'just replace the word "FISH" with "HUMAN"
and could be mis-misleading.... . . . .
For instance, in North West Guyana, up to 16% of the village fishing still prefer to fish with poisons despite the superiority of modern netting materials and prohibition by law (van Andel,2000).
A multitude of plant species are known to possess compounds toxic to
fish, and evidence has emerged that certain plants have different species
specific effects on fishes (van Andel, 2000). In plants, two main classes of poisons, the rotenones and the saponins, account for nearly all varieties of ichthyotoxic substances. The rotenones are mainly produced by leguminous plants (Fabaceae), and the saponins are more diversely distributed among several plant families. In addition, the ichthyotoxic plant family Euphorbiaceae liberates cyanide in water (Béarez, 1998). Saponins are glucoside poisons which destroy the membranes of erythrocytes. Although they are scarcely absorbed by the digestive system, fish directly assimilate saponins into their blood stream through their gills (Elpel, 2000).
Asystasia vogeliana (Acanthaceae) has not been widely studied, thus
information is lacking in the literature. However, this plant is one of the
ichthyotoxic plants commonly used for fishing in the south east of Nigeria.
The genus Tephrosia (Fabaceae) is well known as a rich source of flavo-
noids, comprising the isoflavonoid rotenone among their secondary me-
tabolites (Beal & Anderson, 1993, van Andel, 2000). Tephrosia vogelii Hook F. (Fabaceae) is among the ichtyotoxic plants described by Dalziel (1937) in his account of useful plants of West Africa. Reed et al. (1967, cited in Udolisa & Lebo, 1986) also identified T. vogelii as one of the plants used by fishermen of Lake Kainyi, Nigeria. T. vogelii has a pungent odour which is due to a volatile oil known as Tephrosal (Dalziel, 1937). In addition, the plant contains an active substance known as Tephrosin that is a crystaline substance only slightly soluble in water (Dalziel, 1937). Rotenone is extremely toxic to cold blooded animals and is used also as insecticide (van Andel, 2000; Andrei et al., 2002). The plant derived rotenone is synthesized and used for experimental fishing as well as the elimination of undesirable fish populations in natural waters (Dolmen et al. 1995; Boogaard et al., 1996; Béarez, 1998). Fishermen use macerated leaves of T. vogelii with an amount of approximately 600 mg l-1 wet weight) for fishing. Encircling of fish and loss of balance are observed within 5-10 minutes and death within 1-2 h after application of the substances (Udolisa & Lebo, 1986).
The substances are more effective where the volume and flow of water are at the lowest and fishes are forced into conditions of crowding. Stunned fish, which are not killed, may return to life (Udolisa & Lebo, 1986). The toxic effect of rotenone is caused by the inhibition of mitochondrial electron transport (Budavari et al., 1996). Fish are particularly susceptible to poisoning because rotenone is absorbed via the gill membrane causing immediate death due to suffocation (Reed et al., 1967, cited in Udolesia & Lebo, 1986).
From life cycle studies on fish it has been shown that the early life stages
(embryo, yolk sac-fry and juveniles) are the most sensitive to the impacts of toxicants (McKim, 1977, 1985; Macek & Sleight, 1977; Woltering, 1984).
According to Meinelt & Staaks (1994), early life stages are useful in the
evaluation of effects of toxic substances. However, sufficient information
on the toxicity and the sublethal and lethal effects of the African plants
A. vogeliana and T. vogelii used as fish poisons are not fully documented.
It is therefore the aim of this study to investigate the impact of extracts
from these plants on the early life stages of fish, in order to predict the likely impacts of their continuous use on fish stock.
Materials and methods
Leaves of A. vogeliana and T. vogelii were collected from the fishing community of Ikang in Cross River State (south east of Nigeria). The leaves were thoroughly washed in water to remove sand and debris, air-dried under low intensity sunshine (25°C) for one week, and then oven dried at 40°C for 48 h. Dried leaves were crushed with a morter and a pestle and ground to a fine powder with a manual blender. The powdered samples were freezedried at -54°C for 48 h to ensure complete removal of water. Subsequently, samples were soxhlet extracted (Soxtec System 1043 HT6, Tecator, USA).The ratio of solvent to sample was 50 ml per 1 g of powder. Samples were first extracted with petroleum ether to remove fats and other non-polar substances, and then with methanol. 1 g of each dried methanolic extract was redissolved in 10 ml dimethylsulfoxide D(MSO) and made up to 100 with reconstituted deionized water .
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