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Sargassum City

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One of the goals of this cruise is the research done by Jay Rooker, PhD and his lab on pelagic fishes, such as billfish and tuna.  This ties in the data being collected by DEEPEND with information from the epipelagic zone.  When I picture a billfish what usually comes to mind is the adult stage that recreational fisherman target for sport.  It’s easy to forget that they all start out as zooplankton, and we’ve been finding a few in each of the tows. The image below depicts a sailfish on the top and a blue marlin on the bottom. 


Attached to each of the plankton nets is a flowmeter, which looks similar to a rocket.  


It measures the volume of water flowing through the net.  This allows researchers to calculate the number of fish and other organisms collected per unit volume of water and make estimations about a larger area. 


Since the neuston net is towed at the surface, it frequently contains large amounts of sargassum.  Dr. Rooker explained that areas with high amounts of sargassum usually yield low amounts of these epipelagic fish. We saw evidence of this during our last tow. 

This floating macroalgae is carefully searched for specimens by the group and then weighed.  It’s been very impressive to see the sharp eyes of those around me pick out fish from a heap of sargassum no more than 1 mm long. 


The tiny dot below is on the finger of Maelle Cornic, a PhD student.  Maelle looks at early life stages of tuna in the Gulf of Mexico.  She’s got some of the sharpest eyes on the ship and is consistently finding the smallest critters hiding in the sargassum.  Maelle is from France, where she says being around water sparked her interest in fish from an early age. 


Another PhD student in Dr. Rooker's lab is Mike Dance, an avid fisherman.  Part of his research uses acoustic telemetry, a tracking technology using tags, to monitor the behavior of juvenile red drum in Texas estuaries.  In addition to finding out where the fish are spending their time, Mike determines the type of habitat (oyster bed, seagrass, etc.).  Mike focuses on red drum ages 2-18 months.  Models based on the data collected will shed new insight into the nursery habitats of the fish.


Our last plankton tow of the night once again yielded a diverse group of organisms.  Among those collected was a viperfish, Chauliodus sloani.  Kendall is busy counting the photophores using a stereoscope that projects onto the computer.  This allows pictures and video to be collected of organisms before they're preserved to help with identification later on.  


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  • Guest
    Frank Wednesday, 22 July 2015

    Great photos! It's so interesting to see how such tiny zooplankton develop into grand billfish such as marlin and sailfish!

    Ms. Ehlers, you mention that Dr. Roocker found that in areas where there is a large concentration of sargassum there tends to be lower concentrations of pelagic fish. Why do you think that is?

  • Megan Ehlers
    Megan Ehlers Thursday, 23 July 2015

    It may be because of predators hiding in the sargassum.

  • Guest
    Ryan Thursday, 23 July 2015

    Hi Ms. Ehlers,

    You mentioned sargassum is an algae. Is it something that can be harmful to people or fish, like red tide blooms?

  • Megan Ehlers
    Megan Ehlers Thursday, 23 July 2015

    Great question! No, to my knowledge, sargassum doesn't contain the toxin like red tide has that can kill fish, sea birds, etc. One of the reasons we have to worry about harmful algal blooms (HABs), like red tide, is the toxin that accumulates into the tissue of filter feeding bivalves that we might end up eating. Even if sargassum did contain a toxin, it's not a tiny single celled phytoplankton like the dinoflagellates that make up the HAB blooms.

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