Naturespeak A naturalist's view of the world

May 13, 2009

A Big Fish on the Big D

Filed under: Uncategorized — wykes @ 12:35 pm

If you’ve been keeping track, you will notice that this is the fourth time I’ve brought up the subject of Sturgeon on this blog. Every time I go out on the U.S.F.W.S. research vessel on their Lake Sturgeon survey and stand in the presence of one of these giant fish, my fascination is re-kindled. When I am kindled, my readers are the first to be affected (or infected, depending upon your view point).

Since the subject has been explored before, I won’t go into the fine details of the research other than to mention that it is conducted on the lower Detroit River under the supervision of fisheries biologists Jim Boase and Dr. Bruce Manny. This team has been conducting a long-term study of Lake Sturgeon biology and ecology on the river. As part of their work, it is necessary to gather data from individual fish. From aboard the aluminum hulled vessel Sentinel, set lines, each with 25 baited hooks spaced about ten feet apart, are placed in about 30 foot of water around the vicinity of Fighting Island. The lines are checked daily and the captured fish are weighted, measured, transmitter tagged, and released. My role in this thing is an impassioned volunteer and to assist with the duties.

We captured four sturgeon on my first trip last week. Three of those fish were secured on one set line, which is somewhat unusual. The smallest was about 15 pounds and measured about three feet (only in the world of giants would such a fish be called “small”) while the largest measured about five feet from snout to tail end and tipped the scales at 60.5 pounds (see here). I was quite satisfied with these fish, but Dr. Manny still talks about the seven footer they nabbed last year. “That fish,” he proudly boasts while holding out his hands in an arc, “had a girth of over a meter.” I remember him describing that same fish last year and accused him of upping the size each time he talks about it – he doesn’t, but I like to chide him anyway. I get the feeling that Herman Melville might appreciate this big fish story.

We all hope to re-capture that legendary beast again. On my second trip this year, one of the hooks came up empty and was bent horribly out of shape. The consensus was that  “Moby” was responsible – perhaps.

Content with the fish at hand, however, I made it a point to concentrate on some of the unique features other than size. Sturgeon come from an extremely ancient lineage which dates back some 200 million years. Over all this time, they have changed very little in appearance because their original design left no need for improvement. There are over 20 species of sturgeon today, but only the Lake Sturgeon is found in Great Lakes waters. Like all the sturgeon species, old and new, they are possessed of a cartilaginous notochord, rather than a segmented backbone, and are equipped with bony plates embedded in their scaleless skin. In other words, the only bony part of their internal skeleton is their head capsule.

We didn’t re-capture the “really big one,” but the largest fish of the day was a re-capture. A scar on the left front, or pectoral, fin indicated that this individual had been sampled before (see here). One part of the data collection process is to cut off the first fin ray for aging purposes. When this activity is first executed, the surgery leaves a raw-looking gap. It was good to see how this portion heals over after a few years. Another indication that this fish had been deck-side before was the presence of an embedded transmitter called a PIT tag. The transmitter set off a signal when a detector was passed over the region just back of the head. This fish was missing the old FLOY tag that was once attached to the dorsal fin, but a new one was supplied in its place before the fish was released (see below).

A raw lamprey scar on the body just behind the dorsal fin provided graphic evidence that a Sea Lamprey had fed off of this giant sometime in the recent past (see here). It appears that research scientists are not the only sturgeon samplers out there. Lampreys come from a line of parasitic fish that pre-date sturgeons by some 100 million years, so they have been at their task since day one. One of the unique behaviors of the Lake Sturgeon is that they are known to jump out of the water like breaching whales. It is believed that they may do this in order to dislodge pesky blood-sucking lampreys (or annoying FLOY tags).

In profile, the head of the Lake Sturgeon is a thing of primitive beauty (see above and here). Just ahead of those expressive eyes are the two-parted nostril openings. The nostrils of fish do not open up into the mouth cavity, but instead enter into a sensory pocket in the head. In this case, incurrent water is taken into the first opening, passed over the sensors, and then excurrented out the second. There is a peculiar star-like organ within this chamber that you can see in the first picture.  Dangling from ventral part of the snout, four fleshy barbels, or whiskers if you like, are used to “feel” the bottom and detect potential food items such as invertebrates, snails, or crayfish.

When viewed from below (see above) these barbels share their space with dozens of electro-sensory pores that dot the white flesh surface of the snout.  The entire head is equipped as a super efficient  metal detector and mine sweeper of sorts. Once flipped onto this view, however, the one feature that grabs the eye is the impressive mouth. This whole unit can be extended out – about four inches on large individuals – to vacuum up food and gravel (see below). the gravel is expelled out the gills and the food items are crushed between bony plates in the throat.

One of the data points that needs satisfying is to identify the sex of any given fish. Sexing a sturgeon can be easy to do – or it can be hard. It has nothing to do with make-up or barbel curlers but depends more on the season. During the spring breeding period, the males will emit seminal fluid and the females will emit eggs if prompted to do so. Prompting, in this case, consists of a firm but gentle hand massage from the mid-belly down to the vent opening.  Not wanting to change the rating of this site, I elected not to show you the result of a “spermeating male” but I can show you a massage conducted on a female (see here).

This female turned out to be eggless, but the size of the vent hole was a good indicator of her femininity. I did end up seeing some sturgeon eggs before my river season was over, however. That story will have to wait until Part II (I have Seen the Future).

16 Comments »

  1. Wow, Amazing.
    They do have oddly expressive eyes. I think the big one is thinking “Some days aren’t worth getting out of bed for.”

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