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Since 1974, sea-run Baltic salmon populations have been afflicted byan early life-stage mortality known as the M74 syndrome. The syndrome has been shown to be associated with a thiamine (vitamin B1) deficiency that causes neurological disturbances associated with necrotic brain cells. Treatment with thiamine may counteract development of M74. In this study, eyed eggs of sea-run Baltic salmon were given the thiamine antagonist oxythiamine and the commercial PCB-blend Clophen A50 by means of microinjection into the yolk sac. The aim was to study the effects of an experimentally induced thiamine deficiency and how it affected the biotransformation system CYP1A using the 7-ethoxyresorufin-O-deethylase (EROD) assay. After hatching, we attempted to reverse the deficiency in half of each exposure group by immersion in a thiamine solution and investigated its effect on survival and EROD-activity. Yolk-sac fry from groups of eggs that were injected with oxythiamine, either with or without Clophen A50, demonstrated a loss of coordination, lethargy, exophthalmia, and whitened liver followed by complete mortality (100%). Based on this and the time to death, between124-193 posthatch degree-days (d deg. C), the effects of oxythiaminewere comparable to those of M74-development, however, dissimilarities were also noted. Thiamine treatment of oxythiamine-injected groups delayed mortalities that were reduced to between 64.8 and 91.8%. A dose and time-dependent induction of EROD-activity recorded for ClophenA50 groups was strongly suppressed in oxythiamine groups. Histopathological examination of oxythiamine groups at 103 and 182 d deg. C revealed reduced levels of hepatic glycogen, degenerating hepatocytes and a higher prevalence of necrotic brain cells, all of which are pathological features found in salmon yolk-sac fry affected by M74. Groupsinjected with Clophen A50 demonstrated no histopathological changes. [ABSTRACT FROM AUTHOR]

Sea-run Baltic salmon of the Swedish Dalalven population were subjected to 2 different reconditioning studies in order to determine possibilities to cure the M74 syndrome; namely to produce alevins without M74 mortality. Reconditioning of female salmon was achieved both by feeding and by thiamine injection. In the first experiment, spawned female salmon were given a commercial brood-fish feed until renewed maturation, when they were stripped. The eggs were fertilized with milt from sea-run males. Eggs and alevins were followed to the fry stage. In the second experiment, ascending females were injected with thiamine 1 month before maturation and stripped. Eggs and alevins were handled in the same way as in the first experiment. Feeding sea-run salmon was troublesome and only 13 females accepted dry food. These females improved the thiamine status of their egg tenfold. Thiamine injection of females was also effective, and the thiamine content of their eggs increased by a factor of 13. No M74 mortality was observed in theoffspring of female salmon that were reconditioned. Activity of the hepatic enzyme CYP4501A in salmon alevins seemed to vary with thiamine content and was induced pre-hatch in both M74 alevins and healthy alevins of females reconditioned by feeding, indicating that a good thiamine status is more essential than the burden of xenobiotics. [ABSTRACT FROM AUTHOR]

During recent decades several Baltic fish species have been affectedby poor reproductive success. The sea-run Baltic salmon populations are affected by the M74 syndrome, an early life-stage mortality associated with low thiamine (vitamin B1) concentrations in the offspring.In order to study whether sea-run brown trout with symptoms and mortalities similar to M74 also suffer from low thiamine concentrations, analyses of thiamine were performed. Also, tissues of Baltic cod wereanalyzed for their content of thiamine. This study confirms that Baltic salmon that are affected by M74 suffer from low thiamine levels and that the temporal onset of the development of M74 may be correlated to the thiamine concentration in the eyed egg. Also, thiamine concentrations of muscle and ovaries from females that produced offspring with M74 were significantly lower than those of females that producedhealthy progeny, which indicates that M74 may be a maternally transmitted thiamine deficiency. Preliminary data from sea-run Baltic browntrout showed that yolk- sac fry with symptoms that resembled those of M74-lethargy, darkening of skin and 100% mortality-had low mean thiamine concentrations (0.14 nmol g-1 in 1994 and 0.34 in 1998) compared with those of healthy progeny (4.4 nmol g-1 in 1994 and 3.9 in 1998). This indicates that some family groups of Baltic brown trout may be affected by a reproduction disorder related to thiamine deficiency. In spawning Baltic cod, hepatic and ovarian thiamine concentrations showed great variance, with values ranging from0.56 to 4.7, and 3.8 to 30 nmol g-1, respectively. Whether Baltic cod also suffer from mortality associated with a thiamine deficiency is not known. [ABSTRACT FROM AUTHOR]

Most Baltic salmon populations are affected by the M74 syndrome, which has been linked to the feeding ecology of sea-run fish. A 2-year study in 1995-1997 showed that sprat (Sprattus sprattus) dominated thesalmon diet in the Baltic proper (45-90% of the stomach content by weight), followed by herring (Clupea harengus, 9-39%) and three-spinedstickleback (Gasterosteus aculeatus, 0-14%). These figures are similar to those recorded in the early 1960s. There was a consistent tendency for sprat to make up a higher proportion of the diet in the winter from January to April, and later in the year the proportion of herring and sticklebacks increased. When comparing the eastern and southern parts of the Baltic proper, there was also a tendency in the eastern part in January-September for the diet to have a lower proportion of sprat and a higher proportion of herring. This difference was, however, not significant according to the more conservative of 2 different statistical tests. There were also indications that wintertime feeding by salmon had increased compared with the early 1960s. In a regression study, the incidence of M74 was found to be strongly correlated to several parameters that describe the status of the Baltic sprat stock, two of which were weights of sprat. This suggests that the analysis of factors influencing the development of M74 needs to be expanded to include lower trophic levels as well. [ABSTRACT FROM AUTHOR]

The aim of this project was to determine whether there is an association between a pathogenic microorganism associated with a maternal pathologic lesion, or a maternal metabolic impairment arising from disturbed homeostasis caused by changes in the symbiotic microflora, and the M74 fry mortality syndrome in Baltic salmon. A broad screening approach was taken, using conventional methods for isolating and identifying fungi, bacteria, and viruses. Bacteria were identified using specific sequences of 16S rRNA, and microbe identification based on theanalysis of differences between 2 complex genomes using a DNA subtractive methodology; representation difference analysis (RDA). The selection of tissue sites to be isolated was based on histopathological findings. Bartonella spp., which are known to be associated with inflammatory cardiac diseases in other animals, were not found to be associated with coronary arterial lesions in sea-run Baltic salmon. Duringthe long starvation period during spawning migration, essential substances such as vitamins may be provided by the salmon's symbiotic intestinal micro flora. From the gastrointestinal (GI) tract of adult sea-run Baltic salmon, we were able to isolate Aeromonas sobria, Pseudomonas putrefaciens, Pseudomonas spp., A. hydrophila and Hafnia alvei,all of which can produce vitamin B2 . P. putrefaciens and Pseudomonas spp. isolated from the GI tract also produce small amounts of Vitamin B3. Furthermore, it was found that chemotheraputic treatment of spawning salmon with formalin seems to disturb the symbiotic GI-flora. Upon completion of the present investigations we should be able to conclude whether or not M74 is due to anthropogenic disturbances, caused by chemotherapeutic agents or other anthropogenic pollutants in theenvironment, which compromise maternal health and lead to the activation of a pathogen or the disruption of vitamin production by symbionts. [ABSTRACT FROM AUTHOR]

The results from an investigation performed at the Ar Laboratory, Gotland, Sweden during the period 1994-1998 revealed no reproduction disturbances in brown trout similar to those found in Atlantic salmon from the Baltic sea. Female-specific mortality of yolk-sac fry never exceeded 11%. The mean total thiamine concentration in yolk-sac fry (5.5 1 1.4 nmol g-1; n = 20) well exceeds the threshold limit interval reported from Baltic salmon fry affected by M74 (0.31-0.34nmol g-1). Analyses of astaxanthin in eyed eggs showed levels above values reported from Baltic salmon eggs that later developed into fry with M74. However, the levels are lower than the values reported from the salmon population of Lake Vanern and from the riversLagan and Gotaalv on the Swedish west coast. Total carotenoid levels(mg carotenoid kg-1) in eggs from Gotland spawners decreased significantly with female size. Baltic sea trout from the Swedishriver Ava have experienced high yolk-sac fry mortality rates with the peak mortality in 1992 (35 1 20%). In addition, reproduction disturbances in sea trout are reported from 3 Swedish rivers along the Baltic coast (Dalalven, Indalsalven, and Umealven). Differences in life histories of sea trout and salmon from the Baltic Sea are discussed inrelation to the lower incidence of mortality in yolk-sac fry of Baltic sea trout. [ABSTRACT FROM AUTHOR]

In the present paper we review some factors considered to explain the dramatic decline in the abundance of cod in the eastern Baltic. Environmental conditions, e.g. salinity and oxygen content, are known tohave a considerable impact on recruitment. We conclude that, at present, low recruitment due to deteriorated spawning conditions accompanied by high mortality due to commercial fishing and, thus, a decreasein spawning stock biomass are major determinants for distribution ofcod in the Baltic. Still, a number of additional factors may be important. The impact by sprat and herring predation on eggs and larvae may be considerable under certain circumstances. The temporal shift inspawning time towards summer months has likely resulted in spawning in more deteriorated oxygen conditions. The change in spawning stock structure towards younger fish probably affects the survival of eggs and larvae negatively. Since this change is closely related to high fishing pressure, low recruitment might, to a large extent, be a management issue. The M74 syndrome, a thiamine deficiency known to affect the Baltic salmon, has also been discussed in relation to poor cod recruitment. We conclude that it is unlikely that the Baltic cod suffers from M74 since thiamine levels in gonads from Baltic cod have been found to be, on average, almost 10-fold those found in healthy salmon. Moreover, in contrast to salmon, the survival of cod larvae has been found to be unaffected by thiamine treatment. [ABSTRACT FROM AUTHOR]

The purpose of this study was to describe the histopathological findings during the progression of the M74 syndrome in the Atlantic salmon from the Baltic Sea. We also wanted to evaluate whether there were differences in the histopathological picture between yolk-sac fry from different categories of M74, i.e. family groups with early, intermediate, or late development of disease. In the preclinical stage of M74, no lesions were detected in the yolk-sac fry. In the clinical stage, the dominant findings were necrotic lesions in the brain and a depletion of glycogen in liver and skeletal muscle. Yolk-sac fry with anearly development of disease also exhibited intracranial hemorrhages, hepatocellular lipid accumulation, and hydropic degeneration of sceletal muscle and proximal tubules of the excretory kidney. In the terminal stage, the brain lesions had become more severe and, furthermore, yolk-sac fry with early development of M74 also showed focal hepatocellular necrosis and sometimes hyaline degeneration of skeletal muscle. The findings in yolk-sac fry with intermediate development of disease were generally the same as in yolk-sac fry with early development, even though the lesions were of lesser magnitude. The only major histopathological findings in yolk-sac fry with late development of disease were brain lesions and, in the terminal stage, a dilatation ofsaccus vasculosis. [ABSTRACT FROM AUTHOR]

Muscle, egg and blood samples from female sea-run Baltic salmon fromDalalven were analyzed for neutral and phenol-type organohalogen substances (OHS) to determine their potential associations to the M74 syndrome of wild Baltic salmon. No differences in the concentrations ofDDT and related compounds, polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), hexachlorobenzene (HCB), or methoxylated brominated diphenyl ethers (MeO-PBDEs) in healthy salmon compared to salmon that produced offspring with M74 were observed. A large number of phenol-type compounds (> 100), such as polyhalogenated (brominated and/or chlorinated) phenols, polychlorobiphenylols (OH-PCBs) and OH-PBDEs were identified or indicated. High blood concentrations of PCP were found in both healthy salmon and in salmon that produced offspring with M74. Both hydroxylated and methoxylated PBDEs, in concentrations similar to the PBDE congeners, were found. 2-(2',4'-dibromophenoxy)-4,6-dibromoanisol and the corresponding phenol were identified in the salmon samples. During the spawning period different concentration ratios of the OHS were indicated between muscle, egg, and blood plasma lipids depending on the individual OHS. No differences in OHS concentrations were observed between healthy salmon and salmon that produced offspring with M74. Consequently, the data do not support the theory that the M74 disease is induced by OHS. However, the relation between the disease and phenol type OHS must be further considered since the data available on levels of these compounds are still limited. [ABSTRACT FROM AUTHOR]

Wild salmon populations (Salmo salar) in northern Sweden were at a low level in the period from the 1960s through 1980s, mainly due to extensive exploitation of salmon in the sea. Electrofishing surveys were started in the 1970s and were carried out in 8 wild salmon rivers in the Bothnian Bay area in northern Sweden. There was a high correlation in salmon parr densities between most of the rivers. Abundance ofspawners was also highly correlated between the rivers. This study showed very low salmon parr densities in all rivers for the period 1970 through 1980s. Parr densities increased in the late 1980s and at the beginning of the 1990s, probably because increased numbers of spawners ascended the rivers. From 1992, however, parr densities decreasedagain to low levels, although the number of spawners still remained at a high level. The decrease in parr densities coincided with a highmortality of alevin/fry due to the M74 syndrome found in Swedish salmon hatcheries in the Baltic. M74 mortality versus parr density and corresponding spawning stock (quotient parr/spawners) gave a significant negative correlation in all studied rivers. It is very likely thatM74 mortality has affected salmon alevin/fry survival in wild rivers. The decrease of salmon parr production due to M74 mortality was estimated to be 55-70%. As the factors influencing the development of M74 are poorly understood, it is possible that the incidence of the disease may continue to fluctuate, without any possibility to predict its development. Only a higher number of spawners can counteract the negative effects of M74 mortality in wild salmon populations. The exploitation of wild salmon by fisheries must therefore be kept at a low level. [ABSTRACT FROM AUTHOR]

Antioxidative systems, detoxifying enzymes and thiamine levels were studied in Atlantic salmon from the Baltic Sea that developed M74. Low concentrations of both thiamine and carotenoids were found in roe from family groups that developed M74. Additionally, thiamine was found to influence whether yolk-sac fry exhibited early, intermediate, orlate development of M74. The study revealed new data concerning interactions between oxidative stress and M74. Newly hatched yolk-sac frythat later developed M74, showed an induction of hepatic catalase activity. However, during development the situation became the reverse,whereby yolk-sac fry developing M74 failed to show increasing hepatic catalase activity with increasing age, otherwise shown by healthy yolk-sac fry. Hepatic glutathione peroxidase and glutathione reductaseactivities showed marked increases during development of the disease, indicating that peroxidation mechanisms are involved in the pathogenesis. Hepatic EROD-activities were low at hatching and showed an agedependent increase in healthy yolk-sac fry. In yolk-sac fry developing M74, a decrease in activity was noted between the clinical and terminal stages of disease. [ABSTRACT FROM AUTHOR]