Ocean dredging was an

oceanography Oceanography (), also known as oceanology, sea science, ocean science, and marine science, is the scientific study of the ocean, including its physics, chemistry, biology, and geology. It is an Earth science, which covers a wide range of to ...

technique introduced in the nineteenth century and developed by naturalist Edward Forbes. This form of

dredging Dredging is the excavation of material from a water environment. Possible reasons for dredging include improving existing water features; reshaping land and water features to alter drainage, navigability, and commercial use; constructing d ...

removes substrate and fauna specifically from the marine environment. Ocean dredging techniques were used on the HMS Challenger expeditions as a way to sample marine sediment and organisms.

History

Edward Forbes

Edward Forbes would lay out the dredged material on the deck to examine, preserve and study it. The practice was chronicled in a remembrance of Forbes by William Jerdan in his 1866 book ''Men I Have Known''.

HMS ''Challenger''

Ocean dredging was a common sampling technique used on the

Challenger expedition The ''Challenger'' expedition of 1872–1876 was a scientific programme that made many discoveries to lay the foundation of oceanography. The expedition was named after the naval vessel that undertook the trip, . The expedition, initiated by W ...

. The expedition, led by oceanographer John Murray and chief scientist

Charles Wyville Thomson Sir Charles Wyville Thomson (5 March 1830 – 10 March 1882) was a Scottish natural historian and marine zoologist. He served as the chief scientist on the ''Challenger'' expedition; his work there revolutionized oceanography and led to his ...

, set sail in 1872 and returned to England in 1876. The ship was equipped with 34 dredges and 20 dredge nets, completing 133 dredges at 111 stations during the 4 year long expedition. Thomson and Murray detail the following instructions for surveying dredged organisms: Britannica Dredge and Dredging 17

"Examine mud brought up by dredge from different depths for living
diatom A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...
s; examine also for the same purpose the stomachs of '' Salpae'' and other marine animals."

The expedition successfully dredged, collected, and preserved marine sediments, plants, algae, and invertebrates. The Challenger expedition is attributed to discovering approximately 4,700 new marine species and expanding the current knowledge of ocean sediments and geology.

Seafloor effects

Ocean dredging can negatively affect

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

ecosystems. When dredging equipment is moved along the seafloor, habitat-forming epifauna is damaged or removed. As emergent

coral Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...

sponge Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and a ...

s, and

seagrasses Seagrasses are the only flowering plants which grow in marine environments. There are about 60 species of fully marine seagrasses which belong to four families ( Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the or ...

are damaged there is less habitat complexity for juvenile fishes to find protection in. Dredging also removes the

sand waves A sand wave is frequently defined as a type of usually a large, ridge-like bathymetric feature, called a '' bedform'', that is created by the interaction between underwater unidirectional currents with noncohesive, granular sediment, e.g., silt, ...

in which juvenile

Atlantic cod The Atlantic cod (: cod; ''Gadus morhua'') is a fish of the family Gadidae, widely consumed by humans. It is also commercially known as '' cod'' or ''codling''. The top 2–6 cm of marine substrate is disturbed during dredging, which can have negative impacts on deposit feeders, nutrient flux, and burrowing species. Dredging is often banned or highly restricted within

marine protected areas A marine protected area (MPA) is a protected area of the world's seas, oceans, estuary, estuaries or in the US, the Great Lakes. These marine areas can come in many forms ranging from wildlife refuges to research facilities. MPAs restrict human ...

in order to protect recovering ecosystems.

Equipment used

Dredging in the marine environment can be carried out with a variety of equipment, depending on the purpose of the dredge. If the purpose is to remove sand or redistribute sediment, then a

dredge drag head {{unreferenced, date=July 2018 A dredge drag head is used by a trailing suction hopper dredger to collect sand from the sea floor. The dredge drag head is a steel structure that is connected to the dredger by a suction pipe. Supported by the ga ...

attached to a

trailing suction hopper dredger A trailing suction hopper dredger (TSHD) is a type of ship capable of maintaining Navigability, navigable waterways, deepening the maritime canals that are threatened to become silted, constructing new land elsewhere or replacing sand Erosion, ...

ship is used. A fishing dredge (also known as a scallop dredge) is used for collecting edible species of

oysters Oyster is the common name for a number of different families of Seawater, salt-water bivalve molluscs that live in Marine (ocean), marine or Brackish water, brackish habitats. In some species, the valves are highly Calcification, calcified, a ...

mussels Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, whic ...

scallops Scallop () is a common name that encompasses various species of marine bivalve molluscs in the taxonomic family Pectinidae, the scallops. However, the common name "scallop" is also sometimes applied to species in other closely related famili ...

clams Clam is a common name for several kinds of bivalve mollusc. The word is often applied only to those that are deemed edible and live as infauna, spending most of their lives halfway buried in the sand of the sea floor or riverbeds. Clams h ...

, and

crabs Crabs are decapod crustaceans of the infraorder Brachyura (meaning "short tailed" in Greek), which typically have a very short projecting tail-like abdomen, usually hidden entirely under the thorax. Their exoskeleton is often thickened and ha ...

from the seafloor. PSM V06 D022 Vessels dredging for oysters

References