Salmonid Embryos Research Articles

The sedimentation of salmonid spawning gravels in the Hampshire Avon catchment, UK: implications for the dissolved oxygen content of intragravel water and embryo survival

AbstractThe accumulation of sediment within salmonid redd gravels can have a detrimental impact on the development of salmonid embryos; therefore, redd sedimentation represents a potential limiting factor for salmonid reproduction. The links between redd sedimentation, the dissolved oxygen content of intragravel water and salmonid embryo survival within the upper and middle parts of the Hampshire Avon catchment in southern England are explored. Measurements of surface and intragravel water quality and redd properties were undertaken for artificial redds constructed at known spawning sites. Salmonid embryos were also planted into artificial redds adjacent to the monitoring equipment. The rate of sedimentation of the newly cleaned redd gravels demonstrated a non‐linear decrease over time, which is attributed to a particle‐size‐selective depositional process. The results of the study confirm that low embryo survival and low dissolved oxygen concentrations in intragravel water can be attributed to the accumulation of sediment within the redd gravels. This was found to produce a reduction in redd permeability, which limited the interchange of surface and intragravel water and, therefore, the supply of dissolved oxygen to the intragravel environment. In view of the diminished status of salmonids within many of the UK's chalk rivers and streams, the results highlight the need for management initiatives aimed at reducing redd sedimentation and thereby optimizing salmonid embryo incubation success. Copyright © 2006 John Wiley & Sons, Ltd.

A review of factors influencing the availability of dissolved oxygen to incubating salmonid embryos

AbstractPrevious investigations into factors influencing incubation success of salmonid progeny have largely been limited to the development of empirical relationships between characteristics of the incubation environment and survival to emergence. It is suggested that adopting a process‐based approach to assessing incubation success aids identification of the precise causes of embryonic mortalities, and provides a robust framework for developing and implementing managerial responses.Identifying oxygen availability within the incubation environment as a limiting factor, a comprehensive review of trends in embryonic respiration, and processes influencing the flux of oxygenated water through gravel riverbeds is provided. The availability of oxygen to incubating salmonid embryos is dependent on the exchange of oxygenated water with the riverbed, and the ability of the riverbed gravel medium to transport this water at a rate and concentration appropriate to support embryonic respiratory requirements. Embryonic respiratory trends indicate that oxygen consumption varies with stage of development, ambient water temperature and oxygen availability. The flux of oxygenated water through the incubation environment is controlled by a complex interaction of intragravel and extragravel processes and factors. The processes driving the exchange of channel water with gravel riverbeds include bed topography, bed permeability, and surface roughness effects. The flux of oxygenated water through riverbed gravels is controlled by gravel permeability, coupling of surface–subsurface flow and oxygen demands imposed by materials infiltrating riverbed gravels. Temporally and spatially variable inputs of groundwater can also influence the oxygen concentration of interstitial water. Copyright © 2006 John Wiley & Sons, Ltd.

Effect of Fine Sediment Infiltration During the Incubation Period on Atlantic Salmon (Salmo salar) Embryo Survival

The existence of a negative relationship between fine sediment infiltration during the incubation period and salmonid embryo survival has often been discussed in the literature. However, few studies have specifically addressed this relationship in the field. We conducted a field experiment to examine the relationship between survival of Atlantic salmon (Salmo salar) embryos contained in incubation baskets and the patterns of fine sediment infiltration into the baskets during the incubation period. The results indicate that survival to pre-eyed (STPE), eyed (STE) and hatched (STH) stages of development were all negatively correlated with the percentage of fine sediment entering the baskets. STPE and STE were most strongly affected by silts and clays (<0.063 mm) although this size class represented only a small fraction of the grain size distribution inside the incubation baskets (0.03–0.41%). STH was most strongly correlated with the infiltration of medium sand (0.25–0.50 mm) material. On average, 66% of the implanted embryos survived to the pre-eyed stage of development compared to 63% for the eyed and 48% for the hatched stages of development.

Green Fluorescent Protein Labeling of Primordial Germ Cells Using a Nontransgenic Method and Its Application for Germ Cell Transplantation in Salmonidae1

Transplanting primordial germ cells (PGCs) has a number of potential applications in fish bioengineering. Previously, we established a system to visualize live PGCs in the rainbow trout by introducing the green fluorescent protein (Gfp) gene driven by rainbow trout vasa gene regulatory regions. However, for PGC transplantation to be practically useful in aquaculture, visualization of PGCs using a nontransgenic technique is required. In this study, we demonstrate a method for labeling PGCs from various fish species by introducing chimeric RNAs composed of the Gfp coding region and vasa gene 3'-untranslated regions (UTRs); these sequences play a critical role in stabilizing mRNA in zebrafish PGCs. The GFP chimeric RNAs, including vasa 3'-UTR RNAs from rainbow trout, Nibe croaker, and zebrafish, were microinjected into the cytoplasm of fertilized eggs of several Salmonidae species. All the resulting embryos showed specific labeling in PGCs after the somatogenesis stage, which continued to be visible for at least 50 days. To apply this technique to PGC transplantation, PGCs labeled with chimeric RNA were microinjected into the peritoneal cavity of newly hatched salmonid embryos. The GFP labeling was sufficiently long-lived for the initial stage of donor PGC behavior to be followed in the recipient embryos. Importantly, donor PGCs from brown trout and masu salmon were incorporated into xenogeneic genital ridges in recipient rainbow trout. This nontransgenic method for labeling fish PGCs should be extremely useful for applications of PGC transplantation where the resulting progeny are to be released into the environment, such as PGC cryopreservation for fish stocks and surrogate brood stock technology.

Heat exchanges and temperatures within a salmon spawning stream in the Cairngorms, Scotland: seasonal and sub‐seasonal dynamics

AbstractStream temperatures are often used to predict salmonid embryo development; but there are very few medium‐term studies of the heat exchanges determining water column and bed temperatures. Furthermore, no research exists on the energy balance for sub‐arctic Scottish rivers. This paper reports results of a hydrometeorological study of a Cairngorm stream (Girnock burn, northeast Scotland) over the salmon spawning–hatch season (late October 2001 to mid‐April 2002) that aims: (1) to characterize seasonal and sub‐seasonal stream energy budget and thermal dynamics; and (2) to explain these variations with respect to meteorological and hydrological factors. In terms of average energy flux contributions, sensible heat (38.7%), the bed heat flux (37.0%) and friction at the stream bed and banks (24.3%) are heat sources, while latent heat (73.1%) and net radiation (26.9%) are heat sinks. All energy losses and 38.7% of heat gains occur at the air–water interface; and 61.3% of energy gains (including friction) take place at the water–channel bed interface. Typically, temperatures increase (+1.97°C) and show dampening of thermal response from the water column to depth in the stream bed. The most salient findings include: (1) the stream bed (atmosphere) is the dominant energy source (sink) for heating (cooling) channel water, which may be attributed to inferred heat advection by groundwater up‐welling into the bed of this upland stream; (2) sensible heat is the primary atmospheric energy source due to limited net radiation; (3) friction at the stream bed and banks is an important heat source. Energy budget terms and temperatures exhibit (sub‐)seasonal changes in response to meteorological and hydrological conditions; a schematic diagram is presented to summarize these results. This paper clearly illustrates the need for further medium‐ to long‐term empirical stream energy balance research to characterize heat flux dynamics and, thus, understand and predict water temperature variations over time‐scales of relevance to biological studies. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Second-order Monte Carlo uncertainty/variability analysis using correlated model parameters: application to salmonid embryo survival risk assessment

This paper presents a Monte Carlo uncertainty/variability analysis applied to assessment of salmonid embryo survival affected by the deposition of fine sand in the spawning gravel riverbed. For the system being modeled, the uncertainty originates from the lack of perfect knowledge about the spawning habitat, while the variability arises from the natural diversity in the amount of sand deposit. A second-order Monte Carlo simulation (MCS) procedure consisting of multiple realizations of model parameters and full-range iterations of input variable is used to separately propagate the knowledge uncertainty and natural variability of the system. Given that the three parameters of the present model are highly correlated (correlation coefficients >0.8), we use four multivariate methods (Iman–Conover method, standard normal transformation method, normal copula method, and maximum-entropy copula method) to generate correlated model parameters. The scatter plots of the correlated parameters generated with various methods demonstrate different correlation patterns. An important finding of the present uncertainty/variability analyses and risk assessments is that the outcomes resulting from the four correlated scenarios are very similar, regardless of the diverse correlation patterns. The output of the second-order MCS is a collection of cause–effect relations (spaghetti plot) that can facilitate the analyses of survival-rate uncertainty/variability. The favorable habitat conditions are also defined on the basis of the spaghetti plot. It is revealed that significant values of information are associated with the parameter correlations, especially when higher levels of embryo survival are targeted. The results further indicate that the outcomes of the independent (less realistic) scenario substantially differ from those of the correlated ones, indicating that the effect of parameter correlations should be incorporated into the MCS whenever possible. This work provides a useful paradigm for the systematic uncertainty/variability analyses in the context of ecological risk assessments.

Second-order Monte Carlo uncertainty/variability analysis using correlated model parameters: application to salmonid embryo survival risk assessment

This paper presents a Monte Carlo uncertainty/variability analysis applied to assessment of salmonid embryo survival affected by the deposition of fine sand in the spawning gravel riverbed. For the system being modeled, the uncertainty originates from the lack of perfect knowledge about the spawning habitat, while the variability arises from the natural diversity in the amount of sand deposit. A second-order Monte Carlo simulation (MCS) procedure consisting of multiple realizations of model parameters and full-range iterations of input variable is used to separately propagate the knowledge uncertainty and natural variability of the system. Given that the three parameters of the present model are highly correlated (correlation coefficients >0.8), we use four multivariate methods (Iman–Conover method, standard normal transformation method, normal copula method, and maximum-entropy copula method) to generate correlated model parameters. The scatter plots of the correlated parameters generated with various methods demonstrate different correlation patterns. An important finding of the present uncertainty/variability analyses and risk assessments is that the outcomes resulting from the four correlated scenarios are very similar, regardless of the diverse correlation patterns. The output of the second-order MCS is a collection of cause–effect relations (spaghetti plot) that can facilitate the analyses of survival-rate uncertainty/variability. The favorable habitat conditions are also defined on the basis of the spaghetti plot. It is revealed that significant values of information are associated with the parameter correlations, especially when higher levels of embryo survival are targeted. The results further indicate that the outcomes of the independent (less realistic) scenario substantially differ from those of the correlated ones, indicating that the effect of parameter correlations should be incorporated into the MCS whenever possible. This work provides a useful paradigm for the systematic uncertainty/variability analyses in the context of ecological risk assessments.

In vitro metabolism of pregnenolone to 7α-hydroxypregnenolone by rainbow trout embryos

Tissues taken from rainbow trout embryos at several developmental stages, were incubated in the presence of radioactively-labelled pregnenolone in order to determine the capability of salmonid embryos to metabolize steroids, such as pregnenolone, that are incorporated into the oocyte during gonadal growth and maturation. High performance liquid chromatography was used to separate the steroid products, and gas chromatography–mass spectrometry was applied for the chemical identification of the product. 7α-Hydroxypregnenolone, previously known to be produced only by ovarian tissues, was found to be the sole metabolite of pregnenolone metabolism by rainbow trout embryos. Sulfate and glucuronide conjugated forms of 7α-hydroxypregnenolone were also produced. We hypothesize that this metabolite provides a pathway for excretion of pregnenolone, enabling the embryo to maintain its own steroid milieu, although the possibility of 7α-hydroxypregnenolone also playing a physiological role cannot be excluded.

Failure of interspecies androgenesis in salmonids

Androgenetic development of salmonid embryos was induced in recipient oocytes from the same or other species (intra‐ or interspecies androgenesis). Parameters for induced androgenesis were investigated in brown trout Salmo trutta and brook trout Salvelinus fontinalis. Reciprocal androgenetic and control crosses were conducted among fishes from three genera: Oncorhynchus (rainbow trout, O. mykiss), Salmo (brown trout) and Salvelinus (brook trout), and within two genera: Salmo (brown trout and Atlantic salmon, S. salar) and Salvelinus (brook trout and Arctic charr, S. alpinus). Live hatched androgenetic progenies were obtained in all intraspecies variants, where oocytes and sperm originated from the same species. Interspecies androgenesis resulted in no viable larvae, despite the fact that most hybrid controls and intraspecies androgenetic individuals were viable. When recipient oocytes originated from other genera (interspecific intergeneric androgenesis), embryonic development ceased in early developmental stages, except for haploid controls of brook trout produced in eggs of brown trout. Survival of embryos to the eyed‐egg stage was relatively high in the intrageneric androgenesis experiment. Nevertheless, none of these embryos survived to hatching. Some of the presumed Atlantic salmon individuals developing in brown trout eggs contained maternal DNA, questioning the accuracy of enucleation using irradiation. The inability to induce interspecific androgenesis among the examined salmonid species may have been the result of substantial kariotypical and developmental differences between spermatozoal donors and oocyte recipients, causing an incompatibility between maternal cytoplasmic regulatory factors and the paternal nuclear genome.

Thermal regime in the hyporheic zone of two contrasting salmonid spawning streams: ecological and hydrological implications

The temperatures of stream water and hyporheic water were monitored in spawning areas of an upland stream (Girnock Burn) and a degraded lowland stream (Newmills Burn). Both streams are located in North‐east Scotland and are spawning streams for Atlantic salmon, Salmo salar L., and brown trout, S. trutta L. Monitoring between the spawning and hatching of ova revealed significant differences in the thermal regimes of the two sites. Whilst temperatures in stream and hyporheic water were similar in the Girnock, notable attenuation in the Newmills Burn resulted in increasing moderation of thermal regime with hyporheic depth. This probably reflects contrasting groundwater–surface water interactions in the two systems. In the Girnock an open gravel stream bed and lack of groundwater influence resulted in stream water downwelling through the gravel. In contrast, fine bed sediments and local groundwater inputs resulted in more complex stratification of temperatures in Newmills Burn. However, even in the conditions of the Newmills Burn, temperature based salmonid embryo development models predicted that egg burial depth has an insubstantial effect on ova development.

Cardiovascular responses of eggs, embryos and alevins of Atlantic salmon and rainbow trout to nitric oxide donors, sodium nitroprusside and isosorbide dinitrate, and inhibitors of nitric oxide synthase, N‐nitro‐l‐arginine methyl ester and aminoguanidine, following short‐ and long‐term exposure

Atlantic salmon embryos and alevins Salmo salar that had been exposed to isosorbide dinitrate (ISDN) for 4 weeks, on transfer to fresh water, showed an increase in heart rate. Unexposed embryos and alevins showed a decrease in heart rate following transfer to 100 μmol l−1 ISDN for 4 h. This is in contrast to adult rainbow trout and higher vertebrates where tachycardia occurred in response to nitric oxide (NO) donors. The decreased heart rate in response to ISDN was inhibited by 2 mg 1−1 methylene blue, indicating that NO activates cardiovascular events via guanylyl cyclase and cyclic guanidine monophosphate. Heart rate of rainbow trout alevins Oncorhynchus mykiss exposed to 100 μmol l−1 aminoguanidine responded with a slowly developed but significant bradycardia over 10 min as did those reared in aminoguanidine for 4 weeks then transferred to fresh water. A potentiated increase in heart rate on exposure to the NO donor sodium nitroprusside (SNP), occurred within 1 min in salmon alevins reared in l‐nitro‐arginine methyl ester (l‐NAME) for 4 weeks, indicating up‐regulation of NO receptors. The evidence for down‐regulation of SNP‐reared alevins exposed to l‐NAME was less well defined. The results suggest that both salmonid embryos and alevins have a functional l‐arginine‐NO pathway and that NO has a physiological role in control of cardiovascular events.

Cardiovascular responses of eggs, embryos and alevins of Atlantic salmon and rainbow trout to nitric oxide donors, sodium nitroprusside and isosorbide dinitrate, and inhibitors of nitric oxide synthase, N-nitro-L-arginine methyl ester and aminoguanidine, following short- and long-term exposure

Atlantic salmon embryos and alevins Salmo salar that had been exposed to isosorbide dinitrate (ISDN) for 4 weeks, on transfer to fresh water, showed an increase in heart rate. Unexposed embryos and alevins showed a decrease in heart rate following transfer to 100 μmol l−1 ISDN for 4 h. This is in contrast to adult rainbow trout and higher vertebrates where tachycardia occurred in response to nitric oxide (NO) donors. The decreased heart rate in response to ISDN was inhibited by 2 mg 1−1 methylene blue, indicating that NO activates cardiovascular events via guanylyl cyclase and cyclic guanidine monophosphate. Heart rate of rainbow trout alevins Oncorhynchus mykiss exposed to 100 μmol l−1 aminoguanidine responded with a slowly developed but significant bradycardia over 10 min as did those reared in aminoguanidine for 4 weeks then transferred to fresh water. A potentiated increase in heart rate on exposure to the NO donor sodium nitroprusside (SNP), occurred within 1 min in salmon alevins reared in l-nitro-arginine methyl ester (l-NAME) for 4 weeks, indicating up-regulation of NO receptors. The evidence for down-regulation of SNP-reared alevins exposed to l-NAME was less well defined. The results suggest that both salmonid embryos and alevins have a functional l-arginine-NO pathway and that NO has a physiological role in control of cardiovascular events.

Toxicity of 2, 3, 7, 8‐tetrachlorodibenzo‐p‐dioxin to early life stage brook trout (Salvelinus fontinalis) following parental dietary exposure

AbstractThe toxicity of 2, 3, 7, 8‐tetrachlorodibenzo‐p‐dioxin (TCDD) to the early life stages of F1 generation brook trout (Salvelinus fontinalis) was determined, when dosed by maternal transfer. Effects were compared across six treatments, including a control. The experimental groups, based on TCDD concentrations in freshly spawned eggs (control, 41, 84, 156, 285, and 517 pg TCDD per gram egg [wet weight]), were established by treating 1.5‐year‐old brook trout with TCDD‐dosed food throughout the period beginning with final gonadal maturation and continuing until just prior to spawning. Measurement endpoints assessed at various times throughout the study, included (1) TCDD and lipid concentrations in ovaries, eggs, alevins, and juvenile fish, (2) mortality, (3) fertility indices, (4) growth, (5) juvenile sex ratios, (6) pathology, and (7) cytochrome P4501A1 mRNA (CYPIA1) levels in embryos. The cumulative lethality dose response was determined just prior to hatch, at swim‐up, and at the end of the study. The concentration in eggs causing 50% mortality (LCegg50) at swim‐up and at the end of the study were 138 and 127 pg TCDD per gram egg, respectively. No treatment effects were observed on fertility, growth, or juvenile sex ratios. Pathologies were observed that have been previously reported in TCDD‐exposed salmonid embryos, including edema in free embryos from all TCDD‐exposed treatment groups at hatch. At the end of the test, juvenile fish from all treatments exhibited an increased prevalence of exophthalmia compared to the controls. The ECegg50 for this effect was 117 pg TCDD per gram egg. Histological analysis of the end‐of‐test juveniles, in gonad, nerve, vertebrae, neuromasts, liver, kidney, gut, gas bladder, skin, and blood vessels, did not indicate any significant pathology below the LCegg50. P4501A1 mRNA levels in whole 91‐d postspawn free embryos from the 84 pg TCDD per gram egg treatment were 2.8 times higher compared with control or 41‐pg TCDD per gram egg treatments. Results from this study were consistent with other published results of TCDD effects on salmonids, in which the exposures occurred after fertilization.

Modelling the Growth of Salmonid Embryos

A mechanistic model for the growth of salmonid embryos (prior to feeding) is developed with coupled differential equations describing anabolism and catabolism. The equations model changes in embryo and yolk sac masses in which the flux of nutrients to the embryo is controlled by geometric properties of the embryo and yolk. The rate parameter describing this flux is a well defined function of temperature. Water absorption is also a factor in determining mass. The model, describing the size and time-to-arrival at developmental stages, is fit to chinook salmon growth data from fertilization to complete yolk absorption.

The earliest stages in the development of the circulatory system of the Rainbow Trout Oncorhynchus mykiss.

The older literature suggests that the development of the blood vascular system in teleosts differs from that of other vertebrates. The evidence, however, came mostly from studies of salmonid embryos beyond the stages when blood cells had begun to circulate, which overlooked earlier developmental stages. The development of the blood vessels of the rainbow trout are illustrated from the time of the first heartbeat to the stage of eye pigment formation. Unless they can be injected with a dye solution, the earliest vessels to develop remain invisible until blood flow makes them visible. By the time of the first heartbeat stage, the embryo has a dorsal aorta, caudal artery and vein, a few transverse vessels, and even the beginning of a vitelline network. One feature peculiar to teleosts is the development of the intermediate cell mass, from which the erythrocytes and a temporary capillary network are formed rather than from the yolk sac. Development of the early posterior cardinal and the subintestinal vein occurs much as in other vertebrates. Previous investigators missed these earliest phases of development because of the difficulty of making them visible. Early formation and transformation of the vascular system of the rainbow trout generally conforms to that seen in vertebrates, except as modified by the temporary presence of the intermediate cell mass and the specialized teleostean yolk mass. With the reduction of the intermediate cell mass, the primary circulatory system for yolk utilization is transformed into a secondary one for respiratory and metabolic functions, as happens usually among vertebrates. J. Morphol. 233:215-236, 1997. © 1997 Wiley-Liss, Inc.

Characteristics of current international trade of live salmonid eggs.

World trade in live salmonid embryos (eyed eggs) has grown in response to increased global salmon production, particularly in South America, and parallels international trade in farmed salmonid products. Rainbow trout (Oncorhynchus mykiss), Atlantic salmon (Salmo salar) and coho salmon (O. kisutch) are the most commercially important species. In 1992, the Food and Agriculture Organization of the United Nations estimated world production of rainbow trout at 300,000 tonnes, while the production of Atlantic salmon was estimated at 250,000 tonnes and coho salmon at 50,000 tonnes. One can estimate that roughly 3 billion, 150 million and 30 million eggs, respectively, were required to produce this yield. Broodstock are cultivated world-wide, using a wide variety of water sources, including the marine environment, riverine water containing anadromous fish, and ground water free of migrating fish. As many as 70% of all coho eggs are derived from feral fish. Approximately 50% of all commercial salmonid eyed eggs are produced in Europe, and approximately 15% are produced in the state of Washington, United States of America. Conditions which are ideal for commercial salmonid grow-out are not necessarily ideal for the cultivation of salmonid broodstock; this is one reason why international egg trade is necessary. The trend of current salmonid health regulations is towards facilitating egg commerce on a regional level, in an attempt to control disease transmission. Regulations controlling egg importation often include pathogens which are not vertically transmitted. This serves only to increase egg prices, in compensation for the cost of laboratory tests. Genetic improvements have been the cornerstone of increasing commercial production of all agricultural commodities. Fish health regulations are sometimes instituted in an effort to protect the local industry, but in fact they act more often to restrict the flow of genetic material and may actually serve to reduce industry productivity and profitability.

Evaluation of an Ion Adsorption Method to Estimate Intragravel Flow Velocity in Salmonid Spawning Gravel

Abstract Intragravel water exchange provides oxygenated water, removes metabolic waste, and is an essential factor in salmonid embryo survival. Measurements of intragravel flow velocity have been suggested as an index of gravel quality and also as a useful predictor of fry emergence; however, proposed methods for measuring velocity in gravel are problematic. We evaluate an ion adsorption technique that uses natural streamwater solute chemistry, which therefore does not require introduction of a dye or solute tracer. However, the method requires an access tube for placing an ion exchange resin bag in the substrate. Our studies suggest that the ion adsorption technique is sensitive and reproducible for measuring velocity in free-flowing water but has little utility for measuring velocity in gravel beds. Fine sediment provided a relatively high flux density of adsorbed cations that exchanged readily with the resin, leading to spurious high apparent velocities.

Spatial and temporal variation in dissolved oxygen in natural egg pockets of chum salmon, in Kennedy Creek, Washington

The effects of dissolved oxygen (DO) on incubating salmonid embryos have been studied extensively in the laboratory but there is little information on levels experienced by salmon embryos in complex natural channels. We monitored 33 natural egg pockets of chum salmon Oncorhynchus keta, from shortly after spawning until emergence, and found that DO levels varied considerably among and within egg pockets over time. Egg pocket DO levels varied from 2–10 mg l−1 at the time of spawning and generally declined during incubation but the declines did not occur in all pockets and were not always steady. Much of the variability may be attributed to local channel topography. Pool tailouts had the highest and least variable DO levels whereas levels in lateral bars were generally lower and more variable. Levels in glides and riffles tended to be intermediate between those in pool tailouts and lateral bars. In spite of the variation in DO levels and habitats used by chum salmon, DO levels were not correlated with egg pocket sediment composition (per cent of the sample&lt;1.0 or 4.0 mm diameter) or with the egg pocket's depth.

Spatial and temporal variation in dissolved oxygen in natural egg pockets of chum salmon, in Kennedy Creek, Washington

The effects of dissolved oxygen (DO) on incubating salmonid embryos have been studied extensively in the laboratory but there is little information on levels experienced by salmon embryos in complex natural channels. We monitored 33 natural egg pockets of chum salmon Oncorhynchus keta, from shortly after spawning until emergence, and found that DO levels varied considerably among and within egg pockets over time. Egg pocket DO levels varied from 2–10 mg l−1 at the time of spawning and generally declined during incubation but the declines did not occur in all pockets and were not always steady. Much of the variability may be attributed to local channel topography. Pool tailouts had the highest and least variable DO levels whereas levels in lateral bars were generally lower and more variable. Levels in glides and riffles tended to be intermediate between those in pool tailouts and lateral bars. In spite of the variation in DO levels and habitats used by chum salmon, DO levels were not correlated with egg pocket sediment composition (per cent of the sample<1.0 or 4.0 mm diameter) or with the egg pocket's depth.

Saltwater Exposure at Fertilization Induces Ploidy Alterations. Including Mosaicism, in Salmonid

We found that salt water induces ploidy alterations in salmonid embryos. Flow cytometry analysis revealed significantly higher frequencies of haploids, triploids, heteroploid mosaics, and aneuploids in rainbow trout (Oncorhynchus mykiss) embryos experimentally exposed to salt water from fertilization to the two- and eight-cell stages of development. Heteroploid mosaics have been reported in diploid and triploid salmonid hybrids, although none were observed in the triploid coho salmon or diploid and triploid coho salmon (O. kisutch) × chinook salmon (O. tshawytscha) hybrids we examined. No mosaics were observed in intertidally spawned pink salmon (O. gorbuscha) embryos. Salt water could induce ploidy alterations by causing chromosome segregation errors during meiosis, mitosis, or both. Heteroploid embryos appeared morphologically normal although they may possess physiological deficiencies not immediately apparent.