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Dispatches from the field

Scientists seek out the tiniest catch, larval fish in the Gulf of Alaska


A shrimp caught in a bongo net is a relatively large catch compared to larval fish that are smaller than the width of a baby's pinky finger.  Photo: Ali Deary, NOAA Fisheries

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larval sculpin
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Twelve-hour shifts and sampling our one-hundredth station

May 25, 2017 -- The research cruise has been going very smoothly. Although the seas have been bumpy at times, we have not experienced any serious weather, which makes our tasks much easier to complete. More than a week into our three-week trip, our bodies have adjusted to our shifts and we are getting into the rhythm of sampling. I work from midnight to noon so it has taken a few days to get accustomed to sleeping during the day and waking up at night ready to collect larval fishes. Luckily, the night is not very long in the Gulf of Alaska so the sun sets around 10:30 PM and is back up by 6:00 AM.

We have just finished sampling our one hundredth station. Our goal for this survey is to sample 270 stations; we can sample one station in about an hour. In addition to the critters we catch in our nets, we have also seen humpback whales and Dall’s porpoises while rinsing our nets off out on deck. This is a pretty wonderful view for another day at the “office.”

Our primary sampling gear, which has been a staple of our surveys for more than 40 years, is the bongo net. This net system is aptly named because it consists of two conical nets connected together at the mouth by a central frame, closely resembling a bongo drum. This system is ideal for our type of sampling because it can be fished effectively in a variety of sea states.


larval slender blacksmelt
Deploying bongo nets. The larger nets on the bottom target larval fishes while the smaller diameter nets above target zooplankton. The FastCAT is situated above both pairs of nets.   Photo: Ali Deary, NOAA Fisheries


Generally, we tow our bongo nets in a “V” pattern through the water column. Depending on the depth, we will lower the bongo down to 10 meters off of the seafloor or 100 meters and then bring the net back up to the surface. Whenever we sample our bongo, we are also collecting temperature and salinity data with a type of CTD called a FastCAT.

Depending on the organisms we want to catch, we equip the bongo frame with different sized-mesh nets. For most of our larval fish sampling, we use a net with 505 micron mesh. It is very fine; the space between each thread is the same as a grain of sand. If we want to capture zooplankton, which are larval fishes’ prey, we use an even smaller mesh net, 153 microns. During our current survey, we are interested in looking at the larval fishes and the zooplankton they may be eating so we are actually fishing two sets of bongo nets at the same time at about half of our stations.


larval squid
On deck and taking a sample and appreciating Alaska's beauty, which included a rainbow on this day.  Photo: Ali Deary, NOAA Fisheries


The benefit of the paired net design for us is that we can preserve one side as a strictly quantitative net. The data from this net tell us when, where, and how many individuals of a species were caught during a survey. Since we have been sampling in the Gulf of Alaska for a number of years, we can then compare this year’s data to past years to explore how the number and types of fish larvae and zooplankton have changed over time.

The second side of the net we sort through for fish and zooplankton in our ship’s lab. Because of the second net, we can collect samples for us and other researchers in order to look at genetics of the individuals we catch or how well they are feeding, or even how nutritious the prey are. These additional studies allow us to study how well larvae are growing in the areas we sample and potentially identify the factors that enhance or reduce the survival of larval fishes in addition to determining where each species is found and how many are out there.

Post by: Ali Deary


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