N. ALAM KHANDKER
Department of Zoology
University of Dacca
This study shows that shrimp larvae of Penaeus schmitti enter Lake Unare in July at an average size of 8.5 mm. By October they grow to an average size of 90 mm and enter the commercial catch. Fishing is done in the lake from October to May. The sex ratio is almost 1:1 but the female attains a larger size than the male. Carapace lengh-total length relationship is almost linear. Length-weight relationship is illustrated. Ecology of the lake is also discussed.
QUELQUES ASPECTS DE LA BIOLOGIE DE LA CREVETTE Penaeus schmitti BURKENROAD DANS LE LAC UNARE, VENEZUELA
L'étude indique que les larves de Penaeus schmitti pénètrent dans le lac Unare en juillet à la taille moyenne de 8,5 mm. En octobre, les animaux mesurent en moyenne 90 mm et entrent dans la phase exploitée par la pêche commerciale. La pêche est pratiquée dans le lac d'octobre à mai. Le sex-ratio se rapproche de 1/1, la femelle atteignant une taille supérieure à celle du mâle. Le rapport longueur de carapace/longueur totale est pratiquement linéaire. La communication illustre le rapport longueur/poids, et traite également de l'écologie du lac.
ALGUNOS ASPECTOS DE LA BIOLOGIA DEL CAMARON BLANCO Penaeus schmitti BURKENROAD EN EL LAGO UNARE, VENEZUELA
Según este estudio, las larvas de Penaeus schmitti entran en el lago Unare en julio con un tamaño medio de 8,5 mm. En el mes de octubre este tamaño aumenta a 90 mm, y ya forman parte de la captura comercial. En el lago se pesca de octubre a mayo. La relación de sexos es de casi 1:1, pero la hembra alcanza mayor talla que el macho. La relación longitud total - longitud del caparazón, es casi lineal. Se expone la relación longitud-peso y se estudia también la ecología del lago.
The white shrimp, Penaeus schmitti is found in Cuba, Jamaica, Haiti, the Dominican Republic, Panama, Colombia, Venezuela, the Guianas and Brazil (Boschi, 1963).
P. schmitti is abundant in the shallow waters all along the coast of Venezuela. At present it is the principal species caught in Lake Maracaibo and Lake Unare. This species is also found in the eastern region, in the Gulf of Paria, but the fishery there is not yet well developed (Khandker, 1965). In Lake Maracaibo this species is considerably mixed with other Penaeus spp (Ewald, 1965), but in Lake Unare it is the most predominant one. P. schmitti is also predominant in Lake Piritu, which is adjacent to Lake Unare (Fig. 1).
In 1963 the total shrimp catch from Lake Unare was 48,640 kg. This is of course very insignificant when compared with the catch of 3,842,650 kg from Lake Maracaibo in the same year. However, it has to be borne in mind that the offshore waters near Lake Unare are yet unexploited. Weibezahn (1952) reported that there were possibilities of good catch from that area but that detailed charting of the grounds was necessary before starting commercial fishing.
In Lake Unare fishing for shrimp is carried out from October to May, the maximum catch is normally in January of February. About 200 fishermen do the fishing using cast nets of 2 cm mesh size either from shore or dugout canoes of about 4 m in length. The catch is sold fresh to various fishing companies who send their trucks with ice to collect shrimp.
2.1 Ecology of the Lake Unare
During the rainy season from June to August the lake attains the maximum area of 64 km2 and a depth of 1.5 m. In the dry season the level of water drops down by about 0.8 m, reducing the area by 34 percent. The salinity of the lake varies very much in the different seasons. In 1963 the lowest salinity recorded was 19.32 ‰ in July, and the highest was 77.95 ‰ in April. The temperature varied only slightly, the maximum was 30.6°C in September and the minimum 28.2°C in March.
During the rainy season, fresh water enters the lake through three rivers of which the River Unare is the most important (Fig. 1). The river opens into the sea in between Lake Unare and Lake Piritu. The River Unare is connected with the Lake Unare by two canals, and the lake is connected with the sea only through the river. The mouth of the river at the sea is only open for 6 months from July to December, being closed by silt during the other months.
The animal communities of the lake consist of 34 species of fish and various invertebrates. The most abundant and commercially important fishes are: mullets, Mugil liza and M. curema, mojarras of the genera Diapterus, Eugerres and Eucinostomus and catfishes of the genera Arius and Selenaspis. The invertebrates are mainly crabs of several species, gastropods of the genera Albania and Bittum, bivalves of the genus Tagelus and some oligochaetes.
2.2 Early life history
Plankton samples taken from the sea shore near the lake, in June and July, showed the presence of shrimp postlarvae. Postlarvae began to enter the lake through the River Unare in July. The average size of the postlarvae entering the lake was 8.5 mm. No work has yet been done on the early life history of P. schmitti so larval stages could not be identified definitely. They resembled very closely the larval stages of P. setiferus as described by Pearson (1939). After entering the lake, postlarvae settle down on the bottom and are not found in the plankton samples. The postlarvae grow rapidly and by October attain an average size of 90 mm growing almost 1 mm per day. This rate of growth coincides with that of P. setiferus as reported by Lindner and Anderson (1954).
Fig. 1 Map of Lake Unare.
Fig. 2 Length-frequency distribution of Penaeus schmitti in Lake Unare during 1964.
2.3 Size and weight of adults
Samples of shrimp were collected mainly from fishermen but occasionally some sampling was done with a beam trawl having a mesh of 20 mm. Shrimps were measured and weighed fresh. Total length measured was from the tip of the rostrum to the end of the telson, and carapace length from the orbital notch to the posterior mid-dorsal edge of the cephalothorax.
The length-frequency distribution of shrimps collected and examined between January and May 1964 is given in Fig. 2. A decrease in average size was observed in April compared with that in January and February. In May a shifting of the modal size by 2 mm carapace length was indicated, which corresponds to a growth of about 8 mm in total length. Lindner and Anderson (1954) indicated that in P. setiferus the growth is rapid till October, after which it remains almost stationary until the end of February or March. It appears that in Lake Unare, P. schmitti also grows rapidly till October, after which it slows down.
The relationship between carapace length and total length in male and female P. schmitti is shown separately in Fig. 3. No significant difference is observed between the two sexes. Although there is large variation in the total length for a certain carapace length, the relationship is almost linear. Such linear relationship was also observed in P. duorarum by Iversen and Idyll (1960).
There is a slight size disparity between the two sexes, the female attaining a bigger size. This size disparity is common in other penaeids, as reported by Weymouth Lindner and Anderson (1933) and Iversen and Idyll (1960).
The length-weight relationship of male and female P. schmitti is shown in Fig. 4. No significant difference is observed between the two sexes. In Table I the relationship between carapace length, total length and weight has been summarized combining both sexes.
Length-weight relationship of Penaeus schmitti with both sexes combined
Fig. 3 Relationship of carapace length to total length in males and females of Penaeus schmitti.
Fig. 4 Length-weight relationship in males and females of Penaeus schmitti.
2.4 Sex ratio
Sex ratio was found to be almost 1:1. Although there is a size disparity it probably did not affect the ratio because above the length of 90 mm both sexes are equally retained in the net.
2.5 Maturation and spawning
The maximum size female obtained was 137 mm and at that length the ovary would be naturally underdeveloped, as shown in P. setiferus by King (1948). All ovaries examined in fresh condition and histological sections, were immature. Plankton samples taken in River Unare and Lake Unare in May and June did not show presence of larvae of P. schmitti.
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