Coastal fish

Coastal fish, also known as inshore or neritic fish, are incredibly abundant.

They inhabit the marine waters between the shoreline and the edge of the continental shelf. Because the continental shelf is usually shallow, these fish are mostly epipelagic. This means they live in the sunlit epipelagic zone of the ocean. They are distinct from oceanic or offshore fish. Those fish live in the deep seas far beyond the continental shelves. Coastal fish thrive in diverse areas like tidal pools, fjords, and estuaries. They also live near sandy shores, rocky coastlines, and coral reefs.

These fish exist in habitats with very high biological productivity. This productivity often comes from upwelling. Upwelling is a process that brings nutrients to the surface. Shoreline runoff also brings many nutrients into these coastal waters. These nutrients support forage fish, which are small fish that serve as food. Predators then feed on these forage fish. Some coastal fish are partial residents. They might spawn in streams, estuaries, or bays. However, most species complete their entire life cycles within the coastal zone.

One specific habitat is the intertidal zone. This area is found along rocky shores. It contains indentations called rock pools. These pools trap salty water when the tide goes out. Rock pools are miniature ecosystems. They are home to hardy plants and animals like seaweed and sea anemones. Many fish are only temporary residents in these pools. The tide eventually carries them to a new location. The rock goby is one such small fish. It is about 5 centimeters long and lives in northern temperate waters.

Other fish have developed amazing ways to survive in rock pools. The common blenny is a territorial fish. It can actually crawl on dry land using its paired fins. It has smooth skin covered in soft slime. This slime prevents the fish from drying out. As long as the skin stays moist, the blenny can breathe out of water. The long-spined sea scorpion is another resident. It is a stout fish that grows about 10 centimeters long. It has four long spines on its gill covers. It also has finger-like organs near its mouth to help catch prey.

Estuaries are another vital coastal habitat. An estuary is a partly enclosed body of water. It occurs where a river or stream flows into the sea. This creates a transition zone of brackish water. Brackish water is a mix of fresh and salt water. Estuaries are subject to marine influences like tides and waves. They also feel riverine influences like freshwater flow and sediment. These combined factors provide high levels of nutrients. This makes estuaries very productive natural habitats. Many fish use estuaries as nursery grounds to grow up safely.

Some fish must travel between different water types. These species are called anadromous or catadromous. Anadromous fish, like salmon, live in the sea but swim up rivers to spawn. Catadromous fish, like eels, live in rivers but return to the sea to breed. To survive in estuaries, fish must be euryhaline. This means they are tolerant of a wide range of salinities. In warmer climates, estuaries often feature mangrove swamps. Specialized fish like mudskippers forage on land in these areas. Archer fish even spit at insects to knock them into the water.

Coral reefs are perhaps the most diverse coastal habitats. They occupy less than one percent of the ocean surface. Yet, they provide homes for 25 percent of all marine fish species.

Reef fish have many specializations to survive. Some, like the anthias, are very colorful. Anthias are protogynous hermaphrodites. This means they are all born female. If the dominant male dies, the largest female changes into a male. This change is driven by surging testosterone. They often live in groups called harems. A single male may lead a group of two to 12 females.