Weedlines: A Sargassum Blog

PS-19-05 Daily Log

May 29, 2019 (Day 2)*

*Note: these will be posted on a one-day delay, i.e., this report is for yesterday’s activities. 

We hope you enjoyed yesterday’s blog entry. Remember we also post throughout the day on the @larvalfishlabTwitter account, so follow us there as well!

Now on to today’s action (Day 2), which includes several non-fishy visitors …


Some of our non-fishy visitors (Photo: Sandra Huynh)

We couldn’t have asked for a better start than the one we had today. There’s nothing quite like waking up, walking up to the wheelhouse, and seeing rafts of Sargassum all around. Our decision to take advantage of the relatively calm conditions to the east of yesterday’s sampling region really paid off. Kudos to the R/V Point Sur night crew! Plenty of Sargassum, seas less than 1 foot, and winds around 5 knots–perfect conditions.


The CTD sampling the surface water just below the Sargassum. (Photo: Sandra Huynh)

We started the day with a CTD cast, followed by a neuston tow through the weedline. The neuston tow went well. Shortly after getting the net on board, we drifted along with the Sargassum as we processed the net sample. During this time, a juvenile sea turtle (unknown species) was observed swimming away from a nearby mat towards a more distant Sargassum features. Such encounters serve as a reminder that Sargassum is home to many species, not just fishes. And it’s also the reason we take great precautions when sampling with our nets to avoid any incidental captures of turtles.


(Photo: Sandra Huynh)

Following the neuston tow, we did a standardized hook-and-line sampling set, which for us includes four anglers fishing along side a Sargassum mat for 30 minutes using small hook Sabiki rigs. We do this to collect some of the larger juveniles that typically evade our neuston net. Today it worked very well. So well in fact that we cut our fishing time short; after 15 minutes we had 35 fishes, all Seriola spp. (amberjack species), and most were likely Almaco Jacks (though we will confirm identifications later through morphometric and molecular analyses). These fishes are definitely too fast for our nets, which is why we often use multiple gear types when we sample Sargassum (and other) habitats.


Graduate student, April Hugi, dropping a sabiki line to catch juvenile Sargassum associated fishes. (Photo: Sandra Huynh)

Seas were calm enough to deploy the small boat for additional sampling operations. Using the small boat, we were able to place our floating camera rig into a large Sargassum patch, where it recorded video for approximately two hours. During this time, algal reflectance measurements were also collected from the small boat; these data are used to groundtruth and validate our remote sensing (satellite) observations, as well as improve the algorithms used to detect Sargassum from space.


Dr. Frank Hernandez (Photo: Sandra Huynh)

Having had such a great start, we decided to collect an extra neuston net sample from a very large Sargassum mat at today’s station. Here we picked up several Sargassumfish, Sergeant Majors, and a Sargassum pipefish. As with a few trips last year, we are finding the Grey Triggerfish to be elusive–we see some larger juveniles swimming under the mats at times, yet we have only collected one so far in a net, and none during the sabiki fishing. Hopefully the video survey will provide some abundance estimates for this species; again, a great example of why we use multiple gears!




The maiden voyage of the “deuceton.” (Photo: Sandra Huynh)

After collecting more reflectance data from the Sargassum mat, we set a course for an open water station where we collected water samples and a CTD profile, as well as an open water neuston sample, which was loaded with larval fishes, including larval goatfishes, which are distinctive in their countershading coloration. We also experimented with a new, “double neuston” net, or as we call it, the “deuceton net”. Like our standard neuston net, this frame has a 2 x 1 m opening fitted with a 505 micron mesh net; but the frame also has a smaller, 0.5 x 1 m opening fitted with a 150 micron mesh net. The idea here is that the larger net collects fish larvae and the smaller net will collect zooplankton prey concurrently. And it worked ! Two tows yielded numerous larvae, including several billfish larvae, and a beautiful “blue dragon” nudibranch. We’re hoping this additional net adds to our knowledge of the planktonic food webs of open ocean waters adjacent to Sargassum habitats.

It was a long day to say the least, but a very successful one. Our plan is to steam south-southeast from our current location to take advantage of favorable seas in the eastern Gulf and high Sargassum biomass.


Photo: Sandra Huynh

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