Surface feeding and aggressive behaviour of diploid and triploid brown trout Salmo trutta during allopatric pair‐wise matchings

Diploid and triploid brown trout Salmo trutta were acclimated for 6 weeks on two feeding regimes (floating and sinking). Thereafter, aggression and surface feeding response were compared between pairs of all diploid, all triploid and diploid and triploid S. trutta in an experimental stream. In each pair‐wise matching, fish of similar size were placed in allopatry and rank was determined by the total number of aggressive interactions recorded. Dominant individuals initiated more aggression than subordinates, spent more time defending a territory and positioned themselves closer to the surface food source (Gammarus pulex), whereas subordinates occupied the peripheries. In cross ploidy trials, diploid S. trutta were more aggressive than triploid, and dominated their sibling when placed in pair‐wise matchings. Surface feeding, however, did not differ statistically between ploidy irrespective of feeding regime. Triploids adopted a sneak feeding strategy while diploids expended more time defending a territory. In addition, we also tested whether triploids exhibit a similar social dominance to diploids when placed in allopatry. Although aggression was lower in triploid pairs than in the diploid and triploid pairs, a dominance hierarchy was also observed between individuals of the same ploidy. Dominant triploid fish were more aggressive and consumed more feed items than subordinate individuals. Subordinate fish displayed a darker colour index than dominant fish suggesting increased stress levels. Dominant triploid fish, however, appeared to be more tolerant of subordinate individuals and did not display the same degree of invasive aggression as seen in the diploid and diploid or diploid and triploid matchings. These novel findings suggest that sterile triploid S. trutta feed similarly but are less aggressive than diploid trout. Future studies should determine the habitat choice of triploid S. trutta after release and the interaction between wild fish and triploids during the breeding season prior to utilization of triploids as an alternative management strategy within freshwater fisheries.     

Food partitioning between coexisting Atlantic salmon and brook trout in the Sainte‐Marguerite River ecosystem, Quebec

Food resource partitioning between similar‐sized, sympatric Atlantic salmon Salmo salar and brook trout Salvelinus fontinalis was examined as a possible mechanism enabling their coexistence in a stream (Allaire) of the Sainte‐Marguerite River ecosystem, Quebec, Canada. Fish stomach contents and invertebrate drift were collected concurrently during three diel cycles in August to September 1996. The food and feeding habits of an allopatric brook trout population in a nearby stream (Epinette) were studied for comparison. The diel feeding rhythms of the two coexisting fish species were similar. The composition of their diet, however, showed significant differences. Atlantic salmon predominantly (60–90%) fed on aquatic insects, mainly Ephemeroptera (35–60% of the diet). The brook trout mostly (50–80%) fed upon the allochthonous terrestrial insects (mainly adults of Coleoptera, Hymenoptera and Diptera) which comprised 5–40% of the stream drift. The allopatric brook trout fed opportunistically on the more abundant aquatic insects and terrestrial insects rarely formed 25% of its diet. The allopatric trout fed nearly twice as much as the sympatric brook trout during a day. The results suggest that the differences in feeding by brook trout in the two streams (with and without Atlantic salmon) are the result of inter‐specific interaction with Atlantic salmon and are not related to the differences in food availability between the two streams. Food resource partitioning between Atlantic salmon and brook trout may be viewed as an adaptive response resulting in a greater exploitation of available resources and coexistence.     

Influence of growth rate on white muscle dynamics in rainbow trout and brook trout

In trout, fast growth stimulated white muscle fibre hypertrophy ( P 0·001) and hyperplasia ( P <0·01) in outer fibres but not in deep fibres. Glycogen was most prevalent in outer fibres ( P <0·01) and in brook trout ( P <0·01) that on average had three to four times larger fibres than rainbow trout.     

The critical swimming velocity of diploid and triploid brook trout

No significant difference was found in the critical swimming velocity of diploid and triploid brook trout Salvelinus fontinalis , suggesting that the triploid state is not associated with a reduced aerobic capacity.     

Thermal Tolerance of Diploid Versus Triploid Rainbow Trout and Brook Trout Assessed by Time to Chronic Lethal Maximum

Synopsis An effect of ploidy on thermal tolerance in juvenile trout was assessed in a series of tests comparing time to chronic lethal maximum (CLMax). Diploid and triploid fish were produced from a common spawn for three different groups each of brook trout Salvelinus fontinalis and of rainbow trout Oncorhynchus mykiss. One or two CLMax tests were performed per group, on between 15 and 50 individuals per ploidy within groups. The tests involved exposure of fish to a progressive 2°C day⁻¹ water temperature increase and recording of the time at which each individual fish reached loss of equilibrium (LE). The time to LE data were rank transformed and analyzed as a randomized complete block design. Although relative performance varied among trials, the analysis indicated overall differences due to ploidy were small and nonsignificant among both brook trout and rainbow trout. Size proved to be significantly correlated with time to LE in the brook trout trials, but not in the rainbow trout trials. Two of the six groups included a large proportion of fish which had received a heat shock following fertilization, but were not successfully triploidized. In both cases, thermal tolerance of the heat-shocked diploids was similar to that of the non-heat shocked control diploids, indicating no persistent effect of the heat shock on thermal tolerance.     

The combined effect of feeding time and ration on growth performance and nitrogen metabolism of greenback flounder

Greenback flounder Rhombosolea tapirina ( c. 2 g) fed to satiation had significantly ( P <0·01) higher feed consumption in the evening than in the morning whereas there was no difference between feeding times for flounder fed restricted rations (1 or 2% body weight per day) because they consumed all of the ration. Differences in growth performance were due to feeding time and ration. Carcass moisture, lipid and energy content were significantly ( P <0·001) different between rations; length gain was significantly affected by feeding time ( P <0·05) and ration ( P <0·001); weight gain showed a significant ( P <0·001) interaction between feeding time and ration. The relationship between feed consumption and specific growth rate showed that the exponential gradient was significantly higher ( P <0·01) for the evening fed fish and indicated feed efficiency for evening fed fish increased as feed consumption increased. Urea excretion increased from 12–20 to 58–63% of total nitrogen excretion at the 1 and 3% rations, respectively. Ammonia and urea excretion were significantly affected by ration ( P <0·001) and feeding time ( P <0·05). Fish fed the 2% ration in the evening had higher growth efficiency and significantly ( P <0·01) lower rates of urea excretion than fish fed 2 or 3% ration in the morning. It is suggested that the higher energetic costs associated with differences in ammonia and urea excretion contributed to differences in growth efficiency.     

Endurance swimming of diploid and triploid Atlantic salmon

When groups of diploid (mean ±  s . e . fork length, L _F) 33·0 ± 1·4 cm and triploid (35·3 ± 0·5 cm) Atlantic salmon Salmo salar were forced to swim at controlled speeds in a carefully monitored 10 m diameter 'annular' tank no significant difference was found between the maximum sustained swimming speeds ( U _ms, maintainable for 200 min) where the fish swam at the limit of their aerobic capability. Diploids achieved 2·99 body lengths per second (bl s⁻¹)(0·96 m s⁻¹) and triploids sustained 2·91 bl s⁻¹(1·02 m s⁻¹). The selection of fish for the trials was based on their ability to swim with a moving pattern projected from a gantry rotating at the radius of the tank and the selection procedure did not prove to be significant by ploidy. A significant difference was found between the anaerobic capabilities of the fish measured as endurance times at their prolonged swimming speeds. During the course of the experimentation the voluntary swimming speed selected by the fish increased and the schooling behaviour improved. The effect of the curvature of the tank on the fish speeds was calculated (removing the curved effect of the tank increased the speed in either ploidy by 5·5%). Implications of the endurance times and speeds are discussed with reference to the aquaculture of triploid Atlantic salmon.     

Influence of different rations and water temperatures on the growth rates of shortfinned eels and longfinned eels

Juvenile (12–152 g) shortfinned eels Anguilla australis and longfinned eels A. dieffenbachia caught in New Zealand streams were fed squid mantle Nototodarus spp. 4 days per week in laboratory experiments. A linear multiple regression equation showed the amount of food eaten (0–2·7% w day⁻¹) explained 77·7% of the variation in specific growth rates (–0·60 to +1·07% w day⁻¹) among individual eels, while previous growth rates, water temperature (10·0–20·6°C), and eel weight (12–152 g) explained a further 5·6, 1·4 and 0·8%, respectively. Growth in length ranged from –0·3 to +0·9 mm day⁻¹. Eels which were starved and then given high rations grew substantially faster than expected. Once growth rates were adjusted for differences in ration and other factors, there were no significant differences in growth rates between species or individual fish. Growth of shortfinned eels fed maximum rations of commercial eel food depended on fish size and water temperatures and ceased below 9·0°C. Growth rates in the wild were substantially less than the maximum possible, after seasonal changes in water temperatures were taken into account, indicating that food supplies and not low water temperatures were controlling growth rates in the wild.     

The effects of body mass and feeding on metabolic rate in small juvenile Atlantic cod

Apparent specific dynamic action (SDA) amplitude in young juvenile Atlantic cod Gadus morhua (1 to 8 g wet mass), fed a standardized meal and then exercised in a circular swimming respirometer at a constant swimming speed of 0·5 ± 0·3 body lengths s^-1, occurred within l h after feeding in all juveniles. SDA amplitude were 1·4 to 1·8 times higher in fed fish compared to unfed fish, and rates of oxygen consumption decreased as body mass increased. SDA duration had a tendency to decrease with increasing body mass and was shortest, at 6 h, in the smallest fish (1–1·5 g), but increased to 10–11 h in the largest fish. Apparent SDA in fed fish ( R _r) scaled with a mass exponent of 0·89, while maximum metabolic rate ( R _max) determined by chasing fish to exhaustion and then measuring oxygen consumption for 12 h, and unfed routine metabolic rate (R_r) scaled with a mass exponent of 0·79 and 0·76 respectively. Relative aerobic scope ( R _max– unfed R _r) did not appear to vary over the 1 to 8 g increase in wet mass. These results show that as body mass increased in young juvenile Atlantic cod: (1) apparent SDA ( R _f) increased more rapidly than R _max, and (2) apparent SDA took up >98% of the relative aerobic scope and that young Atlantic cod allocated most of the energy to growth, and left little for other metabolic activities.     

Satiation meal and the effects of meal and body sizes on gastric evacuation rate in brook trout Salvelinus fontinalis fed commercial pellets

Gastric evacuation (GE) experiments were performed on brook trout Salvelinus fontinalis fed commercial food pellets. The experiments included small fish (36 g; 15 cm total length, L_T) fed meals of 0·2, 0·4 and 0·8 g and large fish (152 g; 23 cm) fed meals of 0·8, 2·0 and 4·0 g at temperatures ranging from 15·1 to 18·2° C. The stomach contents were thereafter sampled and weighed at 3 h intervals until the first empty stomach was observed. The course of GE was examined by use of a general power function of the data that revealed that the square‐root function described the GE rate (GER) by the current stomach content mass independently of original meal size. Using the square‐root function, the relationship between GER and fish size was described by a power function of fish length, whereas the effect of temperature was described by a simple exponential function. GER of the commercial pellets fed to S. fontinalis could thus be described by (g h^?1), where S_t is stomach mass (g) at time t (h), L is total fish length (cm) and T is temperature (° C). The result of this study should provide a useful tool for planning of feeding regimes in production of S. fontinalis by optimizing growth and minimizing food waste.     

The effect of ration level and social rank on the development of fin damage in juvenile rainbow trout

The development and severity of fin damage was examined in groups of juvenile rainbow trout Oncorhynchus mykiss of different strength of feeding hierarchies. The development of dorsal and caudal fin damage over time was compared between four groups fed different ration levels (0·25, 0·5, 1·0 and 1·5% body weight day⁻¹) and between individuals of different feeding rank within each group. Dominance hierarchies were assessed from repeated daily measurements of food consumption of individuals using radiography. The feeding and growth data indicated that the strength of the social hierarchy weakened with increasing ration. Caudal fin damage developed with time in all groups whereas dorsal fin damage developed only under limited rations. The severity of both dorsal and caudal fin damage was significantly dependent on the ration size fed to the group, with lower ration groups sustaining more fin damage. The severity of dorsal fin erosion was greater than for the caudal fin. Within the two lower ration groups, subordinate fish suffered the most dorsal fin damage. The results suggested that the severity of dorsal fin damage within groups of juvenile rainbow trout can be used as an indicator of hierarchy Strength.     

Oral administration of cholecystokinin receptor antagonists increase feed intake in rainbow trout

Individual immature rainbow trout consumed 1–2% body weight per day, but significantly more ( P < 0·001) when fed by hand than by demand feeder. When treated with CCK antagonists (L 364, 718; 100 μg kg⁻¹ on day 12 or SR 27, 897; 50 μg kg⁻¹ on day 16), the fish ate significantly more than their mean daily intake on the other days of the experiment. This increase in feed intake was affected by the feeding technique: hand-fed fish increased by 70–80% their feed intake while in demand-fed fish the increase was significantly less (50–60%). However, the increase in feed intake observed on days 12 and 16 was identical for both drugs used.     

Maturation of male age-0 Atlantic salmon following a massive, localized flood

Maturation of male age-0 Atlantic salmon Salmo salar parr in New England, U.S.A. streams is rare (˜5%), but age-0 parr maturation was high (74%) by autumn in the Sawmill River following a massive, localized flood. Maturation was low in two other study streams (3, 7%) in the same year as the flood, and in the Sawmill River (6%) and the other rivers (5%) in the subsequent year, suggesting that high maturation rates were related to the flood. The high age-0 maturation rates appear to have been the result of greater growth opportunity following the flood. Masses of fish in October were two-fold greater in the Sawmill River (13·2 g) than in the other rivers (6·5, 6·9 g). Mechanisms contributing to the fast growth may include community reorganization following the flood and water temperature differences among rivers. The flood caused an age-0 year-class failure for brook trout Salvelinus fontinalis and brown trout Salmo trutta and a large reduction (69%) in the number of salmon compared to the other rivers, possibly reducing competition or agonistic interactions among remaining fish. Average water temperatures were slightly warmer in the Sawmill River (17·0° C) than in the other rivers (15·5, 14·9° C). By influencing community structure and growth of remaining fish, it appears that a strong environmental disturbance can also alter the direction and timing of life histories in Atlantic salmon.     

Effects of malachite green on leucocyte abundance in rainbow trout, Salmo gairdneri (Richardson)

Blood counts from more than 1000 young-of-the-year rainbow trout, Salmo gairdneri (Richardson), were examined to determine if static exposure to malachite green at 1·35,13·5, or 21·0 mg/1 for 25–30 min or 42·0 or 72·0 mg/1 for 5 min caused chronic leucopaenia. The major changes in fish exposed for 25–30 min came during the first 24 h. After an initial lag of 3·4 h, total leucocyte-thrombocyte counts in both treated and control fish rapidly declined. Recovery was essentially complete 1·4 days after exposure, and no leucopaenia was noted after 14 or 28 days. Thrombocytosis developed during the first 24 h in fish exposed to the higher concentrations. Lymphopaenia and neutrophilia also developed, but abated after the 4th post-treatment day. Leucocyte numbers in control and exposed groups were virtually the same by the 14th day. The total leucocyte-thrombocyte counts in fish exposed for 5 min declined after 24 h, but counts were not as depressed as those in fish exposed for 30 min.
Because leucocyte changes similar to those in exposed fish were evident in the controls in both experiments, we believe that the leucocyte changes in rainbow trout exposed to malachite green were a result of a nonspecific vertebrate stress syndrome, rather than of specific leucocytotoxic effect of this chemical.     

Differentiating diploid and triploid individuals using single nucleotide polymorphisms genotyped by amplicon sequencing

Triploidy can occur naturally or be induced in fish and shellfish during artificial propagation in order to produce sterile individuals. Fisheries managers often stock these sterile triploids as a means of improving angling opportunities without risking unwanted reproduction of the stocked fish. Additionally, the rearing of all‐triploid individuals has been suggested as a means to reduce the possibility of escaped aquaculture fish interbreeding with wild populations. Efficient means of determining if an individual is triploid or diploid are therefore needed both to monitor the efficacy of triploidy‐inducing treatments and, when sampling fish from a body of water that has a mixture of diploids and triploids, to determine the ploidy of a fish prior to further analyses. Currently, ploidy is regularly measured through flow cytometry, but this technique typically utilizes a fresh blood sample. This study presents an alternative, cost‐effective method of determining ploidy by analysing amplicon‐sequencing data for biallelic single‐nucleotide polymorphisms (SNPs). For each sample, heterozygous genotypes are identified and the likelihoods of diploidy and triploidy are calculated based on the read counts for each allele. The accuracy of this method is demonstrated using triploid and diploid brook trout (Salvelinus fontinalis) genotyped with a panel of 234 SNPs and Chinook salmon (Oncorhynchus tshawytscha) genotyped with a panel of 298 SNPs following the GT‐seq methodology of amplicon sequencing.     

Differences in feeding between 1+ and 2+ hatchery brown trout exposed to low water temperature

Both 1 + and 2+ brown trout fed during the day and at night when held in hatchery tanks at low temperatures (2·7–3° C). Over 60% of the daily ration was consumed during daylight hours, but there were differences in feeding behaviour between fish of the two age groups: the 1 + trout had greater feeding activity at night than the 2+ fish.     

Morphological comparisons of diploid and triploid hybrid grass carp, Ctenopharyngodon idella♀×Hypophthalmichthys nobilis♂

Morphological and karyological comparisons of 25–28 hybrid grass carp from each of 3 year-classes (1979–1981) indicated that diploid fish resulted during 1979 (82.1%) and 1980 (76.0%) the others being triploid. All of the fish tested from the 1981 year-class were triploid. Most triploid fish differed from diploid fish by having a faster growth rate, fewer scales in the lateral line and transverse series below the lateral line, a relatively longer gut, and fewer deformities.     

Cardiorespiratory status of triploid brown trout during swimming at two acclimation temperatures

At 14° C, standard metabolic rate (75·1 mg O₂ h⁻¹ kg⁻¹), routine metabolic rate (108.8 mg O₂ h⁻¹ kg⁻¹), active metabolic rate ( c . 380 mg O₂ h⁻¹ kg⁻¹), critical swimming speed (U_crit 1·7 BL s⁻¹), heart rate 47 min⁻¹), dorsal aortic pressure (3·2 kPa) and ventilation frequency (63 min⁻¹) for triploid brown trout Salmo trutta were within the ranges reported for diploid brown trout and other salmonids at the same temperature. During prolonged swimming ( c . 80% U _crit), cardiac output increased by 2·3-fold due to increases in heart rate (1·8-fold) and stroke volume (1·2-fold). At 18° C, although standard and routine metabolic rates, as well as resting heart rate and ventilation frequency increased significantly, active metabolic rate and certain cardiorespiratory variables during exercise did not differ from those values for fish acclimated to 14° C. As a result, factorial metabolic scope was reduced (2·93-fold at 18° C v . 5·13-fold at 14° C). Therefore, it is concluded that cardiorespiratory performance in triploid brown trout was not unusual at 18° C, but that reduced factorial metabolic scope may be a contributing factor to the mortality observed in triploid brown trout at temperatures near 18° C.     

Shifting thermal regimes influence competitive feeding and aggression dynamics of brook trout (Salvelinus fontinalis) and creek chub (Semotilus atromaculatus)

The natural distributions of freshwater fish species are limited by their thermal tolerances via physiological constraints and increased interspecific competition as temperatures shift toward the thermal optima of other syntopic species. Species may mediate stress from temperature change physiologically, behaviorally, or both; but these changes may compromise competitive advantages through effects on feeding and social behavior. In the Appalachian Mountains of North America, creek chub (Semotilus atromaculatus) are found in warm‐water and cold‐water streams and overlap in range with brook trout (Salvelinus fontinalis) across lower thermal maxima, where they compete for food and space. As stream temperatures continue to increase due to climate change, brook trout are under increasing thermal stress which may negatively affect their ability to compete with creek chub. To examine the influences of temperature on competitive interactions between these species, we observed feeding behavior, aggression, and habitat use differences at three temperatures approaching brook trout thermal maxima (18°C, 20°C, and 22°C) among dyad pairs for all combinations of species in experimental flow‐through tanks. We also examined feeding and habitat use of both species under solitary conditions. We found as temperature increased, feeding and aggression of brook trout were significantly reduced in the presence of creek chub. Creek chub pairs were more likely to occupy benthic areas and refugia while brook trout pairs used surface water more. Space use patterns significantly changed by pairing treatment. Aggression and space use shifts allowed increased exploitative and interference competition from creek chub when paired with brook trout that was not present in conspecific pairs. The decreased dominance of a top predator may lead to diverse impacts on stream community dynamics with implications for the future range restriction of brook trout and demonstrate possible mechanisms to facilitate competitive advantages of warm water generalist species under thermal stress.     

Influence of dietary protein level on growth performance and body composition of juvenile blackspot seabream, Pagellus bogaraveo(Brunnich, 1768)

Blackspot seabream Pagellus bogaraveo is a potential candidate species for southern European aquaculture. A 12‐week feeding trial was then conducted to evaluate the effect of dietary protein level on growth performance, body composition and feed use efficiency of blackspot seabream juveniles (22·7 g each). Five isolipidic diets (12·5%) with graded levels of protein (20, 30, 40, 50 and 60%) were hand‐distributed, twice a day, to duplicate groups of fish (120 fish per tank), until satiation. Growth (DGC), survival and feed utilization (FCR) were recorded through monthly sampling. Growth performance was significantly affected by dietary protein level ( P  < 0·05), and fish fed at the 50% protein level exhibited the highest final body mass. Feed conversion ratio decreased with increasing levels of dietary protein, fish fed 60% protein being the most efficient. No significant differences were found in whole body composition among treatments ( P > 0·05). Financed by FCT (Project POCTI/CVT/39239/2001, PhD Grant SFRH‐BD‐14068–2003)