 1.8-1.9 times higher than that in the lake water, but changed little from fotegut to feces. The aquarium experiment sbo~d that both carps could collect particles as small as 8-10 /an, which was obviously harmer than the distahoe betv~en their gill rakers. Probably, secretion of mucus plays an important role in collecting such small particles Key world: planktivorous fishes, gut contents and feces, diatoms, valve diameter, empty frustule INTRODUCTION Donghu Lake, 30 ~ 114 ~ is located in Wuhan City close to the middle reaches of the Changjiang River. The total surface area is about 32 km 2 and the maximum depth 4.5 m. According to the production data of the Donghu Lake Fish Farm, the annual fish yield between [1985][1986][1987][1988][1989][1990] varied between 690 and 850 kg/ha, of which over 90% was from two stocked planktivorous species, silver carp (Hypophthalmichthys molitrix) and bighead carp (Aristichthys nobilis). The lake is stocked mainly with fingerlings of the two species every year. No feed is given to the fish once the fingerlings have been released into the like.Since 1985, the dominant phytoplankton were Cyclotella (a centric diatom) and Cryptomonas (a dinoflagellate). The diameter of Cydotella is usually smaller than 20 /ma (the majority 6-15 /an), while the length of Cryptomonas is usually smaller than 30 fan (the majority 10-20 grn). However, the distances between gill rakers of silver carp are larger than 15 /an and those of bighead larger than 30 grn (Liu, 1981;Hampl et al., 1983;Spataru et al., 1983). If the carps can only use the food particles larger than the distances between their gill rakers, Cydotella almost cannot be used by both carps,, and Cryptomonas would only be partly used by silver carp.This work was akned to study phytoplankton, especially diatorm, in the gut No.4 PHYTOPLANKTON IN GUT AND FECES OF TWO CARPS 309contents and feces of bighead and silver carp to clarify whether or not the carps can collect phytoplankton as small as Cydotella.MATERIALS AND METHODS Collection of gut contmts and feces from fishes oaltured in a net cage in Donghu LakeA net cage experiment was conducted during March-May in 1990. The net cage with a surface area of 2.5 m x 4 m was set up near Station 1 in Donghu Lake. The bottom of the net cage reached down to the sediment of the lake. 150 fingerlings of silver carp and of bighead carp were introduced to the net cage on March 2, 1990.Generally, 5-7 of each carp were taken each time from the net cage and dissected to collect their contents. Contents collected fro...

Assessment of phytoplankton species in gut and feces of cultured tilapia fish in Egyptian fishponds: Implications for feeding and bloom control

“…Therefore, Nile tilapia fed on toxic cyanobacteria should be continuously monitored for microcystin levels in their tissues to ensure its suitability for human food. On the other hand, the inability of Tilapia fish to digest diatoms has been demonstrated earlier by Ping & Jiankang (1994) and Grubach (2010) who found seventy-seven percent of the diatom taxa in the fish feces. This means that diatoms are more resistant to digestion in the fish gut than other phytoplankton groups.…”

“…The gut of each fish was removed and fixed with 1 mL of Lugol's solution for microscopic examination. In order to test the digestibility of phytoplankton species by tilapia fish, about 10 tilapias were collected from fishpond 3 on a certain sampling occasion, and introduced into aquarium containing strained fishpond water by filtering through a 20 µm plankton net following the method of Ping & Jiankang (1994).The feces were collected immediately after digestion by means of a pipette, and washed twice carefully in distilled water. They were homogenized with a stirrer for a few minutes, and fixed in Lugol's iodine.…”