Can timing of spawning explain the increase in egg size with female size in salmonid fish?

A common observation is that egg size increases with maternal size within populations. Several hypotheses have been proposed to explain this phenomenon. Most recently, it was suggested that the optimal egg size depends on female size due to correlations between (i) breeding time and egg size selection and (ii) female size and breeding time, and as such, the positive egg size–maternal size relationship represents an adaptive strategy. Here, we test the second of these two premises in two salmonid species, Atlantic salmon (Salmo salar) and coho salmon (Oncorhynchus kisutch) for which we have detailed breeding and egg size data. The body size of spawning females decreased throughout the season in coho salmon, but not in Atlantic salmon. Furthermore, we did not find a direct correlation between egg size and breeding time, which would also be expected if this premise was correct. Accordingly, breeding date does not appear to be as important as other factors in shaping among female differences in egg size.     

Influence of European sculpin, Cottus gobio, on Atlantic salmon Salmo salar, recruitment and the effect of gravel size on egg predation – implications for spawning habitat restoration

Abstract  The study examined if recruitment of juvenile Atlantic salmon, Salmo salar L., was influenced by the presence of European sculpin, Cottus gobio L., and if the spawning substrate size used by salmon influences sculpin predation on salmon eggs. A combination of information on spawning site selection by female salmon and associated densities of juvenile salmon indicated that recruitment of juvenile salmon was 10 times lower in areas where sculpin was present than areas without sculpin. Predation rate on salmon eggs was found to be dependent on substrate size in artificial redds created in stream aquaria using four different sizes of substrate (13, 23, 37 and 62 mm). Predation rate averaged 83% in the aquaria with the largest substrate size, whereas a rate of only 2–3% was observed using smaller substrate sizes. Sculpin may thus be an important factor influencing the recruitment of juvenile salmon. Selecting small enough gravel sizes during restoration of salmon spawning habitat could therefore be important to minimise egg predation.     

Signal crayfish (Pacifastacus leniusculus) predation upon Atlantic salmon (Salmo salar) eggs

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Investigating factors contributing to reduced embryo survival in farm‐raised Atlantic salmon,Salmo salar L.

Embryo mortality of Atlantic salmon, Salmo salar, has been increasing for more than a decade in the State of Maine, a leading producer of this species in the United States. Increasing embryo mortality not only creates a financial bottleneck for farms but also prevents the sale of surplus eggs as an additional source of revenue. Blood and egg samples were collected at three Maine Atlantic salmon farms from female broodstock at the time of spawning over a 2‐year period. Correlative factors for reduced embryo survival were investigated by measuring egg and maternal plasma concentrations of 17β‐estradiol (E2), 11‐ketotestosterone (11‐KT), testosterone and calcium, as well as maternal hepatic ethoxyresorufin‐O‐deethylase (EROD) activity and fork length. Significant positive correlations were found between maternal plasma concentrations of E2 and 11‐KT and embryo survival. Interestingly, there was no correlation with egg concentrations of sex steroids and embryo survival, suggesting that embryo mortality does not likely rest with the maternal deposition of sex steroids into the egg, but with another hormone regulated process related to egg assembly, ovulation or post‐ovulatory ageing.     

Metabolic adaptations of oxidative muscle during spawning migration in the Atlantic salmon <Emphasis Type="Italic">Salmo salar</Emphasis> L.

The adaptability/plasticity of the highly oxidative red muscle in Atlantic salmon was demonstrated during spawning migration. Substrate concentrations and the enzymatic pathways of ATP production were examined in red muscle obtained from Atlantic salmon at different sites along their migratory route in the Exploits River, Newfoundland, Canada. Individuals were chronologically sampled from a seawater site, two sites upstream, and at spawning. The 20% decrease in salmon body weight during the later stages of migration was accompanied by large decreases (mg dry weight⁻¹) in both glycogen (P < 0.01) and total muscle lipid (P < 0.01). In contrast, water content and protein concentration (mg dry weight⁻¹) of the red muscle increased by 25 and 34%, respectively, at spawning. Enzymes of the glycolytic pathways demonstrated a significant (P < 0.001) decrease in maximal activity as migration proceeded whereas enzymes of the oxidative phosphorylation pathways, specifically the citric acid cycle enzymes, exhibited an increase (P < 0.001) in maximal activity at spawning. The antioxidant enzyme superoxide dismutase also demonstrated an increase (P < 0.001) in maximal activity during the latter stages of migration. These adaptations imply that the red epaxial muscle of Atlantic salmon has a more efficient means of oxidizing lipids, while minimizing free radical damage, during the later stages of migration and spawning, thereby potentially increasing post spawning survival.     

Historical changes in age structure of Atlantic salmon, Salmo salar L., in the River Eo, northern Spain

Abstract Variations in the age structure and number of Atlantic salmon, Salmo salar L., running into the River Eo in northern Spain were assessed from catch records of sport fisheries for the period 1949 to 1991. The analysis focused on two periods (1951–1960 and 1981–1991) for which more complete data on fish number, size and age were available. Over the whole period, no evidence for a decline in the total salmon run was found, but the last few years revealed significant changes in both the number and age structure of salmon caught. The catches were mainly composed of multi-sea-winter salmon, and this did not change from the 1950s to the 1980s as the proportion of grilse remained unchanged. However, since 1980, there was a marked decline in both the proportion of 3-sea-winter salmon (versus 2-sea-winter fish) and the frequency of previous spawners among the multi-sea-winter salmon. These changes were probably linked because most of the previous spawners re-entered the river as 3-sea-winter or older salmon. River age of salmon caught in the 1950s was low and constant (about 90% of the catch had smolted at one year) but the mean age of smolting increased in the 1980s.     

The settlement and reproductive success of Lepeophtheirus salmonis (Krøyer 1837; Copepoda: Caligidae) on atypical hosts

The reproductive success of Lepeophtheirus salmonis settled on host and non‐host fish has been compared. Triplicate single species tanks of Atlantic salmon, marine three‐spined sticklebacks, saithe and Atlantic cod were exposed to 10 adult female L. salmonis per tank (n=30 lice per species). Adult female L. salmonis settlement and egg string production occurred only on salmon and cod, with no egg production occurring on saithe and three‐spined sticklebacks. The number of eggs in egg strings, hatching success of eggs and the survival of all larval stages to the copepodid stage were severely affected by the species of fish on which female L. salmonis had settled. L. salmonis settled on cod produced significantly fewer eggs, lower hatching rates and lower survival rates of larvae than females on Atlantic salmon. The production of egg strings by L. salmonis females infecting cod, which successfully hatch and moult through to the infective copepodid stage, albeit in small numbers, is discussed in terms of the implications to aquaculture and salmon and cod farming scenarios.     

Dietary fatty acid enrichment during the spawning season increases egg viability and quality in Hippocampus hippocampus

Control of the reproductive cycle is essential to achieve a successful aquaculture industry. Seahorse reared in captivity often shows eggs of poor quality and low offspring survival due to unsuitable rearing and nutritional protocols. This study aimed to evaluate the effect of dietary enrichment, added exclusively during the spawning season, on the egg quality of Hippocampus hippocampus. The effect of enriching mysis throughout the spawning season was analysed in terms of female fecundity, egg viability, egg size and fatty acid profile. The results showed that fecundity was not affected by the enrichment of the diet, but egg viability was increased when females were fed the enriched mysis (84.9 ± 4.1% compared to 62.2 ± 9.4%). Moreover, the content of several PUFA (20:4n‐6, 18:2n‐6, 22:5n‐3, 22:5n‐6, sum of n‐6) decreased throughout the spawning season, especially in the last clutch, in eggs originated from females fed non‐enriched mysis. Eggs from females fed the enriched diet maintained the fatty acid profile throughout the spawning season. In conclusion, enrichment of the diet of H. hippocampus improved egg viability and avoided a decrease in the quality of eggs by maintaining PUFA content in the last clutches of the spawning season.     

Eggs of Atlantic Salmon (Salmo salar L) and Trout (S. trutta L); Identification by Phosphoglucoisomerase Zymograms*

Salmonid eggs were collected from spawning redds in the Norwegian rivers Alta, Gaula, and Driva in 1982. Their phosphoglucoisomerase (PGI; E.C. No. 5.3.1.9) patterns, as revealed by isoelectric focusing and histochemical staining, were compared to standard zymograms produced by Atlantic salmon (Salmo salar L.) and trout (S. trutta L.) white skeletal muscle extracts, and to the zymogram in salmon eggs of known origin. In salmon, the egg zymogram was identical with that of muscle. Two types of zymograms, identical with either salmon or trout muscle zymograms and thus regarded as species-determinants, were observed among the eggs from wild fish. Their proportions in the three rivers were as expected from current knowledge of species composition. The present findings are related to previous reports on PGI expression in salmonids. The PGI zymograms are probably diagnostic keys to species for eggs from Norwegian stream-dwelling salmonids, including Arctic char (Salvelinus alpinus L.). Some managemental implications of the present results are briefly discussed.     

Interactions between naturalised exotic salmonids and reintroduced Atlantic salmon in a Lake Ontario tributary

Abstract – Atlantic salmon (Salmo salar) was once native to Lake Ontario, however, its numbers rapidly declined following colonisation by Europeans and the species was extirpated by 1896. Government agencies surrounding Lake Ontario are currently undertaking a variety of studies to assess the feasibility of reintroducing Atlantic salmon. We released hatchery‐reared adult Atlantic salmon into a Lake Ontario tributary to examine spawning interactions between this species and fall‐spawning exotic salmonids found in the same stream. Chinook salmon, coho salmon and brown trout were observed interacting with spawning Atlantic salmon in nearly one‐quarter of our observation bouts, with chinook salmon interacting most frequently. Whereas a previous investigation found that chinook salmon caused elevated agonistic behaviour and general activity by spawning Atlantic salmon, the present study found that interspecific courtship was the most common form of exotic interaction with spawning Atlantic salmon. In particular, we observed precocial male Chinook salmon courting female Atlantic salmon and defending the female against approach by male Atlantic salmon. We discuss the potential implications of these interactions on the Lake Ontario Atlantic salmon reintroduction programme.     

Saprolegnia diclina IIIA and S. parasitica employ different infection strategies when colonizing eggs of Atlantic salmon,Salmo salar L.

Here, we address the morphological changes of eyed eggs of Atlantic salmon, Salmo salar L. infected with Saprolegnia from a commercial hatchery and after experimental infection. Eyed eggs infected with Saprolegnia spp. from 10 Atlantic salmon females were obtained. Egg pathology was investigated by light and scanning electron microscopy. Eggs from six of ten females were infected with S. parasitica, and two females had infections with S. diclina clade IIIA; two Saprolegnia isolates remained unidentified. Light microscopy showed S. diclina infection resulted in the chorion in some areas being completely destroyed, whereas eggs infected with S. parasitica had an apparently intact chorion with hyphae growing within or beneath the chorion. The same contrasting pathology was found in experimentally infected eggs. Scanning electron microscopy revealed that S. parasitica grew on the egg surface and hyphae were found penetrating the chorion of the egg, and re‐emerging on the surface away from the infection site. The two Saprolegnia species employ different infection strategies when colonizing salmon eggs. Saprolegnia diclina infection results in chorion destruction, while S. parasitica penetrates intact chorion. We discuss the possibility these infection mechanisms representing a necrotrophic (S. diclina) vs. a facultative biotrophic strategy (S. parasitica).     

Genetic parameters and correlated responses in female reproductive traits in the GIFT strain

The effect of selection for greater growth rate in the GIFT strain on traits related to female reproduction (weight at spawning, number of eggs, number of fry, total weight of fry and number of dead fry) was studied. Available records comprised 10 generations of data from a total of 935 breeding females with known pedigree. Female body weight at spawning was 255 g, whereas the averages for number of eggs, number of live fry at hatching, number of dead fry and total fry weight per spawning were 1444, 908, 64.90, and 7.24 g respectively. Heritabilities for reproductive traits were low to moderate (0.20 for number of eggs, 0.16 for number of hatched fry and 0.11 for fry weight per female at spawning). The genetic correlations between female body weight at spawning and reproductive traits were very low to low (0.01 to 0.31) and not significantly different from zero. The genetic correlations among reproductive traits (i.e. number of eggs, number of hatched fry and fry weight) were high (0.75 to 0.92). Phenotypic correlations were consistent with the genetic correlations, ranging from 0.56 to 0.85. Correlated responses in reproductive traits measured as differences in least squares means between Selection and Control lines were positive and significant (except for the number of dead fry) when age of females at spawning was fitted as a covariate. However, there were no significant differences in all studied traits when body weight of the female at spawning was fitted as a covariate within lines. This indicates that the greater reproductive rate was due to the greater size of females in the Selection line, but the efficiency of reproduction per unit weight of female did not change as a consequence of selection for growth.     

Comparison of population dynamics of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) in a small tributary of the River Scorff (Brittany, France)

The population dynamics of Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) were compared in a small tributary of the River Scorff (Brittany, France) from spawning time to the beginning of the third growing season. The spawning and fry emergence of the two species took place at approximately the same time. In the first autumn, the densityof 0+ juveniles and settling rate from the egg stage were much higher in trout than in salmon. The emigration rate from 0+ population was much higher in trout than in salmon. The size of resident and migrating fish was always smaller in salmon than in trout, whatever the age. The low level of salmon production in the brook, compared with trout, was the result of low survival from egg to 0+ stage in autumn, combined with the small proportion of juveniles migrating after the first growing season. This was not compensated by a high number of migrants the next year. The role of physical habitat, inter-and intraspecific competition, predation and migration dependence on size and early sexual maturity is discussed. Indications are that small tributaries of the type studied are of great value for recruitment in trout but not very productive for juvenile salmon.     

Biofloc technology in intensive broodstock farming of the pink shrimp Farfantepenaeus duorarum: spawning performance,biochemical composition and fatty acid profile of eggs

A 45‐day trial was performed to evaluate the effect of biofloc technology (BFT) with or without fresh food (FF) supplementation during pre‐maturation period on Farfantepenaeus duorarum spawning performance, biochemical composition and fatty acid profile of eggs as compared with conventional clear‐water system (CW+FF). Females raised in biofloc and that received FF supplementation (FLOC+FF) achieved better spawning performance in terms of number of eggs per spawn (49 × 10³), number of eggs per spawn per g of spawner's body weight (2.1 × 10³) and egg size (~275 μm) as compared with CW+FF (23 × 10³, 1.1 × 10³ and 263 μm respectively), but both treatments did not vary from FLOC (P > 0.05). High spawning activity was also observed in biofloc system as compared with clear‐water system as shown in number of spawns per ablated female (2.2–3.0 versus 0.6) and percentage of females that spawn at least once (80–82 versus 25%). Biochemical composition of eggs presented no significant differences among treatments. FA profile of eggs indicated that high spawning activity performed by females in FLOC+FF treatment was reflected in lower mean levels of EPA, DHA and sum of polyunsaturated fatty acids (n‐3) and (n‐6). The better reproductive performance demonstrated by females raised in biofloc justified the application of this technology in F. duorarum broodstock.     

Effect of temperature on the development of eggs and the daily pattern of spawning of round herring <Emphasis Type="Italic">Etrumeus teres</Emphasis>

Effect of temperature on the development of eggs of round herring Etrumeus teres was experimentally examined to construct a temperature-dependent egg development model. Mature fish were collected in the field and their eggs were artificially fertilized onboard. The eggs were incubated at nine temperatures set between 14.0 and 25.0°C. All eggs at the lowest three temperatures, 14.0°C, 15.0°C, and 16.0°C, ceased development and died at various stages before hatching. Durations required to hatching after fertilization ranged from 38.0 h at 25.0°C to 90.0 h at 17.5°C. The temperature-dependent egg development model, i.e., egg age in hours (y _i,t ) at the ith stage and temperature t (°C), was expressed as: y _i,t  = 4.604 × exp(−0.100 × t −0.129 × i) × i ^2.593. From the application of the model to early-stage eggs collected in the field, it is concluded that round herring starts spawning immediately after sunset and almost completes spawning by midnight. The temperature-dependent egg development model and the daily pattern of spawning presented in this study are essential tools for developing the daily egg production method to estimate the spawning stock biomass.     

Effects of Maternal Age on Fecundity,Spawning Interval,and Egg Quality of Nile tilapia,Oreochromis niloticus (L.)

Effects of maternal age on fecundity, spawning interval, and egg quality of Nile tilapia, Oreochromis niloticus (L.), was examined for four age groups (4, 9, 16, and 24 mo) in circular concrete tanks in a recirculating system. Fecundity (eggs per spawn), weight (g), and egg quality were monitored every 4 d for the 168‐d experimental period. Eggs per spawn correlated with maternal age, while it did not correlate with body weight. Eggs per spawn increased by twofold between 4‐ and 24‐mo‐old females, while eggs per female per d did not differ. Implying that eggs per female per d was influenced by spawn per female as well. Eggs per female per d was 1.8 and 2.4 times higher in 9‐mo‐old females than 16‐ and 24‐mo‐old females, respectively. Eggs per female per d showed a decline concurrent with per spawn per female trend after 18‐mo old. Four‐mo‐old females produced eggs that were more uniform in size and weight than others. While egg size (mm) and weight (mg), and their variations within eggs per spawn increased, percent fertilization and hatchability decreased with age of females. Biological optimum age range for reproducible production of good quality eggs was 6–18 mo. This could be adopted in selecting breeders for better seed production.     

A genetics analysis of the performance of three rainbow trout broodstocks

The reproductive performance of three domesticated rainbow trout broodstocks spawning at 2 years of age was analysed using data collected over a period of 5 years. Data were collected for post-spawning body weight, volume of eggs spawned, egg size, egg number and fertility to the eyed egg stage. There were significant differences among the stocks attributable to both genetic and environmental causes. In addition, there were marked differences between year-classes, isolated from each other due to 2-year spawning. The differences appeared to be genetic in origin and demonstrated the effect of sampling the original parents.Heritabilities estimated from full-sib comparisons indicated male weight was more influenced by environmental stress at spawning than was female weight. Heritability estimates for egg volume, egg size and egg number ranged from 0.05 to 0.76 for individual stocks, but when averaged for the three stocks the range was from 0.32 to 0.52. Egg size was found to have the lowest heritability of the three traits. The heritability of fertility was found to be low and was the lowest for the stock with the poorest fertility.Genetic and phenotype correlations among the traits were also estimated. The relationship between body weight and egg volume and size were both positive as was that between egg volume and egg size. It was determined that egg volume was the principal determinant of egg number and that the relationship of number to size could be negative. The latter evidence was obtained by artificially adjusting the egg production traits to a constant female body size. The result suggested that females that produce larger numbers of eggs are likely to produce smaller eggs unless the change is associated with a marked increase in body weight. It is recommended that selection for egg size be included in all selection programs and that egg number be ignored in any selection program designed to increase body size.     

Spawning season,spawning grounds,and egg production of red sea bream in Hiuchi-nada,Seto Inland Sea

The spawning season and grounds of red sea bream in Hiuchi-nada, the central part of the Seto Inland Sea, were described using a new method based on monoclonal antibodies for identifying Pagrus major eggs, and the daily egg production (standardized by the incubation time and survival rate) was estimated. At the peak of spawning (May), the ranges of sea temperature, salinity, and chlorophyll a where red sea bream eggs occurred were 14.8–17.4°C, 32.0–33.0, 0.5–4.4 μg/L, respectively. The main spawning grounds of the red sea bream were confirmed as being the areas near the Geiyo Islands, Misaki Peninsula, Saijyo, Niihama. The spatial spread of red sea bream eggs increased with the egg developmental stage. The ranges of daily egg production in 2005 and 2006 were 0.3–19.3 and 0.2–6.7 × 10⁹ eggs/day, respectively. In Hiuchi-nada, aquaculture farms are located close to the spawning grounds, and the potential spawning population from the aquaculture farms equaled or exceeded that of the estimated spawning population obtained by the egg production method. Red sea bream eggs in Hiuchi-nada might be produced by both wild and aquaculture-based spawning populations.     

Physiological condition and migratory experience affect fitness‐related outcomes in adult female sockeye salmon

Relating fish physiology, behaviour and experience to fitness‐related outcomes at the individual scale is ecologically significant, but presents difficulties for free‐ranging fishes in natural systems. Physiological state (e.g. level of stress or maturity) and experience (e.g. habitat use or exposure to stressors) may alter the probability of survival or reproduction. This study examined the relative influence of physiology and migratory experience on survival, migration duration, reproductive longevity, and egg retention in adult female sockeye salmon (Oncorhynchus nerka) from a Fraser River population. One hundred and thirty‐five females were plasma sampled and tagged with radio transmitters and archival temperature loggers. Fish were tracked 55 km through two natal lakes to spawning grounds, following passage of a hydroelectric dam. For 39 females, we assessed the proportion of time within an optimal temperature (T_optAS) window (13.4–19.5°C), which provides ≥90% of maximum aerobic scope. Females with lower plasma glucose concentrations were more likely to reach spawning grounds. Early migrants spent longer in natal lakes. More time in the T_optAS window was associated with greater reproductive longevity and lower probability of egg retention. Later arriving females had reduced longevity on spawning grounds, as did females that retained eggs. Exposure to higher dam discharge was associated with reduced reproductive longevity and greater probability of egg retention, but not lower survival, indicating a delayed effect of dam passage. Our results underscore the complexity of factors governing fitness‐related outcomes for salmonids, particularly the importance of female experience in the days and weeks prior to spawning.     

Temporal spawning migration patterns of landlocked Atlantic salmon (Salmo salar) in a constructed stream

Landlocked Atlantic salmon (ouananiche, Salmo salar) spawning migration timing varies throughout its range and is influenced by a variety of environmental variables. This study examined the spawning migration time of landlocked Atlantic salmon in a small (4 ha) constructed stream linked to a large reservoir (32,600 ha) in Newfoundland, Canada. Salmon use of the stream was passively monitored using Passive Integrated Transponder (PIT) Tags from 2006 to 2012. Of the salmon tagged throughout the reservoir, 48% entered the stream during the spawning season and remained there for 18–36 days from late September to November before they returned to the reservoir. Most fish entered the stream at dusk (32%). The lunar cycle influenced the timing of migration each year. Mean stream discharge (cubic metres per second) influenced migration timing in some years, and water temperature appeared to have minimal impact on migration timing. Twenty per cent of fish migrated to the stream in multiple years and 51% of these individuals appeared to use the stream on a biennial or alternate year cycle. These results demonstrate that small constructed streams can attract salmon from throughout a large reservoir and can provide valuable habitat in areas where spawning habitat is limited.