The Eel: Biology and Management of Anguillid Eels

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  • ISBN 13: 9780470991992.
  • The Eel: Biology and Management of Anguillid Eels: Biology and Management of the Anguillid Eel!
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Information on the spawning migration, spawning ecology and life history of tropical eels in the Indo-Pacific region is very limited. The physiological and morphological characteristics of tropical freshwater eels, Anguilla bicolor bicolor and A. A total of eels were collected over monthly intervals between February and January and we examined both gonadosomatic index and gonad histology features.

In both species, close positive relationships between advanced maturation stages and eye, fin, gonadosomatic indexes were found in males and females. The final stage of maturation for seaward spawning migration occurred throughout the year in A. These results suggest that year-round spawning in the open ocean occurs in the tropical eel. This non-seasonal spawning ecology is notably different from that of temperate eels, which are known to follow a well-defined spawning season, with spawning migrations generally taking place during autumn months.

Being catadromous, the freshwater eel of the genus Anguilla migrates between fresh water growth habitats and offshore spawning areas. Molecular phylogenic research on freshwater eels has revealed that tropical eels are the most basal species originating from the Indonesian region and that freshwater eels radiate out from the tropics to colonize temperate regions 5. Thus, studying the life history and migration patterns of tropical eels may provide clues to understanding the nature of primitive forms of catadromous migration in anguillid eels and how the migration patterns of this species were established.

Amazing Girl Catching HUGE Eels in Fish-Hole with Fishing Oil

The results may also elucidate the evolutionary pathways of migration for this species and for other diadromous fish species that migrate between fresh water and seawater habitats. The life cycle of the freshwater eel includes five stages: the leptocephalus, glass eel, elver, yellow eel and silver eel stages. Larvae, leptocephali, drift and are transported along ocean currents at the continental shelf. These leptocephali leave oceanic currents after metamorphosing into glass eels and then typically migrate upstream as elvers four to eight months after hatching 6 to grow in fresh water habitats during the yellow stage immature stage.

After undergoing upstream migration, elvers become yellow eels and live in fresh water habitats such as rivers and lakes. Then, during the silver eel stage early maturing stage of the autumn and winter, their gonads begin to mature and they begin their downstream migration into the ocean and back out to the spawning area where they spawn and die. Before migrating downstream to spawn, freshwater eels undergo a transition to prepare for oceanic life and in turn become silver eels 7.

The silver eel migration period is one of the life stages that is still poorly understood. The timing of downstream migration and its periodicity may impact survival rates, and such information may prove crucial to the development of successful protective measures 8. Temperate eels are largely characterized by well-defined spawning and recruitment seasons, long larval durations, and panmictic populations.

The eel; biology and management of anguillid eels [1977]

The spawning migration of temperate eels generally occurs during autumn months between August and December 9 , 10 , 11 in the Northern Hemisphere and between February and May in the Southern Hemisphere 12 , except in a case of the prolonged downstream migration activity between April and December with high temporal variation in the European eel Anguilla anguilla in northeastern Germany The tropical eel spawning period is still not well understood. By contrast, A. Recently, Arai et al. These findings suggest that downstream migration and recruitment characteristics differ between temperate and tropical eels.

The 1st International Eel Science Symposium ‘Science into Management’

The unique year-round spawning ecology and behaviours of tropical eels may facilitate mating between adult eels of different year classes cohorts or from different habitats. Such flexibility in the life history of tropical eels may help to maintain their populations relative to those of temperate eels. However, few studies have confirmed the timing and seasonality of seaward migration for spawning among tropical eel species.

The purpose of the present study was to understand the timing of maturation and downstream migration patterns among tropical freshwater eels Anguilla bicolor bicolor and A. The timing of maturation and downstream migration was examined by investigating fluctuations in both the gonadosomatic index and gonadal histology levels.

THE EEL Biology and Management of Anguillid Eels , Updated, Revised & Extended

Morphometric variables of silver and yellow immature tropical eels were also compared across different maturation stages to understand how morphologies change with physiological changes. These data provide new information on freshwater eels, as knowledge on the maturation and life history of tropical eels is currently very limited.

Penang Island is an island of km 2 located in the Strait of Malacca Fig. The island has a tropical climate, specifically a tropical rainforest climate bordering on a tropical monsoon climate. These results suggest that the temporal variations of water temperature are small in comparison to the spatial variability in the study area.

High precipitation was observed throughout the year with peaks in April and October and April, September and November except in January and February for each year Fig. The approximate locations where eels were collected from Penang Island are shown with closed circles. Average monthly fluctuations in the air a and water temperatures b and the precipitation c in Penang Island in northwestern Peninsular Malaysia, Malaysia 17 , Eels were found to exist throughout the year, with Anguilla bicolor bicolor being the dominant species, constituting For A.

One specimen of A. Female GSIs ranged from 0. The gonadal histology of transverse sections was the same between the examined species, demonstrating that oocyte characteristics of each maturation stage also varied between months from Stage I primary stage to Stage V final stage for spawning Fig.

In Stage I during early stages of oogenesis, gonads were composed of primary germ cells, oogonia, lamellae and primary oocytes. During Stage II, immature cells developed with primary oocytes and a few oil droplets. The early maturation stage of Stage III involved the development of oocytes with cortical alveoli and oil droplets in the cytoplasm Fig. The early vitellogenic stage of Stage IV involved vitellogenic oocyte, nucleus, yolk granule and central yolk platelet development and the oocyte diameter increased drastically while the cytoplasm filled with yolk granules Fig.

During the final stage of maturation Stage V , midvitellogenic oocytes formed in final preparation for spawning; disintegrated nuclei were observed and whole nuclei were no longer visible Fig. Eels occupying more developed stages e. These findings suggest that A. Monthly fluctuations in the gonadosomatic index for female tropical anguillid eels Anguilla bicolor bicolor top and A.

Gonadal histology of Anguilla bicolor bicolor left and A. Each stage was modified according to Lokman et al. Monthly fluctuations in maturation stages of female tropical anguillid eels Anguilla bicolor bicolor top and A. Male GSIs ranged from 0. The gonadal histology of transverse sections was the same between species and revealed that oocyte characteristics of each maturation stage also varied within months from Stages I primary stage to III middle maturation stage in A.

THE EEL Biology and Management of Anguillid Eels , Updated, Revised & Extended

However, unlike the female eels, no male eels of either species reached the advanced maturation stages of Stages IV and V. During Stage I immature testis stage , the development of seminiferous tubules showed a dominance of spermatogonia type A SG-A with early type B spermatogonia SG-EB in tubules with Leydig cells scattered between tubules.

Biology and Management of Anguillid Eels

Spermatocytes are recognizable by their smaller nuclear size and heavily stained chromatin material Fig. These findings suggest that maturation periods of A. Monthly fluctuations in the gonadosomatic index of male in tropical anguillid eels Anguilla bicolor bicolor top and A. Monthly fluctuations in the maturation stages of male tropical anguillid eels Anguilla bicolor bicolor top and A. Significant differences in GSI values between each stage were found for all combinations in both species Fig.

Significantly positive correlations were found between maturation stages and GSI values Fig. The GSI value of A. Relationships between maturation stages and gonadosomatic indexes of female tropical anguillid eels Anguilla bicolor bicolor top and A. Those of female A.

We found significant positive correlations between maturation stages and eye and fin indexes for both species Fig. Relationships between maturation stages and eye top and fin indexes bottom of female tropical anguillid eels Anguilla bicolor bicolor left and A. We found significant positive correlations between maturation stages and TL and BW values in both species Fig. These results suggest that A.

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