How Sebastian Inlet Works as a Fish Nursery — Sebastian, Florida

Fish use Sebastian Inlet in five different ways, according to the Sebastian Inlet District's fish series with biologist Dr. Grant Gilmore: to reach lagoon nurseries, to live on the jetty rocks, to feed, to spawn on both sides, and to feed the predators.

The district built its 2023 series around Gilmore, whose credentials include 32 years at Harbor Branch Oceanographic Institute in Fort Pierce and work at the Kennedy Space Center. In Part One, he outlined five basic life history strategies for the fish of Sebastian Inlet, and each of the five installments takes one group. The district lists the series beside its weekly fishing report.

The passage itself is narrow. The district FAQ calls Sebastian Inlet one of only five navigable channels that connect the Indian River Lagoon to the Atlantic, between Brevard and Indian River counties. Every fish moving between the lagoon at Sebastian and the open ocean in this stretch of coast has to go through it or travel to another inlet.

Groupers, several snappers, sheepshead, porgies, Sailor's Choice grunts and bull sharks are the fish Gilmore names as growing up in the Indian River Lagoon behind Sebastian Inlet before living as adults in the ocean.

Part One calls this group reef fish fishery migrators. Gilmore says gag and red groupers visit the Indian River Lagoon's seagrass beds and oyster reefs as juveniles in spring, while goliath groupers seem to settle more in mangroves. Mutton, lane and gray snappers enter as juveniles to use the seagrass and mangrove nurseries before maturing and returning to ocean spawning sites.

Seagrass carries this nursery. Part One cites the Indian River Lagoon Species Inventory that a single acre of seagrass may support as many as 40,000 individual fish and 50 million small invertebrates, and notes that the district's commission has supported monitoring of the Sebastian Inlet seagrass beds since 2007. The FAQ adds that the district monitors seagrass in six zones on the western flood shoal.

The largest member of the group raises its young the same way. Gilmore says a pregnant bull shark entering the lagoon through Sebastian Inlet stops eating and releases her pups there, between one and 13 of them, which stay in the lagoon five or six years before heading out to Atlantic reefs.

Resident reef fish, a group Gilmore says includes blennies, damselfish, grunts and snappers, spend their whole lives in the jetties and rock structures of Sebastian Inlet.

Part Two explains why such fish live there at all. The jetties and the 3,120-foot-long channel of Sebastian Inlet are armored with granite and coquina rock against tides and waves, and that manmade structure created reef habitat on what would otherwise be unbroken coastline. Fish that are normally found offshore now hold on the rocks a few feet below anglers.

Most of them ignore the bait dangled from the Sebastian Inlet jetties. Gilmore says the hairy blenny, a predaceous little bait-stealer, numerically dominates the group, and adds that this tropical species does not occur on Gulf coast jetties, where the belted sandfish replaces it. The group is rich and diverse, he says, but anglers overlook it because these fish do not necessarily take the bait.

The goliath grouper is the giant exception. Gilmore describes goliaths in the inlet as fairly residential, except during a full moon in August or September when they migrate offshore to spawn. They can reach 400 to 500 pounds and about six feet, eat mostly crustaceans and ambushed fish, and anglers at Sebastian Inlet complain about them stealing hooked snook.

Bluefish, pompano, crevalle jacks and cutlassfish are what Gilmore calls estuarine feeders: predators that come through Sebastian Inlet into the lagoon for days, weeks or whole seasons to eat, without using it as a nursery.

In Part Three, Gilmore says most estuarine feeders are major predators feeding on the Indian River Lagoon's food resources, abundant at one time, as he puts it. Bluefish and crevalle jacks hit a wide range of prey as they move up and down the lagoon, and the crevalle jack's diet runs mostly to smaller schooling fish such as pinfish, mullet, anchovies, Atlantic bumper, croaker and mackerel.

Some feeders are rare elsewhere. Gilmore says the Indian River Lagoon, thanks to Sebastian Inlet, is one of the only Florida estuaries that sees schools of the guaguanche, a tropical barracuda with a gold stripe, and that he does not think any other Florida estuary has a guaguanche migration down the Intracoastal Waterway.

Tarpon straddle two groups. They move through mostly along the beaches hitting schooling mullet, Gilmore says in Part Three, but unlike bluefish and jacks they also use the lagoon behind Sebastian Inlet as a nursery.

Snook, red drum, black drum, menhaden and striped mullet move routinely between the lagoon and the ocean through Sebastian Inlet, and Gilmore believes that since inlets were cut they spawn on both sides.

Part Four calls them estuarine-ocean fishery migrators. Gilmore's view is that without inlets many of these fish evolved to spawn inside the lagoon, and that since inlets were constructed they spawn in both the ocean and the lagoon, all of them using the lagoon as a nursery for larvae and juveniles. Settlers first attempted to cut Sebastian Inlet at its current location in the 1880s.

Red drum are his evidence. Gilmore spent three years mapping red drum spawning grounds in the Indian River Lagoon and says the lagoon spawning sites lie 30 to 50 miles from the nearest ocean inlet, Sebastian or Ponce de Leon, a pattern he traces to the time before the inlet existed.

Snook show the cycle in hours. Part Four says snook spawn near reefs, wrecks and the Sebastian Inlet jetties; their pinhead-size eggs hatch in less than 20 hours, and the larvae settle in the shallow waters of the Indian River Lagoon. The district was required after a 2007 realignment of the inlet channel to monitor flood shoal seagrass for five years, and chose to continue the monitoring beyond the permit.

At Sebastian Inlet the predators feed on what Gilmore calls small estuarine, ocean and reef fish and invertebrate migrators: pinfish, pigfish, mojarras, grunts, small snappers, sardines, anchovies, shrimp, blue crabs and four kinds of mullet.

Part 5 closes the series with this group, which Gilmore says normally does not enter the fisheries but supports them. Mullet show the role most plainly. Every year from September well into November, millions of mullet leave the lagoon through Sebastian Inlet to spawn in the ocean, and tarpon, snook, redfish, jacks and sharks follow the run.

The young come back in winter. Gilmore says juvenile mullet return through Sebastian Inlet to the lagoon around January and February by the millions, silvery and metallic, and saturate the shallow water through early spring, feeding on mangrove detritus that they carry back out to the ocean food chain.

Mojarras are the inlet's specialty. Part 5 says Sebastian Inlet holds the largest population of these fish in the United States and every one of the eleven Florida species, and Gilmore calls the Indian River Lagoon a nursery for the offshore fishery, one that depends on a healthy lagoon and an open inlet.