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 Davison analyzing data while on the SEAPLEX cruise.

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.

Big squid! Josh Jones (left) and Chelsea Rochman inspect an
as-yet-undetermined species of squid, removing parts for later
inspection and identification. The remains of the squid, found floating
off the portside of the ship, featured a mantle that measured 2.5
meters. Adding the tentacles this animal might have been as large as
five or six meters.

Lara writes:

I’m Lara Dickens and I am participating in the Teacher at Sea program during SEAPLEX. In addition to raising awareness about the Pacific Garbage Patch, I will be using this experience to inspire more students to take paths that lead them to science careers. I have already learned a lot and I have loved every minute of our adventure, and it is only the second day.

Practice samplings, equipment calibrations, safety meetings, and safety drills consumed the first 24 hours. We needed to make sure everything was okay before we get too far away to turn back if needed. We also needed to get a control data set to compare to the gyre. The big issue so far: the soda machine in the galley was not functioning well. But it’s all good now.

Things have calmed down now that everyone has a place to work and knows the routine of the ship, and gotten some sleep. I get the pleasure of assisting during the night shift. It is the most exciting time to be up and about. The ocean is a very different place when the sun goes down. Critters of all sorts come to the surface to feed. We collected vampire squid, beautiful jellyfish (we think that it was Periphylla periphylla ), and a rat tail fish just to name a few.

Periphylla periphylla in Monterey Canyon Picture from jellieszone.com

A rat tail fish Picture from pmel.noaa.gov

Biological sampling equipment includes the manta net (water surface), bongo nets (below surface) and Oozeki net (mid water trawl). All three of these pieces of equipment collect specimens at different depths in the water column. The CTD (conductivity, temperature, and depth) equipment measures the biological and physical properties of the water. It also collects water samples from different depths as it rises. Only one piece of equipment can be in the water collecting at a time … so as one comes in, the next goes out and the lab gets busy analyzing and preserving the fresh samples.

The sun is up now and I’m going to bed!

The sun sets on August 3rd as the New Horizon heads west.