Johanna Mendillo: How Well Do You Know Your Pollock? August 4, 2012

NOAA Teacher at Sea
Johanna Mendillo
Aboard NOAA Ship Oscar Dyson
July 23 – August 10, 2012

Mission: Pollock Survey
Geographical area of the cruise: Bering Sea
Date: Saturday, August 4, 2012

Location Data from the Bridge:
Latitude: 62  20’ N
Longitude: 179 38’ W
Ship speed:  0.8 knots (0.9 mph)

Weather Data from the Bridge:
Air temperature: 7.1C (44.8ºF)
Surface water temperature: 8.3C (46.9ºF)
Wind speed: 22.7 knots (26.1 mph)
Wind direction: 205T
Barometric pressure:  1009 millibar (1.0 atm)

Science and Technology Log:

Out of the 30,000+ species of fish on earth, I would now like to introduce you to the fish we follow morning, noon, and night: pollock.

It is time for some fish biology 101!  The scientific name for pollock, also called walleye pollock, is Theragra chalcogramma.  This is a different species from its East Coast relative,  Atlantic Pollock.  They are in the same family as cod and haddock.

Juvenile pollock
Juvenile pollock… aren’t they cute?

AGE & SIZE:  Pollock are a fast-growing species that typically live to approximately 12yrs, but some live longer.  They are torpedo shaped (long, narrow, and with a streamlined body) and have speckled coloring that help them camouflage with the seafloor to avoid predators.  They generally range from 10-60cm in size; we have been collecting pollock generally in the 20-40cm range so far on this cruise.  Here I am holding one of the larger specimens I have seen so far:

One of the larger pollock I have seen so far...
One of the larger pollock I have seen so far…41cm!

WHERE THEY LIVE:  Younger pollock live in the mid-water region of the ocean; older pollock (age 5 and up) typically dwell near the ocean floor.  In order to sample both of these groups, we conduct trawls throughout the water column so we can get representative biological information from all habitats.

Here I am weighing pollock...
Here I am weighing pollock…


Juvenile pollock eat a type of zooplankton called euphausids, otherwise known as krill, copepods, and small fish.  Older pollock feed on other fish…. including juvenile pollock, making them a cannibalistic species!  Pollock play an integral role in the Bering Sea food web and you will help construct that web back at school!

REPRODUCTION:  Pollock are able to reproduce by the age of 3 or 4.  In our work, we have to determine the sex of each fish by slicing it open because no reproductive organs are visible on the outside!  So, in addition to seeing the insides of many, many fish heads, I have now seen many, many fish gonads.  Here is a poster we use in the lab to learn how to identify the ovaries and testes at five different developmental stages (immature, developing, pre-spawning, spawning, and spent).

Poster showing male and female reproductive organs for ages 1-5
Poster showing ovary and testes stages 1-5!
And... it is a female!
And… it is a female!

So, how do you tell, exactly?  On the females, we go by the following guidelines:

Immature female pollock contain small ovaries tucked inside the body cavity, the ovary looks transparent, and there are no eggs visible.

Developing females have more visible and pink-ish ovaries, generally transparent to opaque.

Pre-spawning females contain large bright orange ovaries and eggs are easily discernible inside them

Spawning females have large ovaries bursting with hydrated eggs  (the fish has absorbed large amounts of water at this point), so the eggs look translucent or even transparent!

Spent females have empty flaccid ovaries.

It can sometimes be difficult to identify a female maturity stage by this simple visual scale (this is called macroscopic inspection), due to subjective interpretations of color, ovary size, and visibility of eggs, so fisheries biologists can also collect cell samples to look at gamete stages under the microscope (this is called histological analysis).  For example, a female’s ovaries can be slightly different colors based on her diet.  We are not collecting those types of samples on this cruise, however, but those are often collected during wintertime pollock cruises in the Gulf of Alaska.

These are ovaries in the pre-spawning stage
These are ovaries in the pre-spawning stage     (Photo Credit: Story Miller, TAS 2010)

Regardless of the method used, determining the ratio of different maturity stages in the female pollock population has very important implications for how scientists  calculate spawning biomass estimates, which in turn, are entered into statistical models to determine age class structures, overall population sizes, and, finally, catch quotas for the fishing industry.

On the males, we go by the following guidelines:

Immature male pollock have threadlike testes with a transparent membrane (that can be very hard to see).

Developing males have testes which look like smooth, uniformly textured ribbons.

Pre-spawning male testes appear as larger thicker ribbons.

Spawning males exhibit large testes that extrude sperm when pressed.

Spent males have large, flaccid, bloodshot, and watery testes.

These are the testes of a pre-spawning male
These are testes in the developing stage (Photo Credit: Story Miller, TAS 2010)

As for how they reproduce, pollock, like most fish, do external fertilization, which means they release eggs and sperm into the water, where they come together and fertilize.  For pollock in the northern Bering Sea, this tends to happen in the winter, from January-early April.  It appears that sub-populations in other areas of the Bering Sea and the Gulf of Alaska spawn during shorter time windows throughout the late winter and early spring.

Fish gather in large groups to spawn, and an individual female pollock can release anywhere from 10,000s – 100,000s of eggs in a single season!  They could also be released at one time or in several batches, called batch spawning.  Interestingly, if conditions are not optimal, such as low water temperatures or  poor nutrition, females can reabsorb eggs, in a process called atresia.

Here are several hundred pollock we have to sort from a typical catch!  We toss the  females in the"Sheilas" side and the males in the "Blokes" side!
Here are several hundred pollock we have to sort from a typical catch! We toss the females in the”Sheilas” side and the males in the “Blokes” side!

After spawning and fertilization, the resulting larvae grow into juveniles, the juveniles grow into adults, and the process starts anew!  Overall, scientists still have much to learn about the timing and mechanisms behind the pollock reproductive process— and I have enjoyed learning about it from the NOAA team!

Personal Log:

First, the answer was… 75 dozen eggs!  Those were some pretty close guesses, good job!

Let’s continue our tour aboard the Oscar Dyson!  Now, as you can imagine, safety and training are very important parts of life at sea.  I feel very confident in the crew and officers’ careful preparedness.  Each week, we conduct safety drills.  There are three types: man overboard, fire, and abandon ship.  For each drill, each member of the ship has to report to a certain station to check in.  In addition, you may be assigned to bring something, such as a radio, first aid kit, etc.

One of our many life rings
One of our many life rings

The drill I was most interested in was abandon ship, because not only do you carry your emergency survival (also known as an immersion) suit with you, but sometimes you practice putting it on!  I had seen many pictures of other Teachers at Sea wearing them and wanted the chance to try it on myself!

So, without further ado, here are Allan and I in our suits:

Survival Suit Stylin'
Survival Suit Stylin’

What do you think, do we look like Gumby???

So, how exactly does it work?  Well, it is a special type of waterproof dry suit that protects the wearer from hypothermia in cold water after abandoning a sinking or capsized vessel. It is made of stretchable flame retardant neoprene, and contains insulated gloves, reflective tape, whistle, and a face shield for spray protection.  The neoprene material is a synthetic rubber with closed-cell foam, which contains many tiny air bubbles, making the suit sufficiently buoyant to also be a personal flotation device.

There are various types of immersion suits.  Some contain:

  • An emergency strobe light beacon with a water-activated battery
  • An inflatable air bladder to lift the wearer’s head up out of the water
  • An emergency radio beacon locator
  • A “buddy line” to attach to others’ suits to keep a group together
  • Sea dye markers to increase visibility in water

We keep them in our rooms and there are many others placed throughout the ship in case we are not able to return to our rooms in a real emergency.

I hope that gives you a good feel for life onboard here in week two.  Please post a comment below, students, with any questions at all.

A nice sunny day in the Bering Sea!
A nice sunny day in the Bering Sea!

2 Replies to “Johanna Mendillo: How Well Do You Know Your Pollock? August 4, 2012”

  1. Dear Ms. Mendillo,

    What a fascinating adventure!

    I see you’re sporting an OB t-shirt. A bit more fashionable than the survival suit.

    Dave MacLeod

  2. I can see you have been back to school shopping. Is that red suit a Ralph Lauren? All the kids will want one.

Leave a Reply to Nick Fornaro Cancel reply

%d bloggers like this: