Migration in Fishes – Causes, Types, Methods, Significance

What is Fish migration?

• Fish migration is the large-scale, often cyclical movement of fish from one area or body of water to another. This behavior is a natural phenomenon driven by various biological needs, including feeding, reproduction, and in some cases, to escape unfavorable environmental conditions. The distances fish travel during migration can vary greatly, from a few meters to thousands of kilometers, and the timing can range from daily to annually, depending on the species and the purpose of the movement.
• One well-known type of migration is anadromous migration, where adult fish live primarily in the ocean and migrate to freshwater habitats to spawn. Salmon and striped bass are examples of anadromous fish. In contrast, catadromous migration involves fish that primarily live in freshwater but migrate to the ocean to reproduce. Freshwater eels, like the European and American eel, are classic examples of catadromous species.
• Many fish, especially those in marine environments, migrate between their feeding, spawning, and nursery grounds. These movements are often tied to ocean currents, food availability, and seasonal changes. For example, tuna species follow temperature gradients, moving north or south at different times of the year. The ability of fish like the bull shark to move freely between saltwater and freshwater, or vertical migrations where fish move between surface and deeper waters depending on the time of day, are also important aspects of migration.
• Some fish species are considered highly migratory, such as large pelagic fish that travel across vast oceanic regions, often crossing international boundaries. This classification, under international treaties like the United Nations Convention on the Law of the Sea, emphasizes the importance of understanding migratory patterns for both ecological and commercial reasons.
• Migration is critical for the survival of many fish species, as it allows them to exploit different habitats for specific needs like feeding, reproduction, and shelter. However, human activities such as dam construction can disrupt these traditional migratory routes, particularly for species that rely on moving upstream in rivers to spawn. These disruptions pose significant challenges for the conservation of migratory fish species and the ecosystems they support.

List of Migratory Fishes

Many species of fish are known to migrate between habitats for various biological reasons such as feeding, spawning, or escaping harsh environmental conditions. These migratory behaviors vary greatly among species and often play a crucial role in the life cycles of fish. Below is a list of notable migratory fish along with their scientific names:

• Lampreys – Petromyzon marinus, Entesphenus, Lethenteron, Ceotria, Mondacia
• Eel – Indian longfin eel (Anguilla bengalensis bengalensis), Shortfin eel (Anguilla bicolor bicolor), Common freshwater or European eel (Anguilla anguilla, A. vulgaris), American eel (Anguilla rostrata)
• Hilsa shad – Hilsa (Tenualosa) ilisha
• Toli shad – Hilsa (Tenualosa) toli
• Herrings – Clupea
• Salmon – Salmo salar, Oncorhynchus
• Sturgeon – Acipenser
• Tuna – Thunnus
• Mackerel – Scomber

Causes for Fish Migration

According to various researchers, fish mainly migrate for specific reasons that can be categorized into different types. Below is a detailed list of causes of fish migration:

• Gamecit Migration (Spawning or Breeding Migration):
  • Fish migrate to specific breeding grounds to ensure the survival and proper development of their eggs and larvae.
  • This type of migration is often seen in anadromous species, where adult fish move from the sea to freshwater to spawn.
  • Example: Hilsa ilisha migrates upstream for spawning.
  • Catadromous species, such as eels, migrate from freshwater to the sea for reproduction, although this type of migration is less common.
• Alimental Migration (Feeding Migration):
  • Fish migrate in search of food, often moving to areas with abundant food sources during specific seasons or times of their life cycle.
  • This type of migration helps fish exploit rich feeding grounds, which supports their growth and energy needs.
  • Examples: Chanos and Harpodon species are known for their alimental migrations.
• Climatic Migration (Wintering Migration):
  • Fish migrate to find more suitable climatic conditions, especially to avoid extreme weather conditions such as cold winters.
  • This migration ensures that fish can survive harsh environmental conditions by relocating to regions with more favorable temperatures and environmental stability.
  • Examples: Sturgeons and Atlantic salmons are known for their wintering migrations to escape colder waters.
• Osmoregulatory or Protective Migration (For Water and Mineral Balance):
  • This migration occurs to maintain osmotic balance in fish, especially when they move between environments with different salinities, such as freshwater and saltwater.
  • The movement ensures that fish can regulate their internal water and mineral concentrations effectively, ensuring survival in varying aquatic environments.
  • This type of migration also serves as protection against unfavorable environmental changes that could disrupt the fish’s ability to maintain homeostasis.
  • Example: Fish that migrate between fresh and saltwater environments, like certain species of eels, undergo osmoregulatory migration.

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.

Methods of fish migration

• Overview of Migration Methods: Fish employ diverse strategies for migration, enabling them to traverse different habitats in search of food, spawning grounds, or suitable living conditions. These methods can be influenced by environmental factors, species-specific adaptations, and ecological requirements.
• Methods of Fish Migration:
  • Drifting:
    • In this method, fish are passively carried by water currents without actively swimming.
    • Drifting can occur in various aquatic environments, including rivers, lakes, and oceans, where currents facilitate the movement of fish from one location to another.
    • This method is particularly beneficial for juvenile fish and species that inhabit open waters, allowing them to travel significant distances with minimal energy expenditure.
  • Random Swimming:
    • This migratory strategy involves fish swimming in various directions after being released from a specific point in a homogeneous environment.
    • The movement leads to a phenomenon known as dispersal, wherein fish spread out in all directions.
    • This method promotes uniform distribution of species within a habitat, ensuring genetic diversity and reducing competition for resources.
  • Oriental Swimming Movement:
    • This method entails fish swimming in a specific direction, which can be characterized in two ways:
      • Directional Movement: Fish may swim either toward or away from a particular habitat or stimulus.
      • Angled Movement: Fish may also move at an angle to an imaginary line that connects them to the source of stimulation, such as food or spawning sites.
    • This method reflects an adaptive response to environmental cues, allowing fish to optimize their migratory routes based on external stimuli.

Types fish migration based on needs

Below are the main types of fish migrations based on their needs:

1. Alimentary or Feeding Migration: This type of migration occurs when fish move in search of new feeding grounds, usually when local food resources become scarce or depleted. The migration ensures that fish can find sufficient nourishment to maintain their energy and growth needs.
2. Gametic or Spawning Migration: Fish migrate to specific locations during the breeding season to find suitable spawning grounds. This type of migration is critical for reproductive success, ensuring that fish can lay eggs or spawn in environments that provide optimal conditions for the survival of offspring.
3. Climatic or Seasonal Migration: In response to changing environmental conditions, such as temperature fluctuations, fish engage in climatic migration. They move to areas with more favorable climatic conditions, which may involve traveling to warmer waters during colder months or cooler areas in hot seasons.
4. Osmo-Regulatory Migration: This type of migration helps fish maintain water and electrolyte balance. Fish may migrate between freshwater and saltwater environments to regulate their internal osmoregulatory systems, which are essential for their survival in different water types.
5. Juvenile Migration: Juvenile or larval fish migrate from their spawning grounds to the feeding habitats of their parents. This movement helps young fish access more abundant food sources and ensures that they can grow in a suitable environment before reaching adulthood.

Movement of fishes during the migration

The following are key types of movement during fish migration:

• Drifting Movement: This type of movement is passive, where fish are carried along by water currents. Drifting is often used by larval or small fish that are not strong swimmers. It allows them to conserve energy while being transported to different locations, often to areas rich in food or favorable environmental conditions.
• Dispersal Movement: Dispersal movement refers to the random, non-directional movement of fish from one habitat to various other locations. This type of movement allows fish to explore diverse environments and expand their range. It helps in the colonization of new habitats, especially when resources in their current habitat become scarce.
• Swimming Movement: This movement is a more active, oriented motion in which fish swim either toward or away from a specific stimulus, such as food, light, or temperature changes. Swimming movement enables fish to navigate efficiently, responding to environmental cues that may signal the presence of feeding grounds, predators, or mates.
• Denatant and Contranatant Movement: These are active swimming movements based on the direction of the water current. Denatant movement involves fish swimming with the water current, which may assist in covering long distances with less energy expenditure. On the other hand, contranatant movement is when fish swim against the current, often during migrations upstream to reach spawning grounds, such as in the case of salmon. Both types of movement are vital for navigating various aquatic environments.

Types of fish migration

Fish migration is a regular, innate behavior that allows fish to travel between habitats to meet various biological needs. Migration can be driven by environmental changes, feeding, or reproduction. The types of fish migration are classified based on the habitats involved and the specific needs driving the movement. Here are the key types of fish migration:

• Diadromous Migration: This is the movement of fish between marine and freshwater environments, a unique behavior that allows some species to thrive in both. It is further divided into:
  1. Anadromous Migration: In this type, fish migrate from the sea to freshwater habitats, primarily for spawning. These fish spend most of their lives in the ocean but migrate to rivers during breeding season. Examples include salmon, which undergo physiological changes like color alteration and gonadal maturation during their journey to the spawning grounds. After spawning, the juvenile fish, known as alevins, hatch and eventually return to the sea to grow into adults.
  2. Catadromous Migration: In contrast, this type involves freshwater fish migrating from rivers to the sea for spawning. An example is the eel (Anguilla spp.), which crosses the Atlantic Ocean from European and American rivers to the Sargasso Sea. Before migration, eels accumulate fat, change color, and experience physiological adaptations like shrinking digestive tracts and enlarged eyes. After spawning in the sea, the adult eels die, and the larvae return to freshwater rivers.
• Potamodromous Migration: This is a freshwater migration where fish move within river systems or lakes for spawning or feeding. Fish like carps and catfish exhibit this type of movement to locate more suitable environments within freshwater bodies.
• Oceanodromous Migration: This type occurs entirely within the sea, where marine fish migrate over long distances to locate favorable feeding or spawning grounds. Species like tuna and herring (Clupea) are examples of oceanodromous migrants.
• Latitudinal Migration: Fish migrate north or south based on seasonal changes, often in response to temperature shifts. Swordfish, for example, migrate north in the spring and south in the autumn, adjusting their location according to climate conditions.
• Vertical Migration: This is a daily migration where fish move between different depths in the water column. For example, swordfish often move to deeper waters during the day for feeding and rise to the surface at night. This vertical movement helps them find food and avoid predators.
• Shoreward Migration: This is a temporary movement from water to land, typically seen in species like the eel, which migrate across moist grass to move from one pond to another. Although rare and temporary, this migration type showcases the adaptability of some fish species to diverse environments.

Advantages of Migrations

Below are the advantages of migration:

• Separation of Spawning, Nursery, and Feeding Grounds: Migration allows fish to divide their life cycle across different habitats. By using separate areas for spawning, nursery, and feeding, fish reduce competition for food and space between adult and juvenile members of the population. This spatial division helps ensure that both young and mature individuals can thrive without depleting limited resources.
• Optimizing Environmental Conditions for Reproduction: Fish often return to their natal spawning grounds because these areas offer favorable environmental conditions that they themselves survived during early development. This behavior increases the chances of egg and larval survival, leading to a higher number of successful spawners in the next generation.
• Enhanced Reproductive Success: Migration helps fish gather in specific spawning grounds, ensuring that enough males and females arrive at the same location at the right time. This increases the likelihood of successful fertilization and maximizes the reproductive success of the population. Therefore, migration contributes to the reproductive synchronization required for the continuation of species.
• Exploitation of Abundant Resources: By migrating, fish can take advantage of areas that offer plentiful food resources at different stages of their life cycle. Fish can feed in regions with abundant prey while returning to safer or more optimal conditions for reproduction. This cycle of moving between abundant feeding grounds and ideal spawning areas ensures overall population health.
• Improved Egg and Larval Survival: Returning to specific spawning grounds with favorable conditions helps fish ensure that their offspring are born in environments conducive to survival. These conditions might include suitable temperatures, salinity levels, and reduced predation pressures, leading to higher egg and larval survival rates.
• Increased Population Size and Genetic Diversity: Migration contributes to the dispersion of fish across wide areas during their feeding phase, which can lead to genetic mixing and an increase in population size. This distribution helps spread genetic diversity, which is crucial for the long-term adaptability of species to changing environments.

Significance of fish migration

Here are the primary reasons why fish migration is important:

• Finding Suitable Feeding and Spawning Grounds: Migration enables fish to move between different environments in search of more favorable feeding or spawning sites. By relocating to areas with abundant food or appropriate conditions for reproduction, fish can ensure their survival and the successful development of their offspring.
• Protection from Predators: Many fish migrate to avoid areas with high predator concentrations. This helps maintain fish populations by reducing the likelihood of being preyed upon, especially during vulnerable life stages such as spawning or juvenile growth.
• Surviving Extreme Climatic Conditions: Migration is often a response to unfavorable environmental conditions. Fish move to regions with optimal temperature, salinity, or oxygen levels, which is crucial for surviving harsh seasonal changes or extreme climate fluctuations.
• Increasing Genetic Diversity: By migrating over large distances and mixing with other populations, fish can increase genetic diversity. This genetic exchange helps enhance the adaptability of species to changing environmental conditions and reduces the risks associated with inbreeding.
• Adaptation for Survival and Existence: Migration is an adaptive strategy that has evolved to ensure the long-term survival of fish species. Through these movements, fish can take advantage of diverse habitats and resources, allowing them to thrive in different environments and respond to ecological pressures effectively.