Failure of predator conditioning: an experimental study of predator avoidance in brown trout (Salmo trutta)

Many studies have documented that hatchery‐reared salmonids generally have inferior survival after being stocked compared with wild conspecifics, hatchery and wild salmonids have been observed to differ in their antipredator responses. The response of brown trout (Salmo trutta) juveniles (0+) of differing backgrounds to a live predator was compared in two experiments. First, the antipredator behaviour of predator‐naïve hatchery‐reared brown trout and wild‐exposed brown trout were assessed in behavioural trials which lasted for eight days. Second, predator‐naïve and predator‐conditioned hatchery‐reared brown trout were assessed in identical behavioural trials. Brown trout were ‘predator‐conditioned’ by being held in a stream‐water aquarium with adult Atlantic salmon (Salmo salar) and adult brown trout for two days prior to behavioural trials. Predator‐conditioned hatchery‐reared brown trout spent more time in shelters in the trial aquaria than predator‐naïve hatchery‐reared fish, but did not differ in time spent in the predator‐free area. Predator conditioning may account for the increased time spent in the shelter, but does not appear to have affected time spent in the predator‐free area. However, even if significant alteration in behaviour can be noted in the laboratory, the response might not be appropriate in the wild.     

Differentiated predation risk on hatchery-reared juvenile masu salmon by white-spotted charr with different body sizes

Predation after release is one of the major concerns of hatchery fish propagation. However, size-specific interaction between predator and prey on the survival of hatchery-released salmonid fish is largely unknown. To understand the size-selective predation risk, 24-h predation experiments were conducted on masu salmon Oncorhynchus masou in tanks. Four ranges of fork length (FL) were examined for masu salmon as a prey, in combination with three ranges of FL for white-spotted charr Salvelinus leucomaenis as a predator. The results show that not only predator and prey sizes, but also interaction between prey size and predator size, strongly affected the survival rate of masu salmon. Predation on masu salmon with the FL exceeding 40% of the FL of white-spotted charr was rare in the experiment. A logistic regression suggests that 37% relative FL of masu salmon to white-spotted charr results in the 50% survival of masu salmon. Our results suggest that adjusting relative size of hatchery fish to the size of local fish predators at the time of hatchery release will have a significant impact on the survival of hatchery fish in the wild. From this perspective, site-specific, adaptive management might be important to improve the effectiveness of hatchery fish propagation.     

Ecological processes influencing mortality of juvenile pink salmon (Oncorhynchus gorbuscha) in Prince William Sound,Alaska

Our collaborative work focused on understanding the system of mechanisms influencing the mortality of juvenile pink salmon (Oncorhynchus gorbuscha) in Prince William Sound, Alaska. Coordinated field studies, data analysis and numerical modelling projects were used to identify and explain the mechanisms and their roles in juvenile mortality. In particular, project studies addressed the identification of major fish and bird predators consuming juvenile salmon and the evaluation of three hypotheses linking these losses to (i) alternative prey for predators (prey‐switching hypothesis); (ii) salmon foraging behaviour (refuge‐dispersion hypothesis); and (iii) salmon size and growth (size‐refuge hypothesis). Two facultative planktivorous fishes, Pacific herring (Clupea pallasi) and walleye pollock (Theragra chalcogramma), probably consumed the most juvenile pink salmon each year, although other gadids were also important. Our prey‐switching hypothesis was supported by data indicating that herring and pollock switched to alternative nekton prey, including juvenile salmon, when the biomass of large copepods declined below about 0.2 g m^?3. Model simulations were consistent with these findings, but simulations suggested that a June pteropod bloom also sheltered juvenile salmon from predation. Our refuge‐dispersion hypothesis was supported by data indicating a five‐fold increase in predation losses of juvenile salmon when salmon dispersed from nearshore habitats as the biomass of large copepods declined. Our size‐refuge hypothesis was supported by data indicating that size‐ and growth‐dependent vulnerabilities of salmon to predators were a function of predator and prey sizes and the timing of predation events. Our model simulations offered support for the efficacy of representing ecological processes affecting juvenile fishes as systems of coupled evolution equations representing both spatial distribution and physiological status. Simulations wherein model dimensionality was limited through construction of composite trophic groups reproduced the dominant patterns in salmon survival data. In our study, these composite trophic groups were six key zooplankton taxonomic groups, two categories of adult pelagic fishes, and from six to 12 groups for tagged hatchery‐reared juvenile salmon. Model simulations also suggested the importance of salmon density and predator size as important factors modifying the predation process.     

Do predators,handling stress or field acclimation periods influence the survivorship of hatchery‐reared abalone Haliotis kamtschatkana outplanted into natural habitats?

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Dominance hierarchies in Arctic charr, Salvelinus alpinus L.: differential cortisol profiles of dominant and subordinate individuals after handling stress

We examined the effect of social rank on plasma cortisol dynamics after handling stress in juvenile Arctic charr, Salvelinus alpinus L., reared in groups of 200 individuals in 1.5 m³ circular tanks (rearing density 0.022 kg L^–1). Fish obtained by dip-netting were categorized as either socially subordinate, dominant or as holding an intermediate position on the basis of size, skin coloration and occurrence of bite marks. In undisturbed fish, the highest cortisol levels were found in fish ranked as intermediate and the lowest in dominant fish. In contrast to what has been found under conditions of artificial rearing in small groups (two to ten individuals), cortisol was not significantly elevated in subordinates compared with dominant individuals. In small groups of fish, aggressive interactions and restricted access to food may be significant factors leading to increased stress in socially subordinate individuals. Following stress by dip-netting and transfer, the quickest and largest cortisol response was seen in dominant individuals. These results suggest that hypothalamus–pituitary–inter-renal/adrenal axis responsiveness as well as baseline cortisol production is influenced by social rank in fish, as has been shown in other vertebrates.     

The Effects of Live and Artificial Diets on Feeding Performance of Cultured Winter Flounder,Pseudopleuronectes americanus,in the Wild: Survival,Feeding, Growth,and Nucleic Acid Analyses

This research, which is part of a larger study designed to assess the feasibility of winter flounder, Pseudopleuronectes americanus, stock enhancement in New Hampshire, identifies hatchery feeds that optimize feeding‐related performance of fish once released in the wild. Fish reared on post‐nauplii of brine shrimp, Artemia sp., white worms, Enchytraeus albidus, common burrower amphipods, Leptocheirus plumulosus, and formulated pellets were evaluated post‐release from in situ cages using survival, growth rate, feeding onset and incidence, stomach fullness, diet composition, and nucleic acid‐based condition as indicators of hatchery diet suitability. Amphipod‐reared fish had the highest mean stomach content index of all feed types, including wild fish. Wild and worm‐reared fish exhibited the most similar survival, overall stomach fullness, and diet composition profiles over time. Amphipod‐reared fish ranked highest in overall performance; however, if wild fish performance is viewed as the ideal for a stocked fish, worm‐reared fish performed optimally. This study describes hatchery feeding strategies that may ease the transition of flatfish released into the wild for stock enhancement.     

Efficacy of fluorescent calcein mark retention and identification in endangered Colorado pikeminnow Ptychocheilus lucius

Distinguishing hatchery‐reared fishes from wild conspecifics can be required to quantify the success of augmentation programmes. This study estimated the probability of identifying calcein‐marked, hatchery‐reared Colorado pikeminnow Ptychocheilus lucius Girard from external and internal structures. Both control and marked fish held in the laboratory were correctly identified 80% of the time after 300 days. A fluorometer was more accurate 180 days post‐marking, whereas visual observation was more discriminating at 300 days. There were no differences in detection rates among structures in laboratory fish, and for most structures of fish held outdoors, detection rates were <20%. Overall, a strong negative effect of light on mark persistence was observed. Also, an initial positive effect of fish size on mark fluorescence and a negative effect of fish growth were detected. These results suggest the potential use of calcein as a batch‐marking tool would be aided by future studies that better quantify light exposure and calcein mark persistence in both field and experimental settings.     

Effect of rearing temperatures on the growth and maturation of Arctic charr (Salvelinus alpinus) during juvenile and on‐growing periods

The effect of rearing temperature on the growth and maturation of Arctic charr (Salvelinus alpinus) was investigated. Arctic charr juveniles were reared for 6 months (phase I, October–April, size range 20–500 g) at constant temperatures of 9, 12 and 15 °C and according to two temperature‐step groups (T_step) i.e. fish transferred from 15 to 12 °C or from 12 to 9 °C. All the previous treatments were then reared either at 7 °C or at 12 °C for an additional 4 months (phase II, size range 300–1000 g) and then slaughtered in August 2008. The overall growth rate was the highest at a constant temperature of 15 °C for the first 6 months of the trial, with the fish in this group being 44% and 78% heavier than the fish reared at a constant temperature of 12 or 9 °C respectively. Arctic charr showed a negative response in terms of the growth rate when transferred from higher to lower temperatures, especially for groups previously reared at 15 °C. There was a trend for higher gonadosomatic index values at the end of the experiment for groups of fish that were exposed to higher rearing temperatures during the juvenile phase i.e. 4.18% (±0.79) and 7.29% (±0.89), for the temperature groups of 12 and 15 °C, respectively, compared with 2.49% (±0.74) for the 9 °C group. Our results suggest that for the production of fish >1000 g, moderate or low temperatures (here 9 °C) should be applied during the juvenile phase in order to reduce the negative effects arising from maturation. Farmers with access to heat sources should accordingly choose more moderate rearing temperatures during the juvenile stage, especially if the fish is to be moved down in the temperature regime during the on‐growing period.     

Are hatchery‐reared abalone naïve of predators? Comparing the behaviours of wild and hatchery‐reared northern abalone,Haliotis kamtschatkana (Jonas, 1845)

Abalone populations have declined worldwide, generating interest in enhancement using hatchery‐reared individuals. In many cases, such restoration efforts have met with limited success due to high predator‐induced mortality rates. Furthermore, the mortality rates of outplanted hatchery abalone are often considerably higher than for wild individuals. This study uses northern abalone (Haliotis kamtschatkana) as a case study to determine whether hatchery‐reared abalone behave differently than their wild counterparts. In the field, outplanted hatchery‐reared abalone were significantly less responsive than wild abalone, in terms of number of abalone responding and intensity of response, to nearby movement and to physical contact with an inert probe. Also, when encountering a cue to which all abalone responded (a seastar predator), hatchery‐reared individuals remained subdued. Anti‐predator behavioural deficits in hatchery‐reared abalone were more pronounced in 4‐year‐old individuals than in 1‐year‐old individuals, suggesting an influence of either age or amount of time spent in the hatchery environment. These behavioural differences are expected to increase the vulnerability of hatchery‐reared abalone to predators, and are likely a major cause of their elevated predator‐induced mortality when outplanted.     

Completion of the Pacific bluefin tuna Thunnus orientalis (Temminck et Schlegel) life cycle

Tuna aquaculture is currently dependent on the wild capture of juveniles for production. The development of hatchery technology for bluefin and other tunas would be a major step forward in improving sustainability of their aquaculture. The present study overviews the technology in the life cycle completion of the Pacific bluefin tuna (PBT) Thunnus orientalis (Temminck et Schlegel) under aquaculture conditions in Kinki University, and the problems to be solved for the establishment of tuna hatchery technology. On 23 June 2002, broodstock of PBT that were artificially hatched and reared spontaneously spawned in captivity. The resulting eggs hatched and were subsequently reared to the juvenile stage. The spawning fish were the result of a research project started in 1987 to rear wild‐caught juvenile PBT that were several months old. Fertilized eggs were obtained from these fish in 1995 and 1996. Resulting juveniles (the artificially hatched first generation) were reared to maturity and spawned in 2002. Over the summer of 2002, 1.63 million eggs from these fish were used for a mass rearing experiment, and 17 307 juveniles were produced and transferred to an open sea net cage. Of these artificially hatched second‐generation PBT, 1100 grew to approximately 95 cm total length and 14 kg body weight in 22 months. This procedure means the completion of PBT life cycle under aquaculture conditions, which was first attained among large tuna species. The problems awaiting solution in PBT hatchery production are their unpredictable spawning in captivity, to improve survival during the first 10 days post hatch, to reduce cannibalism in larval and juvenile stages, and to solve collision problem causing high mortality during the juvenile stage.     

Variation in DVM behaviour of juvenile and adult pearlside (Maurolicus muelleri) linked to feeding strategies and related predation risk

In the Norwegian fjord Masfjorden, different developmental stages of the mesopelagic planktivore Maurolicus muelleri form vertically separate sound scattering layers (SSLs) and in late autumn display different diel vertical migration (DVM) behaviour. Post‐larvae and juvenile fish perform normal crepuscular DVM, whereas the large majority of adults remain at depth throughout the diel period. In this study we examined the stomach contents of juvenile and adult fish caught at different times and depths during a 24‐h period in autumn. The different DVM behaviour of these two SSLs in addition to a shallow layer believed to be composed of post‐larvae is explained with a model for visual foraging in aquatic environments that uses gradients in vertical light intensity and copepod density and size as input variables. Field data revealed that vertically migrating juveniles distributed at a higher ambient light intensity and on average consumed 25 times more copepods than non‐migrating adult fish. The model showed that juveniles experienced a 15 times higher prey encounter rate and a higher level of predation risk than non‐migrating adults, and that the energetic benefits for post larvae and juveniles from prolonged feeding in a nearly constant and brighter environment outweigh the associated predation risk. The model also suggests that the visual detection range of piscivore predators is relatively more limited by the turbid surface water than that of their prey, which provide the post‐larva and juvenile life‐stages of M. muelleri a window of reduced visual predation near the surface.     

Substrate type and brightness preference of Schizothorax wangchiachii and Percocypris pingi juveniles

It is advantageous for fish to choose different substrate types and brightness based on their current life cycle stage, foraging requirements, predator avoidance and water conditions. We determined the substrate type and brightness preference of hatchery‐reared and wild juvenile Schizothorax wangchiachii and Percocypris pingi under laboratory conditions. Individuals and groups were exposed to four kinds of substrate types (sand: diameter < 1 mm, small gravel: diameter approximately 2–3 cm, large gravel: diameter approximately 10–15 cm, sand and medium gravel mix: diameter approximately 4–7 cm) and two kinds of substrate brightness (black and white). The results showed that hatchery‐reared and wild S. wangchiachii and P. pingi significantly preferred large gravel and black substrates regardless of the number of fish (p < .05). P. pingi had a significantly higher preference for black substrate than S. wangchiachii both in hatchery‐reared and wild individuals (p < .05). Hatchery‐reared and wild individuals of the two species shared a preference for large gravel and black substrates suggesting that substrate preferences might be genetically based. The findings in the present study could be used to improve the rearing conditions for the two species in hatchery and thereafter to enhance its adaptability after releasing to the wild.     

The effects of ration size on migration by hatchery‐raised Atlantic salmon (Salmo salar) and brown trout (Salmo trutta)

Abstract – The possibility to increase the proportion of migrating hatchery‐reared smolts by reducing their food ration was studied. Lake‐migrating, hatchery‐reared salmon (Salmo salar) and trout (Salmo trutta) smolts were either fed normal rations, based on recommendations from the fish‐farming industry, or reduced (15–20%) rations. They were released into the River Klarälven, western Sweden, and followed as they swam downstream to Lake Vänern, a distance of around 25 km. For both Atlantic salmon and brown trout, smolts fed a reduced ration migrated faster than fish fed a normal ration. Furthermore, a higher proportion of salmon smolts fed reduced rations migrated to the lake than fish fed normal rations in 2007 but not in 2006. This difference between years corresponded to greater treatment differences in size and smolt status in 2007 than in 2006. For trout, the proportion of migrating individuals and smolt development did not differ with ration size. Trout migrants fed a normal ration had a higher standard metabolic rate (SMR) than nonmigrants, whereas there was no difference in SMR between migrating and nonmigrating salmon. These results show that it is possible to use a reduced food ration to increase the migration speed of both Atlantic salmon and brown trout and to increase the proportion of migrating Atlantic salmon.     

Changes in selected stress and immune‐related genes in Atlantic cod,Gadus morhua,following overcrowding

Using Atlantic cod, Gadus morhua, as a model for the stress response in gadoid fish, the changes in the expression of some stress and immune genes as well as the profiles of plasma cortisol were examined. Adult fish were kept at a density of ca. 100 kg m^?3 by lowering the water level in the rearing tank for 1 h and this short‐term crowding stress was repeated thrice over a 12‐h interval period. Blood samples were collected before exposure and at 2, 24 and 72 h post crowding. Plasma cortisol level significantly increased at 2 h post crowding but returned to pre‐crowding levels 24 h after exposure. The relative expression of the stress response genes, glucose transporter‐3 and a putative heat shock protein 70 significantly increased at 2 and 24 h post crowding respectively. Significant up‐regulation of the pro‐inflammatory cytokines, interleukin (IL)‐1β and IL‐8, as well as anti‐bacterial genes, g‐type lysozyme and bactericidal permeability‐increasing protein/lipopolysaccharide‐binding protein (BPI/LBP) was also observed at 2 h and the levels were maintained until 72 h post exposure, except for BPI/LBP which had maximum up‐regulation at 24 h. The present observations have implications with respect to fish welfare and assessment of the health status of the farmed fish.     

Effect of Dietary Minerals and Pigment on Somatic Growth of Juvenile Green Sea Urchins, Strongylocentrotus droebachiensis

This study investigated the effects of dietary minerals and pigments in prepared diets on the somatic growth performance of wild and hatchery‐reared juvenile green sea urchins, Strongylocentrotus droebachiensis, by two feeding trials. In the first feeding trial, a modified Bernhart‐Tomerelli salt mix (BT) at 0, 1.5, 3, 6, and 15% dry mass and a Shur‐Gain/Maple Leaf Foods mineral mix at 3 and 6% dry mass were used to test for mineral effects. Pigment effects were tested by incorporating 1.25% Algro? to the prepared diets (i.e., 250 mg of beta‐carotene per kilogram of diet). Sea urchins (13–15 mm of initial test diameter [TD]) collected from the wild were fed the prepared diets over 154 d. The sea urchins that were fed the pigmented diets had significantly greater test growth than those fed the nonpigmented diets, and mineral concentration in the pigmented diets was directly related to juvenile size at the end of the feeding trial. A sample of juveniles from each treatment group was sacrificed to determine test, gonad, and gut yields and ash concentrations. Ash concentrations in the test and gonad were higher for juveniles fed pigmented diets than for those fed nonpigmented diets with similar mineral concentration, suggesting an interaction between minerals and pigments within the juvenile sea urchins. The second feeding trail used two size cohorts of hatchery‐reared juveniles ranging from 1–2 mm and 2–3 mm of initial TD to compare the growth of sea urchins fed either the pigmented diet with 15% BT (i.e., the best diet in the first feeding trial) or kelp, Laminaria longicruris, over 159 d. Growth performance was similar for both cohorts, indicating no size effect, but the juveniles fed the prepared diet were significantly larger at the end of the feeding trial than those fed kelp. This suggests that prepared diets with pigment and high mineral concentration can outperform kelp, and be utilized for juvenile green sea urchins to increase test growth.     

Maximum handling size,prey size and type selection by snakehead (Channa argus) feeding on juvenile Chinese mitten crab (Eriocheir sinensis)

Laboratory predation trials were conducted to investigate maximum handling size, prey size and type selection by small (35–37 cm in total length, L_T), medium (43–46 cm L_T) and larger (58–60 cm L_T) snakehead (Channa argus) when feeding on a wide size (9–34 mm in carapace width, C_W) range of juvenile Chinese mitten crab (Eriocheir sinensis). The maximum handling size of predator feeding on crabs monotonically increased with predator L_T and mouth gape width, G_W. Snakehead with at least 16.0 cm L_T or 27.7 mm G_W would be capability to consume the smallest size group of crabs, whereas the predator with 72.6 cm L_T or 63.4 mm G_W would be capability to consume all size groups of crabs in these trails. Prey‐size selection trials showed that snakehead has a high preference to the small‐sized crabs, and lower preference to the medium or large size crabs. The preference index was significantly affected by prey size and prey size × predator size interaction, however, was not affected by predator size. In prey type experiments, snakehead consumed lower proportions of juvenile crabs when fed on the crab and crucian carp than when fed on only the crab, which suggests alternative fish prey may reduce predation risk of the crab by snakehead in nature. These results could be useful for improving the fishery management and release strategies for the crab.     

Foraging behaviour of juvenile pink salmon (Oncorhynchus gorbuscha) and size‐dependent predation risk

Two hypotheses related to effects of juvenile pink salmon (Oncorhynchus gorbuscha) foraging behaviour and size on their predation risk were evaluated using field data collected in Prince William Sound, Alaska 1 995–97. My results supported the hypothesis that low macrozooplankton density leads to dispersion of juvenile salmon from shallow nearshore habitats and greater predation risk, but zooplankton type was an important factor. When the biomass of large copepods (primarily Neocalanus spp.) declined, salmon dispersed from shallow nearshore habitats, and mean daily individual predator consumption of salmon increased by a factor of 5. A concomitant five‐fold increase in the probability of occurrence of salmon in predator stomachs supported the notion that increased predation on salmon was caused by a greater overlap between predator and prey when salmon dispersed offshore, not an increase in the number of salmon consumed per feeding bout. The results also generally supported the hypothesis that the timing of predation events modifies the nature of size‐dependent predation losses of salmon to different predator groups (small and large planktivores and piscivores). Size‐dependent vulnerabilities of salmon to predators were a function of both predator and prey sizes. When simulated predation was shifted from May to June, the vulnerability of salmon became more dependent on their growth than initial size. But, the size‐ and growth‐dependent vulnerabilities of salmon differed more among predator groups than between May and June, suggesting that changes in the composition of predator fields could more strongly affect the nature of size‐dependent predation than changes in the timing of predation losses.     

Antipredator responses to overhead fright stimuli in hatchery-reared and wild European sea bass (Dicentrarchus labrax L.) juveniles

In this study, shoals of hatchery‐reared and wild sea bass juveniles (Dicentrarchus labrax L.) were tested for differences in their antipredator responses towards a visual (shadow) and a mechanical (dummy bill) overhead stimulus. Two behavioural variables – distance from the bottom and freezing duration – were measured during post‐stimulus phases of each test and compared between wild and hatchery‐reared shoals, composed of 10 juveniles each. The results showed that in both hatchery‐reared and wild juveniles, stimulus exposure elicited a significant decrease in the mean shoal distance from the bottom. Similarly, individuals from both groups engaged a freezing reaction, but the mean freezing duration was significantly higher in wild‐ than in hatchery‐reared juveniles. Results are discussed in the light of their relevance for the enhancement of restocking programmes.     

The role of dietary L‐ascorbyl‐2‐polyphosphate on the growth and physiological functions of beluga,Huso huso (Linnaeus, 1758)

Effects of dietary ascorbic acid (AA) supplementation were carried out on growth performance and physiological indices in juvenile beluga Huso huso fed different levels of dietary L‐ascorbyl‐2‐polyphosphate as AA source consisting of 0 (control), 100, 200, 400, 800 and 1600 mg AA kg^?1 diet for 16 weeks. No significant differences were found in growth performances between treated groups except for condition factor. Using AA caused significant differences in haemoglobin, number of red blood cells, white blood cells, aspartate aminotranspherase and lactate dehydrogenase, while no significant difference was found in haematocrit, mean corpuscular haemoglobin concentration, differential white blood cells count and alkaline phosphatase activity at the end of the experiment. Lysozyme activity after 8 weeks was significantly affected by dietary treatments with the lowest value in fish fed control diet, but not at completion of 16‐week feeding trial. Liver AA concentration showed a significant increase in correlation with dietary AA increase. Air exposure stress test showed significant differences among the treatments in case of cortisol and glucose concentrations. Also between pre‐ and post stress, significant changes were observed in some of treatments. The lowest and highest levels of cortisol were 9.2 ± 3 (in 200 mg kg^?1) and 16.8 ± 5.1 ng mL^?1 (in control), 6 h post stress (P < 0.05) respectively. Also, the lowest and highest levels of glucose were measured in fish fed 200 and 800 mg AA kg^?1 respectively. The obtained results showed that most growth parameters were not affected by dietary AA, but some physiological and immunological parameters of this species were influenced by this vitamin. We suggest that under rearing conditions, AA should be added to the diet especially during early life stages for improving health status and resistance to stressors.