Denise Harrington: Spotlight on a Blacktip Shark, September 24, 2016

NOAA Teacher at Sea

Denise Harrington

Aboard NOAA Ship Oregon II

September 16-30, 2016

Mission: Longline Survey

Geographic Area: Gulf of Mexico

Date: Saturday, September 24, 2016

Yesterday, I was in the crew lounge, working on my next blog, when Eric Hoffmayer, Research Fishery Biologist, called me out to the fantail to see a large deceased female blacktip shark (Carcharhinus limbatus) brought in that morning.

(deceased) female blacktip shark

The contrast between the gray and white skin caught my eye. The countershading, a dark grey color on top, had a light bronze hue that sparkled in the light. A white band starting at its pectoral fins widened until it merged with the belly at the anal fin.

If there is a mortality, the science team uses the opportunity to dissect the fish, collecting additional information otherwise unavailable.  When we catch a shark, we release it as quickly as possible. The urgency of getting shark back in the water keeps me from carefully studying its detailed characteristics.

While I understand the loss of this particular shark touches many of us on board, understanding the species better through the loss is a practical, necessary approach to  managing the marine environment.  Without an in depth understanding of sharks, their populations, life cycle, and reproduction, there is no way we can sustainably manage fish populations.  Some may find dissection unappealing, and for those folks you may want to skip this blog, but not without first thanking the biologists who do this work compassionately. They keep our fisheries sustainable.

I rubbed my hand from the head to the tail.  It was smooth. Rubbing from the tail to the head felt just the opposite, rough like sandpaper.  Tiny dermal denticles allow sharks to move quickly through the water, an adaptation so amazing, it was put to use by designers of swimsuits in the Olympics and engineers of Navy ships.

Eric, Adam, and Chrissy, placed the shark on the table.  Eric cut the shark and pulled out a long sack that looked like empty sausage casing. At the end of the casing was a tiny shark pup. Trey joined the crew as they took data on each of the six pups.  The shark was pregnant.

The golden colored egg casing is still about six times the size of the pup, giving it plenty of room to grow.
The golden colored egg envelope is still about six times the size of the pup, giving it plenty of room to grow.


Here, Trey stretches out the casing demonstrating the significant amount of room left for the pup to grow,
Here, Fisheries Biologist Eric Hoffmayer stretches out the egg envelope demonstrating the significant amount of room left for the pup to grow. In the background you can see the egg envelop of another pup stretching across the table.

From the number of pups in a brood, to the possibility of immaculate shark conception, the reproduction of blacktip sharks is of interest to fishery biologists.  Without knowing all about shark reproduction, how many, and where sharks reproduce, we cannot sustainably manage this species, or fisheries in general.

Trey takes me through each stage of reproduction. The blacktip shark is viviparous, like humans. They are born alive, “vivi,” and develop within the mother getting nutrients through a placenta.




The shark life cycle begins in the female shark’s ovary with an egg.   Trey hands me an ovary that holds the eggs.  It is a large sack of many small red pinpoint size spheres with about 6 larger marble like balls from the high in the body cavity. These eggs wait to mature until the conditions are ideal for reproduction. At that time, the follicle ruptures, and the egg comes out.

Shark eggs are fertilized inside the female’s body.  The male fills his siphon sacs with seawater, and then flexes his abdomen to shoot the seawater and semen into the female shark through his clasper.

Now I understand why we spin the clasper of a male shark to determine its maturity.  I was able to rotate this male Gulf smoothhound shark (Mustellus sinusmexicanus) clasper 180 degrees and reported it as an adult male.


The male blacktip shark is often ready to mate in April to May but the females are often not ready to reproduce until June or July.  With many sharks, blacktip sharks included, the sperm can remain inside the female until she is ready to reproduce.  When that moment arrives, the egg slips through the ostium, down the anterior oviduct, and into the oviducal gland where it is fertilized by the sperm. For the blacktip shark, usually 4-6 eggs will be fertilized and develop into shark pups.  Females usually reproduce every other year.


Note that different sharks have different modes of reproduction.  For example, Cuban dogfish (Squalus cubensis) reproduce through aplacental viviparity or ovoviviparity. The tiny pups you see here nourish themselves with the yolk “ovo” and have no placental connection to their mother.  They are born live “vivi,” and able to feed and protect themselves. Some sharks are oviparous, which means they lay eggs  that hatch later.

Initially,  the blacktip shark embryo uses the nutrients from a yolk sac for about 10-11 weeks. For the remaining time inside the mother, the pup increasingly gets nutrients from the mother through a placenta.  They are viviparous and remain inside the mother for approximately 10 months until they can survive on their own.  I held a pup, still connected to its mother by the umbilical cord. The similarities between human reproduction and blacktip shark reproduction surprised me so much I began to question the classification of viviparous sharks as fish.


I held a pup, still connected to its mother by the umbilical cord.

Immature Shark/Juvenile

For approximately two months after it is born, the immature shark has an umbilicus (like a bellybutton) that is still open.  During this phase of the life cycle it is called a neonate, or newborn.  It is otherwise just like a miniature adult blacktip shark.  It can hunt and hide from predators (including its mother).

Here, Eric and Evan Pettis, Texas Parks and Wildlife Fisheries Biologist, tag, measure, and release an immature blacktip shark.

 Mature Shark/Adult

Individual sharks even within a species mature at different rates, just like humans.  Generally, a male blacktip shark matures between 4-5 years of age, and females between 7-8 years.

This 1385 mm male mature blacktip shark was brought in our second day of the survey.

How does the shark’s life cycle affect fisheries?

Evolutionarily speaking, placental viviparity gave the blacktip shark and others like it an advantage; the shark is born able to survive independently.  But this adaptation has also has a downside:  the females only produce a small brood, unlike other fish that use broadcast fertilization.

During gestation, the female shark we caught most likely migrated to our current location just off the coast of the Mississippi from deeper waters.  Called the Fertile Fisheries Crescent, the Mississippi Sound is one the most productive seafood areas in the nation.  Another risk to this species is pollution and over-fishing in the fragile estuarine habitat, the juvenile shark’s nursery.

There is demand for the high quality blacktip shark meat, the fins, and even the carcasses for fishmeal. The work NOAA Fisheries does to collect information about shark populations over time and over a wide geographic area not only helps keep blacktip shark populations sustainable, it also gives us valuable information about the ocean’s health in general.


Personal Log

Today I reached the half way point in my time on the longline crew.  I finally feel like I am getting into the groove, finding my way around the ship, and meeting people beyond my fishing buddies.  Valerie  McCaskill, Chief Steward, and her cousin, Ava Speights cook amazing seafood, grilled veggies, and au gratin everything. Ava showed me a great piece of exercise equipment, Jacob’s Ladder, to allow me to enjoy the great food guilt free.

Each station, each day, a new adventure.

Rebecca Himschoot, June 26, 2007

NOAA Teacher at Sea
Rebecca Himschoot
Onboard NOAA Ship Oscar Dyson
June 21 – July 10, 2007

Mission: Summer Pollock Survey
Geographical Area: North Pacific Ocean, Unalaska
Date: June 26, 2007

Weather Data from Bridge 
Visibility: .5 nm (nautical miles)
Wind direction: 80° (ENE)
Wind speed: 10 knots
Sea wave height: 1 foot
Swell wave height: 1 feet
Seawater temperature: 4.4°C
Sea level pressure: 1018.8 mb (millibars)
Cloud cover: stratus

Deck crew of the OSCAR DYSON retrieving sensors from a buoy.
Deck crew of the OSCAR DYSON retrieving sensors from a buoy.

Science and Technology Log: Data buoy retrieval and replacement 

Luckily we had calm weather today to retrieve two data buoys that were deployed in 2006, and replace them.  These buoys contained an Acoustic Doppler Current Profiler, a marine mammal voice recorder, and sensors for other data such as water temperature, nitrates, and salinity.  Because the sensors are on a stationary buoy, the information is collected at depth (much of this same information is collected on board the OSCAR DYSON continuously, but at the surface), and over a long period of time.

Life Cycle of the Walleye Pollock  
(Interview with Dr. Mikhail Stepanenko, scientist from TINRO)

Dr. Mikhail Stepanenko is assisting in the summer pollock survey from his home institution, the Pacific Research Fisheries Center (TINRO), which is located in Vladivostok, Russia. Dr. Stepanenko graduated with a degree in fish biology in 1968, the year before an agreement was signed for scientists in the Soviet Union and the United States to cooperate to help manage international fisheries.  Dr. Stepanenko took some time to share what he knows about the life history of the walleye pollock. According to Dr. Stepanenko, walleye pollock are found throughout the Bering Sea, and south into the Gulf of Alaska. Their range extends as far west as Russian and Japanese waters, and east to the Eastern Aleutians.  These fish can live up to 25 years, however the average age of a walleye pollock is 10-12 years. Pollock are related to the cod family.

Scientist Bill Floering with some of the new sensors deployed today from the OSCAR DYSON.
Scientist Bill Floering with some of the new sensors deployed today

Pollock begin spawning around age 4, although the most productive spawning years for both males and females is between 5-8 years of age.  Dr. Stepanenko has observed pollock spawning in an aquarium setting.  The male will swim next to a female to show his interest.  If she is also interested in that male, the female will swim next to him with sudden, short bursts of speed for several hours before they spawn. If she is not interested, she will continue to swim normally until the male gets the message.

Mature pollock spawn annually in nearshore areas, mostly in the Bering Sea and Gulf of Alaska (98% of pollock spawn in US coastal waters). Although the females will spawn only once annually and then move to the edge of the spawning area to feed, the males will spawn 4-5 times during the annual spawning season.

The eggs will hatch about 25 days later, or longer if the water temperatures are colder.  The annual survival rate of the eggs and larvae is highly dependent on the sea conditions and salinity.  At the correct salinity, the eggs sink and then are suspended at a certain depth due to a thermocline at that depth.  Poor sea conditions or a dramatic shift in salinity can result in higher mortality for the eggs or the larvae. They must also survive predators such as jellyfish and other small fish.

Dr. Mikhail Stepanenko processes walleyed Pollock
Dr. Mikhail Stepanenko processes walleye Pollock

Directly after hatching the pollock larvae have enough yolk reserve to survive a few days, but they must find food within the first three days of hatching if they are to survive. The larvae are approximately 3.5 mm long when they hatch, and with enough food will grow several centimeters in the first year of life. Only two of the 30-40 types of plankton in the Bering Sea are small enough to serve as prey for these tiny fish.  Harsh sea conditions, salinity changes, and scarce food resources in the first year contribute to a survival rate of only about .1% of pollock eggs. Adult pollock eat euphausids, as well as smaller fish such as capelin or smelt.  In times of scarcity, pollock are given to cannibalism.

The international pollock fishery targets four-year-old fish, and the total Bering Sea harvest of pollock is around two million metric tons annually.  Pollock is used in frozen seafood products, such as fish sticks, and as a central ingredient in surimi.

Personal Log 

We have been in an area where there are very few fish, so much of my time has been spent learning about pollock and the work that is done here on board.  The sea has been pretty rough at times, and I have continued to take some seasickness medication. We’re getting back into places with fish, so soon we’ll be collecting more data.

Question of the Day 

Answer to the last question about the controlled variables in the summer pollock survey: (The scientific method includes controlling the variables in an experiment.  What are some examples of variables the science team from the AFSC is controlling in the summer pollock survey?)

One example is the calibration of the acoustic equipment at the beginning of each leg of the survey. Another example is that the OSCAR DYSON cruises the same area of the Bering Sea during each summer pollock survey on transects of the same basic lengths and directions. The survey is conducted at the same time every year, as well.

Today’s question: Scientists use Latin names for each animal or plant they find, even though Latin is no longer a living language. How do scientific (Latin) names get selected and why are they important?

Walleye pollock
Walleye pollock