A new method to evaluate egg‐to‐fry survival in salmonids, trials with Atlantic salmon

A new method to estimate the survival of salmonids from egg fertilization to fry emergence is described. Fine mesh screen cylindrical capsules, 12 cm³ in volume, filled with batches of 10 eggs of Atlantic salmon Salmo salar were implanted in the substratum using small removable guiding tubes inserted with a metal spike. The method was compared with two other commonly used techniques, capping redds with fry‐traps and fine mesh screen incubation‐emergence boxes buried into the gravel. Egg‐to‐fry survival was recorded for the three methods run in parallel in artificial redds created on three sites of the Nivelle River watershed (south‐west of France), two in the stream and one in an experimental spawning channel. In the channel, survival to the eyed stage and to hatching in capsules and incubation‐emergence boxes was also compared. The implant of capsules proved easier and faster to use than other methods, the structure of the surrounding substratum was less disturbed and the capsules were less vulnerable to spates. This technique provided survival values largely free of the bias induced by other methods and intermediate between that of incubators and of traps.     

Effects of fish farm effluents on egg-to-fry development and survival of brown trout in artificial redds

Egg-to-fry development and survival of brown trout Salmo trutta were compared in two rivers of the Pyrenean piedmont, the Nive d'Arnéguy, with few human activities, and the Nive des Aldudes, with many anthropogenic activities including 11 fish farms and two sewage treatment plants (STPs). Survival was estimated between spawning (early December) and emergence time (early March) by means of capsules, 7 cm³ in volume, filled with green eggs then inserted into the gravel of artificial redds at spawning sites. In the Nive des Aldudes, three sets of conditions were studied: a fish farm near the springs, a fish farm and an STP in a village, and a series of fish farms and an STP in a village. In each situation, two artificial redds were created upstream and two others downstream from the fish farms. In the Nive d'Arnéguy, four sites were equipped: each of the two downstream sites with two artificial redds, and the two upstream sites with one redd. Substratum characteristics (proportion of fine particles) and the quality of surface and interstitial water (oxygen content, ammonia and nitrite nitrogen) were periodically measured. There was no redd substratum difference between sites upstream and downstream of fish farms. Survival to fry emergence was higher in the Nive d'Arnéguy (63·5%) than in the Nive des Aldudes (47·7%). In this latter river, the nitrogen released from the STPs was 0·5% that from fish farms. Fish farming impaired survival close to fish farm effluents (31·6%), as compared to survival upstream (63·6%), and induced a development delay during the yolk-sac fry stage. These differences were mainly linked to a drop in the dissolved oxygen content in interstitial water induced by the nitrogen flow in surface and interstitial waters.     

An individual-based model for predicting the emergence period of sea trout fry in a Lake District stream

The objective of this study was to predict interannual fluctuations in the emergence period of sea trout fry, using models developed from field data for 70 excavated redds, and laboratory data on egg and alevin development at 30 constant temperatures (range 1·5–10·5° C with 100 naturally fertilized eggs at each temperature). Egg weight and numbers per redd both increased with female length; a power function described the relationship. Early spawners were the largest females laying the largest and most numerous eggs, whilst late spawners were the smallest females laying the smallest and least numerous eggs; middle spawners being intermediate between these two extremes. Mean values for egg weight and numbers of eggs per redd were obtained for these three groups. Hatching and emergence times in the laboratory decreased with increasing temperature. Of five models tested for hatching time, the best fit was provided by a three-parameter hyperbolic model which formed the asis of the individual-based model used to predict egg hatching and fry emergence. Model development was described in detail and the final equations predicted the times taken for 5, 50 and 95% of the fry to emerge, and hence the period over which 90% of the fry emerged. Analogous models were obtained for egg hatching. All models were excellent fits to the laboratory data. Hatching times for eggs kept in perforated boxes in the stream were almost identical to those kept at similar mean temperatures in the laboratory. Model predictions of fry emergence times were validated by field data for 8 years (1967–1971, 1974, 1975, 1980). The chief objective was therefore fulfilled, and predictions for the 30-year study (1967–1996) revealed a large variation in the timing of emergence (extremes: 11 March–4 April 1989, 15–20 May 1979). Most of the variation in median emergence date was due to variations in water temperature, with spawning dates as a secondary factor; the latter, however, had a greater effect on the length of the emergence period.     

Change in allometric length–weight relationship of Salmo trutta at emergence from the redd

In their life cycle, especially at early life stages, fishes pass through distinct growth stanzas with different length‐weight relationship patterns. Salmonid fish fry emergence from the redd gravel is a crucial moment in their life history. This study presents an ontogenic change in allometric length–weight relationship scaling of sea trout Salmo trutta at the time of emergence from the natural redds in a small lowland stream of western Lithuania. The paper also gives shrinkage rates and correction factors for emergent Salmo trutta fry after protracted formalin preservation.     

Differential dispersal patterns of male and female masu salmon fry

Behavioural experiments using artificial and natural stream channels were undertaken to determine whether there were differences in dispersal between newly emerged male and female masu salmon. Eyed eggs from a cross with wild spawners were planted in the middle pool of an artificial channel. After emergence, more males than females moved into an upstream trap, while fewer males moved downstream. In a natural stream, eyed eggs were marked with alizarin complexone to distinguish them from naturally spawned eggs and these were planted into artificial redds. More newly emerged male fry remained at the planted site than female fry. In contrast, more females moved downstream than males. These results imply that differences in dispersal patterns between male and female masu salmon fry are genetically controlled.     

Size characteristics and diet of emergent chinook salmon in a small, stable, New Zealand stream

The emergence timing, quality and diet of chinook salmon, Oncorhynchus tshawytscha , fry were studied as they emerged from ten natural redds. The impact of selected environmental factors on emergence was studied also. Samples of fry were obtained daily using redd traps.
The mean period of total emergence for redds with more than 450 fry emerging was 44.8 days (range 25–57 days), but for 80% of the fry in each redd to emerge the mean time elapsed was 10.8 days (range 6–21 days). No fry sampled had a visible yolk sac. The effect of temperature and light on emergence timing was weak, but stream flow had a strong influence. Fry length was constant for each redd throughout emergence, but fry weight tended to decline; consequently condition factor declined also. There was evidence that fry fed before emergence, but they did not grow. The major prey item, Deleatidium spp. (47.4% of the contents of all stomachs sampled), was the most abundant invertebrate in stream benthos and drift. Empty stomachs were prevalent at the start of emergence, with full stomachs becoming more common as emergence progressed.
The time distribution of numbers of fry emerging was usually unimodal, but occasionally bimodal. Over one third of fry emerged during the best 2 days of sampling.     

Predicting fluctuations in the size of newly emerged sea-trout fry in a Lake District stream

The objective was to predict interannual fluctuations in the size of sea-trout fry when they emerged from the redd, using models developed from field data for 70 excavated redds (≥three per year), and from experimental data on egg and alevin development at 30 constant temperatures in the laboratory (range 1·5—10·5) C with 100 naturally fertilized eggs at each temperature). Egg weight increased with female length and also with the number of eggs laid in a redd, both relationships being well described by a power function. Early spawners were the largest females laying the largest and most numerous eggs, whilst late spawners were the smallest females laying the smallest and least numerous eggs, with middle spawners being intermediate between these two extremes. Mean values for egg weight and number of eggs per redd were obtained for these three groups. The numbers of early, middle and late spawners for each year of a 30-year study and the mean values from the excavated redds were used to estimate weighted means for the number of eggs per unit area and egg weight. Mean values varied considerably between years (30-year ranges: 518–7964 eggs per 60 m2; 112–138 mg wet weight). In the laboratory, mean weights of newly hatched alevins and newly emerged fry were both related positively to mean egg weights. Alevin and fry mean weights were independent of the number of days required for 50% of the eggs to hatch or fry to emerge. Models described in a previous paper formed the basis of those used to predict fry weights over the emergence period. Model predictions were validated by field data for the whole emergence period in 8 years (1967–1971, 1974, 1975, 1980), and by pre-fry weights on single dates in 21 years (1967–1987). As pre-fry densities on these single dates were very similar to egg densities for the same year class, mortality in the egg and alevin stages was very low. The chief objective was therefore fulfilled, and the extent of interannual fluctuations for the 30-year study showed some variation in mean fry weight (30-year ranges: 153–193 mg for both the whole emergence period and the date on which 50% of fry emerged) but a progressive decrease in fry weight through the emergence period. Possible reasons for this variation are discussed, and it is concluded that the size of the female spawners is the dominant factor.     

An experimental study on the natural removal of dead trout fry in a Lake District stream

The chief objective was to discover why few moribund or dead Salmo trutta fry were observed on the stream bed when mortality within the stream was known to be high ( c . 13 000 dead fry year ⁻¹ for whole stream). Newly dead fry were placed in 20 boxes embedded in the stream bed (20 fry of known total weight per box) and arranged in pairs with one box open and one closed. One pair was removed every 2 days, the fish remains being weighed and the invertebrates in the open box being identified and counted. The experiments were performed from late April to early May in 1967, 1968, 1969 and the results were similar in each year. Both wet and dry weights of fry decreased exponentially but the rate of decrease was much higher in the open boxes; detectable fish remains were about 55% of initial weights after 20 days (end of experiment) in closed boxes but zero after 16 days in open boxes. Invertebrate scavengers were responsible for the higher loss rates in the open boxes and showed a definite succession with caddis larvae and carnivorous stonefly larvae dominant at first, but then being replaced by detritivorous stonefly larvae and freshwater shrimps. These experiments show clearly why dead fry disappear rapidly from the stream bed.     

Redd superimposition by introduced rainbow trout,oncorhynchus mykiss, on native charrs in a japanese stream

Spawning redd superimposition of introduced, spring-spawning rainbow trout,Oncorhynchus mykiss, on native, fall-spawning Dolly varden,Salvelinus malma, and white-spotted charr,S. leucomaenis, were examined in a small stream in Hokkaido, Japan. The stream reaches in which Dolly Varden and white-sported charr redds were observed in fall 1997 greatly overlapped with the reaches in which rainbow trout redds were recorded in spring 1998. Spawning microhabitats were also similar between trout and the two charr species. Thirteen and 3% of Dolly Varden and white-spotted charr redds, respectively, were superimposed by rainbow trout redds. The eggs or alevins in the disturbed charr redds were potentially damaged because charrs were not likely to have emerged from the redds before the superimposition occurred. In sufficiently great abundance, introduced rainbow trout may negatively impact native charr populations by dislodging the latter’s spawning redds.     

Egg-to-fry survival of the sea trout in some streams of Gotland

The quality of the surface water of Gotland (Sweden) streams was good, and did not limit sea trout egg-to-fry (ETF) survival. The oxygen concentration of the interstitial water was positively correlated to the permeability of the stream bed, and to the geometric mean diameter of the substratum. When the oxygen at 15 cm inside the stream bed was undiminished from surface values, the permeability of the stream bed was at least 6000 cm h^–1 and the geometric mean diameter of the substratum at least 15.0 mm. When the interstitial oxygen concentration increased, the interstitial ammonium concentration decreased, the NO₂ concentration remained stable and the NO₃ concentration increased. The ETF survival up to hatching was highly dependent upon the oxygen concentration of the interstitial water. For survival from hatching to emergence, the geometric mean diameter of the substratum was the most important factor. ETF survival >50% was observed where the interstitial oxygen concentration averaged at least 10mg I-¹, the substratum permeability at least 2000cm h-¹ and the substratum geometric mean diameter at least 15 mm. Other factors, such as overdigging and drying out of the spawning grounds could increase the ETF mortality.     

Survival and emergence pattern of sea trout fry in substrata of different compositions

The emergence of the sea trout Salmo trutta fry of Gotland, Sweden, was studied in the laboratory. The main limiting factor for survival up to hatching was the interstitial oxygen concentration, which depends on the gravel permeability, therefore on the substratum granulometry. Fry emergence was possible in substrata in which the geometric mean particle diameter ( d _g) was >15 mm and in substrata in which d _g was <6 mm. In the substrata of low d _g, emergence took place earlier than in the other ones. As a consequence the fry were smaller and their yolk sac only partially resorbed. In an unfavourable environment, fry started to emerge as soon as they reacted positively to light, about 2 weeks after hatching.     

The emergence period of sea trout fry in a Lake District stream correlates with the North Atlantic Oscillation

The date of fry emergence over 30 years in a sea trout nursery stream, predicted by an individual-based model, correlated significantly ( r =0·660, P <0·001) with an index of the North Atlantic Oscillation. Water temperature is the main driving variable in the model and stream temperature also correlated significantly ( r =0·662, P <0·001) with the index, providing a probable causal link. Therefore, the inter-annual variations in emergence may not be unique to this one stream, but may be typical of other trout streams with similar climatic conditions.     

Temporal and spatial segregation of spawning in sympatric populations of Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L.

Based on data from Norwegian streams with sympatric populations of Atlantic salmon and brown trout, it is suggested that temporal segregation is the main mechanism segregating Atlantic salmon and brown trout during spawning. Peak spawning of trout was about 15 days earlier than that of salmon. Physical factors, such as water depth, water velocity and distance from the river banks segregate spawning sites of salmon and trout poorly. Gravel sizes of the redds of salmon and trout were significantly different, though with a considerable overlap, and mean egg depth of salmon and trout were 0.18 and 0.12 m, respectively, probably attributable to the different size of spawners of salmon and trout. None of the temporal or spatial parameters analysed segregate spawners of salmon and trout completely. Species determination of eggs and alevins from the redds showed no interspecific superimposition of redds. It is, therefore, concluded that low survival of hybrids after hatching does not explain the low frequency of hybrids observed in sympatric populations of salmon and trout.     

Brown trout Salmo trutta redd superimposition by spawning Lampetra species in a lowland stream

Reproductive interaction between sympatric lampreys and salmonids was studied. The superimposition of brown trout Salmo trutta redds by spring-spawning river lamprey Lampetra fluviatilis and brook lamprey Lampetra planeri was examined in a small lowland stream of western Lithuania. A high superimposition rate of S. trutta redds by both L. fluviatilis (up to 83%) and L. planeri (up to 48%) was found, when the spawning intensity of Lampetra spp. was high. The occurrence of this phenomenon is the result of the overlap in the spawning habitat preferences at the reach-scale and at the microhabitat scale for the three species. One of the main requirements for Lampetra spp. spawning site selection was the negative streambed slope, an essential trait of the pool-riffle transitional zone. The structure of the salmonid redd created a considerable negative microhabitat slope suitable for Lampetra spp. spawning, which put the redds under a higher susceptibility to be superimposed. The timing of Lampetra spp. spawning overlapped closely with the emergence of S. trutta fry, suggesting a probable ecological effect of superimposition on S. trutta in the pre-emergent and emerging stages.     

Aquarium studies on the consumption of small animals by O–group grass carp, Ctenopharyngodon idella (Val.)

In aquarium experiments common invertebrates from streams and ponds were offered to O-group grass carp, Ctenopharyngodon idella (Val.) in the presence of palatable plants. When there was no cover for the prey to hide under, the carp ate many of the invertebrates, but when stones were provided, considerably more invertebrates escaped. Rainbow trout eggs were not eaten, but trout fry were taken as soon as they had emerged from artificial spawning redds. While searching for food, grass carp never disturbed the stones covering possible food organisms.     

The consumption of rainbow trout (Salmo gairdneri Richardson) eggs by macroinvertebrates in the field

SUMMARY. 1. Perforated boxes containing live, dead or no eggs of the rainbow trout, Salmo gairdneri Richardson, were buried in a natural spawning redd in a stream in North Wales for the duration of the natural incubation period.
2. A significant difference ( P <0.001) was found between the mean number of invertebrates colonizing perforated boxes containing live eggs, dead eggs and no eggs.
3. The number of eggs retrieved from boxes originally containing dead eggs decreased as the number of invertebrates colonizing the boxes increased.
4. Using a serological method of prey identification it was demonstrated that Leuctra hippopus (Kempny), Gammarus pulex (L.) and Ptychopter-idae sp. scavenged on trout eggs in the field.
5. There was some evidence that Rhyacophila dorsalis (Curtis) and Diura bicaudata (L.) preyed on trout eggs, but in view of their scarcity, it is suggested that they were a minor mortality factor.     

The influence of water velocity on the downstream movement of alevins and fry of brown trout, Salmo trutta L.

Trout eggs were planted in four experimental stream channels, each channel being run at a different but constant discharge. Survival of the eggs to hatching was low and apparently unrelated to surface water velocity. However, movement of young trout out of channels was affected by water velocity, the higher the velocity the greater the proportion of trout that were lost. Virtually all fry moved out of a channel with a mean surface water velocity of 0.73 m s⁻¹. The rate of loss was not constant over the experiment but increased as the young trout entered the free-feeding stage. At the end of the experiment loss of fry, after abrupt increases in discharge, was demonstrated in the lower velocity channels.     

The critical-period concept for juvenile survival and its relevance for population regulation in young sea trout, Salmo trutta

An example of density-dependent regulation is provided by a long-term investigation (1966-present) of a population of migratory trout (estuarine and sea trout), Salmo trutta L., in a Lake District stream. Evidence for the concept of a critical period for the survival of young fish is briefly reviewed and found to be rather equivocal. The concept is, however, relevant to the trout population. Loss rates were high before but low after a critical survival time ( t_c days after fry emergence) that varied between year-classes (range 33-70 days) and was inversely density-dependent on egg density. Survivor density and loss rates were strongly density-dependent on egg density before t _c, but proportionate survival with stable loss-rates occurred after t _c. Some trout established feeding territories soon after emergence and the number of fish without territories decreased from a high initial value to a negligible value at t _c. Fish size at t_c was not constant but increased as t _c increased. The range of t _c for the different year-classes was similar to that for survival times of unfed fry in the laboratory. A new stock-recruitment model, incorporating t _c, has been developed for the trout population and shown to be related to the model (Ricker curve) used in the long-term study. The critical time can also be regarded as the critical age for survival in young trout; this concept may be relevant to other fish species.     

Lake sturgeon spawning on artificial habitat in the St Lawrence River

In 1996, lake sturgeon (Acipenser fulvescens) spawning was documented for the third consecutive year on an artificially placed gravel bed in the St Lawrence River. Two distinct spawning periods were observed in 1996. Spawning initially commenced on 17 June, when water temperature reached 15°C. A second spawning event was documented from 28 June to 1 July (16°C). Sturgeon egg densities were monitored in three transects on egg trays, on the gravel surface, and within interstitial spaces in the gravel. Counts of developing eggs in the gravel bed during both spawning periods were used to estimate a total of 275 000 eggs on the study area (0.075 ha). Average egg density was highest in the transect with the highest water velocities. Lake sturgeon fry were first observed in the gravel on 24 June (15.5°C), and first emergence from the gravel was documented on 28 June. Hatching following the second spawning event commenced on 3 July. Based on assessment of average embryo viability (61.6%) and egg‐to‐emergent fry survival (17.6%) an estimate of about 171 000 sturgeon eggs hatched, producing over 49 000 emergent fry. Current velocity, substrate particle size, depth of substrate, and maintenance of sediment‐free interstitial spaces are important considerations in planning future spawning habitat enhancement projects.     

Elevated summer temperatures delay spawning and reduce redd construction for resident brook trout (Salvelinus fontinalis)

Redd (nest) surveys for resident brook trout (Salvelinus fontinalis) were conducted annually in a mountain lake in northern New York for 11 years with multiple surveys conducted during the spawning season in eight of those years. Repeated surveys throughout the spawning season allowed us to fit an individually based parametric model and estimate the day of year on which spawning was initiated, reached its midpoint, and ended during each year. Spawning phenology was then assessed relative to (1) mean of maximum daily air temperature and (2) mean of maximum daily water temperature at the lake bottom during summer in each year using a linear model. Elevated temperatures in summer were correlated with a delay in spawning and a reduction in the total number of redds constructed. Increasing the summer mean of maximum daily air temperatures by 1 °C delayed spawning by approximately 1 week and decreased the total number of redds constructed by nearly 65. Lake spawning brook trout select redd sites based on the presence of discharging groundwater that is relatively constant in temperature within and across years, leading to relatively consistent egg incubation times. Therefore, delayed spawning is likely to delay fry emergence, which could influence emergence synchrony with prey items. This work highlights non‐lethal and sub‐lethal effects of elevated summer temperatures on native resident salmonids in aquatic environments with limited thermal refugia.