Migration in Fishes – Causes, Types, Methods, Significance

What is Fish migration?

Fish migration is the active, regular movement of fish populations from one place to another, often over long distances and various environments. This behavior is critical to the survival of fish and is a function of varied ecological, environmental, and biological factors.

Fish migration is specific to different fish species, ranging from small daily movements within one water body to extensive movements between different aquatic ecosystems. The migration is motivated by spawning and feeding, while other stimuli include predator avoidance and refuge seeking. Migration for spawning and feeding is common among many fish species.

Anadromous fish, such as salmon and striped bass, are residents in the marine but move to freshwater to spawn. Similarly, catadromous fish, such as the Anguilla eel, live in freshwater but migrate to the sea to spawn. Synchronization of the spawning movement is essential to ensure that fish reproduce under optimal conditions. Feeding is another motivation for fish migration, with feed mainly the marine forage fish migrating between spawning, feeding, and nursery areas. Migratory movements are predominantly based on oceanic currents and availability of food.

Others involve daily diel vertical migrations when fish move to the water’s surface during the night to feed before returning to deeper layers during the day. The vertical movement minimizes the risk of predation while optimizing opportunities for feeding. Refugeseeking and predator avoidance is another major driver of fish migration, with juvenile fish and species migrating to safer environments. Nevertheless, there is a risk of tradeoff with low food availability when creating predator-free environments. Fish migration also presents other challenges, including natural and artificial barriers.

Dams create barriers that separate feeding and spawning grounds contributing to fish populations’ low completion of mature life cycles. Fragmentation of rivers negatively affects population sizes and could lead to species extinction. It has led to the creation of structures, such as ladders and bypass channels, that facilitate fish movement around these barriers.

Fish migration also affects the ecological interactions and population dynamics of aquatic ecosystems. Movements changes the available resource base and species composition. Proper management of fish migrations is vital in supporting a sustainable fisheries sector as many species are vulnerable to human activities and environmental changes due to their migratory nature.

List of Migratory Fishes

Different migratory fish species based on their life stages and habitats. They can be classified according to the groups of the location of their migration and the time. Here are some examples of fish species and their migratory behavior.

1. Anadromous Fish
   • Definition: These fish spend most of their time at sea but return to freshwater to spawn.
   • Examples:
     • Atlantic Salmon (Salmo salar): They migrate to rivers from the sea to spawn.
     • Sea Trout (Salmo trutta): Essentially the same migrating habits as the salmon.
     • Shad species: Allice shad and Thwaite shad, are shad known to migrate into rivers to breed.
2. Catadromous Fish
   • Definition: These fish live mostly their lives in freshwater but swim to the sea to reproduce.
   • Examples:
     • European Eel (Anguilla anguilla): Spawns in Sargasso Sea and migrates to rivers of Europe in juvenile stage, returning to sea as adults to spawn.
3. Oceanodromous Fish
   • Definition: Fish that migrate, based to ocean distance the fish do migration for consumption and stream.
   • Examples:
     • Herring (Clupea harengus): These fish migrate long distances in the open ocean.
     • Tuna Species (Thunnus) (e.g. Bluefin tuna migratory routes driven by seasonal feeding and spawning needs.
4. Potamodromous Fish – We have specific knowledge on fish species that naturally migrate in freshwater rivers or lakes, usually for feeding or spawning reasons.
   • Examples:
     • Potamodromous fish include carp and catfish, which also migrate within freshwater habitats.
5. Diadromous Fish
   • Definition: A broad category with both anadromous and catadromous fish. These fishes migrate between freshwater and saltwater habitats.
6. Latitudinal Migration
   • Definition: This quasi-migratory pattern moves in a north-south direction based on seasonal changes.
   • Examples:
     • Swordfish travel north in the spring and south in the autumn.

Causes of Migration

Fish move from one place to another for important reasons. They do this to stay alive and to find the best conditions for growing, reproducing, and feeding. Their movements are affected by their body’s needs, the environment, and their life cycle.

• Gametic Migration (Spawning Migration)- Fish travel to breeding areas to help their eggs and baby fish grow properly and survive. This type of migration is primarily driven by the need to find optimal conditions for reproduction.
  • Anadromous Migration- Fish like salmon travel from saltwater to freshwater to spawn.
  • Catadromous Migration- Less common, seen in species like eels, where fish move from freshwater to saltwater for breeding.
• Alimental Migration (Feeding Migration)– Fish move to find food sources, which may be abundant in certain areas at different times of the year. Species like Chanos and Harpodon are fish species that migrate particularly for foraging and meeting their nutritional requirements.
• Climate Migration or Wintering Migration – Fish move to warmer climate regions, especially to escape extreme temperatures. Species such as Sturgeons and Atlantic Salmon move to warmer waters during the cold seasons to experience comfort and survival.
• Osmoregulatory or Protective Migration – This type of migration helps the fish to keep correct water and mineral balance. The migration pathways of fish change to help the control of osmoregulation. Fish move to locations that help them maintain the proper concentrations of salt and water.

Movement of Fishes During Migration

Fish migrate due to different physical and environmental forces. Different types of migration are done for different reasons while others are brought about by varied conditions. These kinds of movements in fish are elaborated below.

• Drifting Movement- A type of drifting movement where a fish goes by drifting with the current instead of swimming actively. This movement helps fish travel long distances while using little energy. They depend on water currents to move them from one place to another.
• Dispersal Movement– This is a random movement where fish go away from one type of habitat to many different areas. This kind of movement does not aim at a destination. Fish swim in all directions without an apparent place to go, mainly due to their environment or search for food or a breeding ground.
• Swimming Movement- A directional movement where fish swim either towards or away from something particular. This movement is active and purposeful. Fish may swim upcurrent to find food, to reach warm water, or to get to areas where they wish to breed. Conversely, they may swim downstream to avoid danger or to leave bad conditions.
• Denatant and Contranatant Movement-
  • Denatant Movement: Fish swim upstream.
  • Contranatant Movement: Fish swim downstream.
  • Both forms of movement are important to the migration mode of fish. Denatant movement conserves energy as fish swim in the current of water, and contranatant movement helps fish swim against the current, allowing them to reach a place, such as where they mate.

Types of migration in fish Based on Needs

Fish migration is driven by various biological needs. Each type of migration serves a specific purpose, whether it’s for food, reproduction, or adapting to environmental conditions. Here’s a breakdown of the main types of fish migration based on their needs:

1. Feeding Migration (Alimentary Migration):
   Fish migrate in search of food when the supply in their current habitat becomes insufficient. The availability of food fluctuates, making migration necessary for survival. This type of migration ensures that fish can grow faster, mature more effectively, and produce more eggs due to the better food sources they encounter.
   Examples: Cyprinids, Salmonids, Percids.
2. Spawning Migration (Gametic Migration):
   Spawning migration is driven by the need to reproduce. Fish travel to specific locations that provide the ideal conditions for laying eggs. These migrations often involve large numbers of fish aggregating in one area to spawn.
   Examples: Salmonids, Osmeridae, Cyprinids, Catostomidae (suckers), Smelt.
3. Climate & Refuge Seeking Migration (Climatic Migration):
   This type of migration occurs in response to extreme climate conditions. Some fish species migrate to avoid harsh environmental factors such as cold temperatures, ice, or unfavorable water conditions.
   Examples: Arctic grayling (Thymallus arcticus), Arctic char (Salvelinus alpinus), European eel (Anguilla anguilla). These species move to more favorable habitats to avoid freezing waters and extreme cold.
4. Contranatant Migration:
   This type of migration involves the movement of fish against the current of water. Fish travel upstream or in the opposite direction of the water flow to reach their destinations, such as breeding grounds or feeding areas.
5. Detenant Migration:
   In contrast to contranatant migration, detenant migration refers to fish moving with the current, traveling downstream or with the flow of water to reach their new habitat or breeding area.
6. Juvenile Migration:
   Juvenile migration refers to the movement of young fish, typically larvae, from their spawning grounds to feeding habitats. This migration is essential for their growth and survival, ensuring they reach areas with better food availability.
7. Osmo-regulatory Migration:
   Some fish migrate to maintain their water and electrolyte balance. These species move between fresh and saltwater to regulate their internal conditions, such as the European eel (Anguilla anguilla), which migrates between the sea and freshwater environments.

Types of migration in fish Based on Direction

Fish migration is not only driven by biological needs but also follows specific directional patterns. These patterns can vary based on the fish species, environmental factors, and the purpose of the migration. Below are the types of fish migration classified by their directional movement:

1. Diadromous Migration:
   This type of migration involves fish moving between freshwater and saltwater. While many fish are restricted to one habitat or the other, some species migrate regularly between the two, maintaining a balance of osmotic pressure.
   Diadromous migration can be broken down into two subcategories:
   • Anadromous Migration:
     In this migration pattern, marine fish travel from the sea to freshwater to spawn. These fish spend most of their lives in the ocean and only migrate to rivers during the breeding season.
     Examples: Salmon, Hisla, Lamprey.
     Characteristics:
     • Fish stop feeding during their migration.
     • Their bodies change color from silver to a dull reddish-brown.
     • Gonads mature, preparing them for spawning.
     • A saucer-like nest is made for the female to lay eggs, which are fertilized by the male. The larvae, known as Alevins, eventually develop into Parr and then mature into adults before returning to the sea.
   • Catadromous Migration:
     In this pattern, freshwater fish migrate to the sea to spawn. Fish like the European eel and American eel travel long distances from rivers to the Sargasso Sea for reproduction.
     Examples: European eel (Anguilla anguilla), American eel (Anguilla rostrata).
     Characteristics:
     • Fish deposit fat reserves to sustain them during the journey.
     • Their bodies undergo a color change from yellow to silvery gray.
     • Digestive systems shrink and feeding stops.
     • Sensory organs, including eyes, become more sensitive.
     • Gonads mature for reproduction.
     • After spawning, the adult eels die, and the larvae return to freshwater habitats.
2. Potamodromous Migration:
   This migration occurs entirely within freshwater systems. Fish migrate between different freshwater habitats for feeding or spawning purposes.
   Examples: Carps, catfish.
3. Oceanodromous Migration:
   Oceanodromous migration refers to the movement of fish within the sea, primarily in search of suitable feeding or spawning grounds. These migrations happen entirely in marine environments without crossing into freshwater habitats.
   Examples: Clupea (herrings), Thunnus (tuna).
4. Latitudinal Migration:
   Fish that engage in latitudinal migration move between the northern and southern parts of the ocean, typically in response to climate changes. This type of migration is driven by temperature shifts, with fish moving to find better feeding or spawning conditions.
   Examples: Swordfish, which migrate north in the spring and south in the autumn.
5. Vertical Migration:
   Vertical migration involves daily movements of fish between the surface and deeper waters. This migration pattern is driven by the need for food, protection, or spawning. Fish often migrate from the deep, where it’s safer, to the surface to feed.
   Examples: Swordfish, which move vertically to greater depths in search of food.
6. Shoreward Migration:
   Shoreward migration occurs when fish move from water to land, but it is a temporary migration. These movements are typically short-distance migrations, often linked to spawning or other environmental factors.
   Examples: Eels, which may migrate from one pond to another across moist meadows.

Orientation and Navigation During Migration

Fish have sophisticated methods of navigation, which help in the long migrations and accurate homing. It is all formed by environmental stimuli, sensory capabilities, and biologic characteristics.

• Homing Behaviour and Natal Stream Return– Salmon exhibit homing behavior wherein they return to their natal streams to breed after migrating out to sea. This is evidence that fish make use of definite cues for orienting and recognizing sites.
• Orientation in Various Ecosystems– With open-water migration, orientation direction is among 360° and thus the task is always challenging. Riverine migrants swim by currents; hence they possess either positive or negative rheotaxis.
• Sensory Mechanisms for Orientation– Various studies have confirmed that fish utilize the sun, polarized light and geomagnetic field to navigate. Strong cues are furnished by polarized light, more so at dawn and dusk when patterns appear clear.
• Magnetic Imprinting and Migration– Research demonstrated magnetite crystals in fish including rainbow trout and Atlantic salmon that aid in geomagnetic detection. Sockeye salmon imprint magnetic field variations when migrating to the sea. They use magnetic intensity changes in order to find coastal sites upon returning.
• Olfactory Cues in Stream Identification– Olfactory imprinting suggests juvenile fish imprint on chemical compounds unique to their natal streams, derived from soil, vegetation, and site-specific odors. During migration, adult fish recognize these “stream bouquets” to locate their natal streams.
• Pheromone Theory in Stream Selection – Migrating adults may follow population-specific pheromones released by younger relatives in natal streams. In lampreys, bile acid pheromones guide adults to spawning streams, although they do not return to their natal stream.
• Challenges in Open-Water Navigation Debate continues on fish using a “map and compass” for navigation due to their imprecise biological clock. Their keen sense of smell is crucial for navigation and homing.

Fish navigate using environmental and sensory cues, adapting to varied aquatic challenges. These mechanisms highlight their capability for migration and population continuity.

Mechanisms of Migration

Fish migration is a process that requires the species to shift habitats, meaning they need changes in behavior, physiology, and morphology for survival.

• Morphological Adaptations– Streamlined bodies reduce drag, which enables fish to swim for longer distances. Sockeye and brook trout have their bodies change for better energy efficiency during migration. In the salmonids, smoltification results in a streamlined body and smaller pectoral fins to improve migration. Long-distance migrators, including Fraser River sockeye, have more fusiform bodies with shorter lengths to reduce transportation costs. All these changes support improved performance and survival on various migratory routes.
• Osmoregulation During Migration– Diadromous species such as the Atlantic salmon exhibit physiological changes in response to differences in salinity that cause osmotic stress. Osmoregulation takes place in gills, kidneys, gut, skin, and bladder, although most changes happen prior to the transfer between habitats.
• Salmonids are adapted to enable them to have higher euryhalinity; they can tolerate a range of salinities. With these adaptations, the fish survives in an aquatic environment of diverse salinities.
• Long-Distance Migration and Species Variations– Fishes of the Amazon and Mekong Rivers are migratory and move long distances. These species are adapted to many different aquatic habitats, ranging from fresh to saline. The Brachyplatystoma flavicans and Prochilodus lineatus are examples of migratory fish, traveling long distances for breeding. Some of the potamodromous species are the Pangasius krempfi in the Mekong River, showing the influence of the region on the migration of the fish.

Factors Influencing Migration

The success, route, and timing of fish migrations are influenced by many factors, whether physical or biological. These may either facilitate or hinder migration in species and given environmental conditions.

• Physical Factors
  • Temperature is the main factor in fish migration. Warm seawater triggers the migration of salmon. In freshwater, fish move upstream with rising temperatures, typically for spawning.
  • Light Intensity and Photoperiod: Daylight influences migration. When the days start getting longer, fish change their migratory patterns. Among them, the Pacific salmon and the sea lamprey are very sensitive to light shifts for seasonal migrations.
  • Water Current: The strong currents could either help or hinder the fish. Increased flow during snowmelt can trigger migration. Strong currents may aid in finding habitats or spawning, but may displace eggs or larvae.
  • Turbidity and Depth: Turbidity of high levels in fast waters assist smolts in downstream migration by reducing predators’ visibility. Changes in depth influence fish to move to improved habitats depending on the water condition.
• Chemical Factors
  • Salinity influences fish movement. For instance, diadromous species, such as salmon, are greatly affected by the changes in salinity, adapting to them well. On the other hand, freshwater species cannot handle salinity fluctuations and tend to remain within freshwater.
  • pH, odor, and taste: The changes in water chemistry, such as pH, odor, and taste, affect the performance of fish during migration because they try to reach good feeding, spawning, or sheltering sites.
• Biological Factors
  • Food availability: Fish migrate to find food, traveling long distances to suitable feeding areas for survival and growth.
  • Sexual Maturity: Fish migrate to spawn once they have reached sexual maturity.
  • Predators and Competitors: To avoid predators or competitors, fish move to safer areas, thereby reducing risks to their survival.
  • Hormones: The main thrusting force of fish migration is the change in hormones. These hormones propel fish towards breeding grounds at the time of breeding.
• Anthropogenic Impacts
  • Developmental Structures: Migratory ability is hindered by man-made structures like dams as they cause an inability to reach habitats as well as create changed routes resulting in increased mortality. Fish ladders can bypass, but the degree of success can vary by species and strength of the current.
  • Overfishing in migration or spawning areas reduces fish populations and disrupts migration, leading to declines in species that depend on these areas for reproduction or feeding.
  • Habitat Degradation: Pollution from urbanization and agriculture degrades migration habitats. Heavy metals and toxic chemicals can disrupt fish navigation, metabolism, and survival.
  • Climate Change: Climate changes fish migration patterns. Changes in flow, temperature, and habitat may alter patterns. Species will likely be threatened with decreased freshwater supply, sea-level rise, and changing ocean conditions, potentially decreasing survival.

Methods of Migratory Movements

Migratory movements in organisms can follow different patterns, primarily influenced by the environment and the species’ adaptations. These movements can occur in various directions and speeds, often categorized into distinct types based on the organism’s interaction with water currents and environmental factors.

1. Denatant and Cotranatant Movements
   • Denatant: Movement along with the water current.
   • Cotranatant: Movement against the water current.
   • Some species may exhibit a combination of both types, depending on environmental factors and their life stages.
2. Drift
   • This refers to passive movement driven by water currents.
   • Commonly observed in larvae, drift allows them to travel without active propulsion.
   • Rarely seen in adult forms of species.
3. Swimming
   • Swimming can be categorized into two types:
     • Random Locomotory Movement:
       • Movement in random directions, which may result in the scattering or aggregation of species.
       • This movement often depends on environmental factors like light or temperature gradients, influencing the direction and extent of the migration.
     • Orientated Locomotory Movement:
       • Movement directed by specific stimuli, such as environmental changes or the presence of certain cues.
       • This results in the species migrating toward or away from the source of the stimulus.
       • Speed plays a crucial role in the efficiency and direction of migration.

Each of these methods provides distinct advantages based on environmental conditions and the specific needs of the migrating species. The use of water currents and active swimming mechanisms are critical factors in how organisms execute migratory behavior.

Speed of the Fish

The speed at which fish migrate is determined by a combination of their physiological capabilities and environmental factors. Different species of fish exhibit various maximum and sustainable speeds, each suited to their migratory patterns and survival needs.

• Maximum Speed
  • The maximum speed a fish can reach is roughly ten times its body length per second.
  • This speed is not sustainable for long periods; fish can only maintain it for about one minute.
  • After reaching their peak speed, fish need to slow down to recover stamina before they can reach maximum speed again.
• Maximum Sustainable Speed
  • This refers to the speed that a fish can maintain over extended periods.
  • It is about three times the fish’s body length per second.
  • For example:
    • A 25 cm herring can sustain a speed of 75 cm per second (25 x 3).
    • An 80 cm cod can maintain a speed of 240 cm per second (80 x 3).
• Migratory Species and Their Speed
  • Some fish species, like salmon, cod, and eel, exhibit long-distance migration.
  • These species breed in one location but migrate to different areas for feeding and growth.
  • Migration distances can be substantial, sometimes exceeding 700 miles between feeding and spawning grounds.

What is a Migratory Trigger?

Migratory triggers are crucial environmental and biological factors that initiate the migration process in various species, particularly in fish. Understanding these triggers can enhance our comprehension of ecological dynamics, fish behavior, and conservation strategies. Migration is a complex phenomenon that encompasses various stimuli, which can be broadly categorized into extrinsic and intrinsic triggers. Both categories play vital roles in signaling fish to migrate, influencing their movement patterns significantly.

• Definition and Importance: A migratory trigger is any external or internal factor that prompts a species to migrate, similar to how a trigger on a firearm initiates action. In the context of fish, these triggers can be environmental conditions or biological signals.
• Extrinsic Triggers: These are external environmental factors that can initiate migration. The interaction between various extrinsic triggers complicates the study of migration, as multiple factors often correlate, making it challenging to isolate a single trigger.
  • Water Level, Current, and Discharge:
    • Changes in water levels, currents, and discharge often signal fish to migrate, particularly during seasonal changes such as the monsoon.
    • For instance, 26 different fish species have been identified that migrate in response to changes in water levels and current speeds.
    • Examples include species like Bangana behri and Cyprinus carpio.
  • Precipitation:
    • In tropical regions, the onset of the rainy season leads to elevated water levels, triggering reproductive migrations.
    • Certain species, such as Tenualosa thibaudeaui and Barbonymus gonionotus, initiate upstream migrations following the first rainfall of the season.
  • Lunar Cycle:
    • Some fish species exhibit migration patterns linked to lunar phases, which may be perceived through tidal effects or visual cues.
    • Notable examples include eels that migrate downstream during new moons and several other species that respond to full moon cycles.
  • Insect Apparition:
    • The emergence of insects, such as mayflies and dragonflies, often coincides with fish migration.
    • Fish may surface to feed on these insects, establishing a connection between food availability and migratory behavior, seen in species like Pangasius pleurotaenia.
  • Turbidity and Water Color:
    • Changes in water clarity and color can also serve as migratory triggers for fish like Tenualosa thibaudeaui.
  • Photoperiod:
    • Light intensity and duration are critical for synchronizing migration activities among fish species.
    • For example, Pacific salmon and lampreys depend on photoperiod cues to time their spawning migrations effectively.
  • Temperature:
    • Fish respond to temperature changes in two primary ways: seeking cooler habitats during extreme heat or migrating when temperature conditions shift to support growth or reproductive needs.
    • An example is the sea lamprey, which migrates when stream temperatures exceed a specific threshold.
  • Oxygen Concentration:
    • Low dissolved oxygen levels can trigger migration, particularly in centrarchid species and others facing hypoxic conditions.
  • Fish Density:
    • A minimum population density may be necessary to initiate migration, as fish often gather in sufficient numbers before moving.
  • Food Availability:
    • The search for food is a significant motivator for migration, prompting large-scale movements toward nutrient-rich areas.
• Intrinsic Triggers: These are internal biological factors that drive migration behaviors.
  • Instinct:
    • Migration can be an innate behavior, encoded in the genetic makeup of certain species, influencing their migratory patterns from birth.
  • Physiological Factors:
    • Key hormonal changes play a pivotal role in migration, particularly in relation to reproduction.
    • The hypothalamo-pituitary-gonadal (HPG) axis regulates gonadal maturation, facilitating long-distance migrations, such as those observed in salmon.
  • Hypothalamo-Pituitary-Thyroidal (HPT) Axis:
    • Hormones related to olfactory learning and homing behavior significantly influence migration.
    • During critical developmental phases, such as Parr-smolt transformation, juvenile salmon imprint on environmental odors that guide their migratory paths later in life.
  • Growth Factors and Hormones:
    • Insulin-like Growth Factor-1 (IGF-1), prolactin, and growth hormone are hormones that support maturation and migration readiness, with significant increases in these hormones noted as fish approach spawning grounds.

Advantages of Migration in Fish

Fish migration is of great importance from ecological, biological, and economical points of view. It enables survival, reproduction, and adaptation to new habitats as a result of movement, a factor that affects healthy fish population and ecosystems as well.

• It increases feeding opportunity.- Migrating fish seek areas of rich food. Through migration to nutrient-rich waters, they access seasonal food, which is very vital for growth and survival at various life stages.
• Improved Reproductive Success – Fish migration plays a significant role in reproduction. The salmon species moves to freshwater breeding sites, which have conditions favorable for egg laying and for the survival of their offspring.
• Genetic Diversity – Fish migration results in the increase of genetic variability by mixing two populations, essential for their proper health and survival. This variance helps in adjusting environmental changes, minimizing the risk of inbreeding, and amplifying resilience.
• Adaptability to Environmental Variations– Migration helps fish adjust to changing environments, such as temperature changes or habitat loss. As conditions deteriorate, migratory fish can move to better habitats, which would help them survive in the face of climate change or habitat loss.
• Fewer Competitors– Migration decreases competition due to the movement of fish towards areas where there is minimal competition in terms of food or space, hence reducing overcrowding and promoting healthy populations and efficient resource utilization.
• Wider Habitat Range- Migration through diverse environments allows fish to occupy more habitats, thus helping species populations and maintaining balance across ecosystems.
• Cycling Nutrients– Migratory fish play a very important role in the cycling of nutrients in aquatic ecosystems. These fish transfer nutrients and organic matter across habitats and thus support other organisms’ health and increase productivity in the ecosystem.
• Ecosystem Health– Migratory fish indicate a healthy ecosystem. These keep the food webs going since they serve as food for other, larger predators. In addition to these functions, the dynamics in the ecosystem balance, because it enhances nutrient flow and recycling, hence stabilizing it.
• Economic benefits– It is very important for commercial fishing that these fish migrate. Such species as tuna and herring provide a certain level of stability in income to fishermen during a particular season. These migrations create sources of income for many local economies, which sustain the dependent fisheries.

Disadvantages of Migration in Fish

• Higher risks of mortality– The migratory fish are more exposed to predators. Thus, there is a likelihood of them getting killed. Their populations reduce due to easy hunting during their transit through different habitats.
• Energy Expenditure – The migrating fish has to travel long distances, which is very energy-consuming. Inadequate food can also exhaust them which may inhibit further successful reproduction after travel.
• Obstacles of Migration– The most significant impacts occur from human infrastructure such as dams and weirs that block fish migration routes. It means preventing the spawning and exacerbating the declines of fish populations. Amongst the most impacted species, it is Atlantic salmon whose access to traditional spawning rivers is being curtailed.
• Habitat Degradation- Such habitats of migratory fish are being threatened by pollution, encroaching urbanization, and climatic change. Degraded habitats often lead to poor water quality and food materials, resulting in improper fish migration and poor spawning.
• Loss of Navigational Cues– Environmental cues fish use to migrate depend on temperature and currents. Human activities or climate change may also disorient fish, thus resulting in failed migrations or even reduced survival rates.
• Cannibalism and Competition– Migration can cause fish to run into their larvae or other conspecifics in crowded patches, resulting in increased competition for food or even cannibalization of smaller fish. Genetic Isolation
• Genetic Isolation– Genetic diversity requires migration. In its absence, the fish populations get isolated, and hence the ability to exchange genetics is reduced. The risk of diseases and less adaptability towards environmental changes also increases.

Significance of migration in fishes​

This movement is crucial to the survival and success of the aquatic species for population continuity and ecosystem health.

• Finding Food and Spawn Sites– Migrating fish seek the areas with rich food supplies to grow better. The spawning migrations make sure that the fish have an opportunity to access ideal habitats where eggs can be laid and young are nurtured.
• Protection from Predators – Migration helps fish evade high predator zones, especially during vulnerable life stages. Some species move to less dangerous areas to boost offspring survival.
• Surviving Extreme Climatic Conditions– Movement between habitats allows fish to dodge extreme conditions like high temperatures or low oxygen. Seasonal migrations correspond with water temperature changes, keeping fish in livable areas.
• Enhancing Growth and Reproductive Success– Moving to environments more favorable in terms of resource availability enhances the growth of the fish. Reproduction will be successful if the fish spawned when the environment for eggs and the juveniles was favorable.
• Adaptive Traits for Survival– This is a sort of adaptation which has helped the fish to cope with the environment. This mechanism of movement supports the resilience of the species over generations.
• Promoting Genetic Diversity– Interbreeding across large distances encourages genetic diversity. Genetic diversity boosts populations’ resistance to diseases and environmental shifts.