Early Life Stage Mortality Syndrome in Fishes of the Great Lakes and Baltic Sea

<em>Abstract</em>.—Since 1974, feral salmon <em>Salmo salar </em>populations of the Baltic Sea have suffered from a yolk sac fry mortality known as the M74 syndrome. Mortality rates of 40–95% have been recorded during the 1990s in compensatory rearing stations along the east coast of Sweden. The M74 syndrome has been linked to the offspring of specific females and associated with low thiamine (vitamin B₁) concentrations in both female tissues and their progeny. This study evaluated the effect of thiamine treatments on mortality and thiamine concentrations in progeny with and without M74. Eggs and newly hatched yolk sac fry were immersed in water containing thiamine at concentrations of 100, 500, or 2,000 mg/L. Hardening of eggs in water containing thiamine at 500 or 2,000 mg/L completely eliminated M74-related mortality, whereas treatment with thiamine at 100 mg/L only partially reduced M74 mortality. The mean thiamine concentrations at the yolk sac fry stage (21–23 d after hatching) in untreated normal and M74-affected groups were between 0.70–1.0 and 0.19–0.26 nmol/g, respectively. At the same sampling, the mean thiamine concentrations in groups in which eggs were waterhardened in thiamine at 500 or 2,000 mg/L were between 0.8 and 9.4 times higher than the concentrations in the untreated groups. A thiamine threshold limit interval of 0.34–0.47 nmol/g was estimated for the development of M74 in yolk sac fry.

Early Life Stage Mortality Syndrome in Fishes of the Great Lakes and Baltic Sea

<em>Abstract</em>.—Thiamine concentrations in representative Great Lakes prey fish, including alewives <em>Alosa pseudoharengus</em>, rainbow smelt <em>Osmerus mordax</em>, slimy sculpin <em>Cottus cognatus</em>, bloater chub <em>Coregonus hoyi</em>, and lake herring <em>Coregonus artedi</em>, and their major dietary items, including mysids <em>Mysis relicta</em>, amphipods <em>Diporeia hoyi</em>, and net macroplankton, were measured to assess their potential involvement in depressed thiamine concentrations in lake trout <em>Salvelinus namaycush </em>of the Great Lakes. Mean thiamine concentrations in all biota were greater than the recommended dietary intake of 3.3 nmol/g for prevention of effects on growth, although the adequacy of these concentrations for reproduction is not known. Mean thiamine concentrations decreased in the order alewives > bloater chub, herring > smelt and differed from the order of associated egg thiamine concentrations published for lake trout feeding on these species (herring > alewives, smelt). As a result, these data strongly implicate the high thiaminase content, rather than the low thiamine content, of alewives and smelt as being responsible for the low egg thiamine concentrations of Great Lakes lake trout stocks that feed heavily on these species. Variations in thiamine content among prey species did not appear to be related to levels in their diet, because thiamine concentrations in <em>Mysis</em>, <em>Diporeia</em>, and macroplankton showed little consistency between group or between lake variation. There was no lake to lake variation in mean thiamine concentrations of prey species, but considerable within species variation occurred that was unrelated to size.

Early Life Stage Mortality Syndrome in Fishes of the Great Lakes and Baltic Sea

<em>Abstract.—</em>Populations of Baltic salmon <em>Salmo salar </em>and cod <em>Gadus morhua </em>are facing acute threats because of poor reproduction. The salmon is afflicted with high yolk sac fry mortality, and the incidence of cod larvae mortality is high. There are also indications that anadromous Baltic brown trout <em>Salmo trutta </em>populations are affected by reproductive disorders. These top predators have significant ecological, economic, and socioeconomic importance. Other species are also suffering from poor reproductive success and declining populations. Burbot <em>Lota lota </em>populations are locally affected by inadequate sexual maturation, resulting in a failure to spawn; gonad anomalies have also been described in roach <em>Rutilus rutilus</em>. High egg mortality has been recorded for whiting <em>Merlangius merlangus</em>, flounder <em>Platichtys flesus</em>, and herring <em>Clupea harengus</em>. Attempts have been made to discover the cause of reproductive disorders in Baltic fish species, but the available data suggest several possible causes, both abiotic and biotic. Species with pelagic eggs such as cod and flatfish are dependent on salinity and oxygen concentrations, factors that often limit the volume of reproduction in the Baltic Sea. A variety of biotic causes (i.e., infectious diseases, parasitism, and toxic algae) have been shown to affect species such as roach and herring. There are indications that nutritional factors (i.e., thiamine and astaxanthin) are involved in the cause of the yolk sac fry mortality syndrome affecting the Baltic salmon. Furthermore, anthropogenic activities causing both local point sources (i.e., metals and persistent organic pollutants) and long-range transport and deposition of acidic rain and pesticides must also be considered as potential threats to Baltic fish species.

Early Life Stage Mortality Syndrome in Fishes of the Great Lakes and Baltic Sea

<em>Abstract</em>.—Dietary amprolium, a thiamine antagonist, was fed to lake trout <em>Salvelinus namaycush </em>broodstock from April to October before spawning to determine its effect on egg and tissue concentrations of thiamine, thiamine monophosphate, and thiamine pyrophosphate. The thiamine concentration of eggs from fish fed no amprolium was 61.8 nmol/g, whereas the concentration of thiamine in fish fed 0.05 and 0.10% amprolium was 4.02 and 1.71 nmol/g (<EM>P </EM>< 0.01), respectively. In lake trout fed 0.10% amprolium beginning in August, egg free thiamine concentration was reduced to 11.6 nmol/g. No sign of early mortality syndrome was observed in sac fry from eggs in this study, which suggests that thiamine concentrations in the egg were not low enough to be below a critical threshold or that factors other than thiamine are involved in early mortality syndrome.

Early Life Stage Mortality Syndrome in Fishes of the Great Lakes and Baltic Sea

<em>Abstract</em>.—Since 1974, feral Baltic salmon <em>Salmo salar </em>populations have suffered from yolk sac fry mortality caused by the M74 syndrome. This syndrome affects yolk sac fry originating from specific females, and the mortality in affected family groups is usually 100%. Since 1990–1991, disturbances in the behavior of spawning migrating Baltic salmon have been observed. This study found a strong correlation between wiggling behavior in adult female Baltic salmon and the development of M74 in their offspring. Moreover, the ovarian thiamine concentrations in wiggling females were found to be significantly lower than those of females with normal behavior. In addition, eyed eggs and yolk sac fry that subsequently developed M74 contained only 13% (0.24 nmol/g) and 6% (0.11 nmol/g), respectively, of the thiamine concentrations detected in viable progeny sampled during the same periods. Eyed eggs with thiamine concentrations below a threshold limit interval of 0.36–0.77 nmol/g were found to have a high risk of developing M74 at the yolk sac fry stage.

Early Life Stage Mortality Syndrome in Fishes of the Great Lakes and Baltic Sea

<em>Abstract</em>.—Reproductive success and vitamin B₁(thiamine pyrophosphate, thiamine monophosphate, and free thiamine) concentrations were assessed in feral female lake trout <em>Salvelinus namaycush </em>from Lake Ontario and Lake Manitou. We monitored fertilization success, survival to hatch, incidence of blue-sac disease, other anomalies, and lake trout early mortality syndrome (EMS). Fertilization and hatching success were high, whereas mortality from blue-sac disease and other anomalies was low in egg batches from both lakes. There was no mortality from EMS in families from Lake Manitou. However, EMS occurred after hatching in the offspring of 48% of the females collected from Lake Ontario. We measured thiamine in liver, red blood cells, eggs, and developing embryos. Relative to fish collected in reference lakes, females in Lake Ontario had depressed hepatic, red blood cell, and egg thiamine concentrations. Although more extensive investigation of thiamine balance is required, it may be possible to use red blood cell thiamine pyrophosphate as a predictive index for EMS susceptibility in offspring. Total thiamine concentrations in developing embryos declined by 50% between fertilization and swim-up. Free thiamine reserves declined most rapidly, whereas levels of thiamine pyrophosphate increased between the eyed embryo and hatch stages. A high proportion (67%) of lake trout families in which the initial egg free thiamine reserves or embryonic concentrations of thiamine pyrophosphate levels were <0.8 nmol/g exhibited EMS. Below this threshold (0.8 nmol/g), the occurrence of EMS was variable (0–100%) and only weakly related to free thiamine concentrations (<em>r</em>² = 0.32, <EM>P</EM> = 0.014). This observation implies the possibility of additional interactions with other factors.

Early Life Stage Mortality Syndrome in Fishes of the Great Lakes and Baltic Sea

<em>Abstract.—</em>Muscle and egg samples from returning adult female Lake Michigan coho salmon <em>Oncorhynchus kisutch </em>were collected for thiamine analysis. Three groups of five females having low (2.5%), medium (42.4%), or high (92.6%) mean fry survival were selected for this study. Egg and muscle samples were collected at spawning and analyzed by high-performance liquid chromatography analysis for free thiamine, thiamine monophosphate (TP), and thiamine pyrophosphate (TPP). Egg concentrations of ascorbic acid, iron, zinc, magnesium, and potassium were measured. Twenty-five contaminants were also measured in muscle tissue of adult females. Total thiamine levels in eggs were similar between the medium and high survival groups but significantly lower in the low survival group. Eggs from the high and medium survival groups had higher levels of free thiamine and TP (<EM>P </EM>< 0.01) than eggs from the low survival group. There were no significant differences among the three groups in egg TPP. Muscle concentrations of TPP, TP, and total thiamine were similar among the three survival groups (<EM>P </EM>> 0.10). Correlations between fry survival and egg free thiamine (<em>r </em>= 0.61) and TP (<em>r </em>= 0.52) were observed. Fry survival was not correlated with adult muscle concentration of any form of thiamine or contaminant measured. Among the three groups, no differences in egg concentration were found for ascorbic acid, dehydroascorbic acid, iron, magnesium, zinc, and potassium. This research supports the hypothesis that low egg thiamine is an important factor in early mortality syndrome.

Early Life Stage Mortality Syndrome in Fishes of the Great Lakes and Baltic Sea

<em>Abstract</em>.—Juvenile and adult lake trout <em>Salvelinus namaycush </em>that were fed semipurified, thiaminedeficient diets or alewives <em>Alosa pseudoharengus </em>containing thiaminase, a thiamine-destroying enzyme, showed no overt symptoms of thiamine deficiency. Growth rates and ovulation rates were similar among all treatments. However, liver thiamine pyrophosphate (TPP), a biochemical indicator of impending thiamine deficiency, in juvenile lake trout fed thiamine-deficient diets was reduced to 35 pmol/ g compared with 59 pmol/g in control groups. Blood TPP in adult female lake trout fed alewives was one-third of that in controls fed a commercial diet. Adult lake trout from Lake Michigan had blood TPP levels similar to those of fish fed the alewife diet in the laboratory. Lake Superior lake trout had TPP levels similar to those of fish fed the control diet in the laboratory. Thiamine synthesis occurred in the intestine of lake trout. At least 81% of thiamine in the posterior intestine was synthesized, presumably by bacteria, when a ¹⁴C-labeled thiamine diet was force-fed to lake trout. Thiamine had a long retention time in the lake trout: at 27 weeks after fish were injected with radioactive thiamine, blood cells retained 11% of the radioactivity that was present at 2 d and liver tissue retained 34% of the 2-d level. Lack of self-sustaining lake trout reproduction by Lake Michigan fish may be related to their lower blood thiamine levels. Thiamine deficiency may cause early mortality syndrome, which is common in Lake Michigan but not Lake Superior fish with higher blood thiamine levels.

Early Life Stage Mortality Syndrome in Fishes of the Great Lakes and Baltic Sea

<em>Abstract</em>.—Thiamine pyrophosphate, thiamine monophosphate, and thiamine were measured by reversed phase high-performance liquid chromatography in tissues of lake trout <em>Salvelinus namaycush </em>and alewife <em>Alosa pseudoharengus</em>. Mean assay sensitivity for thiamine and its phosphates was 0.012 pmol. Average recoveries of low and high doses of thiamine compounds added to tissue samples ranged from 91.4 to 104.5%. Average coefficients of variation for between assay reproducibility ranged from 4.8 to 12.8%. The predominant form of vitamin B₁was unesterified thiamine in eggs and plasma of lake trout. Thiamine pyrophosphate was the predominant form in red blood cells, liver, muscle, and kidney. The stability of thiamine forms in fish tissues was temperature and species dependent. Thiamine levels were markedly depressed in lake trout collected from Lake Ontario relative to levels in fish captured from Lake 468 at the Experimental Lakes Area in northwestern Ontario.