Our first post of the day comes from Pete Davison, a PhD student at Scripps Institution of Oceanography. Today, Pete writes about mesopelagic (midwater) fish, which are the subjects of his thesis work. This is Pete’s second post to the SEAPLEX blog.
Mesopelagic fishes are small fishes, about 2 to 10 centimeters (0.8 to 4 inches) in length, that live everywhere in the deep ocean. They are very common. It is estimated that there are as many as 600 million tons of these fishes
worldwide. This averages out to 1.7 grams per square meter of ocean surface (about one fish per square meter). In fact, the lanternfish family is the most common vertebrate on Earth (by weight).
These fish are very important ecologically, and are consumed by predators such as squid, birds, and bigger fish. Mesopelagic fishes spend their lives in very dim light in order to avoid predation. This means that they must remain at least 200 meters (about 656 feet) deep during the day. At night, many of the fish migrate to the surface to feed. They primarily eat zooplankton such as krill and copepods, which are most abundant in shallow water. The daily movement between their deep predation refuge and their shallow feeding ground is called “diel vertical migration.” It is the largest migration on Earth.
The vertically migrating lanternfish (Tarletonbeania crenularis) is a common fish that we captured at SEAPLEX Station 1 in the California Current. Note the light-emitting photophore organs along its belly. The light emitted by these organs provides counter-illumination camouflage from predators that attempt to seek prey from below by silhouetting their target against sunlight. The dark back prevents the fish from being seen from above, and the silvery sides help it avoid being seen from the side. The pearleye (Benthalbella dentata) is an example of a predatory fish that has upward looking telescopic eyes that it uses to find prey above it.
On the SEAPLEX cruise, we are capturing mesopelagic fishes with a large trawl net. The “Oozeki Trawl” has a five square-meter (about 54 square feet) mouth opening, and we pull it at about three knots (3.5 miles per hour) through the water. We tow it on a wire 1.8 kilometers (about 1.1 miles) behind the ship to about 800 meters (2625 feet) in depth so that we can catch mesopelagic fishes during the day while they are in deep water. These captured animals will be used for multiple purposes.
We will be dissecting the midwater fish that we catch to see if they are eating the small fragments of plastic that are found in the water here. It is important to see if plastic is entering the food chain because it adsorbs chemical pollutants such as DDT from the water. These pollutants accumulate in predatory fish such as tuna, which are in turn eaten by people. So, even though we do not eat these little mesopelagic fish, their diet may still affect us.
Deploying the Oozeki trawl to catch midwater fish.
The lanternfish (Tarletonbeania crenularis) is a common midwater fish. It swims from as deep as 700 meters (2297 feet) to the surface of the ocean every day!
The pearleye (Benthalbella dentata) is a predatory midwater fish. Its eyes look upward so that it can see prey swimming above it.