Emily Cilli-Turner: One Fish, Two Fish….Pollock Counting Techniques July 29, 2018


NOAA Teacher at Sea

Emily Cilli-Turner

Aboard NOAA Ship Oscar Dyson

July 24 – August 11, 2018

 

Mission: Pollock Acoustic-Trawl Survey

Geographic Area of Cruise: Eastern Bering Sea

Date: July 29, 2018

 

Weather Data from the Bridge:

Latitude: 57° 10.46 N

Longitude: 171° 58.29 W

Wind Speed: 11.16 knots

Wind Direction: 77.54° (NW)

Air Temperature: 10.1° C (Manual Reading from the Bridge)

Barometric Pressure: 992.7 mb

Visibility: 6 nautical miles

Sea Wave Height: 3 feet

Sky: Overcast

 

Science Log:

How do the scientists aboard NOAA Ship Oscar Dyson estimate the number and biomass of pollock in the Eastern Bering Sea? By using the science of statistics, of course! When political strategists want to determine what percentage of voters support a specific candidate or issue, they take a sample from the population of all registered voters. Voters in this sample are then asked about their preferences and statistical techniques are employed to extrapolate the results from the sample to the entire population and measure the margin of error.  Similar statistical techniques are employed by the scientists on NOAA Ship Oscar Dyson, but as you can imagine it is more difficult to sample pollock than voters that can be called on the phone!

Before each pollock survey begins, a set of transects is created for the Eastern Bering Sea.  These transects are paths for the ship to follow along which the scientists sample the pollock.  As you can see below, the transects for this survey are a fixed distance apart and cover the entire area of interest.  Generally, the transects are straight lines created to be perpendicular to the ocean depth grade. This allows for the scientists to encounter a variety of species as well as different ages of pollock to gain a robust picture of the ocean life in the area.

transect

The transects for this survey leg can be seen as the straight lines. The other markings are places where the trawls have been done and other scientific instruments have been deployed.

The NOAA Ship Oscar Dyson follows the transects during daylight hours, continuously recording water column acoustic backscatter data using EK60 instruments mounted on the bottom of the centerboard.  Scientists monitor the backscatter images, and when they observe sufficient pollock or other fish aggregations they  use the trawling nets to take a random sample of the fish and other ocean life they observed.  The trawling net is 140 m long with a vertical mouth opening of 25 m and horizontal mouth opening of 35 m. The net is deployed from the back of the ship and dragged at a fixed depth for an amount of time determine by the lead scientist to ensure a large enough sample. Once the trawling net is hauled in, the sample of marine fish and invertebrates is processed in the wet lab and entered into a database. Later the pollock numbers and weights by length are combined with  recorded acoustic data to create a robust estimate of the pollock population in the Eastern Bering Sea.

After the catch comes in, the first job in processing the sample is to sort the specimens from the trawling net.  The first part of the net to come in is called the pocket net. This small net, also called a recapture net,  has a fine mesh and is designed to capture small species such as krill, age 0 pollock and jellyfish which slip through the meshes of the large trawl.  After the pocket net is processed, we process the codend, the closed end of the net and the main section where larger fish enter and are captured.  The fish in the codend are sorted by species.  The scientists can choose to measure the length of all the pollock in the haul or, if it is a particularly large catch, split the haul and measure length of a subsample of pollock.  Other species are also identified and their length is measured for later estimates of the total biomass that pollock make up as compared to other species.  Smaller species such as krill are weighed in aggregate instead of individually.

codend

The codend of the trawling net.

Sample analysis consists of measuring the lengths of approximately 200-400 adult pollock in the catch using the magnetic length board.  This is just one of the numerous software and instruments created by the MACE (Midwater Assessment and Conservation Engineering) group at NOAA in Seattle to make analysis easier and more automated.  The length distribution of the adult pollock helps scientists determine the approximate age distribution of pollock in the sample and it also helps them compare this distribution to other samples taken in the Eastern Bering Sea.  A subsample of about 50 pollock from the haul is taken to get more in-depth measurements. From these pollock, we measure both the length and weight and a subsample from the 50 is taken to determine the gender, measure maturity (i.e. what stage in the life cycle the pollock is at), and collect the otolith (ear bone), which gives a more accurate measurement of the pollock’s age.

Personal Log:

At this point, I am getting used to life at sea and have a nice routine.  The beginning of my shift, from 4am to a little past 7am, starts at sunrise and during which we resume our path along the transect.  No trawling operations are conducted at night, but there is still excitement.  If the underwater acoustics show that the pollock are at an appropriate depth, we can go pole fishing off the boat.  NOAA scientist Mike Levine is interested in post-capture mortality of pollock and the feasibility of tagging pollock.  Thus, he would like to catch pollock using a fishing pole, which puts much less stress on the pollock and increases the chance of their survival after the catch, instead of the trawling nets.

fishing

NOAA scientist Mike Levine with a pollock caught with a fishing pole.

As an instructor of mathematics, I have little knowledge of fish biology, but the scientists are great teachers!  I have been given a crash course on fish anatomy using specimens from the catch and I have learned how to sex the fish as well as how to collect the ovaries and the otoliths (ear bones).  If you asked me a week ago if I ever thought I would know so much about pollock after just a couple days on board, I would have laughed.  It has been great being the student and being able to learn so much in such a short time with real hands-on experience!

Did You Know?

Most of the personnel that are responsible for piloting and maintaining the ship are part of NOAA Corps, which is one of the seven uniformed services of the United States.

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