Field Deployment Report: Bottom-Mounted ADCPs in the Tsugaru Strait near Hakodate

Deployment Notes: Hakodate Coastal Waters, October 2023

The wind was biting as we pushed off from the Hakodate harbor just before 05:00. The air had that sharp, salty sting typical of Hokkaido in autumn. As we headed toward the deployment site, the water looked deceptively calm, but the radar showed the Tsugaru Warm Current pushing hard against the local tide. This is the chaos of Hakodate's coast: you have the warm, nutrient-rich flow from the south colliding with the cold depths of the strait, creating a vertical mixing zone that makes standard current measurements a nightmare.

The site was tricky. We were working in an area where the underwater topography shifts violently from deep trenches to sudden ridges. The water was a murky green, thick with organic matter. I could tell just by looking at the surface chop that the tidal range was peaking. In this part of the Tsugaru Strait, the currents don't just flow; they swirl. If you don't account for the bathymetry, your data is useless.

What We Found

The data came back with a spike that caught us off guard. We clocked peak velocities that far exceeded the regional averages for October. The Tsugaru Warm Current was hammering through the channel, but the real story was the shear. We saw a massive velocity gradient between the bottom bins and the surface. In some profiles, the surface water was racing northeast while the deeper layers were practically stagnant or even reversing. It's a classic case of coastal trapping, but the intensity was higher than my previous models predicted.

I’ll be honest: the surface drifters we ran as a sanity check were a disaster. The wind was blowing a steady 15 knots from the northwest, pushing the buoys right off course. They gave us a 'surface current' that was actually just wind-drift. It proves why you can't trust a floating buoy in the strait. You need a fixed reference on the seabed to get the truth. Once we looked at the ADCP's bottom-track data, the noise cleared up. The real current was moving slower than the buoys suggested, but with a much more complex vertical structure.

Equipment Performance

We used a 600kHz ADCP for this run, and it held up well, though we fought some bin contamination near the seabed. Because the bottom is rocky and uneven around Hakodate, the 'blanking distance' needs to be set carefully, or you get echoes from the terrain bleeding into your first few velocity bins. I noticed some signal attenuation in the mid-water column, likely due to the high biomass—squid and salmon are thick in these waters this time of year. Still, the 600kHz unit provided a clean signal where a 300kHz unit would have been too coarse for the shallow coastal shelf. The battery life remained stable despite the cold water temperatures, which is always a worry in Hokkaido.

Recommendations for Future Deployments

If you're heading back to the Tsugaru Strait, don't wing it. The environment is too volatile for a 'set and forget' approach. I suggest the following:

• Increase the sampling rate during spring tides to capture the rapid acceleration of the ebb flow.
• Use a heavy-duty tripod mount with a wide footprint to prevent the unit from tilting on the uneven rocky bottom.
• Set a larger blanking distance (at least 1.5 meters) to avoid seabed interference in the lower bins.
• Avoid relying on GPS drifters for ground-truthing; use a secondary moored current meter if you need a second opinion.
• Check the salinity gradients before deployment, as the mixing of the Warm Current can create pycnoclines that bend your acoustic signal.

Measuring flow in Hakodate isn't about having the most expensive gear; it's about knowing where the water is fighting itself. If you ignore the topography, you're just guessing. We got the signal we needed, but it took some aggressive filtering to strip out the noise from the biological layers. Next time, I'd deploy two units in a tandem array to see if we can map the horizontal shear across the strait more accurately.

Field report by Dr. Kenji Sato. Dr. Sato is a specialist in underwater acoustics and oceanographic instrumentation with 20 years of experience in high-energy coastal environments.