Thyroid hormones in eggs of various freshwater,marine and diadromous teleosts and their changes during egg development

Thyroid hormone concentrations in unfertilized eggs of 26 species of various freshwater, marine and diadromous teleosts were examined, together with changes in their concentrations during egg development in some species. Significant quantities of both thyroxine (T₄) and triiodothyronine (T₃) were found in eggs of all species examined. Mean T₄ and T₃ concentrations in eggs varied from 0.04 (marbled sole) to 15.00 ng/g (chum salmon), and from 0.07 (goldfish) to 9.95 ng/g (Pacific herring), respectively. T₄ concentrations were significantly greater than T₃ concentrations in eggs of most freshwater fishes, whereas T₃ concentrations were greater in seawater fishes. During the course of development, thyroid hormones in eggs decreased markedly before hatching. These findings suggest that thyroid hormones are consistently present in teleost eggs, and thus may play an important role in the egg development.     

Comparison of factors affecting recruitment variability of walleye pollock Theragra chalcogramma in the Pacific Ocean and the Sea of Japan off northern Japan

The Japanese Pacific stock (JPS) and the northern Japan Sea stock (JSS) of walleye pollock Theragra chalcogramma are mainly distributed in the Pacific Ocean and the Sea of Japan off northern Japan, respectively. This paper summarizes and compares the factors affecting the recruitment variability of these two stocks. Spawning season is from December to March for both stocks. JPS recruitment has a positive relationship with the water temperature in January and February, whereas that of JSS has a negative relationship with the water temperature in January, February, and April. One possible reason for this is that pollock larvae have an optimum growth temperature of approximately 5 °C in the field. Drift of early life stages also appears to be an important influence on the recruitment of both stocks. Because the current generated by the northwest wind carries eggs of JPS into the main larval nursery ground, JPS recruitment is enhanced in years when the northwest wind is predominant in February. On the other hand, early life stages of JSS are transported into the nursery ground by the Tsushima Warm Current. However, this current also carries early life stages into the Sea of Okhotsk and offshore, resulting in poor JSS recruitment in years when this current is strong in March. In contrast to JPS, the recruitment of which is significantly impacted by cannibalism, young pollock have not been found in the stomachs of adult JSS. Warm temperatures in the Sea of Japan seem to induce the separation of young and adult pollock, and the shape of the stock–recruitment relationship also suggests that cannibalism is not important for JSS. Based on this knowledge, and on the hatch date distributions of larvae and juveniles, we propose mechanisms that can explain the recruitment fluctuations for JPS and JSS pollock.     

Larval performance of matrinxã, Brycon amazonicus (Spix & Agassiz 1829), after maternal triiodothyronine injection or egg immersion

This study compared the larval performance of matrinxã, Brycon amazonicus, after maternal triiodothyronine (T₃) injection or egg immersion of T₃. In the first experiment, three groups of females (n=4) induced to spawning received pituitary extract (CPE) and a corn oil injection (control), or CPE plus 10 mg or 20 mg kg^?1 bw T₃ dissolved in corn oil (experimental). Larvae were sampled for body weight and length measurement at hatching (0 h) and 12, 24, 36, 48 and 60 h thereafter. Hatching time, hatching success and abnormal development were monitored. In the second experiment, fertilized eggs from four females were immersed in T₃ solutions (0, 0.01, 0.05 and 0.10 mg L^?1) and larvae were sampled at hatching (0 h) and 6, 18, 30, 42, 54, 126 and 198 h thereafter. Hatching time was not affected by either means of hormone treatment. Abnormalities decreased as the T₃ concentration increased in larvae from T₃‐treated broodfish but the number of dead larvae increased proportionally. Larvae from T₃‐injected females had higher weight from 24 h after hatching and greater length from hatching, while the weight of larvae produced from T₃‐immersed eggs changed at 198 h and length from 126 h of rearing. Both routes of T₃ administration affected the early growth of matrinxã but the effect was observed earlier when broodstock females were injected.     

Changes in thyroid hormone levels in eggs and larvae and in iodide uptake by eggs of coho and chinook salmon,Oncorhynchus kisutsch andO. tschawytscha

Developmental profiles of thyroxin (T₄), triiodothyronine (T₃) and radioactive iodide uptake were established for eggs and T₄ and T₃ profiles were established for larvae (whole-body, yolk-only and body-only) of coho and chinook salmon. T₄ and T₃ were consistently present in all samples. In eggs, hormone levels remained fairly constant in all cohorst for at least the first three weeks of incubation, but then fluctuated in both directions in some sample groups. Large increases in T₄ (from 9 ng/g to 245 ng/g) were seen in 1985 chinook eggs 28 days after fertilization. Radioactive iodide uptake (which was used as a possible indicator of thyroxinogenesis) increased at least 10-fold in both 1986 coho and chinook eggs from 23–30 days after fertilization. T₄ (62 ng/g) and T₃ (393 ng/g) were found in the bodies of 28-day-old 1986 chinook embryos. In whole larvae, hormone levels varied depending upon the cohort studied. In general, initial body-only concentrations of both T₄ and T₃ decreased as body weight increased, but before yolksac resorption was completed, both thyroid hormone content and concentration increased (except for chinook T₃). T₄ and T₃ content in larval yolk stayed constant as yolksac size decreased, resulting in increased thyroid hormone concentration in the yolksac. All of these data suggest that the initial source of thyroid hormones in coho and chinook salmon eggs is maternal, but that by approximately 3–4 weeks after fertilization, the developing embryos begin to produce their own thyroid hormones. After hatching, increases in tissue T₄ and T₃ concentration coupled with constant T₄ and T₃ content in diminishing yolksacs suggest that larvae also produce their own thyroid hormones; yolksac content then may reflect both the original maternal hormones and the larva-producted hormones.     

Salinity and temperature effects on whole-animal thyroid hormone levels in larval and juvenile striped bass,Morone saxatilis

Whole-animal thyroxine (T₄) and 3,5,3′-triiodothyronine (T₃) levels were measured in larval and juvenile striped bass, Morone saxatilis, reared for 10 days at one of three levels of salinity (equivalent to fresh water (FW), one-third seawater (1/3 SW), and seawater (SW) and two temperatures (15°C and 20°C). The striped bass were pre-metamorphic larvae, metamorphic larvae or juveniles. The short-term effects of seawater on plasma T₄ levels of juvenile striped bass were also measured. Higher salinities increased T₄ levels in premetamorphic larvae. In metamorphic larvae, SW and 1/3 SW increased T₄ levels and SW increased T₃ levels at 20°C. This response was eliminated in those at 15°C. Whole-animal thyroid hormone content was unaffected by salinity or temperature in juvenile striped bass, although significant fluctuations in plasma T₄ levels occurred in those transferred to 1/3 SW and SW. The thyroid axis of striped bass responds to salinity and temperature as early as in the pre-metamorphic stage. Thyroid hormones may mediate the beneficial effects of salinity on larval striped bass growth and survival.     

Stock assessment and management for walleye pollock in Japan

We review the stock assessment strategies and management procedures for walleye pollock Theragra chalcogramma in Japan. In Japan, walleye pollock is classified into 4 stocks. Because biological data, fishing conditions, etc. are different for each stock, the stocks are assessed by different methods. Harvest strategies aiming at stock recovery are proposed for the Northern Japan Sea stock and the Nemuro Strait stock, which are currently in poor condition. For the Japanese Pacific stock and the Southern Okhotsk Sea stock, which are in good condition, harvest strategies for current fishery operations are proposed. In Japan, fisheries co-management has traditionally been carried out, and in recent years a total catch limitation system called the total allowable catch, a resource recovery plan, and a resource management plan have also been implemented. Although a plan is devised that accounts for the stock conditions of walleye pollock, it is also necessary to consider socioeconomic factors, ecosystem factors, and so on. However, we consider that the main focus of stock management for walleye pollock will still be maintaining fishing pressure at an appropriate level, which includes regulating fish size and price during the fishing season.     

Integrating seabird dietary and groundfish stock assessment data: Can puffins predict pollock spawning stock biomass in the North Pacific?

Information on the annual variability in abundance and growth of juvenile groundfish can be useful for predicting fisheries stocks, but is often poorly known owing to difficulties in sampling fish in their first year of life. In the Western Gulf of Alaska (WGoA) and Eastern Bering Sea (EBS) ecosystems, three species of puffin (tufted and horned puffin, Fratercula cirrhata, Fratercula corniculata, and rhinoceros auklet, Cerorhinca monocerata, Alcidae), regularly prey upon (i.e., “sample”) age-0 groundfish, including walleye pollock (Gadus chalcogramma, Gadidae) and Pacific cod (Gadus microcephalus, Gadidae). Here, we test the hypothesis that integrating puffin dietary data with walleye pollock stock assessment data provides information useful for fisheries management, including indices of interannual variation in age-0 abundance and growth. To test this hypothesis, we conducted cross-correlation and regression analyses of puffin-based indices and spawning stock biomass (SSB) for the WGoA and EBS walleye pollock stocks. For the WGoA, SSB leads the abundance of age-0 fish in the puffin diet, indicating that puffins sample the downstream production of the WGoA spawning stock. By contrast, the abundance and growth of age-0 fish sampled by puffins lead SSB for the EBS stock by 1–3 years, indicating that the puffin diet proxies incoming year class strength for this stock. Our study indicates connectivity between the WGoA and EBS walleye pollock stocks. Integration of non-traditional data sources, such as seabird diet data, with stock assessment data appears useful to inform information gaps important for managing US fisheries in the North Pacific.     

Survival and growth of reciprocal crosses between two stocks of the Hong Kong oyster Crassostrea hongkongensis (Lam & Morton, 2003) in southern China

The Hong Kong oyster, Crassostrea hongkongensis, is one of the most economically important and well‐known oyster species in southern China. To explore the possibility of improving the performance of the Hong Kong oyster, complete diallel crosses between two oyster stocks, the Zhuhai stock (Z: fast growth line F4) and the Maowei Sea stock (M: resistant line F3), were conducted using pooled gametes. Three replicates, each consisting of two pure stocks (ZZ and MM) and two reciprocal crosses (ZM and MZ), were successfully generated. High fertilization rate and hatching level were observed among all the experimental groups, suggesting that there was no sperm–egg recognition barrier between geographic populations. Reciprocal crosses had higher survival rate compared to the pure stock crosses and the rate of survival increased with progeny growth. Growth heterosis became obvious both in larval and adult stages, and was primarily influenced by the egg origin and mating strategy at the larval stage. Also, the phenotypic traits of all progeny differed amongst the culture sites, suggesting a significant environmental effect. The Zhuhai site was more suitable for oyster aquaculture than the Maowei Sea site. Thus, our results demonstrated that crossbreeding between Zhuhai and Maowei Sea stocks of Hong Kong oyster to produce superior heterosis represents a promising means to improve the fishery yield of this species in southern China.     

Improvement of Matrinxã, Brycon amazonicus,Larviculture by Exposing Eggs to Triiodothyronine

This work evaluated the effect of triiodothyronine (T₃) on larviculture of matrinxã, Brycon amazonicus. Oocytes of three females were pooled, fertilized with pooled semen of two males and separated in four batches that were immersed in triiodothyronine solutions as follows: M₁ (control – water); M₂ (0.01 mg/L T₃); M₃ (0.05 mg/L T₃); and M₄ (0.1 mg/L T₃). Triiodothyronine did not affect fertilization rate and number of hatched larvae. Weight of hatched larvae was significantly higher in treatments M3 and M4, as well as among larvae sampled at Day 12 in all treatments. After 12 d of rearing, biomass gain was higher in the hormone treatments (M₁ 688 ± 569 mg; M₂ 2436 ± 562 mg; M₃ 3572 ± 569 mg; and M₄ 4129 ± 770 mg). In general, coefficients of variation of weight (CV_w) and length (CV_l) did not differ among treatments and cannibalism was registered between 36 and 72 hours post‐hatching (h.p.h.) without differences among treatments. Larval survival increased in the hormone treatments (M₁ 26.5%; M₂ 37.6%; M₃ 40.6%; and M₄ 40.8%). The results indicate that the immersion of matrinxã eggs in triiodothyronine can promote beneficial effects to its larviculture and indicate promising perspectives for culture of this tropical species.     

Walleye pollock egg distribution and mortality in the western Gulf of Alaska

We examine the distribution and mortality of walleye pollock ( Theragra chakogramma ) eggs in the western Gulf of Alaska. Most pollock eggs were found in mid-water, with low proportions in the neustonic and epibenthic layers during all years of sampling. A silhouette camera towed through a high egg density region provided new information on small-scale spatial distributions and provided density estimates at two depth layers similar to those of depth-discrete net sampling. Annual egg production curves and natural mortalities were estimated for 1987-92 based on the abundance of several cohorts relative to their production rate. Production during 1989 and 1990 was lower than in the other four years but 1988 was the only year to show markedly different (higher) mortality than the rest. Fertilization rate was generally very high (>99%) but several collections early in the season contained a substantial fraction of unfertilized eggs. Invertebrate egg predation was mainly due to euphausiids and was variable among locations and years. Egg cannibalism by adult pollock on the spawning grounds was inconsequential (< 1% for all years) compared to invertebrate predation.     

Hormonal profile of growing male and female diploids and triploids of the blue tilapia,Oreochromis aureus,reared in intensive culture

Triploidy as a result of thermal shock exposure of fertilized eggs decreases the growth rate ofOreochromis aureus as compared to their diploid controls, but this is due to the higher female ratio present in triploids (86%) and the lower growth rate of females. When females and males are considered separately, the growth rate is not significantly different in diploids and triploids. Since triploidy results in a malfunctioning steroidogenesis in females (mainly testosterone (T) and 17β-estradiol (E₂)), but does not affect the growth rate, it is concluded that female gonadal steroids do not influence growth unless in pharmacological concentrations. These low levels of gonadal steroids are generally accompanied by higher levels of gonadotropin (GtH), but the difference is not always significant. Despite their lower growth rate diploid females have higher plasma concentrations of growth hormone (GH) during several months compared to the triploid females and diploid males. 3,5,3′-triiodo-L-thyronine (T₃) levels, however, are comparable between diploid and triploid females (except for 1 month), but higher in diploid males in 4 of the 5 months studied. 11-ketotestosterone (11kT) is always higher in males. These results indicate that the higher growth rate of males may be related to the high circulating levels of T₃ and 11kT.     

Investigations into the development of the pituitary gland-thyroid tissue axis and distribution of tissue thyroid hormone content in embryonic coho salmon (Oncorhynchus kisutch) from Lake Ontario

Total organism content of L-thyroxine (T₄) and triiodo-L-thyronine (T₃) were measured in the early developmental stages of a stock of Lake Ontario coho salmon from the egg to the yolk absorption stage. Whole organism T₄ levels were constant between the egg and pre-hatch embryo stages, but fell progressively during yolk absorption. T₃ levels were low from egg to eye-pigment appearance, but then increased prior to hatch and fell again during the post-hatch yolk absorption period. When expressed as ng/tissue, T₄ content of the body compartment rose progressively between days 67 and 87 post-fertilization, whilst T₄ content of the yolk compartment fell progressively during the same period; the pattern was not evident for tissue T₃ content. When expressed as ng/g dry weight of tissue, the inverse relationship was found for T₄, and T₃ content of the body and yolk compartments decreased progressively and increased progressively, respectively during the same period, suggesting that thyroid hormones were selectively retained in the yolk compartment. Intensely “immunostained” (using anti-human β-TSH antibody) thyrotropic cells were present in small numbers in the pars distalis of the embryonic pituitary at the eye-pigment appearance stage, and the numbers increased markedly until the pre-hatch period. Administration of either bovine thyrotropic hormone (bTSH) or ovine growth hormone (oGH) had no effect on thyroid hormone content of larvae challenged during the yolk absorption period, suggesting that the thyroid tissue was not responsive to exogenous bTSH challenge at this time, and that oGH-sensitive 5′-monodeiodination was either not present or at levels that were too low to cause an elevation in total T₃ content, or that the substrate levels were insufficient to permit a measureable increase in whole body T₃ content.     

Role of thyroid hormones in tilapia larvae (Oreochromis mossambicus): I. Effects of the hormones and an antithyroid drug on yolk absorption,growth and development

Treatment of one-day old yolksac larvae of tilapia (Oreochromis mossambicus) by immersion in 0.05 ppm T₄ or 0.01 ppm T₃ significantly accelerated the differentiation and growth of all the fins, particularly pectoral and tail fins. Both the treatments also significantly accelerated yolk absorption and transition to free-swimming activity in the larvae. The treatments also significantly accelerated the growth of the larvae, with T₃ at 0.01 ppm having a greater effect than T₄ at 0.05 ppm. The yolk conversion efficiency was found not to be significantly affected by the hormone treatments but the treated larvae exhibited an increased heart beat, suggesting metabolic stimulation by the hormones.On the other hand, yolk absorption and free-swimming activity were significantly delayed in tilapia larvae immersed in 25 ppm solution of an antithyroid drug, phenylthiocarbamide (PTC). PTC also retarded the growth of the larvae. T₄ (0.05 and 0.10 ppm) or T₃ (0.01 and 0.02 ppm) therapy removed the PTC-inhibition,albeit not completely, suggesting that thyroid hormones are involved in the larval growth and development of tilapia.     

Predation on walleye pollock (Theragra chakograrnma) eggs and yolk-sac larvae by pelagic crustacean invertebrates in the western Gulf of Alaska

Immunological detection of yolk protein was used to assess predation by pelagic amphipods (gammarid and hyperiid), mysids, and euphausiids on eggs and yolk-sac larvae of walleye pollock Theragra chalcogramma during 1988 and 1989. Consumption estimates were made on the basis of frequency of positive immunoassays, assay detection times (gut clearance time), predator abundance, and spatial overlap of predators and prey. From our results gammarid amphipods and euphausiids were important predators on eggs and yolk-sac larvae, respectively. Gammarid amphipods alone consumed about 14% of the standing stock of pollock eggs in 1989. These results were compared with those from clearance rate experiments of predators feeding on pollock eggs in 300-1 bags. In general, clearance rate estimates of egg consumption were lower than those determined from gut contents.     

Determination of 3,5,3′-triiodo-L-thyronine (T3) levels in tissues of rainbow trout (Salmo gairdneri) and the effect of low ambient pH and aluminum

Tissue T₃ (3,5,3′-triiodo-L-thyronine) concentrations were measured in rainbow trout, Salmo gairdneri, after digestion by Pronase or collagenase and extraction with ethanolic ammonia (99:1, v/v) followed by 2N NH₄OH and chloroform. Recoveries of [¹²⁵I]T₃ administered in vivo or in vitro were high and consistent and there was close parallelism between sample dilutions and the radioimmunoassay curve, but recoveries of unlabeled T₃ administered in vitro were low and variable. Alternatively, trout were brought to isotopic equilibrium by [¹²⁵I]T₃ infusion for 96 h, the extracted [¹²⁵I]T₃ determined by gel filtration and the tissue T₃ content calculated from the specific activity of plasma [¹²⁵I]T₃. By the latter method, tissue T₃ concentrations were: intestine (4.2 ng/g), kidney (2.5), liver (2.8), stomach (1.5), heart (1.0), muscle (0.7), gill (0.6) and skin (0.3). Muscle (67% of body weight) comprised the largest tissue T₃ pool (82% of all tissues examined). Seven days exposure of trout to water acidified with H₂SO₄ (pH 4.8) or acidified water containing aluminum (21.6 mM), decreased tissue T₃ content generally and particularly in muscle (14% of controls). In conclusion, skeletal muscle is the largest T₃ tissue pool and seems highly responsive to altered physiologic state.     

Role of thyroid hormones in tilapia larvae (Oreochromis mossambicus): II. Changes in the hormones and 5′-monodeiodinase activity during development

Thyroid hormone profiles and 5-monodeiodinase activity were determined in tilapia at different stages of early development. The results showed that both T₄ and T₃ were present in significant amounts in fertilized eggs. There was a steady decrease in both T₄ and T₃ levels during embryonic development. The levels continued to decline after hatching until around 7 days later when most of the yolk had been absorbed. The T₄ level started to rise then, suggesting that the larval thyroid had begun to produce T₄ at this time, which coincided with the period of faster growth of the larvae. The T₃ level remained fairly constant until around 20 days after which it rose significantly. In vitro determination of 5-monodeiodinase activity (5-D activity) in the whole-body homogenates of larvae showed that the enzymatic conversion of T₄ to T₃ was not detectable in eggs and 3-day-old larvae but detected in 5-day-old and older larvae. There was a gradual increase in the V_max as development proceeded indicating increasing 5-D activity during larval development. The K_m values did not differ significantly in the different stages of development. These results are discussed in relation to the growth and development of the larvae.     

Total and free thyroid hormones in plasma of tropical marine teleost fish

Plasma levels of L-thyroxine (T₄) and 3,5,3-triiodo-L-thyronine (T₃) and the percentage of plasma T₄ and T₃ present in the free (dialyzable) form (%FT₄ and %FT₃) were measured in 16 species (11 families) of tropical marine teleosts from an inshore Barbados reef. Mean plasma T₄ varied from 0.2 ng/ml to 42 ng/ml; mean plasma T₃ varied from < 0.2 ng/ml to 50 ng/ml. The highest T₄ and T₃ levels were recorded in parrot-fish and the lowest levels in filefish. The %oFT₄ and %FT₃ varied from 0.05–3.41%. Estimated levels of plasma free T₄ and free T₃ levels ranged from 0.4–466 pg/ml. The extremely wide inter- and intra-species ranges in levels of free T₄ and T₃ do not support a previous suggestion, based on temperate freshwater salmonid species, that free T₄ and T₃ levels in fish may fall within a relatively range narrow comparable to that of homeothermic vertebrates.     

Efficacy of garlic (Allium sativum) extract applied as a therapeutic immersion treatment for Neobenedenia sp. management in aquaculture

Garlic, Allium sativum L., extract administered as a therapeutic bath was shown to have antiparasitic properties towards Neobenedenia sp. (MacCallum) (Platyhelminthes: Monogenea) infecting farmed barramundi, Lates calcarifer (Bloch). The effect of garlic extract (active component allicin) immersion on Neobenedenia sp. egg development, hatching success, oncomiracidia (larvae) longevity, infection success and juvenile Neobenedenia survival was examined and compared with freshwater and formalin immersion. Garlic extract was found to significantly impede hatching success (5% ± 5%) and oncomiracidia longevity (<2 h) at allicin concentrations of 15.2 μL L^?1, while eggs in the seawater control had >95% hatching success and mean oncomiracidia longevity of 37 ± 3 h. At much lower allicin concentrations (0.76 and 1.52 μL L^?1), garlic extract also significantly reduced Neobenedenia infection success of L. calcarifer to 25% ± 4% and 11% ± 4%, respectively, compared with 55% ± 7% in the seawater control. Juvenile Neobenedenia attached to host fish proved to be highly resistant to allicin with 96% surviving 1‐h immersion in 10 mL L^?1 (15.2 μL L^?1 allicin) of garlic extract. Allicin‐containing garlic extracts show potential for development as a therapy to manage monogenean infections in intensive aquaculture with the greatest impact at the egg and larval stages.     

Fecundity, egg size and total egg volume differences in 12 stocks of rainbow trout, Oncorhynchus mykiss Richardson

Abstract. Measurements of egg number, egg size and total egg volume in relation to post-stripped weight, made on the eggs stripped from 12 separately maintained farm stocks of 4-year-old rainbow trout. Oncorhynchus mykiss Richardson, revealed highly significant differences between populations when analysed by regression and covariance statistical techniques. All of the stocks showed increases in fecundity with increasing fish size, with 61% (coefficient of determination or r²) of the variation in fecundity of the 12 stocks explained by the common regression on parental fish size. Comparison of the regressions of the different stocks revealed that the rate of increase of fecundity with increasing fish weight was constant throughout the weight range (i.e. the regressions had similar slopes) although there were highly significant differences in elevation, with some stocks producing almost twice as many eggs as some of the others. By contrast, the relationship of egg size with fish weight was much more variable with three of the stocks showing no significant regression of egg size on fish weight and a further two stocks with data poorly fitted to the regressions. Of the remaining seven stocks only one showed a significant difference in slope of the regression with modest increases in fish weight being associated with large increases in egg size. Only four of the remaining stocks of fish showed significant differences in the elevation of their regressions, i.e. the egg sizes were significantly different after the potentially conflicting influence of fish size was partitioned using covariance techniques. Assessments of total egg volume produced far improved regressions on post-strip weight with 75% (r²) of the variation in the pooled data of the 12 stocks being described by the line of the common regression. In general, there appeared to be a “trade-off” between egg number and size with the most and least fecund fish producing somewhat smaller and larger eggs respectively. All 12 regressions of totai egg volume on fish weight showed a common slope. However, there were significant differences in elevation of the regressions, with some of the stocks of trout producing up to a 55% greater volume of eggs. There appeared to be no relationship between the differences in egg number, size or total egg volume and the type or geograpfiical location of the farms on which the stocks were held. Although it remains to be determined to what extent these differences in reproductive performance are Inheritable, it is recommended that these characteristics should be taken into account by hatcheries in tbe design of long-term broodstock selection programmes for rainbow trout.     

Effects of thyroxine administration on the growth and survival of pike silverside (Chirostoma estor) juveniles

The pike silverside has high aquaculture potential despite its slow growth. In this study, thyroxine (T₄) concentrations (0.13, 1.3 and 13 nM), a control with no hormonal supplement and a negative control containing 4.5 mM methimazole (MMI) were tested to evaluate the growth of this species. Juveniles (0.2 g) were exposed by immersion to these treatments for 8 h every second day for 120 days, and growth evaluations were performed monthly over the entire trial period. In addition, tissue samples from each treatment group were assayed for triiodothyronine (T₃) and deiodinase type 2 (D2) activity . The survival rates in all T₄ groups were high, and a significant increase in growth was observed (average of 58%). The MMI treatment caused an increase in mortality and a reduction in the final body weight compared with the control. T₄ administration did not affect the tissue levels of T₃, and it decreased muscular D2 activity only after 30 days of exposure. These results demonstrated that low concentrations of T₄ in the culture environment could improve the growth of this species without affecting tissue hormone levels. The technique may have useful applications for early‐stage aquaculture of this and other economically important species.