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Discussion in 'Dive Dry with Dr. Bill' started by drbill, Oct 17, 2020.

  1. drbill

    drbill The Lorax for the Kelp Forest Scuba Legend

    # of Dives: 2,500 - 4,999
    Location: Santa Catalina Island, CA

    I reached my interim goal of 150 dives for the year in mid-October. Not bad for an old geezer battling cancer and a 2 1/2 month closure of our dive park due to COVID-19! With 2 1/2 months left in 2020, I may reach 200. Of course the number of dives is an important marker, but even more important is what I see on those dives... or, in today's case, what I don't.

    One thing I've noticed this year is a lack of a certain flatfish known as a C-O sole (Pleuronichthys coenosus). Although not extremely common in the dive park, I normally would see dozens of these bottom-dwelling fish each year. They prefer sandy bottoms but can also be found in areas covered with rocks.Wikipedia states it is found in "subtropical" waters from Sitka, Alaska in the north to San Quintín, Baja California in the south. I guess they have a different definition of "subtropical" than the one I use!

    The C-O sole is a fairly small flatfish, reaching a maximum length of about 14 inches. Coloration is highly variable and ranges from an off-white to very dark brown and in some areas of Baja California they are almost black. Most of the ones I've seen in the past have been a mottled tan to brown. I've even seen them appearing to mimic encrusting red algae when living on rocky bottoms. They can be readily identified by the presence of a dark circle and a dark crescent-shaped mark on their caudal or tail fin. It is from these markings that they get the common name of C-O sole.

    The year 2020 certainly has been a strange one with COVID-19, massive wildland fires and hurricanes. In my experience it is also unusual in the fact that I have only seen one C-O sole in our local dive park all year. On recent dives I did observe a small California halibut and later a hornyhead turbot swimming mid-water. As a marine ecologist, I look out for changes in species composition and abundance in our local kelp forests, especially the dive park.

    I put on my scientist's cap and tried to come up with hypotheses to explain their relative absence this year. A few divers have stated they've seen them in the dive park, but in very low numbers. Given their so-called "subtropical" distribution from as far north as Alaska, I assume their temperatures are quite unlike my own and that they favor cooler water.Water temperatures this year have been pretty warm. I've recorded surface water as high as 78° F and even down at 95 feet as high as 71° F.

    My first hypothesis was that these fish were heading into deeper, cooler waters given their preferred temperature range and the warm conditions within the depth limits of the dive park. This seemed a reasonable explanation. However, other divers were reporting them with greater frequency in normal depths elsewhere around the island, including sites reasonably close to Avalon and the dive park. Maybe this wasn't the answer.

    When I conducted my Ph.D. research, I initially made an hypothesis about the depth distribution of giant kelp (Macrocystis pyrifera) along the entire coast of Catalina Island. I analyzed it on both the leeward and windward coasts. The data proved that my initial hypothesis was in error so I had to come up with another to explain what the data revealed.

    I opened my mind to other possibilities regarding the C-O sole. What was different this year that might explain the relative absence of this interesting fish in our dive park? I have had a lot of fun filming our wreckfish (giant sea bass, Stereolepis gigas) throughout the late spring and summer. I have seen these gentle giants with fish tails (yellowtail and kelp bass) sticking out of their mouths. They are suction feeders that reportedly can slurp up a lobster while it is sheltering in the rocks.

    So could predation by wreckfish explain the low numbers of C-O sole? That certainly seemed to be a possibility. The more I thought about it, the more I questioned this hypothesis (as a good scientist should). Wreckfish have used the dive park as a courtship aggregation site for at least two decades. I've seen more C-O sole during those years than I've seen this year. I don't doubt the wreckfish suck up these exposed flatfish out on the sandy bottom. However, that should have happened every summer over the last two decades.

    So, I'm stuck without a firm answer to the mystery of the missing sole. yet another mystery for this denizen of the deep to solve. After all, I'm a "Sole Man."

    © 2020 Dr. Bill Bushing. For the entire archived set of over 875 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page

    Image caption: C-O sole in various color variants.

    DDDB 882 C-O sole sm.jpg
    АлександрД likes this.

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