Weedlines: A Sargassum Blog

PS-19-05 Daily Log

June 3, 2019 (Day 7)*

*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!

Another scorcher today on the water. The sun was unrelenting, and there was barely a breeze out today. The only upside was the near glass-like sea state, which makes for nice working conditions, especially for our small boat operations. It’s hard to imagine a vast area like the Gulf of Mexico having such conditions, but the water’s surface was so smooth it was actually difficult to locate Sargassumin the distance, as there were no wave crests and troughs to break the horizon. Fortunately, we found ourselves surrounded by Sargassum yet again at the northernmost extent of the Loop Current in an area located approximately 70 nautical miles south-southeast of the Louisiana birdsfoot. So of course we got right down to work.


The neuston collecting sargassum in the beautiful blue water (Photo: Sandra Huynh)


PhD student, Zabe Premo, filters water in the R/V Point Sur lab. (Photo: Sandra Huynh)

First, we collected our CTD profile and water samples at the edge of a large Sargassum weedline, and with no wind it was relatively easy to stay on station without disrupting the algae. As the sun rose higher in the sky, the ‘blueness’ and clarity of the water became apparent, and we could see schools of adult Mahi Mahi swimming at depth from patch to patch. We then towed our neuston net along the same weedline and collected numerous juvenile fishes, including Whitespotted Filefish, Grey Triggerfish, Scrawled Filefish, and small jacks, among other species. The small boat team departed to take reflectance observations in Sargassumand open water habitats, and those remaining on board completed a Sabiki hook-and-line set. For the first time during the cruise, we did not catch many fishes with the Sabiki rigs, perhaps only five or so amberjacks. We also did not see a lot of larger juveniles swimming around–perhaps due to those adult Mahi cruising the weedlines…..?


R/V Point Sur Marine tech, Josh Bierbaum, was able to snap this beautiful picture of dolphins leading the small boat back to the mother ship. (Photo: Josh Bierbaum)

Or perhaps due to another set of visitors that arrived shortly after our fishing set. While fishing we could see a small pod of dolphins swimming, leaping, and all. around having a good time in the distance. About a half hour or so after our fishing set, they had made their way to the vessel, and as best as we could tell these were rough-toothed dolphins (we have lots of pictures if any experts want to help us out!). One of the dolphins seemed to be having a particularly good time, and we watched as it repeatedly tossed an object in the air and retrieve it. It looked to be a small fish, and once we captured a few pictures (thanks to Casey Hurt’s excellent photography skills) we could actually identify it as a Grey Triggerfish, one of our target species! We’re not sure if the dolphin ever consumed the triggerfish, or if it just toyed with it for a while. We did see some of the larger dolphins in this group chase and feed on adult Mahi Mahi, which was exciting. We often encounter dolphins at sea during our cruise, but mostly as they are riding our bow as we transit between stations. We’ve not seen them forage within the mats before, though given the number of potential prey within Sargassum, this is not surprising. And whether dolphins use Sargassum habitats regularly or opportunistically is unknown. In any event, today was once again a reminder of how much there is yet to learn about the role of Sargassum in offshore environments.


The talented R/V Point Sur Second Mate and photographer, Casey Hurt, was able to snap this amazing picture of a dolphin playing with a trigger fish. 

After all of the marine mammal excitement, and a quick lunch, we decided to collect another neuston sample in this region. This was one of those decisions that seemed like a good idea at the time, but given the heat and the amount of time it took to process a second neuston sample, the science team was pretty worn out at day’s end. After a well-deserved dinner break, we collected our open water station samples, which are not nearly as time-consuming and laborious, as well as a pair of deuceton net samples (and more billfish larvae!). There was a particularly nice sun set this evening, which always puts a smile on your face.


Hernandez lab members spend countless hours rinsing and sieving down the sargassum to preserve for analysis back in the lab. (Photos: Sandra Huynh)

It’s our seventh straight day of sampling, and the weariness is starting to set in, though I know we’ll all be sad when we start steaming for home tomorrow evening. Tomorrow we will likely sample again near the birdsfoot region, perhaps along the ‘tide line’, or Mississippi River plume edge, where there is usually a convergence zone.

So please join us again as we begin to wrap up our final cruise.

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