Fish undertake remarkable journeys, often traveling vast distances across oceans, rivers, and lakes. This mass relocation, known as migration, is a regular phenomenon for many species, occurring daily, annually, or over longer cycles. Distances covered can range from a few meters to thousands of kilometers. This behavior is fundamental for the survival of numerous aquatic species.
The Reasons for Fish Migration
Fish migrations are primarily driven by the needs for survival and reproduction. Reproduction, specifically spawning, is a common reason, involving journeys to specific breeding grounds. Salmon, for instance, undertake upstream migrations from the ocean to freshwater rivers where they were born to lay their eggs. While some species, like Pacific salmon, die after this reproductive effort, others, such as Atlantic salmon, return to the sea.
Feeding is another motivation for these movements, as fish relocate to areas offering abundant food sources. This often involves predatory fish following the movements of their prey, such as baitfish. Forage fish also migrate between spawning, feeding, and nursery grounds.
Temperature regulation also prompts migration, with fish seeking optimal water temperatures. As water temperatures change, many fish species shift their distributions, moving toward the poles or deeper waters to find suitable thermal habitats. This helps them avoid conditions that disrupt their physiology and reproductive cycles. Fish also migrate to safer environments to avoid predators, especially during vulnerable life stages. Young salmon, for example, benefit from starting their lives in rivers where they are less exposed to predators found in the open sea.
Classifying Fish Migration Patterns
Fish exhibit migratory patterns, categorized by the environments they traverse. Anadromous fish live primarily in saltwater but migrate to freshwater environments to spawn. Examples include salmon, striped bass, and American shad, which undertake upstream migrations to their natal rivers for reproduction. They spend most of their adult lives in the ocean, returning to freshwater to lay eggs.
In contrast, catadromous fish spend most of their lives in freshwater but migrate to saltwater to spawn. Eels, such as the European and North American eels, exemplify this type of migration, traveling to marine spawning grounds like the Sargasso Sea. Their life cycle is the inverse of anadromous species.
Oceanodromous fish migrate entirely within marine environments. Species like tuna and herring are examples, moving across ocean basins in search of food or suitable breeding grounds. Tuna may migrate annually between northern and southern waters, following temperature gradients. Herring populations exhibit unique migratory behaviors and spawning seasons within the sea.
Potamodromous fish migrate exclusively within freshwater systems. This category includes species like some carp, trout, and the Colorado pikeminnow. These migrations are shorter than those involving transitions between fresh and saltwater, often occurring between different parts of a river system or between lakes and streams for spawning.
Navigating the Waters: How Fish Find Their Way
Fish employ sensory mechanisms to navigate their migratory routes. One method involves olfactory cues, using their sense of smell to detect chemical signatures. Salmon, for example, imprint on the chemical composition of their natal waters as juveniles, later using this “smell map” to locate their spawning grounds as adults. This allows them to return to the rivers where they were born.
Many fish also possess a geomagnetic sense, enabling them to use Earth’s magnetic field as a navigational aid. This sense helps them maintain a consistent direction and determine their location, acting much like an internal compass and map. Salmonids use this magnetic sense for oceanic navigation and for pinpointing their return to natal areas.
Beyond these, fish may also rely on celestial cues, such as the position of the sun or patterns of polarized light, especially during migrations closer to the surface. Water currents also play a role in their navigation. While eggs, larvae, and young fish can drift passively with currents, adult fish often actively swim against the current when moving toward their breeding grounds, demonstrating directional control.
Pressures on Their Journeys
Migratory fish populations globally face pressures, with freshwater migratory species experiencing an average decline of 81% between 1970 and 2020. Habitat alteration and loss are major threats, accounting for about half of the issues impacting these species. Dams and culverts, for instance, block migration routes, preventing fish from reaching spawning and feeding grounds. The fragmentation of rivers by such barriers directly impacts population sustainability by reducing available spawning areas.
Pollution compounds these challenges, as chemical contaminants and plastic debris can impair fish health and disrupt their navigational abilities. Nutrient runoff also degrades water quality, leading to oxygen-depleted “dead zones” inhospitable to aquatic life. These environmental changes force fish to adapt or abandon traditional migration routes.
Climate change affects migratory patterns, primarily through rising water temperatures. Warmer waters can force fish to shift their distributions toward cooler, polar regions or deeper waters, disrupting established ecosystems. Changes in temperature can also misalign the timing of spawning with the availability of food sources, impacting reproductive success. Ocean acidification, from increased carbon dioxide absorption, can impair fish larvae development.
Overfishing is another pressure on migratory fish populations. Unsustainable fishing practices deplete fish stocks, and the removal of older, more experienced fish can disrupt the collective knowledge and migratory patterns within schools. These combined pressures threaten the survival of these species, as well as the food security and livelihoods of millions worldwide.