Swimming performance and morphology of juvenile sockeye salmon, <Emphasis Type="Italic">Oncorhynchus nerka</Emphasis>: comparison of inlet and outlet fry populations

We raised two populations of sockeye salmon fry from fertilized eggs in the laboratory and tested the hypothesis that outlet fry populations, fish which must migrate upstream to reach rearing lakes after yolk-sac absorption, have better swimming ability and morphological characteristics conducive to enhanced swimming performance than inlet fry populations, fish which migrate downstream to rearing lakes. Despite being of identical age, fry from the outlet population were larger (approx. 6.7% longer, ~5 mm on average) and more laterally compressed than inlet fry at the time of our initial experiments. Using an open-top box flume, we found that the burst-swimming performance (in cm s⁻¹) of the outlet population was 31% better. We found no differences between populations in prolonged-swimming performance. We were unable to find any direct relationships between measures of swimming performance and size or shape variables, suggesting that the larger, more robust morphology of outlet fry was not responsible for the superior burst ability. Recent biochemical studies indicate outlet fry may be metabolically better provisioned for burst swimming than inlet fry. It is possible that the morphological differences between the populations of fry reflect adaptations needed by adults during their migration and spawning.     

Age,Experience and the Development of Adaptive Variation in Anti-predator Responses in Three-spined Sticklebacks (Gasterosteus aculeatus)

Stickleback fry (approx. 10 mm in length) were collected from two sites in Scotland, one with abundant predatory fish and birds and the other free of predation. The fry were reared in the laboratory until they reached a length of about 30 mm, at which time fry of the same length were caught from the two study sites. 10 fish from each size and rearing category (10 mm fry, 30 mm labreared fry and 30 mm wild-caught fry) and from each population were exposed singly to a silhouette of an avian predator moved above the surface of the tank. On average 50% of the fish jumped away on sighting the model, and there were no differences between the groups in the proportion of fish giving this response. Other aspects of the anti-predator repertoire of sticklebacks (in particular, remaining motionless after an encounter) were absent in 10 mm fry from both sites. Protective responses appeared (in fish from the predated but not the unpredated site) by the time they reached a length of 30 mm, regardless of whether they were laboratory-reared or wild-caught. Thus adaptive behavioural differences between these two populations arise as sticklebacks from the heavily predated site develop responses that fail to appear in those from the unpredated site and that are independent of direct experience of predatory attack.     

Role of diet and experience in the development of feeding behaviour in largemouth bass, Micropterus salmoides

Two groups of largemouth bass, Micropterus salmoides , were reared in the laboratory. One group was reared on an artificial, passive diet (frozen brine shrimp) whereas the second was reared on a natural, active diet (cultured zooplankton). Observations on the development of feeding behaviour indicated that the motor patterns and duration (number of weeks in the behavioural repertoire) of the feeding acts did not differ between fry reared on the two diets. While feeding on their respective diets, natural-diet fry performed significantly more orientations and bites, the two major early feeding acts, than did the artificial-diet fry. When tested with live fish prey, fish reared on the natural diet performed fewer orientations and strikes and captured more prey per fry than did the artificial-diet fry. Natural-diet fry had a significantly better net efficiency (captures minus strikes minus orientations) than did artificial-diet fry. Diet, experience, and length (T.L.) of fry affected their predator efficiency significantly. We argue that providing hatchery-reared bass fry with an opportunity to prey on live forage fish once or twice before their release would enhance their survival and eventual recruitment into natural populations.     

Recruitment variation in a stream galaxiid fish: multiple influences on fry dynamics in a heterogeneous environment

1. The relative importance of density‐dependent and density‐independent processes in explaining fluctuations in natural populations has been widely debated. In particular, the importance of larval supply and whether it may control the type of regulatory processes a population experiences has proved contentious. 2. Using surveys and field experiments conducted in streams in Canterbury, New Zealand, we investigated how variation in the survival of non‐migratory Galaxias vulgaris fry was affected by density‐dependent and density‐independent processes and how this variation influenced recruitment dynamics. 3. Fry populations with high settlement densities experienced a 70–80% reduction in population size from density‐related mortality during the first fourteen days after peak settlement but thereafter the influence of density‐dependent processes on fry was weak. The impact of environmental conditions on fry populations was dependent on fry size and the magnitude of the perturbation, such that flooding effects on fry survival were most severe when fry were small. 4. In streams not affected by flooding, the size and density of introduced trout (Salmo trutta and Oncorhynchus mykiss) were the most significant factors determining the abundance of eventual recruits. A field experiment manipulating brown trout access to fry populations revealed that trout as small as 110 mm may be capable of greatly reducing and possibly preventing galaxiid recruitment. 5. Overall, the results indicated density‐dependent population regulation was only possible at sites with high native fish densities because trout were likely to be suppressing the number of potential recruits at sites with low native fish numbers. Whilst density‐dependent processes had a strong effect on fry survival following the period of peak fry abundance, density‐independent processes associated with flow and predatory trout influences on fry survival largely determined recruitment variability among galaxiid populations. Focusing conservation efforts on improving habitat to increase fry retention and reducing the impacts of trout on galaxiids would ensure more native fish populations reached their potential abundance.     

Reduced anti-predator responses in multi-generational hybrids of farmed and wild Atlantic salmon (<Emphasis Type="Italic">Salmo salar</Emphasis> L.)

Cultured organisms undergo genetically-based behavioural changes that may reduce their ability to survive in the wild. This has raised concerns that interbreeding between escaped cultured and wild organisms will generate hybrids exhibiting maladaptive behaviours which may ultimately reduce the fitness of the wild counterpart. We compared anti-predator responses in Atlantic salmon (Salmo salar) from two wild North American populations, the major farmed strain used in regional aquaculture, and their wild-farmed hybrids (F₁, F₂, and wild backcross). Anti-predator responses of fry (age 0+ parr) were measured under common environmental conditions, using a model of a natural predator (belted kingfisher, Ceryle alcyon). Farmed fry exhibited significantly reduced anti-predator responses relative to fry from both wild populations. The anti-predator responses of wild-farmed hybrid fry were intermediate to those of the parental populations (pure farmed or wild). The magnitude by which wild-farmed hybrids differed in anti-predator responses from pure wild fish also depended on the wild population. These results suggest that: (1) the observed behavioural differences have a genetic basis; (2) wild-farmed hybrids have, on average, reduced anti-predator responses relative to wild fish; and that (3) the effects of wild-farmed interbreeding on anti-predator responses will differ between wild populations. Our study is consistent with the general hypothesis that continual farmed-wild interbreeding may have detrimental effects on the fitness of wild organisms.     

Relation between exploitation and production in experimental populations ofPoecilia reticulata

In order to make researches on the effect of exploitation upon fish populations, six populations of guppy were maintained during a period of 331 weeks under conditions of space, light, temperature, pH, and food controlled as much as possible. The six populations were divided into three groups two by two: a control group without exploitation, a 10% rate of exploitation group and a 20 and 33% rate of exploitation group. All the populations repeated an increase and a decrease in number and in biomass about once a year. The exploitation affected the age composition in the populations: in the non-exploited population adults occupied the largest part, and the more intensive the exploitation the larger the part fry occupied. The exploitation decreased the biomass of exploited populations, but productivity increased with increasing of of rate of exploitation. The relationship between adult and fry could be represented byRicker's reproduction curve. The form of the curve was changed by an intensity of exploitation. It was thought that exploitation acts as a preventer of aging of populations by removing old fish and results increasing number of fry and productivity.     

Population responses of plant-associated invertebrates to foraging by largemouth bass fry (Micropterus salmoides)

The effect of foraging by largemouth bass fry (Micropterus salmoides) upon invertebrates associated with aquatic macrophytes was determined using six 4 m² exclosures in Cochran Lake, Michigan during June 1978. The cladoceranSida crystallina rapidly declined in exclosures with fry, but increased in control exclosures without fry. Chironomids and chydorids showed little change in the exclosures.Invertebrate populations and foraging by fry were also monitored in the lake during 1976 and 1979.Sida declined rapidly in June of both years. In 1979, a decline from 2.2 × 10³ to 0.3 × 10³ individuals per m² and a sharp drop in the proportion of adults between 19 and 22 June coincided with the entry of a large school of fry into the study site on 19 June. Chironomids also declined during June of both years, while chydorids became increasingly abundant in mid-summer and showed no evidence of depletion by the fish.These results suggest that populations of certain prey, which are found locally in high densities in the littoral zone, may be highly susceptible to brief episodes of intense predation by fish fry.     

Temporal constraints on the potential role of fry odors as cues of past reproductive success for spawning lake trout

Deciding where to reproduce is a major challenge for most animals. Many select habitats based upon cues of successful reproduction by conspecifics, such as the presence of offspring from past reproductive events. For example, some fishes select spawning habitat following odors released by juveniles whose rearing habitat overlaps with spawning habitat. However, juveniles may emigrate before adults begin to search for spawning habitat; hence, the efficacy of juvenile cues could be constrained by degradation or dissipation rates. In lake trout (Salvelinus namaycush), odors deposited by the previous year's offspring have been hypothesized to guide adults to spawning reefs. However, in most extant populations, lake trout fry emigrate from spawning reefs during the spring and adults spawn during the fall. Therefore, we postulated that the role of fry odors in guiding habitat selection might be constrained by the time between fry emigration and adult spawning. Time course chemical, physiological, and behavioral assays indicated that the odors deposited by fry likely degrade or dissipate before adults select spawning habitats. Furthermore, fry feces did not attract wild lake trout to constructed spawning reefs in Lake Huron. Taken together, our results indicate fry odors are unlikely to act as cues for lake trout searching for spawning reefs in populations whose juveniles emigrate before the spawning season, and underscore the importance of environmental constraints on social cues.     

The relationship between fecundity and fertility in the mouthbrooding cichlid fish Tilapia leucosticta

The development of fry in the mouth of the female mouthbrooder Tilapia leucosticta Trewavas is described. The egg production by fish of various lengths is plotted and a curvilinear regression for standard and total length fitted. The number of eggs produced is approximately equal tothe square of the total length in centimetres of the parent fish. The brood numbers of 460 femalesare plotted against length, and linear regressions for mean brood number and mean fry number fitted. The relationship between the expressions for numbers of fry brooded and egg production gives anestimate of brooding efficiency that decreases with increasing parent length. Individual female fish show signs of reripening and the timing of this process is estimated from gonad state and from the length of fry brooded in the mouth. The relationship between fecundity (egg production) and fertility (fry brooded) is considered and the effect of maturation size, growth rate and spawning frequency assessed.     

Interspecific relationships between emerging Atlantic salmon, Salmo salar, and coho salmon, Oncorhynchus kisutch, juveniles

Interspecific relationships between Atlantic salmon and coho salmon were studied at early life stages in laboratory and semi-natural stream channels. During emergence, the survival and dispersal patterns were similar for the two species in single or mixed populations. Survival of Atlantic salmon fry was reduced in the presence of older coho fry. However, no predation was observed. Microdistribution differed between the two species, with Atlantic salmon fry more numerous in riffles when coho were present.
Coho juveniles had a pelagic and gregarious distribution, in contrast to the benthic behaviour of the Atlantic salmon. In laboratory streams, Atlantic salmon fry moved out or adopted a subordinate cryptic behaviour which allowed them to escape predation while negatively affecting their growth.     

Effect of sex and size on copper susceptibility of the common guppy, Lebistes reticulatus (Peter)

Multiple regression analyses were conducted to relate rate of toxic action, body weight and concentration for fry, male and female populations of Lebistes reticulatus exposed to copper solutions. For each of these three populations, copper susceptibility decreased as body weight increased. On the basis of the rate of toxic action adjusted with the size factor, the copper susceptibility differed significantly among the fry, male and female populations at the ratios of 2.20:1.55:1.00. When the size factor was not included in calculation, the rate of toxic action or median survival time did not show such a difference between the males and females.     

Looking for a needle in a haystack: Discovery of indigenous Atlantic salmon (Salmo salar L.) in stocked populations

Microsatellite analysis of Atlantic salmon fromfive Danish rivers was performed to determinethe stocked or indigenous status ofindividuals. Genetic variation at six highlypolymorphic microsatellite loci was assayed andused for individual based analyses (assignmenttests). Contemporary samples of adult returningspawners and fry were compared to baseline datafrom: 1) historical DNA samples (from oldscales) representing the indigenouspopulations, 2) samples from another Danishpopulation (Skjern River) used for stocking,and 3) five exogenous populations used forstocking. Assignment power was high. Thepercent of stocked salmon correctly assigned topopulation of origin ranged from 83% to 99%and the percent of indigenous salmon correctlyassigned to population of origin ranged from83% to 90%. For two of the riverssignificantly more individuals were assigned tothe indigenous populations than expected frommisclassification alone, suggesting that someremains of the indigenous populations hadpersisted. Still, many fish were of exogenousorigin. Simulated hybrids among releasedexogenous salmon and between exogenous andreleased Danish salmon (Skjern River) revealedthat natural hybridisation among released fishwas not likely to be the source of the fryclassified as indigenous, however, thepossibility of hybridisation among indigenousand released fish could not be dismissed.Several full sib groups were found amongindigenous natural fry ruling out one or a fewmatings as the source of the indigenous fry.These results show that some native populationsmay persist even after years of introductionand environmental perturbation; geneticinformation can be used to identify thesepopulations and identify individualsrepresenting these populations for use inrestoration programs.     

The relationship between fecundity and fertility in the mouthbrooding cichlid fish Tilapia leucosticta

The development of fry in the mouth of the female mouthbrooder Tilapia leucosticta Trewavas is described. The egg production by fish of various lengths is plotted and a curvilinear regression for standard and total length fitted. The number of eggs produced is approximately equal to the square of the total length in centimetres of the parent fish. The brood numbers of 460 females are plotted against length, and linear regressions for mean brood number and mean fry number fitted. The relationship between the expressions for numbers of fry brooded and egg production gives an estimate of brooding efficiency that decreases with increasing parent length. Individual female fish show signs of reripening and the timing of this process is estimated from gonad state and from the length of fry brooded in the mouth. The relationship between fecundity (egg production) and fertility (fry brooded) is considered and the effect of maturation size, growth rate and spawning frequency assessed.     

The influence of habitat availability and adult density on non-diadromous galaxiid fry settlement in New Zealand

This study investigated factors controlling settlement and initial density of two species, Galaxias vulgaris and Galaxias paucispondylus (Galaxiidae), in New Zealand streams. Factors affecting maximum fry abundance were assessed at 10 sites (eight streams) differing in habitat characteristics and the presence or absence of introduced brown Salmo trutta and rainbow trout Oncorhynchus mykiss . Regression analysis indicated that conspecific adult numbers, extent of backwater habitat and brown and rainbow trout density were the most important factors determining initial fry density. Experimental additions of backwater habitat increased fry settlement tenfold compared with control reaches, demonstrating that the area available for fry settlement had a substantial effect on fry abundance. Results from this study indicate that fry settlement may be a critical limiting phase for some non-diadromous galaxiid populations. Increasing settlement area to enhance fry retention offers a practical conservation tool for fisheries managers that may aid conservation of threatened fishes like galaxiids.     

Biology of young grey mullet, Mugil cephalus L., populatioins in a coastal lagoon in Sri Lanka

The biology of young grey mullet, Mugil cephalus L. populations of a coastal lagoon—the Negombo Lagoon on the West Coast of Sri Lanka was studied from October 1976 to March 1978. The relative abundance of fry were found to vary from month to month, with peaks occurring in December-January, May and September-October, coinciding with the end of rainy seasons. It was found that the young fry tended to migrate into the lagoon at a size of about 10–20 mm, most frequently between 15–20 mm in length, by which time they were fully scaled. The young fry after a period of growth in the shallow areas of the lagoon, when they reached a size of 30–35 mm, emigrated into the deeper waters of the lagoon. The rate of growth, as computed from length frequency distribution studies, was found to be around 0.24 mm day⁻¹. Results of diurnal surveys showed a movement of the larger size fry into shallower waters during the hours of darkness and this movement appears to be independent of the tidal cycle. It is postulated that the cause of this migration is to escape from predators, chiefly piscine. Investigations of the meristic characters indicated that the young fry which migrated into the lagoon at different times of the year probably originated from a single spawning stock.     

Effects of sea trout stocking on the population genetics of landlocked brown trout, Salmo trutta L., in the Conwy River system, North Wales, U.K.

The effect of the introduction of fry of anadromous sea trout, Salmo trutta L., on the genetic integrity of landlocked brown trout populations was evaluated. Samples were taken from six brown trout populations from streams above impassable waterfalls in the Conwy river system (North Wales, U.K.) in 1989 and 1990. Three of these streams had no known stocking history and three had been stocked with sea trout fry from the lower Conwy system over the last few years. Representatives of these sea trout were collected from two streams in the lower Conwy system and from a hatchery. Allele frequencies at 13 loci, six of which were polymorphic, were determined by starch gel electrophoresis.
The stocked populations were intermediate in their allele frequencies between unstocked brown trout and sea trout samples. A principal component analysis suggested significant numbers of hybrids in all of the stocked streams. This shows that some of the introduced sea trout did not migrate down the falls to the sea, but stayed in fresh water and hybridized with the local population. The significance of this finding for the conservation of the genetic resource of brown trout stocks is discussed.     

Within-stream variation in early life-history traits in brown trout

Significant additive genetic variance for most early life-history traits was found in brown trout Salmo trutta living in both allopatry above an impassable waterfall and sympatry (below the waterfall in the same stream) with alpine bullhead Cottus poecilopus. These traits included length, mass and yolk sac volume at hatching, and size at'button-up' (the time when yolk is enclosed within the body cavity). There were small differences in size at hatching and size at button-up among populations (adjusted for egg size). However, sympatric fry grew more rapidly and experienced lower mortality rates during the period of first feeding than allopatric fry. This might indicate behavioural differences between brown trout from the two populations. It is suggested that these phenotypic differences may be a result of adaptation to living in sympatry with alpine bullhead.     

Does among‐population variation in burst swimming performance of sockeye salmon Oncorhynchus nerka fry reflect early life migrations?

Using a fixed‐speed test, burst swimming performance was found to vary among nine populations of emergent sockeye salmon Oncorhynchus nerka fry reared in a common‐garden environment. No consistent relationship was, however, detected between difficulty of fry migration (upstream v. downstream) to rearing areas and total burst swimming duration or bursting rate.     

High mortality in a large-scale zebrafish colony (Brachydanio rerio Hamilton & Buchanan, 1822) associated with Lecythophora mutabilis (van Beyma) W. Gams & McGinnis

Zebrafish (Brachydanio rerio) have become an important model system for studying vertebrate embryonic development and gene function through manipulation of genotype and characterization of resultant phenotypes. An established research zebrafish colony without substantial disease problems for more than 7 years of operation began experiencing appreciable mortalities in November of 1997. Young fish (fry), from five to 24 days after hatching, spontaneously developed elongate strands of organic material protruding from the mouth, operculum, and anal pore, leading workers in the laboratory to describe the infected fish as "bearded." Unlike typical freshwater fish fungal infections, the skin surface did not have evidence of fungal colonization. The disease was associated with progressive lethargy, reduced feeding, and subsequent mortality. From 10 to 100% of the fry in a given tank were affected. Initial examination indicated that the biofilm around the head of affected fry consisted of bundles of septate fungal hyphae, large numbers of mixed bacterial populations, and protozoans. Environmental samples of air and water in the laboratory were obtained to ascertain the source of the infective agent and to isolate and identify the fungus. A fungus identified as Lecythophora mutabilis was isolated repeatedly from infected fish and water samples from infected fish tanks, and from the main laboratory water supply tanks, but not from laboratory air. Some biofilm beards on fish were found to consist of relatively pure bacterial populations, and beards on occasional fish examined in the later part of the study consisted of hyphae and spores of the oomycete genus Aphanomyces. Lecythophora mutabilis did not invade tissues; however, elimination of the epizootic correlated with reduction in the number of L. mutabilis conidia in the water following modification of the laboratory water system by use of new filtration and sterilization systems. We conclude that the dense hyphal strands of L. mutabilis composing the predominant biofilm type, along with mixed bacteria and protozoa, contributed to the die-off in young fry by occluding the oral cavity and/or gills, leading to starvation and/or asphyxiation.     

Predator‐induced hyperventilation in wild and hatchery Atlantic salmon fry

Following exposure to a predator stimulus (a brown trout Salmo trutta ), the opercular rate of Atlantic salmon Salmo salar fry increased by 35·3 ± 11·0%(mean ± 95% CI). The time taken for opercular rate to decline to baseline levels depended upon the occurrence of any associated locomotory activity. Opercular rates of fish that dashed when exposed remained elevated for 38·2 ± 20·6 min, whereas those of individuals that did not move ('freezers') recovered within 7·2 ± 2·9 min. The duration that opercular rate remained elevated was positively correlated with the magnitude of the elevation, which was higher in 'dashers' than freezers. The maximum opercular rate in 'freezers' was similar between wild fry and hatchery‐reared fry (from wild parents). There was a significant delay, however, in hatchery compared with wild fry in the time until peak ventilatory response and onset in the decline phase. This difference in opercular response suggests that hatchery fish were slower to realize fully the potential danger from the predator. Any delay in response could be directly attributed to the effect of hatchery‐rearing environment, rather than domestication or hatchery selection effects.