Marine fish visitors to freshwater springs

It is not uncommon to see different species of marine fish in many of Florida's freshwater springs. Many visit for days, weeks or even an entire season! Some seem attracted to the warmer spring water when the marine waters and surface fresh waters have cooled in winter. It is theorized that others can shake certain parasites by moving into a different salinity environment, or that habitats of differing salinity offer new or more food, or escape from predator. Some fish, like sturgeon & shad must visit freshwater environments to complete their life cycle.

Springs nearer the ocean (eg Homosassa Springs and Hospital Hole Springs, both on separate river systems but only 5 miles from the Gulf of Mexico) are more often visited by marine fish. On any given dive in Homosassa Springs, you are likely to see schools of gray snapper, tarpon, jacks, snooks, sheepshead and pipefish.

Gray Snapper Lutjanus griseus at Hospital Hole

Sheepshead Archosargus probatocephalus at Hospital Hole

Spring-influenced rivers, like the Suwanee River (with more than 300 freshwater springs!), boasts more marine visitors, and these visitors get further upstream. Common marine visitors spotted in the freshwater springs and river runs off the Suwanee River include striped mullet, Southern flounder, Gray Snapper, Atlantic Needlefish and Gulf Pipefish.

Rainbow River is another example of a spring fed river, that has lots of marine fish enjoying the clear, 72 degree springs water....

Shad at Rainbow River

Some marine fish access freshwater springs by very circuitous routes. The St John's River is primarily a blackwater system, but many of its tributaries are spring runs. American Shad travel 100 miles up the St John's River to the Oklawaha River, which they follow to the Silver River to finally arrive at Silver Springs. Soon after spawning they reverse their journey and travel 100 miles back downstream to the ocean again!

Some fish travel even further to get to the freshwater springs, schools of mullet and tarpon are regularly spotted at Blue Springs, Volusia County, having traveled a long way up along the St. Johns to get there!

Mullet at Blue Springs

Tarpon at Blue Springs

Needlefish make the long journey to Alexander Springs...

Needlefish at Alexander Springs

Hogchoker at Fanning Springs

Diadromous is a term describing fish that spend part of their life cycle in fresh water and another part in marine water in order to complete their life cycle (as oppose to "occasional visitors" mentioned above). Moving into an environment of different salinity causes many problems for fish. Some fish do not have the tools to survive this transition, and simply stay one in particular habitat, these are called a stenohaline creatures (adapted to a narrow range of saltwater concentrations). Fish that move to and from environments of varying salinities are called diadromous or euryhaline creatures (can tolerate a range of saltwater concentrations) show a unique adaption to help them move between environments of different salinities.

All fish survive by striking a precise balance between the amounts of salt and water in their bodies and in the surrounding water. Wherever a fish is, it needs to keep its blood at a constant salinity to survive. That optimum level is actually much less saline than seawater but a lot more saline than freshwater, so all fish need to actively regulate and balance their salt & water levels (this process is called osmoregulation).

Fish that live in a seawater are constantly loosing water from their bodies (the high salt content of the ocean causes water to constantly flow out of their body through the fish's gills). Due to all this water loss, fish need to drink lots of surrounding to stay hydrated, and as the only water available is seawater, this increases their body salinity. Their kidneys are adapted to excrete a small volume of fluid with a high concentration of salt to keep their blood salinity levels at an optimum.

Freshwater fish face the opposite problem; water constantly moves into their bodies, diluting their body salinity level. Their kidneys are adapted to excrete a large volume of dilute urine which is low in salt to keep their blood salinity constant.

Most fish are restricted to one environment (fresh or marine water) because they cannot change the way they osmoregulate, but the diadromous species are able to spend periods of their life in very different salinities because they are able to switch between the two methods of osmoregulation. Although many diadromous fish can tolerate rapid extreme changes in salinity, most require a gradual shift from one environment to the other to allow their bodies to make the necessary adjustments.

Diadromous fish can be separated into:

* Catadromous fish are born or hatch in marine water but migrate to freshwater areas where they spend the majority of their lives growing and maturing, and as adults they return to the sea to spawn. The American eel Anguilla rostrata is a typical species occurring in north Florida that exhibits this life history pattern.

American eels Anguilla rostrata, at Vortex Springs

American eels begin their lives as eggs, hatching in the middle of the Sargasso Sea. The tiny larvae are then washed by ocean currents towards the coast of the United States, where they migrate inland to freshwater lakes, rivers and springs. This journey can take many years, with some eels traveling more than 3,500 miles! American eels spend their adult lives in freshwater, then return to the Sargasso Sea to spawn. They do not eat while they are at sea, and die once their life cycle is complete.

* Anadromous fish, those that are born or hatch in freshwater, migrate to the ocean to grow up and then return to freshwater to spawn and complete their lifecycle. Striped bass Morone saxitilis and several species of shad Alosa spp are species in Florida that exhibit this life history.

Diadromous fish comprise less than 1% of worlds fish fauna, but their value far exceeds this portion. Many diadromous fishes such as salmons, sturgeons and shads are not only economically important, but they also serve as crucial links for energy flow between fresh and marine environments.

Unfortunately, like so many other fish populations, many diadromous fish populations have decreased significantly over the last few decades. Most species were heavily exploited before major industrialization and the physical alteration of many waterways; but continued watershed alterations due to human population expansion and climate change have further increased their habitat loss.

Like many of their close relatives, these travelling fish need our help to survive in an ever changing world.....