08/14/09 – The Night Watch
By Tara Casazza
Although the primary objective of this cruise is to use the Johnson-Sea-Link submersible to collect deep-sea organisms during the day, we also have a group of scientists on board that work all night long. It is not cost effective, nor could we sample the entire water column if scientists worked only during the day, so the night watch begins work at 6 pm and finishes at 6 am. Five scientists are on the night watch and are responsible for collecting animals from the midwater (200–1000 m) and surface waters using special nets. The nighttime is an optimal time to target particular fishes such as midwater fishes that migrate to the surface at night, and surface fishes that are attracted to lights, such as flyingfishes. Night sampling helps us obtain a more complete picture of what is going on in the water column and provides data that we could not obtain otherwise.
During this cruise, the midwater community is sampled by towing a 1 m MOCNESS through the water column to collect midwater organisms. A MOCNESS is a Multi Opening/Closing Environmental Sampling System — essentially a specialized series of nets that you can open and close from the ship. The environmental monitors on the net measure the temperature, pressure and conductivity so that you can determine if the net is in the area that you wish to sample. Up to seven nets can be “fished” at a time. We are interested in midwater fishes because they provide a link between deep-sea and surface fish communities, consuming zooplankton at the surface at night and then migrating to deeper depths during the day. Many of the midwater fishes, such as lanternfishes, have light organs (photophores) which may provide an advantage to these organisms that live in deeper waters without light. Some of the objectives of this study are to describe the midwater fish community and determine what these fishes are eating. By sampling the midwater community with the MOCNESS we are able to accomplish these objectives.
Sampling the surface community at night involves attaching bright lights (500 W and 1000 W) to the deck of the vessel and pointing the lights into the water. We also turn on all of the deck lights on the vessel to help attract fishes. Some fishes are attracted to light and swim into these well lit areas where we collect them with dip nets. Fishes that appear to be attracted to light are flyingfishes, halfbeaks, needlefishes and dolphinfish. Dolphinfish are recreationally important, whereas flyingfishes and halfbeaks are an important food for recreationally and commercially important species (dolphinfish and tuna). Thus far, we have collected at least seven flyingfish species, three halfbeak species, two needlefish species and juvenile dolphinfish by nightlighting. Additionally, an adult Ocean Triggerfish was collected at the surface at night by dip net.
The night watch works unusual hours and has different but complementary objectives to the science crew that works during the day. If sampling was not conducted during the night, a fairly substantial gap would exist in our data. Luckily for us, some of the fish species we are targeting are more active at night or they are attracted to lights at night, making it a little bit easier to collect them. The samples collected by the night watch provide invaluable information about the targeted species of fishes and help us gain a more complete understanding of the distribution of fishes in the water column.
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