Functional response of juvenile pink and chum salmon: effects of consumer size and two types of zooplankton prey

Feeding rate experiments were conducted for pink salmon Oncorhynchus gorbuscha fry [mean fork length ( L _F) 39 mm], juveniles (103–104 mm L _F) and juvenile chum salmon Oncorhynchus keta (106–107 mm L _F). Fishes were presented with small copepod ( Tisbi sp.) or larger mysid shrimp ( Mysidopsis bahia ) prey at varying densities ranging from 1 to 235 prey l⁻¹ in feeding rate experiments conducted at water temperatures ranging from 10·5 to 12·0° C under high light levels and low turbidity conditions. Juvenile pink and chum salmon demonstrated a type II functional response to mysid and copepod prey. Mysid prey was readily selected by both species whereas the smaller bodied copepod prey was not. When offered copepods, pink salmon fry fed at a higher maximum consumption rate (2·5 copepods min⁻¹) than larger juvenile pink salmon (0·4 copepods min⁻¹), whereas larger juvenile chum salmon exhibited the highest feeding rate (3·8 copepods min⁻¹). When feeding on mysids, the maximum feeding rate for larger juvenile pink (12·3 mysids min⁻¹) and chum (11·5 mysids min⁻¹) salmon were similar in magnitude, and higher than feeding rates on copepods. Functional response models parameterized for specific sizes of juvenile salmon and zooplankton prey provide an important tool for linking feeding rates to ambient foraging conditions in marine environments, and can enable mechanistic predictions for how feeding and growth should respond to spatial-temporal variability in biological and physical conditions during early marine life stages.     

Lacustrine growth of juvenile pink salmon and a comparison with sympatric sockeye salmon

The growth rates of naturally sympatric juvenile pink Oncorhynchus gorbuscha and sockeye Oncorhynchus nerka salmon were compared in a common lacustrine environment in south‐west Alsaka, an unusual opportunity given the normal disparity in freshwater residence time of these two species. Fork length ( L _F) frequency distributions of juvenile pink salmon caught in the lake during the summer in 1991 and 1999–2003 indicated a growth rate of 0·54 mm day⁻¹, 54% greater than the estimated growth rate of juvenile sockeye salmon sampled from 1958 to 2003 (0·35 mm day⁻¹). Examination of daily growth rings on otoliths indicated that pink salmon in Lake Aleknagik grew an average of 1·34 mm day⁻¹ in 2003 but sockeye salmon grew only 0·63 mm day⁻¹(average specific growth rates were 3·0 and 1·8% day⁻¹, respectively). Pink salmon increased from c . 32 mm L _F and 0·2 g at emergence to 78 mm L _F and 3·0 g within 3–4 weeks. After experiencing these rapid growth rates, the pink salmon appeared to leave the lake by late July in most years. The diets of pink and sockeye salmon in the littoral zone of the lake were very similar; >80% of the stomach contents consisted of adult and pupal insects and the remainder was zooplankton. This high degree of diet overlap suggested that the observed differences in growth rate were not attributable to variation in prey composition.     

Diet of Atlantic cod in the southern Gulf of St Lawrence as an index of ecosystem change, 1959–2000

Major temporal changes in the importance of euphausiids and Atlantic herring Clupea harengus in the diet of Atlantic cod Gadus morhua , 10–75 cm L _F, occurred in shallow waters (<100 m depths) of the southern Gulf of St Lawrence between 1959 and 2000. Euphausiids represented 6–70% of prey mass for Atlantic cod 31–60 cm L _F from 1959 to 1987 but only trace amounts were detected in stomachs collected from 1990 to 2000. Mysids and gammarid amphipods (for Atlantic cod ≤45 cm L _F), and Atlantic herring (for Atlantic cod >45 cm L _F) largely replaced euphausiids in the Atlantic cod diets from 1990 to 2000. This diet change suggested there has been a major perturbation of the food web of the southern Gulf of St Lawrence. The importance of fishes (mostly Atlantic herring) in the diet of Atlantic cod >45 cm L _F increased significantly between the periods 1959–1980 and 1987–2000. Atlantic herring comprised 0–4% (mean 1.3%) of prey mass of Atlantic cod 46–60 cm L _F from 1959 to 1980 and increased to 2–42% (mean 19.6%) of the diet from 1987 to 2000. Atlantic herring comprised 0–25% (mean 9.4%) of the prey mass of Atlantic cod 61–75 cm L _F from 1959 to 1980 and increased to 42–81% (mean 54.2%) of prey mass from 1987 to 2000. This increased consumption of Atlantic herring was consistent with observed changes in abundance of Atlantic herring in the ecosystem between the late 1970s and 2000. The large changes in consumption of euphausiids and Atlantic herring represent highly significant changes and would need to be included in the development and interpretation of ecosystem-based management models for this ecosystem.     

Return migration of Atlantic salmon in the River Tana: the role of environmental factors

Multi-sea-winter Atlantic salmon (75–115 cm fork length L _F, 2–4-winter fish) were radio-tagged in the Tanafjord (700 N), Norway, in 1992–1993, and 130 fish entered the large subarctic River Tana (Teno). They entered the fresh water at any time of the tidal cycle but more so during the high and ebbing tides. No diel rhythm was detected in river entry under polar day conditions. There were no differences in the change of flow between days when salmon moved and when they did not, but during active migration increasing discharge was associated with increased swimming activity of salmon, especially later in the summer. Increasing air temperature was also associated with enhanced migration activity. Low river flow was associated with increasing delay in salmon passing the first riffle area of the river, 35 km from the sea.     

Temporal and spatial variation in growth of juvenile Atlantic salmon

Spatial and temporal variation in length‐at‐age and environmental factors affecting variation in growth rate of juvenile Atlantic salmon Salmo salar were studied using data from a long‐term study in the River Stjørdalselva, central Norway. Mean annual instantaneous growth rate among 1+ and 2+year juvenile Atlantic salmon varied between 0·59 and 1·50 g g⁻¹ year⁻¹ and mean instantaneous daily growth rate of young‐of‐the‐year (YOY) varied between 0·013 and 0·033 g g⁻¹ day⁻¹. Between year variation in growth was larger than the within year intra‐watercourse spatial variation. For YOY and 1+year Atlantic salmon, a major part of the observed between year variation in growth rates was explained by variation in mean daily water discharge and spring temperature. For 2+year juvenile Atlantic salmon, mean daily water discharge and cohort density were the only variables to significantly explain variation in growth rates. A large part of the within water‐course spatial variation could not be explained by temperature variations and juvenile Atlantic salmon in the uppermost areas of the river, experiencing the lowest ambient temperatures during the growth period, displayed the highest growth rates. Within the baselines set by temperature, biotic and abiotic factors connected to water flow regime and variation in food availability are suggested to be a major determinants of the temporal and spatial variation in juvenile Atlantic salmon growth rates.     

Influence of size, winter duration and density on sexual maturation of Atlantic salmon (Salmo salar) juveniles in Little Codroy River (southwest Newfoundland)

Factors affecting sexual maturation of Atlantic salmon ( Salmo salar L.) juveniles in fresh water were analysed under wild conditions in Little Codroy River (southwest Newfoundland). Large size after the first year of life and short winters favoured maturation of age 1 + male parr. Incidence of maturing 1+ males was greater where high densities of parr (>0.2 individuals m ²; fish >6cm fork length) were recorded in the second season of growth. These results were discussed according to current knowledge on early maturation in Atlantic salmon.     

Lifetime consequences of variable 0 year group length in riverine populations of chub Leuciscus cephalus (L.)

The fork length ( L _F) of individual chub Leuciscus cephalus in English riverine populations at the end of their first growth season varied considerably, ranging from <25 to >70 mm. This had a significant influence on the subsequent growth of individuals over their lifetime. Chub of small L _F at age 1 year generally produced smaller annual growth increments throughout life than those of longer L _F at age 1 year, although they had the potential to attain greater ultimate L _F. This variability in L _F at age 1 year resulted, at least in part, from multiple spawning events over a protracted period, that caused variation in the growing seasons for 0 year group chub of the same year class. Since the adult population contained individuals that only attained L _F of <25 mm at the end of their first growing season, it is possible that L _F at age 1 year may not be such a major precursor to strong chub recruitment as previously thought.     

Allozyme heterozygosity, date of first feeding and life history strategy in Atlantic salmon

The relations between allozyme heterozygosity, relative date of first feeding and life history strategy in juvenile Atlantic salmon Salmo salar were examined using eggs obtained from a 400 family cross (20 male × 20 female adult Atlantic salmon). Multilocus heterozygosity, through its positive associations with the timing of first feeding and growth rate, was correlated with life history strategy in juvenile Atlantic salmon, albeit under genotype × environmental (temperature, food availability) regulation. Under hatchery conditions, a 10 day difference was observed in the relative date of first feeding between early and late first feeding Atlantic salmon. Early first feeding Atlantic salmon exhibited a significantly higher mean heterozygosity, grew faster at ambient water temperature (April to November) and a significantly higher proportion adopted the early freshwater maturation (age 0+ years, male fish) or early migrant (age 1+ years, mainly female fish) strategies compared to late first feeding Atlantic salmon. Elevated water temperatures over the winter (December to April, >10·5° C) provided additional growth opportunity allowing previously mature male parr (mainly early first feeders) and lower modal group parr (mainly late first feeders) to adopt the early migrant strategy by the following spring.     

Evidence that growth hormone-induced elevation in routine metabolism of juvenile Atlantic salmon is a result of increased spontaneous activity

Growth hormone (GH) significantly increased the growth in mass and length of Atlantic salmon parr. It also significantly increased routine oxygen consumption ( M O₂). This change in routine M O₂ could be attributed to an increase in activity, but not resting M O₂. Any elevation in resting M O₂ due to GH treatment is much lower than previously suggested.     

Habitat selection by juvenile Atlantic salmon: the interaction between physical habitat and abundance

The effect of physical river habitat variables on the distribution of juvenile Atlantic salmon Salmo salar L. in the Rivière de la Trinité, Québec, Canada, was examined using generalized additive modelling. A survey of Atlantic salmon fry and parr densities and habitat variables (flow velocity, water column depth and substratum size) was conducted in the summer months from 1984 to 1992. Clear patterns of habitat use existed: specific ranges of habitat variables were selected, with parr preferring greater velocities, depths and substratum sizes than fry. There was a large variation, however, in juvenile densities for given velocities, depths or substratum sizes, with this variation being greatest in optimal habitats. On examination of an individual year, interaction between the variables was found to explain some of the variation. On a year‐to‐year basis the juvenile Atlantic salmon population was found to exhibit an 'Ideal Free Distribution', which resulted in greatest variation in optimal habitats with year‐to‐year changes in population abundance.     

Sublethal effects of the pesticide Diazinon on olfactory function in mature male Atlantic salmon parr

Diazinon, an organophosphate pesticide, had a sublethal effect on the olfactory system of mature male Atlantic salmon parr. The olfactory responses of the parr to prostaglandin F_2a (PGF_2a) were studied after exposure of the epithelium to different concentrations of Diazinon in water. Electrophysiological recordings from the epithelium indicated that the responses to this prostaglandin were significantly reduced at nominal concentrations as low as 1.0μg l⁻¹ and the threshold of detection was reduced 10-fold at 2.0 μg 1⁻¹ . Mature male salmon parr exposed for a period of 120 h to Diazinon (nominal concentrations 0.3, 0.8, 1.7, 2.7, 5.6, 13, 28 and 45 μg 1 ⁻¹) also had significantly reduced levels of the reproductive steroids, 17,20β-dihydroxy-4-pregnen-3-one, testosterone and gonadotrophin II in the blood plasma after priming with ovulated female salmon urine. Both prostaglandin F_2a and ovulated female urine are known to have important roles in synchronizing reproductive physiology and behaviour in salmonids as well as other fish species. The results are therefore discussed in relation to the possible sublethal effects of Diazinon on reproduction in the Atlantic salmon and possible effects on populations of salmonids.     

Is Atlantic salmon production limited by number of territories?

Thirty Atlantic salmon Salmo salar parr (mean 135 mm total length, L _T) in the River Alta, Norway, were radio‐tagged and tracked during a 11–13 day period. The parr stayed within defined home ranges with a 95% probability of localization within an average area of 1286 m²(241–3484 m²). On average, each parr had overlapping home ranges with 7·7 other parr, and the overlap between pairs of fish covered on average 24% of their home range areas. Mean length of the river stretch used was 90 m (22–383 m). On average, during 38%(16–77%) of the tracking surveys, the fish had moved >10 m since the previous survey. Mean total distance moved during the whole study was 402 m (208–862 m). The Atlantic salmon were most often recorded in riffles, but 27% of the parr alternated between riffles and pools. The extensive movements, flexible habitat utilization and relatively large home ranges, together with the fact that all the parr had home ranges partly overlapping with other radio‐tagged parr, indicate a flexible and variable behaviour and, thereby, a more complex social structure than a rigid defence of a fixed territory. Thus, Atlantic salmon production seems not to be limited by the highest potential number of territories for large parr in a given area.     

Evaluation of a new semi-natural incubation technique for Atlantic salmon Salmo salar fry

The incubation environment had a significant effect on fork length ( L _F) and body mass ( M ) of Atlantic salmon Salmo salar fry at the time of emergence. Fry that were incubated in a new incubator design, that mimics the conditions in a natural redd (the Bamberger-Box), achieved significantly greater and attained a significantly higher M than those reared in conventional hatchery troughs for control. Fewer fry from the Bamberger-Boxes had visible deformities compared with those from the hatchery troughs. Results were consistent for five consecutive seasons using both wild and domesticated broodstock from genetically different origins. Survival from the eyed embryo stage in the Bamberger-Boxes and hatchery troughs was >93% during normal climatic conditions. Only larvae reared in Bamberger-Boxes, however, survived abnormally high water temperatures during one test season. The results demonstrate that the Bamberger-Box is an effective alternative to the conventional incubation technology.     

Gonadal sex differentiation in Atlantic halibut

The process of gonadal sex differentiation in 338 Atlantic halibut Hippoglossus hippoglossus larvae, ranging in size from 10 mm L_s to 230 mm L _F, is described histologically. Gonadal sex differentiation occurred by 38.0 mm L _F, which coincided with the weaned, post-metamorphic, settled stage in the life cycle. This was a gradual process that coincided with other organogenesis in the developing larvae.     

Age, growth and reproduction of the barrelfish Hyperoglyphe perciformis (Mitchill) in the western North Atlantic

Otoliths ( n = 847) and gonads ( n = 817) were collected from barrelfish Hyperoglyphe perciformis that were captured by commercial fishermen in the waters off South Carolina and Georgia in 1995, 1997 and 2001–2006. Of the otoliths collected, 97% were aged successfully, and specimens sampled ranged from 5 to 85 years, with a median age of 12 years. The von Bertalanffy growth parameters yielded the equation: L_t = 857·8{1 − e^{−0·0985[ t −(−8·95)]}}, where L_t is fork length ( L _F) at time t . Through histological examination, 94% of the gonads assessed were assigned to a sex and reproductive class. Females spawned from September to May with a peak from November to January. Males spawned year round, but had a peak from September to April. The sex ratio (M:F) for this population was 1:1·34. The smallest mature female was 605 mm L _F and the youngest immature female was 697 mm L _F. Estimates of L _F and age at 50% maturity ( L ₅₀ and A ₅₀) for females were 660 mm L _F (95% CI = 633–667 mm L _F) and 6·08 years (95% CI = 3·50–7·27 years), respectively. The youngest mature male was 575 mm L _F and the oldest immature male was 762 mm L _F, and no estimates of L ₅₀ or A ₅₀ were made for males. It was determined that barrelfish exhibit the typical characteristics of long life span, slow growth and high age at maturity seen in other deepwater fishes, and that care should be taken to manage this species accordingly.     

Gait transition speed as an alternate measure of maximum aerobic capacity in fishes

This study demonstrated that the transition from a steady to an unsteady locomotory gait ( U _STmax) in juvenile brook trout Salvelinus fontinalis can be measured easily using a new tilting raceway design and a simple experimental protocol. It was found that U _STmax increased linearly with fork length ( L _F), and that this relationship was statistically identical in fish that swam volitionally in the raceway and those that were forced to perform, although slightly different data processing methods were needed in the latter to achieve this result. Furthermore, the relationship between L _F and U _STmax was statistically identical to that between L _F and critical swimming speed ( U _crit), although L _F in the former relationship explained 83% of the variance compared to 37% in the latter. This finding indicates that gait transition speed can be used to estimate maximum aerobic capacity, with less unexplainable variance than U _crit. Gait transition speeds were also determined from U _crit tests; however, this required measuring and incorporating ground speed into the analysis. U _STmax as determined in the U _crit tests was not significantly different from that measured in the raceway, suggesting that gait transition speed can be measured in raceways or swim tunnel respirometers.     

Reproductive biology of gag in the southern Gulf of Mexico

Aspects of the reproductive biology of gag Mycteroperca microlepis in the southern Gulf of Mexico were studied by following seasonal variations in the gonado‐somatic index and through histological examination of gonads. Gag were collected from inshore and offshore waters of the Campeche Bank, Yucatan, Mexico, between April 1996 and December 2001. This species is a protogynous hermaphrodite, and appeared to be depth‐size distributed. The smallest gag (9–49 cm L _F) collected were all juvenile females, and were caught in inshore waters (1–10 m depth), while the largest (49–116 cm L _F), mainly adult females, males and transitionals, were captured in offshore waters (33–167 m depth). Overall the offshore male to female ratio was female‐biased (1 : 3·3) and differed significantly from unity. The species spawns at depths of c . 50–53 m, from early winter to mid‐spring, with peak spawning activity occurring between January and March. Fifty per cent of females reached first maturity at 72·1 cm L _F. At 103 cm L _F, 50% of sampled females had changed into males. Gag can be considered a monandric species, and sexual transition for this grouper seemed to occur in fish distributed within a narrow size range (85–111 cm L _F). The results are compared with those of other authors for gag stocks from the south‐east Atlantic coast of the U.S.A. and the north‐east Gulf of Mexico.     

Life-history strategies and protein metabolism in overwintering juvenile Atlantic salmon: growth is enhanced in early migrants through lower protein turnover

The specific growth rate (length) of early migrant juvenile Atlantic salmon Salmo salar was significantly greater than that of delayed migrants in November and February. However, there were no differences in the fractional rate of protein synthesis ( k_s ) of white muscle tissue between the migrant groups at any time. Early migrants had significantly greater fractional rates of protein growth ( k_g ) of white muscle in February and significantly lower fractional rates of protein degradation ( k_d ) in May. These results suggest that, compared to delayed migrants, Atlantic salmon adopting the strategy of early seaward migration maximize their overwinter growth by minimizing their rate of protein turnover.     

Reproductive biology of the threatened golden galaxias Galaxias auratus Johnston and the influence of lake hydrology

Golden galaxias Galaxias auratus (31–235 mm fork length, L _F) were collected monthly from littoral habitats in Lakes Crescent and Sorell, Tasmania, Australia, between July 2000 and December 2002. Spawning habitats were identified and monitored in both lakes, and surveyed in Lake Crescent. Trends in gonado-somatic indices and reproductive stages of development indicated that gonad development in both sexes begins in midsummer and peaks in late autumn to early winter. Males mature at smaller sizes (50% at 52 mm L _F) than females (50% at 76 mm L _F), larger individuals are predominately females (95% of fish ≥138 mm L _F), and overall male to female ratios are female biased ( c . 1:2). Spawning occurs late autumn to early spring (water temperatures = 1·4–9·7° C) with peaks in spawning activity in winter (mean water temperatures <5° C). Demersal adhesive eggs ( c. 1·5 mm diameter) were found on cobble substrata ( c. 20–250 mm diameter) in littoral areas ( c. 0·2–0·6 m deep) and fecundity of fish 71–181 mm L _F ranged from 619 to 14 478 eggs. The rate of change in water level over the 20 days prior to monthly sampling was important in explaining the occurrence of spent fish and this accounted for temporal differences in spawning between the populations. Lake hydrology influences the reproductive cycle of G. auratus by possibly providing a stimulus for spawning and it controls the availability of spawning habitat in Lake Crescent. Seasonal hydrological cycles ( i.e. rises during late autumn to winter) and a minimum water level of 802·20 m Australian Height Datum in Lake Crescent during autumn (above which littoral areas of cobble substratum are inundated) are critical to G. auratus populations.     

Comparison of growth rate and formation of otolith increments in age-0 red snapper

For wild red snapper Lutjanus campechanus , mean otolith increment deposition rate after marking with oxytetracycline dihydrate (OTC) was daily (0.97 increments day⁻¹) when growth rates were fast (0.63 mm fork length, L _F day⁻¹), but were not daily (0.82 increments day⁻¹) when somatic growth was slow (0.2 mm L _F day⁻¹). For reared larvae ( n =8), increment deposition rates were daily (0.99–1.03 increments day⁻¹), and growth rates ranged from 0.6 to 0.9 mm L _F day⁻¹. Growth rate affected increment deposition rate as a threshold function, i.e. when growth rate was <0.3 mm L _F day⁻¹, deposition was less than daily, but above this level increment deposition did not exceed a daily rate. As growth rates increased increment widths increased. Examination of a sub-sample ( n =8) of the otoliths from the slowest growing wild fish by scanning electron microscopy did not increase increment counts. Because L. campechanus are late spring-early summer spawners, young fish can expect maximum growth due to warm summer temperatures. Thus, daily ageing methods should be well suited to this species.