Samples collected with the Matsuda-Oozeki-Hu trawl (MOHT) are used to ground truth the multi-frequency acoustics.
While the use of acoustics provides a real-time high resolution technique to study aquatic organisms, it is an indirect technique and the scatterers present in the water column need to be identified. Because of this, net samples need to be collected to ground truth the acoustic data. The MOHT is a net with a 5 m2 mouth area and a cambered V-type depressor designed to sample juvenile fishes. It can be trawled at speeds of up to 4.5 knots under variable weather conditions (Oozeki et al. 2004).
2. Data Collection
2.1. Three types of oblique tows are conducted:
|Shallow tows:||the net samples to 150 m depth. This is carried out at night to sample migratory mesopelagic organisms as well as epipelagic organisms.|
|Deep tows:||the net samples to 400-600 m depth and is carried out during the day to sample mesopelagic organisms (migrating and nonmigrating).|
|Targeted tows:||sampling is carried out targeting an acoustic feature of interest in the echogram. The net will sample to the depth where the acoustic feature (school or layer) is located.|
Before the net is deployed the TSK flowmeter is “zeroed” and a Temperature & Depth Recorder is attached to the net’s main frame.
Once the net is deployed, deployment time is recorded (when the flowmeter hits the water) as well as the location of the deployment (lat/long)
Ship and wire rates:
Deployment: vessel speed = 4knots, wire speed = 50m/min
Retrieval: vessel speed = 3knots, wire speed = 20m/min
After retrieval, the time when the net is out of the water is recorded as well as the flowmeter reading to estimate water volume filtered.
Data are downloaded from the temperature & depth recorder to make sure the net reached the desired depth. If the net did not sample to the desired depth, then the amount of wire out is adjusted accordingly.
The sample collected is sorted and all fish are separated from the plankton component on board the vessel. Sardine and anchovy larvae and juveniles are separated from the rest of the fish and preserved in ethanol. The rest of the sorted fish and the plankton component of the sample are preserved in 5% formalin.
3. Sample Processing
Plankton samples are split using a Folsom splitter into a fraction small enough for sorting, measuring, and counting.
From the split, organisms are separated into major plankton groups (copepods, chaetognaths, mysids, euphausiids, etc) and counted. Euphausiids are the only group identified to species level.
The length and width of a maximum of 30 individuals of each group are measured.
All fish from samples are identified to species level (unless the individual is in bad condition), measured (total length), weighed, and counted. They are transferred into isopropyl alcohol for preservation soon after.
Biomass and abundance are calculated for each tow using the volume filtered estimated from flowmeter readings and the number of individuals and/or total weight obtained from samples.
Length distributions per group are generated for each group
Length data will be used to estimate the acoustic backscatter cross-section from each individual and to predict the total backscatter using the appropriate acoustic models
- Matsuda-Oozeki-Hu trawl net, frame and cambered V-type depressor
- TSK flowmeter (Tsurumi Seiki Co., LTD.)
- Wildlife computers temperature & depth recorder
- Folsom splitter
- Stereo microscope with fitted reticule
- Weighing scale for fish
- Fish measuring board
- Sample jars of different capacity
- 37% Formaldehyde
- Sodium Borate with filtered seawater for a saturated solution
- Isopropyl alcohol
- Oozeki, Y., F.X. Hu, H. Kubota, H. Sugisaki, and R. Kimura. 2004. Newly designed quantitative frame trawl for sampling larval and juvenile pelagic fish. Fisheries Science 70: 223-232.