Perhaps the most well-known function of estuaries, such as Charlotte Harbor, is their role as nursery grounds for growing 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 to settle out and grow within. Settling young fish and crustaceans utilize a number of different survival strategies, but common to all is a quest to not be eaten.
Redfish, sand sea trout, southern kingfish and spot larvae for example, when ready to settle as juveniles, look for low salinity waters associated with river mouths. Low salinity waters tend to support less 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, river mouths associated with the upper harbor are good places to be – that is, if 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, (sometimes 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, rip rap and seagrass beds.
Snook in their quest to not be eaten seek out back country areas, at land margins, 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 predator 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 back country juvenile snook feed on what’s available, and what’s available is mostly mosquito fish and sailfin mollies.
When snook reach a size of about 5-6 inches in length they begin to lose their tolerance for low dissolved oxygen and are forced to move out of the back country. At this size, they are also in pursuit of food and they 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 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 requires stable, high salinity conditions. As such, they settle out in the first suitable habitat encountered, generally seagrass beds found in Gasparilla & Pine Island sounds, as well as in the mangrove prop roots in these same areas. These habitats allow juvenile gag to hide from predators.
Like the other fish mentioned, juvenile gag’s priority at this stage is to not be eaten. As such, they are feeding upon 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 (white bait) and silver perch. Other species that settle out in sea grasses as juveniles include mangrove snapper, grunts and spotted sea trout.
Like gag, permit also settle out in areas of higher salinity waters. 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 it will be found. How, and for how long fish utilize the estuary varies by species. Gag will only spend about a year in the estuary before moving offshore. Redfish stay in the estuary for 3-4 years before joining adult populations offshore. And, spotted sea trout complete their entire life cycle within the estuary environment. Adult snook are pretty complex in that segments of the population exhibit very different migration tactics. Portions of the population will use freshwater, many will 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 sport fish species. Of course, it is much more complicated in the real world. Estuaries and life cycles are intricate and complicated, which of course is why they are so fascinating.
Adams and Blewett. 2004. Spatial patterns of estuarine habitat type use and temporal patterns in abundance of juvenile permit, Trachinotus falcatus, in Charlotte Harbor, Florida. Gulf and Caribbean Research Vol 16(2), 129–139.
Blewett et al. 2006. Feeding habits of common snook, Centropomus undecimalis, in Charlotte Harbor, Florida. Gulf Carib Res, 18: 1-16.
Casey et al. 2006. Habitat use by juvenile gag, Mycteroperca microlepis (Pisces: Serranidae), in subtropical Charlotte Harbor, Florida (USA). Gulf Carib Res, 19, 1-9.
Stevens et al. 2007. Variable habitat use by juvenile common snook, Centropomus undecimalis (Pisces: Centropomidae): Applying a life-history model in a southwest Florida estuary. Bull Mar Sci, 80(1): 93-108.
Ichthyology at the Florida Museum of Natural History, http://www.flmnh.ufl.edu/fish/
Stevens. 2018. Email communication.