Hey fellow anglers! Ever wonder why some of the best fishing for species like salmon, striped bass, or shad happens at certain times of the month? It’s not just luck. Many of these species are anadromous fish—fish that spend most of their lives in the saltwater ocean but migrate into freshwater rivers to spawn. And one of the biggest forces guiding this incredible journey is the Moon. Its gravitational pull influences tides, currents, and even the timing of these migration runs. Let’s take a closer look at how the Moon plays a silent but powerful role in when and how anadromous fish make their way upriver.
What are “Anadromous Fish” Anyway?
First things first, what’s an anadromous fish? It’s a fancy scientific word for a very cool type of fish.
“Anadromous” means “upward running.”
These fish live most of their adult lives in the saltwater ocean.
But when it’s time to have babies, they make an incredible journey upstream into freshwater rivers and streams to spawn.
Think of it like us going on a road trip from our home to a special birthing center! Famous examples include Atlantic and Pacific Salmon, Striped Bass, Shad, River Herring, and Sturgeon.
The Moon’s Invisible Hand: Tides, Currents, and Cues
So, how does the Moon, millions of miles away, tell a fish it’s time to move? It all comes down to tides and currents.
The Moon’s Gravity = Tides: The Moon’s gravity pulls on the Earth, especially on our oceans. This pull creates the daily rise and fall of ocean water we call tides.
Tides Create Currents: As water moves in and out with the tides, it creates strong currents, especially in river mouths and estuaries (where rivers meet the sea).
Anadromous fish use these tidal cues like a highway system, telling them when to start their journey and even helping them conserve energy!
The Power of Moon Phases: Spring Tides and Neap Tides
The Moon doesn’t just create tides; its phases change how strong those tides are. This is where it gets really interesting for fish migration.
New Moon & Full Moon = Spring Tides (Strongest Tides!)
When the Sun, Earth, and Moon line up perfectly (during a New Moon and a Full Moon), their combined gravity creates extra-strong pulls.
This means higher high tides and lower low tides.
These powerful tides create stronger currents pushing water into river mouths.
Why it matters for fish: Many anadromous fish, especially species like Shad and Striped Bass, use these strong currents as a “push” to help them swim upstream. The higher water levels also give them easier access over obstacles and into shallower spawning grounds.
Angler’s Tip: Often, the days leading up to and right after the New and Full Moons can be prime times for observing big pushes of migrating fish!
Quarter Moons = Neap Tides (Weakest Tides)
When the Sun and Moon are at right angles to each other (during the First Quarter and Last Quarter Moons), their gravitational pulls work against each other.
This results in weaker tides – lower high tides and higher low tides.
The currents are less intense.
Why it matters for fish: During neap tides, migrating fish might slow down their journey or hold in deeper pools, waiting for the stronger spring tides to return. Some species might prefer these calmer periods for certain parts of their migration, but often the big pushes are tied to stronger tides.
Angler’s Tip: Fishing might be a bit slower during neap tides for actively migrating fish, but it can be a good time to target fish that are resting or feeding in specific spots.
More Than Just Tides: Other Lunar Cues
While tides are the biggest factor, the Moon might also provide other signals:
Moonlight for Night Travel: For fish that migrate at night, the amount of moonlight can be a factor. A bright full moon might offer more visibility for some, while a new moon’s darkness might provide cover for others.
Biological Clocks: Fish have internal “clocks” (like ours that tell us when to sleep and wake up) that are incredibly sensitive. These clocks might be “set” or adjusted by the subtle changes in the Moon’s light or gravitational pull over time, helping them prepare for their big journey.
Case Studies: Fish and the Moon
Salmon: Many salmon species time their river entries with higher tides, which help them navigate river mouths and overcome rapids. The increased water flow during spring tides can be crucial for reaching upstream spawning grounds.
Striped Bass: “Stripers” are highly tidal. They often use incoming tides to move into estuaries and up rivers to feed and spawn. Big schools can be observed pushing upriver on strong spring tides.
Shad: American and Hickory Shad are famous for their strong spring runs. These fish also heavily rely on strong tidal pushes from the ocean to enter and ascend rivers, especially during the spring full and new moons.
Put It to Practice: Your Angler’s Advantage!
Understanding the Moon’s influence gives you a powerful edge:
- Check the Moon Calendar: Before your next trip, look up the Moon phase. If it’s near a New or Full Moon, prepare for potentially strong currents and active fish runs.
- Target Spring Tides: If you’re chasing migrating anadromous fish in tidal waters, aim your efforts around the stronger spring tides (New and Full Moons). This is often when the biggest pushes of fish occur.
- Watch the Water Movement: Pay attention to how the tide is flowing. Fish often move with the strongest part of the current, especially in river mouths. An incoming tide can bring new fish into your area.
- Observe Local Runs: Every river system is a little different. Keep a fishing log! Note the Moon phase, tide, weather, and your success. Over time, you’ll start to see patterns specific to your favorite fishing spots.
Osmoregulation: How Anadromous Fish Survive the Journey
One of the wildest things about anadromous fish is that they can move between saltwater and freshwater — something most fish cannot survive. This superpower comes down to something called osmoregulation, which is just a fancy word for how fish control the balance of salt and water inside their bodies.
Here’s the simple breakdown:
- In the Ocean: Saltwater pulls water out of a fish’s body.
So anadromous fish drink lots of seawater and use their gills and kidneys to actively remove extra salt. - In Freshwater: Freshwater pushes water into their bodies.
So the fish stop drinking, and instead their kidneys flush out excess water while their gills help keep important salts inside.
Their bodies literally reprogram themselves during migration.
This switch isn’t random — it’s triggered by:
- Changing day length
- Water temperature
- And yes… the Moon-timed migration window
Their internal biology prepares them just before they enter the river, like changing gear before climbing a hill.
Why this matters:
This ability allows salmon, shad, striped bass, and others to:
- Grow big in the nutrient-rich ocean
- Then return to protected freshwater environments to spawn safely
It’s one of the most incredible survival strategies in the natural world — a built-in life cycle upgrade powered by evolution, instinct, and environmental cues.
Beyond the Fish: Why This Matters for Ecosystems
The Moon’s role in fish migration is more than just a cool fishing fact. It’s vital for entire ecosystems.
Food for Wildlife: When fish like salmon or herring run upstream, they become a crucial food source for bears, eagles, ospreys, and other animals. Without these migrations, many animals would struggle to find enough food.
Nutrient Cycling: The bodies of fish that spawn and then die in freshwater rivers release valuable nutrients back into the ecosystem, feeding insects, plants, and other life forms.
Healthy Rivers: Strong, healthy runs of anadromous fish are a sign of a clean, connected river system. Protecting these fish and their ability to migrate is super important for the health of our planet.
The Takeaway
The next time you’re out on the water, take a moment to look up at the Moon (or imagine it there during the day!). Remember that its silent, distant pull is orchestrating one of nature’s most incredible journeys beneath the waves. By understanding this lunar connection, you not only become a smarter fisherman but also gain a deeper appreciation for the amazing, interconnected world we live in.