Abstract
This article synthesizes findings from multiple studies to provide a comprehensive analysis of brown trout (Salmo trutta) diets, focusing on how diet is affected by factors such as developmental stage, environmental conditions, and genetic differences between trout populations. The information presented here is compiled from research examining the effects of dietary transitions from pelleted feed to live prey fish in hatchery settings and the natural feeding behaviors of wild trout populations across various ecosystems.
Introduction
The brown trout (Salmo trutta) is a widely distributed salmonid fish native to Europe and western Asia, but introduced to other continents. It is a popular species for both recreational and commercial fisheries. Brown trout exhibit diverse life history strategies, resulting in various ecological morphs, including river trout (S. trutta m. fario), which remain in freshwater, and sea trout (S. trutta m. trutta), which migrate to coastal seawater.
Conserving brown trout populations is increasingly important due to habitat degradation and loss of genetic diversity. Hatchery-reared trout are often used for stocking programs, but these fish may face challenges adapting to wild environments, including altered foraging behaviors. Understanding the dietary needs and feeding habits of brown trout is crucial for improving reintroduction practices and ensuring the survival and reproduction of stocked fish.
This article explores various aspects of brown trout diets, including the impact of prey availability, size selectivity, ontogenetic shifts, and the influence of environmental and genetic factors on feeding behavior. By synthesizing findings from multiple studies, this analysis aims to provide insights into the complex interplay between brown trout and their food resources.
Dietary Transition from Pelleted Feed to Live Prey Fish
One study assessed the effects of a four-week dietary transition from pelleted feed to live prey fish on two morphs of brown trout: river trout and sea trout. The trout were divided into four groups:
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- Group C: Received the same pelleted diet as before.
- Group E: Switched to an experimental pellet.
- Group EF: Received both the experimental pellet and prey fish for two weeks, then only prey fish for another two weeks.
- Group F: Received only prey fish throughout the four weeks.
Histological measurements were performed on visceral samples obtained after two and four weeks, using various staining methods (HE, AB-PAS, Perl’s Prussian blue). The results indicated that fish from groups EF and F had shorter intestinal folds, smaller hepatocytes, and less vacuolization of hepatic parenchyma compared to fish from groups C and E. Hepatocyte nuclei and splenic melanomacrophage centers were larger in fish from group F, with sea trout having larger centers than river trout.
These findings suggest that transitioning brown trout to a live prey fish diet can lead to significant histological changes in the digestive tract and associated organs. The shorter intestinal folds observed in groups EF and F may reflect an adaptation to the different nutrient composition and digestibility of live prey compared to pelleted feed. Similarly, the smaller hepatocytes and reduced vacuolization in these groups may indicate a shift in liver metabolism in response to the altered diet.
Natural Diet and Piscivory in Brown Trout
In their natural habitats, brown trout predominantly prey on zoobenthos and insects. Dietary preferences often shift from benthic to surface organisms as trout grow, typically around a body length of 8 cm. Older trout may also predate on smaller fish, although piscivory (fish-eating) is facultative and influenced by factors such as seasonal food availability, diversity, spatial distribution, and morphological traits of local populations.
Piscivorous behavior in brown trout typically becomes noticeable at a total length of 13 cm, with a more pronounced shift at 25-30 cm. Larger trout exhibit flexibility in adjusting to available prey fish, and their trophic position and somatic growth benefit from a habitual switch to fish-eating.
A study examining piscivory in brown trout populations across different ecosystem types (riverine, lacustrine, and marine) found that the prevalence of piscivory increased from lotic (flowing water) to lentic (still water) ecosystems. Fish community configuration and brown trout population structure were identified as the most influential drivers of variation in piscivory. The number of sympatric fish species (species living in the same geographic area) also positively correlated with piscivory, particularly in lacustrine ecosystems.
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The study also revealed that temperature-related geographical variables, such as latitude and elevation, influenced piscivory. However, the relationship between piscivory and latitude varied depending on the ecosystem and fish community type, with piscivory increasing with latitude only in multi-species, prey fish systems of lacustrine ecosystems.
These findings highlight the complex interplay between environmental factors, community structure, and brown trout feeding behavior. The increased prevalence of piscivory in lentic ecosystems may be attributed to differences in prey availability and community composition compared to lotic systems.
Prey Size Selection and Ontogenetic Shifts
Brown trout exhibit size-selective feeding behavior, with a preference for prey items of certain sizes. One study found that brown trout preferred prey items between 4 and 6 mm in length, although they could consume prey larger than 10 mm. The similarity between the size frequency distribution of prey in the environment (benthos and drift) and the diet of trout was considerable in some cases, indicating that trout tend to feed on the most abundant prey sizes available.
The study also explored ontogenetic shifts in prey size, with the expectation that larger trout would consume larger prey items. However, the results revealed that the 4- to 6-mm size category was generally dominant in all age groups. Differences in average prey size among age classes were only observed in two of the eight rivers studied, where prey size increased with increasing fish age.
These findings suggest that while brown trout exhibit size-selective feeding behavior, the relationship between prey size and trout size may not always be consistent. Other factors, such as prey availability and energetic considerations, may also influence prey size selection.
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The Relationship Between Prey Size, Prey Numbers, and Stomach Fullness
The relationship between prey size and the number of prey items consumed by brown trout has been investigated. It was hypothesized that prey numbers should be low when the predator feeds on large prey items, and vice versa. Indeed, the results revealed that as food size decreased, prey numbers increased, suggesting that trout adjust their feeding strategy based on the size of available prey.
This flexible feeding strategy allows trout to optimize their energy intake by either consuming a few large, energy-rich prey items or many small, less energy-dense prey items. The study also explored the relationship between prey size and stomach fullness, but no clear relationship was found.
Metabolic Rate and Food Restriction
Metabolic rate, or the rate of energy use, is a critical factor influencing an organism's performance and survival. Studies have shown that food restriction can reduce baseline metabolism in brown trout, but metabolic traits also vary by sex and population factors.
One study examined the effects of long-term food restriction on standard metabolic rate (SMR) and maximum metabolic rate (MMR) in offspring from two wild brown trout populations that differ in migratory tactics (anadromous vs. non-anadromous). Both populations showed decreased SMR under low food conditions, indicating that trout can reduce their baseline energy requirements in response to food scarcity.
The anadromous population had higher MMR than the non-anadromous population, suggesting that migratory trout may have a greater capacity for aerobic metabolism to support their long-distance migrations. The MMR difference was greater than SMR, resulting in a higher aerobic scope (AS) in the anadromous population. Males also had higher MMR and AS than females.
These findings suggest that different components of metabolic rate can vary in their response to environmental conditions and intrinsic factors such as population background and sex. Populations may also differ in their flexibility of metabolic traits, potentially due to life-history differences related to migratory tactics.
Implications for Conservation and Management
Understanding the dietary habits and metabolic responses of brown trout is crucial for effective conservation and management strategies. By providing hatchery-reared trout with a transitional diet of live prey fish, managers can improve their foraging skills and increase their chances of survival in the wild. Additionally, considering the influence of environmental factors, community structure, and genetic differences on brown trout diets can help inform habitat restoration efforts and stocking programs.
Conserving and restoring riverine habitats and spawning areas is essential for maintaining healthy brown trout populations. Protecting the genetic diversity of brown trout populations is also crucial for ensuring their long-term survival in the face of environmental changes.