Posted by: Alison Cawood | August 12, 2009

SEAPLEX Day 11 Part 1: Midwater Fish

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.

Pete writes:

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.

Pete (Large) (Large)Pete Davison analyzing data while on the SEAPLEX cruise.

MOHT 094 (Large)Deploying the Oozeki trawl to catch midwater fish.

Tarletonbeania crenularis 035b (Large)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!

Benthalbella dentata 138b (Large)The pearleye (Benthalbella dentata) is a predatory midwater fish.  Its eyes look upward so that it can see prey swimming above it.

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Responses

  1. This comment is for Chelsea Rochman.
    As a sport fisherman on the shore of the Gulf of Mexico, I can tell you that plastic is not the only thing to wash up during southerly winds. By walking the beach or marshes from Galveston to Florida whole items can be seen, with almost no life clinging because it hasn’t been in the water long enough. Stomach flip-flop? You bet. Enjoy your trip. I’ll be with you vicariously. Godspeed.

  2. Hi folks…

    fascinating stuff… keep up the great updates!

    Out of interest- after you collect a bunch of little mesopelagics (or any other live samples for that matter) how are they rendered – um – fit fo dissection? I imagine a employing a “fishermans’ priest” to do the honours would be rather counterproductuve ;D

    hope you have some cold ones aboard for after your long days/nights of work!

    Cheers!

    • When the fish are pulled up in the net, they are going through about a 100x change in pressure, being thrust into lots of light, and it is significantly warmer at the surface than at several hundred meters below the surface. All in all, they have a really tough day, and many of them do not make it to the surface alive. The others are put into preservative soon after coming to the surface, so they don’t live long. Also, no alcohol is allowed on US research vessels, so no happy hour to unwind after a long day!

      >

  3. This truly is a one of the most amazing scientific adventures that I have seen.

    Thank you for your efforts and the time you spend posting on this blog.

    Be safe and God bless.

  4. It looks like it has googles on.

  5. *Goggles. Guess you know where my mind was?

  6. Amazing creature! I’ve never seen that kind of fish before. BIG EYE for such a small fish. The expedition never ease to amaze me with all sea creatures they discovered on the ride up there. Ocean is quite an exciting place to be at. Keep up the good work! Love the photos!

  7. Dear Pete
    Being a marine biologist myself working on population genetics of crustaceans, i would be highly interested in knowing how you visualize the ingested plastic by the fish.

  8. [...] “SEAPLEX Day 11 Part 1: Midwater Fish « SEAPLEX”. Seaplexscience.com. 2009-08-12. Retrieved [...]


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