Charlotte Harbor is the second-largest estuary in Florida. Perhaps the most well-known function of estuaries is their role in the lives of young fish, shrimp and shellfish. Very few marine species spawn in estuaries, but estuaries are used extensively as nursery grounds.
Most fish and crustaceans (crabs, shrimp, etc.) spawn offshore. The eggs are typically planktonic (free-floating). Eggs develop into larvae that depend upon tides and currents to transport them to suitable habitats where they will settle out and grow. Settling young fish and crustaceans utilize a number of different survival strategies, but common to all is a quest to not be eaten.
When ready to settle as juveniles, larvae of redfish, sand seatrout, southern kingfish (usually called whiting) and spot look for low-salinity waters associated with river mouths. Low-salinity waters tend to support fewer predator fish than higher salinity waters. In fact, three times as many piscivores (fish-eaters) are found in lower Charlotte Harbor — where the water is deeper and saltier — than are found in the upper Harbor.
So, if your survival strategy is not to be eaten (and again, that’s a very solid strategy), river mouths associated with the upper Harbor are good places to be — assuming you can tolerate low salinities.
River mouths are also areas of high productivity. These areas concentrate phytoplankton algae, which in turn attract tiny animals such as copepods and mysids. These animals are eaten by small fish and predatory invertebrates such as comb jellies (also called sea walnuts), which are then fed upon by slightly larger fish. For juvenile sportfish, river mouths mean ample food and less predators. Important habitats associated with river mouths include the mangrove shorelines, oyster reefs, riprap and seagrass beds.
Baby snook seek out backcountry areas at land margins, places that are only accessible to us by kayak or across marshes on foot. These areas are very shallow, low in salinity and low in dissolved oxygen — a perfect environment for keeping predatory fish at bay. Juvenile snook have a very high tolerance for low dissolved oxygen, but they will lose that tolerance as they grow. While they are in the backcountry, juvenile snook feed on what’s available, and what’s available is mostly mosquitofish and sailfin mollies.
When snook reach about 5 to 6 inches in length, their oxygen requirements increase and they’re forced to move out to more open waters. They also require deeper water due to their increased size. Snook at this size will hang out at the entrances of creeks for a while, where they are afforded some protection from predators. They are now eating bigger food items, such as juvenile pinfish and pink shrimp. Adult snook have a very diverse diet, but at the top of their prey foods are pinfish, pigfish and swimming crabs.
Not all juvenile fish utilize low-salinity waters to escape being eaten. Juvenile gag, for instance, need stable high-salinity conditions. As such, they settle out in the first suitable habitat they encounter in the Harbor, generally seagrass beds in Gasparilla and Pine Island sounds, or in the mangrove prop roots in these same areas. These habitats allow juvenile gag to hide from predators.
Like the other fish mentioned, a little gag’s priority at this stage is to not be eaten. They don’t go far to feed, eating whatever is common in the environment and appropriate for their size range. Young juveniles feed primarily on pinfish and shrimp. As they get bigger, they feed on pinfish, pigfish, scaled sardines (whitebait) and silver perch. Other species that settle out in seagrasses as juveniles include mangrove snapper, grunts and spotted seatrout.
Permit also settle out in areas of higher salinity. Juvenile permit establish along estuary beaches where they are easily camouflaged against the sand. Small juveniles feed upon copepods, mysids and shrimp. As they get larger, they shift to mole crabs, coquina clams and barnacles. Adults feed upon sea urchins, bivalves and crabs.
As fish increase in size, habitat and diet demands largely determine where they will be found. How, and for how long, fish utilize the estuary will vary by species. Most gag will only spend about a year in the Harbor before moving offshore. Redfish stay in the estuary for 3 to 4 years before joining adult populations offshore. Spotted seatrout complete their entire life cycle within the estuary environment.
Snook are probably the most complex, in that segments of the population exhibit very different migration tactics. Some move to fresh water, others remain the estuary year round, and a smaller contingent probably lives on nearshore reefs.
This is just a very basic look at the early life history of some of our common sportfish species. Of course, it’s much more complicated in the real world. Estuaries and life cycles are intricate and complicated, which of course is why they are so fascinating.
Betty Staugler is the UF/IFAS Extension Charlotte County agent for the Florida Sea Grant Program. She is active in many areas relating to boating, fishing, and watershed/coastal living. Sea Grant supports research and education activities that help Florida’s shoreline communities, industries and citizens wisely use the state’s coastal and marine resources. Contact her at firstname.lastname@example.org or 941-764-4346.