Growth and salinity tolerance in hybrids within Salmo sp. and Salvelinus sp.

Attempts were made to complete all possible interspecific hybrid crosses between Atlantic salmon (Salmo salar), rainbow trout (Salmo gairdneri), brook trout (Salvelinus fontinalis), lake trout (Salvelinus namaycush), and Arctic char (Salvelinus alpinus). Survival was appreciable only in (giving female first) lake × brook, salmon × char, brook × char and char × brook hybrids. Growth rates over a weight increment of 3–30 g averaged 2.14% wet weight/day for all species and hybrids (14.5–17.0°C — excess rations). The highest growth rate, 2.74%, was obtained with the brook (female) × char (male) hybrid and the lowest, 1.63%, in lake trout. Salmon (female) × char (male) hybrids grew faster than salmon; brook trout (female) × char (male) hybrids grew faster than brook trout. Salmo species grew no faster than Salvelinus species but became silvery and had greater salinity tolerance at a smaller size and earlier age. Intergeneric hybrids between female Salmo spp. and male Salvelinus spp. more closely resembled the female parent in processes related to smoltification (silvering and salinity tolerance).     

Genetic Introgression Between Arctic Charr (Salvelinus Alpinus) and Brook Trout (Salvelinus Fontinalis) in Bavarian Hatchery Stocks Inferred from Nuclear and Mitochondrial DNA Markers

Nuclear insulin-like growth factor 2 gene (IGF-2), growth hormone 1 gene (GH-1) and internal transcribed spacer 1 (ITS-1) of the ribosomal DNA as well as the mitochondrial NADH-3 and NADH-4 dehydrogenase genes (ND-3/4) exhibited species-specific restriction fragment patterns and three microsatellite loci (Sfo18, Ssa85 and Ssa197) had non-overlapping allele size ranges in Arctic charr and brook trout and were used as diagnostic markers for testing genetic purity of hatchery stocks and wild populations of Arctic charr and brook trout in Bavaria, Germany. Screening of four wild populations (three in Arctic charr and one in brook trout) revealed only a single hybrid (back-cross to brook trout) individual in L. Starnberg. In contrast, in three (out of five) hatchery stocks of Arctic charr and in both hatchery stocks of brook trout hybrids were detected with the frequency from 3 to 100%. Three hatchery stocks (SS2, SA and BS1) represent a hybrid swarm because they contained a very high proportion of hybrids (from 83 to 100%) and most or all hybrid individuals had alien alleles at only one or a few of six unlinked diagnostic loci, indicating that post-F₁ hybrids represent the majority of individuals in these stocks and introgression has taken place. Release or escape of introgressed individuals from hatcheries into natural water bodies should be avoided in order to protect the biological diversity and genetic integrity of native fish populations.     

Survival, growth and dressing traits of triploid hybrids between rainbow trout and three charr species

Triploid hybrids between female rainbow trout Oncorhynchus mykiss and male brook charr Salvelinus fontinalis, Arctic charr S. alpinus and lake charr S. namaycush, together with diploid and triploid rainbow trout controls from the same dams, were tested in freshwater farming up to their fourth year of life. All hybrids displayed lower survival rates than the controls, the weakest genotype being the Arctic charr hybrid. Mortalities were mostly observed at the embryonic and larval stages and at the adult stage as a consequence of male sexual maturation. Growth of all hybrids was hindered (compared with controls) during the first year, but only moderate differences remained after 3 years. Sexual maturation resulted in a weight inferiority of males in all genotypes. As to carcass traits, female hybrids displayed a slightly higher dressing percentage than female triploid rainbow trout, as a result of lower visceral losses. These results are discussed with reference to hybrid resistance to rhabdoviruses from the viewpoint of fish farming improvement.     

Survival, growth, sexual maturation and reproduction of brook charr, Salvelinus fontinalis (Mitchill), Arctic charr, Salvelinus alpinus L., and their hybrids

Survival of hybrids (FA, AF) between brook charr, Salvelinus fontinalis (Mitchill), (FF) and Arctic charr, Salvelinus alpinus L., (AA) were compared at three fish farms. Survival of hybrids and Arctic charr was impaired in warm water. In cold water, brook charr and hybrids showed similar survival percentages until the spawning period. Each spawning period was followed by an increase of mortality, but to a much higher degree in brook charr. Growth was studied in the best (cold water) fish farm. During the first 2 years of rearing, brook charr had the highest relative weight. The weight of the two hybrids approached that of brook charr in the third year because of the drastic decrease of growth rate in brook charr after the spawning period. No differences between genotypes of either females or males could be detected in the eviscerated weights, the dressing percentages, or the gonado-somatic indexes. The two hybrids matured sexually, but their sperm quality was inferior to that of the pure species. Crosses between the 4-year-old breeders from the F1 population (FF, FA, AF and AA) were achieved. Survival from the eyed stage to 6 weeks post-hatching was significantly lower in F2 progeny than in back-crosses or pure species crosses. Even though the survival following spawning periods and the total biomass produced were better in hybrids than in brook charr raised in cold water, we believe the farming of this hybrid should not be carried out. The full development of sexually mature hybrids does not resolve problems linked with precocious sexual maturation in brook charr farming carried out in eastern Canada.     

Histopathology of gonadal neoplasms in cyprinid fish from the lower Great Lakes of North America

Abstract. The hybrids of carp, Cyprinus carpio L., and goldfish, Carassius auratus (L.), collected from the lower Great Lakes between 1978 and 1981 exhibited epizootics of gonadal neoplasms which were rare in the parental species. The gonadal histology of male and female carp captured for this study was typical of a successfully reproducing population but the hybrids appeared to be sterile. In male hybrids meiosis was often completed, but spermatids usually degenerated in early spermiogenesis. In female hybrids, vitellogenesis was frequently completed but there was no evidence of final maturation or ovulation. Proliferations of undifferentiated germ cells and stromal cells were evident in the tumour nodules of both the male and female hybrids and were considered to represent neoplastic transformations. Proliferations of Sertoli cells were common in males and in the male portion of intersex gonads, which were present in some hybrids. The Sertoli cells of these individuals were well differentiated, formed tubules and accumulated large stores of lipid. Extensive degeneration of germinal cells was often apparent in areas of Sertoli cell proliferation as were numerous, large, melano-macrophage centres. Gonadal stromal cell tumours did not form tubules or accumulate lipid. The spindle cells of these neoplasms could be found in homogeneous sheets or intermixed to varying degrees with the early stages of germ cells and/or connective tissue elements.     

Sex reversal in salmonid culture: Part III. The production and performance of all-female populations of brook trout

Oral administration of 17β-oestradiol at 20 mg/kg diet for a period from 0 to 60 days following “swim-up” resulted in the sex reversal of male brook trout and the production of essentially all-female populations. The gonads of these animals were indistinguishable from the ovaries of untreated fish.Oral administration of this hormone at the same concentration but for the period from 0 to 40 days resulted in a population which comprised 67% females, 21% males and 12% bisexual animals.Within the control group, the growth rate of males was significantly greater than that of females. Growth in the all-female stocks produced by dietary oestradiol administration (0–60 days) was significantly lower than that of the untreated population taken as a whole but was identical to that of the control females.The growth of brook trout during the first year was closely comparable to that of rainbow trout reared under identical conditions. During the second year, the growth of the rainbow trout exceeded that of the brook trout. At 19 months of age, all the animals in the control and oestradiol treated brook trout populations were fully mature; maturity in the rainbow trout was substantially confined to males.     

Induced diploid gynogenesis and polyploidy in crucian carp, Carassius auratus gibelio (Bloch), x common carp, Cyprinus carpio L., hybrids.

Abstract. Results of a 16 year (1977–1992) study of hybrids obtained from crossing two cyprinid fish, crucian carp, Carassius auratus gibelio (Bloch) (bisexual form), and common carp, Cyprinus carpio L., are presented. The ability to produce diploid eggs in F₁ females (resulting from chromosome endoreduplication in early oogenesis) was responsible for the high yield of spontaneous diploids in gynogenetic progenies of F₁ females and for triploidy in hybrids from back-crosses of F₁ females with males of the parent species. Different types of hybrids, including diploid and triploid gynogenetic, tetraploid (amphidiploid) and androgenetic progenies, were obtained. Diploid and some triploid hybrids (females only) were reproduced inter se by induced gynogenesis in several subsequent gynogenetic generations. Amphidiploid hybrids were obtained by crossing triploid F₈ females with diploid common carp males or by crossing diploid gynogenetic females with diploid inverted hybrid males producing diploid sperm. Results of cytogenetic investigations and fish culture properties in hybrids are presented. The scientific and practical significance of the data obtained are discussed.     

Occurrence of two forms of Y chromosome in rainbow trout (Oncorhynchus mykiss) males from Rutki strain

Morphologically differentiated sex chromosomes are found in only few salmonid species. Some populations of rainbow trout, Oncorhynchus mykiss, exhibit chromosomal polymorphism related to sex. We found sex-related chromosomal polymorphism in fish from the synthetic Rutki strain, Poland, in approximately 85% (n = 22) of examined males (XY-like) whereas the remaining males (n = 4) possessed chromosomes similar to these observed in females (XX-like). To investigate whether males possessing XX-like chromosomes were genotypic males or genotypic females with altered phenotypic sex, androgenetic progeny of four males (representing both XY-like and XX-like forms, n = 2 + 2) was examined. Androgenetic progeny (F₁) of all four fathers consisted of both phenotypic females and males. F₁ male progeny of two fathers showed XX-like chromosomes whereas F₁ male progeny of the other two fathers possessed YY-like (supermale) chromosomes. F₁ were reared further until they were sexually mature. Two males from each of four F₁ families were used to produce F₂ androgenetic and control F₂ generation. All F₂ individuals, androgenetics and control, were phenotypic males at sexual maturation. The results indicate that males possessing XX-like chromosomes are genetic males and they are not sex-reversed females. Thus, the Y chromosome can exist in different morphological forms in farmed rainbow trout. The Y^S chromosome (shorter form, unlike the X chromosome) has a shorter or absent p-arm and lacks the pericentromeric cluster of AT-rich chromatin and the 5S rDNA sequences that are found in the Y^L chromosome (longer form, like the X chromosome).     

Effects of estradiol-17β and 17α-methyltestosterone on gonadal differentiation in the coho salmon, Oncorhynchus kisutch

Methods are described for controlling sex differentiation in the coho salmon, Oncorhynchus kisutch. Eyed eggs and alevins were immersed in solutions of estradiol-17β and 17α-methyltestosterone and fed these steroids in the diet for a period of 10 weeks postswim-up. In seven out of 10 groups that received estradiol all fish resembled normal females when sampled at 4 months posthatch. In fish treated with methyltestosterone at all except the lowest dose (25 and 50 μg/l) the gonads resembled neither normal males nor normal females; these gonads were composed largely of connective tissue with only occasional germ cells. Various proportions of male, female and intersex gonads were observed in groups which received androgen or estrogen only in the diet. The implications of the work for salmon culture include the production of sterile fish and increasing the proportion of female salmon in hatchery populations.     

Characterization of triploid hybrid groupers from interspecies hybridization (Epinephelus coioides ♀ × Epinephelus lanceolatus ♂)

Interspecies hybridization is widely used in aquaculture as a beneficial strategy. Diploid and triploid hybrids have been detected from the interspecies hybridization of Epinephelus coioides ♀ × Epinephelus lanceolatus ♂. This is the first report of triploidization through hybridization in grouper. Confirmation has been obtained through flow cytometry, karyotyping and erythrocyte nuclei measurement. The chromosome numbers of E. coioides, E. lanceolatus, diploid hybrid grouper are 48 and triploid hybrid grouper are 72. Measurements of erythrocyte nuclei indicate that triploid fish have a larger nuclear surface than the diploid groupers, and the average ratio of triploid to diploid surface area is 1.59. During the first 1.5 years, triploid hybrid groupers grow faster than diploid hybrid groupers or either parent species. The average growth rate of triploid hybrids is 1.61 times greater than that of diploid hybrids at 6 months of age and 1.43 times greater at 18 months of age. The triploid hybrid groupers are inferior in gonadal development, with no primary‐growth‐stage oocytes appearing in the gonads at 18 months of age. Morphological studies indicate that triploid hybrid groupers have distinctive differences in snout length, eye diameter, body trunk shape, and tail shape development compared with diploid hybrid groupers. Triploid hybrid groupers have an advantage in growth ability, and artificial breeding of triploid groupers might be of great potential use in the grouper aquaculture industry.     

No difference in survival, growth and morphology between offspring of wild-born, hatchery and hybrid brown trout (Salmo trutta)

Abstract –  We studied survival, growth and morphological characters in the offspring of native hatchery and wild-born anadromous brown trout ( Salmo trutta ) and their hybrids (wild-born female × hatchery male and wild-born male × hatchery female) in a 1-year field experiment. We also conducted laboratory studies where we examined social interactions between the offspring of the same hatchery and wild-born trout. All offspring were raised in a hatchery and nose tagged before being released into the stream. In total, 1125 individuals were released into the stream (1999) and a total of 614 individuals were recovered (2000). We found no differences in growth and survival between the offspring of hatchery, wild-born and hybrid trout. Morphology was also similar among groups, where only 38% females and 36% males were classified into the right category, which were only 12% better than random classification. In the laboratory experiment, we compared only the offspring of hatchery and wild-born trout with respect to growth, dominance, aggressiveness, feeding and activity. We found small differences between the offspring of hatchery and wild-born fish with respect to growth but this effect was not found in the field experiment. Our result suggests that the offspring of hatchery trout and hybrids between hatchery and wild-born trout performed equally well to the offspring of wild-born trout.     

Ultrasound evaluation of the gonadal structure in sex-reversed rainbow trout females

Ultrasound approach has been used to assess morphology of the gonadal structure in the sex-reversed rainbow trout females (neomales) provided in the course of 11β-hydroxyandrostenedione treatment applied within gonadal differentiation period. Eighteen matured individuals (in range 38.0–48.6 cm of body length and 802–1644 g of body weight) were scanned using digital ultrasound apparatus (DP-6900 model) Mindray Ltd., during the spawning season. After screening, fishes were killed to validate the morphological configuration and position of the gonads in the body cavity. The favorable place for cross-sectional imaging of gonadal lobes was half of the distance between the pectoral and pelvic fins below lateral line of neomales. Most of the examined specimens (61 %) had properly shaped, paired testis. Moreover, presence of individuals with asymmetrical gonads (33 %) and one bisexual fish was confirmed. There were no differences between total volume of sampled semen and sperm motility, but statistically significantly different in sperm concentration within selected (with symmetrical and asymmetrical testis) groups of neomales. It was confirmed that ultrasonic imaging is an efficient and accurate method to determine the state of gonads of mature sex-reversed rainbow trout females during spawning season and offers the opportunity for non-invasive detection of any morphological anomalies in their gonads.     

半滑舌鳎Shh基因的克隆与表达及甲基化分析

通过RACE方法获得半滑舌鳎(Cynoglossus semilaevis)Hh基因家族的Shh基因,该基因全长1 922 bp,其中5′UTR 266 bp,3′UTR 360 bp,ORF1 296 bp,编码431个氨基酸,经预测该多肽的相对分子质量为47.28 k D,理论等电点6.95,具有Hh基因家族特有的保守结构域。氨基酸序列分析表明,半滑舌鳎Shh与牙鲆的同源性最高,为82%,与其他鱼类同源性为74%~81%,与非洲爪蟾的同源性为61%,与人和鼠的同源性为63%~64%。甲基化结果显示,Shh基因在1龄卵巢中的甲基化程度普遍低于雄鱼与伪雄鱼精巢。基因表达分析显示,Shh基因在胚胎期的囊胚期表达显著高于其它时期(P0.05),在雌雄成鱼8个组织器官均表达,在雌性性腺分化的关键期孵化后50 d,雌性性腺中表达量较前期升高,且显著高于同时期雄性性腺中的表达(P0.05),在雄性性腺分化的关键期80~95 d,Shh基因在雄性中的表达水平显著升高(P0.05),在8月龄、1龄及2龄雌鱼性腺中表达显著高于雄鱼与伪雄鱼。本研究表明,半滑舌鳎Shh基因在进化中高度保守,与胚胎分化、组织器官形成、雌雄性腺分化及性腺发育密切相关。     

美洲红点鲑和白斑红点鲑杂交子代及其亲本早期生产性能比较

采用完全双列杂交、群体间杂交及群体内自繁等方法对美洲红点鲑（Salvelinus fominalis）和白斑红点鲑（Salvelinus leucomaenis）进行了配组繁殖,成功地获得了4类组合的子一代群体。比较了各交配组合的受精率、孵化率、成活率等生存力,结果表明：正交子一代（白斑红点鲑♀×美洲红点鲑♂）组合的受精率明显高于父本和反交子一代组合,与母本子一代组合差异不显著（p〉0．05）,孵化率和成活率与双亲自繁对照组差异不显著（P〉O．05）,高于反交子一代（美洲红点鲑♀×白斑红点鲑♂）组合。杂交子一代和亲本后代140d室内生长试验表明：不同发育阶段的正交子一代表现出杂种优势,体质量的杂种优势率为27．09％～35．66％,体长的杂种优势率为8．47％～11．25％,在体质量、体长增长量均高于双亲子一代。反交子一代群体生长发育始终没有表现杂种优势（H〈0）。     

Alterations in iron, zinc and major plasma proteins of rainbow trout, Oncorhynchus mykiss (Walbaum), and brook trout, Salvelinus fontinalis (Mitchill), with sterile peritonitis induced by oil-adjuvanted multivalent bacterin vaccination

The early humoral responses of rainbow trout, Oncorhynchus mykiss (Walbaum), and brook trout, Salvelinus fontinalis (Mitchill), with sterile inflammation induced by intraperitoneal Lipogen Triple vaccination were compared to determine if genetic differences in susceptibility to furunculosis in salmonids correlated with different acute phase responses to vaccination. Similar severe acute sterile peritonitis occurred in response to Lipogen Triple in both species. Both species also had a rapid transient reduction in plasma iron concentration at 3 days. Moderate hypoferraemia persisted to day 14 in brook trout, but returned to normal by day 7 in rainbow trout. Plasma zinc decreased sharply 3 days after vaccination in rainbow trout and returned nearly to control levels by day 10; however, plasma zinc did not change in brook trout. Two-dimensional sodium dodecyl sulphate polyacrylamide electrophoresis of plasma proteins revealed that increased amounts of a 48-kDa protein group coincided with the hypoferraemic response in rainbow trout. In addition, a modest elevation in a 16-kDa protein group also occurred in rainbow trout. These studies demonstrated the rapid changes in plasma iron in both species and mild elevation of two putative acute phase plasma proteins associated with vaccine-induced inflammation in rainbow trout.     

The purification and development of a quantitative enzyme linked immunosorbent assay (ELISA) for the measurement of vitellogenin in diploid and triploid brook trout (Salvelinus fontinalis)

Vitellogenin (VTG) was purified from the plasma of estradiol-17β (E₂) treated male brook trout (Salvelinus fontinalis; bt) using gel filtration and anion exchange chromatography. An antiserum to bt-VTG was raised in rabbits, then used to detect bt-VTG by Western blot analysis. Purified bt-VTG and its antibody were then used to develop an antibody-capture, competitive enzyme-linked immunosorbant assay (ELISA). Confirmation of the purified protein as bt-VTG was based on the characteristics of high molecular weight (∼562 kDa), dominant plasma protein in vitellogenic females and induction by exogenous E₂ treatment in males. The antiserum recognized a major 184 kDa polypeptide as well as minor bt-VTG degradation products (150–66 kDa) in purified bt-VTG and in plasma from vitellogenic females. Low levels of antibody cross reactivity were shown with plasma from nonvitellogenic females, control rabbit serum, and several other antisera known to not contain VTG. The ELISA had a minimum detection limit of 8 ng ml⁻¹ and intra- and inter-coefficients of variation less than 9% and 15%, respectively. The ELISA demonstrated parallel binding slopes among dilution curves of purified bt-VTG standard and plasma from diploid and triploid brook trout females. Using the ELISA, maximum plasma VTG levels of 93.5±33.6 mg ml⁻¹ were detected in vitellogenic diploid females, whereas only 0.18±0.15 mg ml⁻¹ were detected in triploid females of the same age (n=5 for each). Diploid and triploid males cohabitating with vitellogenic females showed measurable levels of plasma VTG during vitellogenesis in females (i.e., 0.17 and 0.06 mg ml⁻¹, respectively (n=1)). This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of the estrogen mimic genistein as a dietary component on sex differentiation and ethoxyresorufin-O-deethylase (EROD) activity in channel catfish (<Emphasis Type="Italic">Ictalurus punctatus</Emphasis>)

A number of aquaculture species, including channel catfish Ictalurus punctatus, are fed high proportions of soybean meal in their diet. We have investigated the potential for the most common phytoestrogen in soybean meal to alter phenotypic sex during sexual differentiation in channel catfish. Channel catfish were fed four dietary concentrations of the phytoestrogen genistein (0, 2, 4, and 8 mg g⁻¹) to determine its effect on gonadal sex differentiation. The four treatment diets were fed to sexually undifferentiated channel catfish between 5 and 140 days post hatch (dph) and between 60 and 150 dph. Phenotypic sex was determined by histological examination of the gonads. Ethoxyresorufin-O-deethylase activity was not significantly different among the treatment and control groups. Phenotypic sex was significantly dependant on dietary phytoestrogen concentration (P = 0.01). Additionally, logistic regression showed a significant relation between genistein concentration in the diet and gonadal sex (P = 0.02). Intersex individuals were present at all treatment concentrations, with increasing proportions of intersex fish as the genistein concentration increased for individuals fed treated diets between 5 and 140 dph. Increased proportions of phenotypically male individuals resulted from chronic dietary exposure to the estrogen mimic genistein. There were no significant differences in the proportions of males and females between feed treatment durations. These findings establish that dietary concentrations of genistein can alter sex ratios in cultured channel catfish populations and demonstrates the need to further understand the actions of this and other prominent phytoestrogens in aquaculture species.     

Comparison of growth rate in Atlantic salmon,pink salmon,Arctic char,sea trout and rainbow trout under Norwegian farming conditions

Growth rates of Atlantic salmon, pink salmon, Arctic char, sea trout and rainbow trout were compared under Norwegian farming conditions. During the juvenile, freshwater period, growth was fastest in pink salmon, followed by rainbow trout and Arctic char. Freshwater growth of sea trout and, especially, Atlantic salmon, was slow. After transfer of smolts or fingerling to sea water, Arctic char failed to survive the autumn. Sea water growth of sea trout was slow, but the three species, rainbow trout, Atlantic salmon and pink salmon, all grew rapidly through all seasons. When in sea water, rainbow trout and pink salmon were regularly attacked by vibriosis, while Atlantic salmon were rarely attacked, and sea trout never. It is concluded that, for commercial farming in Norway, rainbow trout are of value for production of fish of any size up to 3–4 kg, and pink salmon for production of small fish of 0.5–1.5 kg. Atlantic salmon is the only species suitable for production of a very large salmonid, i.e., more than 4–5 kg.     

Maternal variation in juvenile survival and growth of triploid hybrids between female rainbow trout and male brown trout and brook charr

Related sib‐groups of rainbow trout × brown trout and rainbow trout × brook charr triploid hybrids and monospecific diploid and triploid rainbow trout controls were obtained from a common set of rainbow trout dams. On the basis of hybrid juvenile performances, 10 sib‐groups were selected and the corresponding diploid rainbow trouts were raised up to adult stage. Females from each group of rainbow trout were used to produce a second generation of hybrid progeny, the performances of which were analysed for grandmaternal variation and relation with first‐generation relatives. Results showed that hybrid traits (alevin yield and weight, survival and growth of fingerlings) were strongly influenced by maternal origin, and could be correlated to those of rainbow trout controls, but that maternal abilities had a low rate of inheritance. It was concluded that little improvement can be expected through selective breeding within parental populations.     

Differentiation of gonad maturation in sibling precocious males of the sea trout (Salmo trutta m. trutta L.) in their first year of life

Age at maturity and dynamics of gonadal cycle vary among individuals in a standard population; the extent of that variability in a group of siblings is not known for any species. Therefore, the aim of the study was to find out how the differentiation of gonad development varies in a group of sibling fish. The study involved 176 seven-month-old sea trout parr (mean fork length of 11 cm) collected at a single time point during their first spawning season (late October) from the brook of Pniewa in Western Pomerania (Poland). Gonad maturation was determined from histological analysis which was interpreted according to specially developed criteria.

The stage and forms of gonad maturation were diverse in the sibling group studied. Gonads of 52.8% males were not maturing (Class I), with only spermatogonia A present. The remaining males (47.2%) had gonads differing widely in the development, from beginning of maturation to completion of the reproductive cycle (Classes from II to VIII). In addition to the classes described, some males were observed to feature attempted maturation. It is difficult to provide an unequivocal estimate of the proportion of precocious males (mature at a young age, prior to the first descent to the sea) in the first year of life because of difficulties in categorizing males in Class II. It was predicted that 32.4% of the population would complete the cycle during the first spawning season. The body length and weight of the maturing males was significantly smaller (P < 0.05) than that of the non-maturing males.

Three forms of gonad maturation were identified. Gonads of a large group of precocious males (70%) developed incompletely (only some germ cell developed during the reproductive cycle), some of those gonads assumed the form of attempted maturation and partial maturation. Testes of the remaining 30% of the males underwent complete maturation. Gonads that undergo attempted maturation do not spermiate.

It is only from microscope-aided analysis of the testes that the class of gonad development can be accurately evaluated and the extent of precocious maturation assessed, particularly in fish maturing for the first time. It is impossible to correctly identify maturing males of the underyearling sea trout on the basis of morphological examination of gonad appearance, abdominal massage to detect spermiation or gonadosomatic index value.