The NOAA Ship Bell M. Shimada is now steaming back to Newport. Two science parties have spent 30 days at sea evaluating the distribution and biology of Pacific hake (and coming up with more questions), but our sea time is up. The survey ends on 12 February.
2017 was the 2nd year of winter work, and this year was a different beast in terms of both hake distribution/densities and survey conditions. What fun would it be if 2017 was exactly like 2016? As with reading/writing, for this survey, “There is no real ending. It’s just the place where you stop the story.” (Frank Herbert)
What was accomplished during the 2017 survey?
We completed ~3,200 nautical miles (nmi) of acoustic data along transects. Eleven midwater trawls were done, with 10 of the trawls containing hake. Fifty-four casts were made with the Conductivity-Temperature-Depth (CTD) rosette to measure water properties, and zooplankton were collected at 25 of those locations. Water samples for eDNA (i.e. environmental DNA) were collected at 13 stations. Fish (or fish parts) were collected for collaborators working on topics ranging from aquaculture to genetics to toxicology. We redesigned, revised, improvised, and made decisions on the fly when needed. The paper map in the Acoustics Lab – decorated by proposed transects, stations, advice, and funny sci quotes – was always changing.
How was 2017 different from 2016 (and why)?
We saw many dense layers of hake in 2016, but in 2017 the hake were harder to find and were in weaker (i.e. less dense) aggregations. Another interesting observation was that in 2016 hake were in the northern and middle portions of the survey area, but in 2017 they were predominantly in the middle and southern portions of the survey area. We can’t say for sure why 2016 and 2017 were different. Maybe some of the hake decided this year to go way offshore, or down to Mexico, for reasons that made sense in their hake-brains. It is possible that differences in conditions (e.g. temperature, salinity, currents), even if subtle to us, influenced the distribution and densities of hake. We’ll pick up these questions back on land as we start the harder part of our work, the analysis.
Were your science parties for legs 1 and 2 total rock stars?
Yes! Both legs 1 and 2 kept their humor no matter what, joined in on the speculation about where we would find hake, and pitched in to help with everything we did. What we accomplished this year, in spite of the weather, the uncooperative hake, and other factors plotting against us, is a testament to the dedication of the science parties. Even when tired, frustrated, or sick (not naming names, but I will remind *someone* that terrestrial birds are pretty interesting), the scis always kept me laughing and reminded me of why we do this crazy stuff.
Thanks for following along during the at-sea portion of the 2017 Winter Hake Survey adventure. As possible, I’ll post on what we’re finding during the on-land part of our work.
The acoustics lab on the Bell M. Shimada is full of computers and screens. The photo shows the aft (i.e. rear) portion of the lab with 25 screens! Not all screens were being used at the time of the photo, but all are operational and have a purpose. What kind of information are we monitoring or working with that we need this many screens?
As you can imagine, this many screens (and the computers that are attached to them) generate a lot of heat. The acoustics lab can be “68°F and breezy” with all the vents blowing.
We continue to watch the monitors (all of them!) as we head north toward Newport, hoping to find more hake.
During a trawl offshore of San Francisco, where the seafloor was at a depth of ~3800 m, we finally caught female hake that showed signs of having spawned. Score! These may not be the females that co-produced the larvae that we caught in the bongo net (we are 290 nmi north of that spot now), but it’s still exciting.
Unlike the full ovary shown in the previous post, the ovaries from these females were not full. They were loose, baggy, deflated. Interestingly, they do still contain lots of eggs that are developing, likely for an upcoming spawning event. Ovaries from these females were collected and are unique among our samples – we haven’t seen any other females (or males for that matter) that showed evidence of recent spawning. The histological analysis of these samples is going to be so interesting and increase our understanding about hake biology and reproduction.
In the previous post, I mentioned that we found larval hake, and wondered when we would find the adults that might be responsible for those larvae. Imagine how excited we were when we found adult hake just 54 nmi NNE of where we caught larvae in the bongo tow!
Our excitement was short lived.
The adults we caught, with an average nose-to-tail length of 37 cm, had not spawned. The male fish were more developed than the females, something we frequently see. But, while these fish might be close to spawning, neither sex had spawned. Huh.
Assessing the maturity of Pacific hake is done visually at sea. As part of the sampling, the Wet Lab Biologists assess the development of hake ovaries (female) and testes (male). The photo shows the ovary from a 49 cm female who isn’t quite ready to spawn. Some of the ovaries and testes are dissected out, preserved in formalin, and brought back to the lab for closer histological analysis with a microscope. The histological work-up can confirm the at-sea maturity assessment, but also gives a closer look at details like how many eggs a female can produce or whether female hake spawn all of their eggs at once or a little at a time. Important information to know for such an important species.
Sometimes, even after surveying thousands of nautical miles of the ocean (on this trip, and over the years), we see something that piques our curiosity. If weather cooperates, and we have time, we stop and check it out.
This morning, offshore of San Diego, was one of those times. The shallow layer was thick from the surface down to 100 m. What caught our interest (especially that of Chelsea and Chu) was the pattern across the frequencies – it was almost absent on the 18 kHz, very strong at 38 kHz, and then much weaker at 120 kHz.
We guessed that the layer was not fish given the frequency pattern, so opted to deploy the bongo net. This net has two 60-cm rings and 333 um (i.e. micrometers) mesh, so it’s good at catching small things. The bongo is lowered into the water and then towed toward the surface at a 45° angle.
When the bongo tow on our mystery layer was done, we looked into the jar and saw lots of little eyes looking back – larval fish! Given their size, these fish likely hatched from eggs less than two weeks ago. In the close-up photo you can see the distinct dark band on the tail and fins and the dark pigment near the eyes. We’re pretty sure these are larval hake, but need to get confirmation from an ichthyoplankton (i.e. larval fish) specialist.
The big question for us – if these are larval hake, where are the adults?
We collect lots of things at sea – otoliths (“ear bones”) for ageing fish, fin clips to see if our hake are from the same genetic stock, ovaries/testes to see how close to spawning our fish are, stomachs to see what our hake just ate, and tissue samples to see what a hake average diet is like over time.
Perhaps the coolest thing we are collecting from fish on this trip is an x-ray image of them. Dr Dezhang Chu (NWFSC-FRAM) is leading this work. Unlike humans, the fish x-rays aren’t to look at bones, but rather to look at their swimbladders.
(Break for a quick review of acoustics. In fisheries acoustics, also known as “active acoustics,” we send a pulse of sound into the water. That sound energy bounces off anything with density and sound speed that are different than water. The more different the object is than water, the stronger the returned energy back to our equipment. A fish’s swimbladder, which is often filled with air, is very different than water.)
Chu is taking x-ray images of fish (see the adult hake example) to see:
1. How large the swimbladder is, as this affects the amount of energy that is returned
2. How the swimbladder sits in the fish’s body relative to how the fish usually swims. If the swimbladder is very flat in a “normal” swimming orientation, it will reflect more sound than if the swimbladder is at a sharp angle in the body
How can we use this information? The main application is for estimating fish density/biomass. If we understand how much sound energy an individual fish reflects, we can more accurately calculate how many fish are present – simply, the total amount of energy from all fish divided by the average energy we expect to get from one fish. Chu is also interested in understanding how variable swimbladders are from fish to fish, and species to species, to improve models of expected energy returns.
Another application of the x-ray images is to determine how sound energy reflection changes as a fish grows. It is typically assumed that energy return (i.e. target strength) increases with fish size, but that may not be the case for all species. Relationships between target strength and fish length are sometimes developed based on measurements from dissected fish, but the dissection process itself may affect the size/shape of the swimbladder. The x-ray images that Chu is taking will help to more quantitatively describe the swimbladders of hake, as well as other fish species.
So far, we have completed three trawls during Leg 2. This brings our total for the survey to six. Go, Leg 2!
Our first Leg 2 trawl was about 125 nmi offshore of Morro Bay, over 4,000 m bottom depth. We caught hake with an average length of 43 cm and these fish were likely 3 or 4 years old. We had previously seen fish of this size 120 nmi offshore of San Francisco during Leg 1.
The second and third trawls of Leg 2 were just west of Point Conception. One was on a thin layer (<20 m top to bottom) of fish up in the water column. The other was on an aggregation of fish that was near the bottom. Acoustically these two fishing locations looked very different, but both trawls revealed the identity of these fish to be 1 year olds (21 cm and 20 cm mean lengths for the two trawls).
In June 2016, the Northwest Fisheries Science Center’s Prerecruit Survey (Lead Ric Brodeur, Fish Ecology Division) would have caught these 1 year olds when they were age-0s. Ric and his team had been surprised at how far north they found age-0s in 2016, and here we are catching them so far south.
In a few days we will start working our way north again and maybe we’ll find more of our 1 year olds. If we do find the age-1s up north, then perhaps they don’t (always) do a substantial southward winter migration. But, if we don’t find age-1 hake when we go north, then maybe these little fish move longer distances than we think!
Between the weather and delays, we are behind where we’d planned to be at the start of leg 2. Knowing that we still want to acoustically survey the original area, trawl, and do side ops (CTD and zooplankton), a redesign was in order.
In redesigning the survey, needs for all sampling elements (acoustics, trawls, side ops) have to be considered. And then reconsidered. Here is how the redesign calculations worked.
The first (and most important) question is, “How much time do we have?”
Answer: 11 (24-hour) days at sea
The acoustic transects and travel take the most time. We have to decide how far inshore-offshore we want to go, how much distance transiting to/from/between it requires, and make assumptions about ship speed. If we want to cover the same inshore-offshore area as the original transects at a coarser resolution, how much time does that require?
Answer: 7.4 days to cover 1,765 nmi at 10 kt
Trawling is such an important part of our work, that it gets considered next. I’m optimistically budgeting for 3 trawls plus accompanying CTD casts on each of the red diagonal pairs. From start to finish we need up to 4 hours, so how does the math work out?
Answer: 1.5 days to do 9 trawls and 9 CTD casts
Next up are the oceanographic and zooplankton side ops. We have new stations coming up and hope to pick up stations we missed during leg 1. Some stations are CTDs, some CTD and zooplankton vertical net, and some have CTD, vertical net, and bongo. There are 21 side stations we would like to complete. These are identified, based on logistics and discussions with collaborators, as “definitely” or “if time” stations. How much time will that take?
Answer: 1.3 days for “definitely” and 0.4 days for “if time”
Sum total for the above plan: 10.6 days at sea
So long as the weather behaves and operations go well, the redesign is ambitious but achievable. I’ll be tracking our progress as we go to see if we need to reconsider any elements of this plan!
On another note - I wanted to thank the High School Explainers from the Exploratorium for the totally awesome card! I had a great time showing you around the Shimada and am sure that I’ll see some of you out at sea with us in the future.