Aspects of the Biology of Juvenile Freshwater Eels (Anguillidae) in New Zealand

<p>The early freshwater life of the two species of New Zealand freshwater eels, Anguilla australis schmidtii Phillipps and A. dieffenbachii Gray was studied involving an examination of 8131 glass-eels, 5275 migratory elvers, and 4291 resident eels of less than 26 cm. Most eels were collected from the Makara Stream, Wellington by set-net, hand-net and electric fishing. These extensive samples together with subsidiary collections from elsewhere in New Zealand show that glass-eels of both species arrive in fresh-water from July to December. Their otoliths indicate a marine larval life of about 18 months but it is not possible as yet to locate the precise oceanic spawning areas. Migratory movements of glass-eels are in two phases: an invasion of fresh-water from the sea and an upstream migration. The former occurs only at night with a periodicity corresponding to the daily ebb-flood tidal rhythms. There is a seasonal reversal in this response which is attributable to the onset of the behavioural transition taking place prior to the second migratory phase. Increased pigmentation and changes in response to light, flowing fresh-water and schooling tendencies characterise this latter migration which occurs primarily at spring tide periods. Such juvenile eels show specific habitat preferences and a high degree of olfactory differentiation of water types. This behaviour, together with pigment development and physical tolerances, was studied in the laboratory. Measurements of invading glass-eels show that mean length, weight and condition all decline throughout the season of arrival but mean vertebral numbers remain constant. An upstream migration of small eels (elvers) occurs each summer and is readily observed at many hydro-electric stations. These migrations, comprising eels of mixed sizes and age groups, penetrate progressively further upstream each year. In both species, scales begin formation at body lengths of 16.5-20 cm. All features of scale formation, including the number of scale rings, are related to length with relative differences in rate of development occurring between the species. In contrast to scale rings, otolith rings are annual in formation and become visible after grinding or burning the otolith. Growth rates established for 273 eels to 29 cm in length from the Makara Stream, Wellington, are slow, with mean annual increments of 2.2 and 2.1 cm respectively for shortfins and longfins. In contrast, shortfins from a coastal lake near Wellington reach 26 cm in their third year of freshwater life. Length-weight relationships for small eels are given together with mean monthly condition factors. Growth studies on elvers held in a multiple tank unit in which temperature, density, and amount and frequency of feeding could be controlled, show that young eels grow more slowly than normal under such conditions. However, growth appears optimum at 20 degrees C with a feeding rate of 5-7% body weight per day. Feeding efficiency decreases with higher temperatures. At both glass-eel and elver stages, shortfins adapt and survive better under artificial conditions.</p>

Aspects of the Biology of Juvenile Freshwater Eels (Anguillidae) in New Zealand

<p>The early freshwater life of the two species of New Zealand freshwater eels, Anguilla australis schmidtii Phillipps and A. dieffenbachii Gray was studied involving an examination of 8131 glass-eels, 5275 migratory elvers, and 4291 resident eels of less than 26 cm. Most eels were collected from the Makara Stream, Wellington by set-net, hand-net and electric fishing. These extensive samples together with subsidiary collections from elsewhere in New Zealand show that glass-eels of both species arrive in fresh-water from July to December. Their otoliths indicate a marine larval life of about 18 months but it is not possible as yet to locate the precise oceanic spawning areas. Migratory movements of glass-eels are in two phases: an invasion of fresh-water from the sea and an upstream migration. The former occurs only at night with a periodicity corresponding to the daily ebb-flood tidal rhythms. There is a seasonal reversal in this response which is attributable to the onset of the behavioural transition taking place prior to the second migratory phase. Increased pigmentation and changes in response to light, flowing fresh-water and schooling tendencies characterise this latter migration which occurs primarily at spring tide periods. Such juvenile eels show specific habitat preferences and a high degree of olfactory differentiation of water types. This behaviour, together with pigment development and physical tolerances, was studied in the laboratory. Measurements of invading glass-eels show that mean length, weight and condition all decline throughout the season of arrival but mean vertebral numbers remain constant. An upstream migration of small eels (elvers) occurs each summer and is readily observed at many hydro-electric stations. These migrations, comprising eels of mixed sizes and age groups, penetrate progressively further upstream each year. In both species, scales begin formation at body lengths of 16.5-20 cm. All features of scale formation, including the number of scale rings, are related to length with relative differences in rate of development occurring between the species. In contrast to scale rings, otolith rings are annual in formation and become visible after grinding or burning the otolith. Growth rates established for 273 eels to 29 cm in length from the Makara Stream, Wellington, are slow, with mean annual increments of 2.2 and 2.1 cm respectively for shortfins and longfins. In contrast, shortfins from a coastal lake near Wellington reach 26 cm in their third year of freshwater life. Length-weight relationships for small eels are given together with mean monthly condition factors. Growth studies on elvers held in a multiple tank unit in which temperature, density, and amount and frequency of feeding could be controlled, show that young eels grow more slowly than normal under such conditions. However, growth appears optimum at 20 degrees C with a feeding rate of 5-7% body weight per day. Feeding efficiency decreases with higher temperatures. At both glass-eel and elver stages, shortfins adapt and survive better under artificial conditions.</p>

Studies on the Reproductive Biology of New Zealand Freshwater Eels

<p>Macroscopic and histological observations of the gonads from 1,739 non-migrant freshwater eels, the shortfin Anguilla australis schmidtii Phillipps and the longfin A. dieffenbachii Gray, showed that they pass through seven stages of development. Shortfins become sexually differentiated at body lengths of 35.0cm to 56.9cm and longfins at lengths of 50.0cm to 67.0cm. No intersexual stage was present, as in A. anguilla L., and although 1% of 350 migrating longfin males examined contained ribbon-like testes, the typical lobed organ of Syrski (testis) can be used as diagnostic of maleness. Histologically, the maximum stage of development attained in the non-migrant, immature stage, was spermatogonia in the males and vacuolated oocytes in females. At the time of seaward migration, based on gonad histology, gonadosomatic indices and ova diameters, migrating longfins were more sexually developed than shortfins. These differences may relate to the location of different oceanic spawning areas: that for the longfin possibly being closer to New Zealand. The autumnal migratory runs, from March to May, of the sexually maturing adults in the Makara stream showed no particular species or sex sequence. The movement of eels was coincident with a rise in stream level and the second half of the lunar cycle. Other relevant environmental factors are discussed. In Lake 0noke peak catches of seaward migrating shortfins were made before the longfins and movements of eels occurred throughout the lunar cycle. Once at sea, the eels apparently disappear. A published note is included on the first eel of the New Zealand species, a longfin female, to be caught at sea. Age determinations from 995 eels were made by otoliths, which were burnt lightly to intensify the growth zones for reading purposes. Shortfin males are younger than females at migration. Longfins are older than shortfins at migration but the males are younger than the females. In the non-migrant stage, sexually undifferentiated shortfins grow more slowly relative to the males, and males relatively more slowly than the females. Similar but less significant differences in growth occur in longfins. Migrant males held in seawater were induced to mature and spawn with injections of mammalian hormones or carp pituitaries, over temperatures of 11.8 degrees C to 28 degrees C. The maturation period was dependent on temperature. Testes of experimental eels that survived maturation regressed to the pre-migrant or migrant stage. Two eels that had regressed were induced to mature a second time. Females held at 20 degrees C and injected with mammalian hormones showed significant increases in sexual development but died before maturity. Females injected with carp pituitaries matured and spawned. Mature longfin eggs, 0.9mm to 1.2mm in diametar, and mature shortfin eggs, 0.9mm to 1.2mm in diameter, are translucent and contain one to many oil globules. A blastodisc formed in water hardened eggs but attempts at fertilization were unsuccessful. Gametogenesis, observed from non-migrant, migrant and hormone injected eels is similar to that described for other teleosts. Electron microscope observations showed parallel features of spermiogenesis in both species. Mature spermatozoa have crescent shaped heads with an anteriorly placed mitochondrion. A flagellum of the unusual 9 + 0 pattern arises from the posterior region of the head, and a short, striated rod-like structure is positioned adjacent to the main flagellum. A complex of subfibrils which extend along either side of the head to the mitochondrion arise from the proximal centriole.</p>

Studies on the Reproductive Biology of New Zealand Freshwater Eels

<p>Macroscopic and histological observations of the gonads from 1,739 non-migrant freshwater eels, the shortfin Anguilla australis schmidtii Phillipps and the longfin A. dieffenbachii Gray, showed that they pass through seven stages of development. Shortfins become sexually differentiated at body lengths of 35.0cm to 56.9cm and longfins at lengths of 50.0cm to 67.0cm. No intersexual stage was present, as in A. anguilla L., and although 1% of 350 migrating longfin males examined contained ribbon-like testes, the typical lobed organ of Syrski (testis) can be used as diagnostic of maleness. Histologically, the maximum stage of development attained in the non-migrant, immature stage, was spermatogonia in the males and vacuolated oocytes in females. At the time of seaward migration, based on gonad histology, gonadosomatic indices and ova diameters, migrating longfins were more sexually developed than shortfins. These differences may relate to the location of different oceanic spawning areas: that for the longfin possibly being closer to New Zealand. The autumnal migratory runs, from March to May, of the sexually maturing adults in the Makara stream showed no particular species or sex sequence. The movement of eels was coincident with a rise in stream level and the second half of the lunar cycle. Other relevant environmental factors are discussed. In Lake 0noke peak catches of seaward migrating shortfins were made before the longfins and movements of eels occurred throughout the lunar cycle. Once at sea, the eels apparently disappear. A published note is included on the first eel of the New Zealand species, a longfin female, to be caught at sea. Age determinations from 995 eels were made by otoliths, which were burnt lightly to intensify the growth zones for reading purposes. Shortfin males are younger than females at migration. Longfins are older than shortfins at migration but the males are younger than the females. In the non-migrant stage, sexually undifferentiated shortfins grow more slowly relative to the males, and males relatively more slowly than the females. Similar but less significant differences in growth occur in longfins. Migrant males held in seawater were induced to mature and spawn with injections of mammalian hormones or carp pituitaries, over temperatures of 11.8 degrees C to 28 degrees C. The maturation period was dependent on temperature. Testes of experimental eels that survived maturation regressed to the pre-migrant or migrant stage. Two eels that had regressed were induced to mature a second time. Females held at 20 degrees C and injected with mammalian hormones showed significant increases in sexual development but died before maturity. Females injected with carp pituitaries matured and spawned. Mature longfin eggs, 0.9mm to 1.2mm in diametar, and mature shortfin eggs, 0.9mm to 1.2mm in diameter, are translucent and contain one to many oil globules. A blastodisc formed in water hardened eggs but attempts at fertilization were unsuccessful. Gametogenesis, observed from non-migrant, migrant and hormone injected eels is similar to that described for other teleosts. Electron microscope observations showed parallel features of spermiogenesis in both species. Mature spermatozoa have crescent shaped heads with an anteriorly placed mitochondrion. A flagellum of the unusual 9 + 0 pattern arises from the posterior region of the head, and a short, striated rod-like structure is positioned adjacent to the main flagellum. A complex of subfibrils which extend along either side of the head to the mitochondrion arise from the proximal centriole.</p>

Influence of environmental parameters on habitat use by sympatric freshwater eels, Anguilla marmorata and A. japonica on Yakushima Island, Japan

It is known that environmental parameters, such as water depth, size of substrate material, and current velocity influence the organization of stream fish communities. However, few studies have investigated the effects of these parameters on sympatric freshwater eels. Here, the habitat use of two anguillid eels, Anguilla marmorata Quoy and Gaimard, 1824 and A. japonica Temminck and Schlegel, 1846, coexisting in the same river systems in Japan was investigated. It was found that the tropical species A. marmorata exclusively utilized habitats with larger substrate materials, fewer fallen leaves and less leaf detritus, higher current velocity, and lower turbidity than the temperate species A. japonica, and the distributions of the two anguillid eels were clearly segregated. Thus, when both species co-occur in the same river systems, clear flowing mainstems and tributaries are preferred habitats for A. marmorata, while stagnant muddy estuaries, backwater areas, irrigation channels, and reservoirs are preferred habitats for A. japonica. These habitat segregations were consistent through body-size increases and life-stage developments from elver to yellow eel and did not show ontogenetic shifts. The findings indicated that both species had strong habitat preferences, and these environmental parameters must be considered in the conservation of anguillid eels in sympatric zones.