On the Rocks-Value of Rocky Habitat

By Alison Verkade, NOAA Fisheries Greater Atlantic Regional Fisheries Office
from The Fishing Wire

Subtidal rocky gravel habitat recovering from fishing gear impacts on Georges Bank. Photo credit: NOAA-NURP

Marine rocky habitats provide Essential Fish Habitat for many fish species – places where fish can feed, breed and grow. They are places for fish to hide from predators and find food. They are among the most structurally complex habitats in the ocean.

Rocky habitats are three-dimensional, providing both height in the water column and crevices between rocks. These crevices provide fish shelter from predators and strong water currents. Rocky habitats also contain a diversity of sessile (non-mobile) animals and algae that serve as food and cover for young fish.

Disturbed and undisturbed site with Atlantic cod swimming among dense invertebrate coverage. Photo credit: Institute of Marine Research, Norway.

These habitats include gravel, cobble and boulders. You might expect that a habitat made up of rocks would be rugged enough to withstand a lot of disturbance. That is not always the case. It’s really a matter of how much disturbance occurs. Natural or man-made disturbances that cause sedimentation, turbidity, water quality degradation, or directly contacts rocky substrate can harm or change these habitats.

Natural disturbances from typical storm events are usually not destructive because sessile organisms like barnacles and sea squirts are adapted to high-energy environments. They attach to rock surfaces and can remain upright in the moving current to take advantage of the influx of food and nutrients. However, this adaptation also makes them vulnerable to other disturbances they cannot escape. For instance, beach nourishment projects, where sand is used to restore beaches after major storms, can threaten these important habitats. Some of the sand placed on the beach may run off into the surrounding water. The runoff can cover nearby rocky habitats — filling crevices used by fish, or smothering the animals that live there. Other human activities like undersea gravel mining, construction and navigation dredging can actually scrape away the rocks and animals that live there, resulting in a loss of structural complexity.

Atlantic cod taking shelter in rocky habitat. Photo credit: NOAA Fisheries

What do changes to rocky habitats mean for the fish and other marine life that use them? Scientific studies have shown that algal cover and attached marine organisms in rocky habitats are important in the survivorship of commercial and recreational species like juvenile Atlantic cod, and forage fish like blueback herring, alewife and shad.

For centuries, Atlantic cod supported important commercial and recreational fisheries along the New England coast. Today, the Gulf of Maine cod stock is in poor condition. The condition of the Georges Bank cod stock is not much better, with few young fish being born each year. Blueback herring and alewife, collectively known as river herring, and shad are key components of the marine food chain. While the numbers of alewife seem to be increasing in some rivers, both species’ overall populations are much lower than historic levels. Protecting rocky habitats is important to helping all of these species recover.

Cod and rocky habitat. Photo credit: NOAA/SBNMS

How does NOAA Fisheries protect these important habitats?

We make recommendations to other federal agencies that fund or permit beach nourishment, mining or construction projects. We want to help ensure that projects can move forward in a responsible way. The key is to both protect the rocks that make up the habitat and the attached organisms and macroalgae that are essential for fish.