ROUNDED GOBIES - LAKE ERIE - CENTRAL BASIN - AVON POINT
The photograph
below is of rounded gobies
In 1997, after
the reef had seemed to reach a type of equilibrium a second great change
took place, one that may be as profound as the coming of the Zebra mussels.
I'm not sure when the fish known as the rounded goby first appeared on
the reef. Before their arrival a type of fish called a sculpin would occasionally
be found and they, to my untrained eye, looked much like a goby. The goby
became a major factor in the reef's ecosystem in 1996. Since 1997, they
are found at a density of 5 to 10 fish per square meter in the rocky areas.
The Gobies demonstrate behavior showing a great
curiosity about their environment. When underwater I have held myself motionless,
just above the bottom, to watch the gobies that would leave the safety
of the undersides of the rocks and gather in a circle around me. At times
I have counted as many as seventy-five of them. They would stare up at
me from the bottom with their big bulging eyes making me feel as if I was
suppose to entertain them, perhaps do a song and dance. Their curiosity
makes it hard to get a true sense at what their population density is.
There is no
doubt that these creatures are creating havoc with the ecosystem but they
are great fun to swim with.
CHANGES SINCE THE COMING OF THE GOBIES
The following
as a both an account of the changes going on the Lake's bottom and speculation
on why it is happening. Bare in mind as to the question of why things are
occurring I am just speculating, there is a lot of “ post hoc, ergo
propter hoc –“ in the following.
Four of the many animals I have tried to track over
the years are: 1) Triclads of the genus Dugesia, 2) five species of snails,
3) a midge larvae of the genus Ablabesmyia 4) a bryozoan or ectoproct of
the genus Fredericella.
Triclads are commonly known as planarians or flatworms.
They are about a quarter inch in length and live on the underside of rocks.
Below is a chart of the reef's planarian population density.
# per sq. meter
1994 1995
1996
May
22 214
0
June
221 287
30
July
318 352
260
Aug
539 635
390
Sept
681 413
420
Oct
664 387
200
(The chart
assumes that loose rocks having spaces under them for the animal to live
cover the entire square meter. At some locations on the reef this is the
case but it might be more accurate to divide the above numbers in half
to account for the areas where rock cover is not complete)
(The picture below is
of rounded gobies, note the way they all hover close to the bottom)
As is easily seen the number of planarians
began declining in 1996, in 1997 this animal had almost completely disappeared
from the reef. The only planarians found in the samples in 1997 were seen
on Aug 26. Underwater I found planarians on two rocks on Oct 5.One rock
had eight planarians the other had ten. These were the only planarians
seen the entire summer.Since 1997 I have stumbled across a few planarian
but their numbers are still negligible.
There are five species of snails that live on the
reef: 1) Pleurocera acuta, 2) Elimia livescens, 3) Amnicola limosa, 4)
Ferrissia (sp?) 5) Physa (sp?). These animals have had a decrease in population
every year for the last ten years. A sixth species that used to be found
up until 1991, Gyraulus parvas, is now totally extinct in this area. When
I first began diving on the reef one of the most impressive sights was
the hundreds of E. livescens slowly making their way along the blue clay
flats of the Lake's bottom.Every year since the population of snails has
steadily decreased. This year the snails of all five species have almost
vanished.
The midge larva, Ablabesmyia, lives in tubes it
constructs either on the rocks surface or in the strands of the Cladophora
algae that grows on the rocks. This animal is a detritus scrapper. As organic
matter rains down from the moving waters unto the rocks, the midge scrapes
it up and uses it for food (I think it also uses it this material, as well
as it own secretions, to build the tube it lives in). In all the years
previous to 1997 the midges built most of their nest in the algae strands
or in tunnels on the rocks surface. In 1997 the midges seemed to be building
most of their nest on the undersides of rocks attaching the nest to the
rocks itself.
(Below is a
photo of the midge larvae Ablabesmyia, they range in size from 1 to 4 mm.)
Bryozoans, one of the most beautiful creatures on
the reef, have also been one of the rarest of animals to find. Some summers
only one or two of their colonies were found and none would appear on any
of the rocks sampled. In 1997 this changed. From the bryozoan, Fredericella,
being one of the rarest creatures on the reef it has now become very common.Why
the bryozoans are now commonly found perhaps is because the rounded gobies
have eaten all the planarians. Planarians are known to feed on bryozoans
and with the thousands of planarians the reef once held the odds of a bryozoans
colony escaping these voracious predators was slim. Another predator of
bryozoans are snails and these too have all but disappeared from the area.
In the case of snails though, determining the cause of their population
decline since the great mollusk die-off in the winter of 1991 is, at least
to me, unknown. Their population decline began long before the rounded
gobies started living in this area and whether the gobies help hasten the
snails decline is unknown. This slow disappearance of the snails worries
me. With the Unionicean clams, while I deeply regretted their disappearance,
at least I could point to a cause of why they were lost. With the snails
it is different. I can see no reason why they should all be disappearing
and this troubles me when I wonder about the long-term state of Lake Erie's
ecosystem.
The midge larvae,
Ablabesmyia, display not so much a decrease or increase in population density,
as they appear to have changed their preference in their selection of home
sites. Before the gobies were found on the reef the midges overwhelmingly
built their tubes either on the rock's upper surfaces or in the strands
of the algae Cladophora growing on the rocks. Since the gobies arrival
the midges appear to have shifted the site of their tube building to the
underside of the rocks. This is again, I think, is perhaps directly related
to the disappearance of the planarians. The planarian species found in
this area are extremely light sensitive animals and always move away from
any light source. Midges that built their nest on the rock tops were then
relatively safe from predation and those living in the algae strands were
safer still for the planarians could not climb the algae stalks. There
appears to be two real disadvantages in living in the algae though. One
is that the Cladopora, the long algae strands that is found growing on
rocks and piers, as part of its life cycle, releases its hold on the rocks
and drift in the open waters of the Lake. The other disadvantage is that
when strong wave generated currents sweep through the area the algae can
be torn off the rocks and much of it is thrown up on the beaches. Any larvae
nested in such algae are doomed. Therefore there is a real advantage in
living on the rocks surface but I can't really imagine why living on the
rock's underside would be better than living on the upper side. In fact
living on the top side would appear to have the advantage of having more
edible organic material settling on it providing more food for the midges
but with the elimination of the Planarians the midges are building far
more nest on the rock bottoms then in any years since this study began.
One more point about the midges and planarians; there is a possibility
the midges (and also the mayflies) will be able to increase greatly in
numbers if the
Below is a photo of a mayfly nymph of the genus Stenonema. This mayfly
is different then the burrowing mayfly of the genus Hexagenia, the Hexagenia
are the mayflies that make up the vast swarms found along the Lake's shores
in early summer. The Stenonema are found in far fewer numbers, they live
on
the underside of rocks were they scurry about like scorpions. They live
for at
least to years in this stage before taking their mating flights. Their
bodies are
between 1 to 3 mm. long and their three prong tail matches their body length.
planarian population remains so extremely reduced. I would imagine that
the planarians ate the eggs of both these insects, since planarians seem
to eat anything they can subdue, so with their elimination more eggs should
be able to hatch. The changes that have been written on above cannot be
thought of as negative or positive in terms of the Lake's ecosystem, they
just are. One result of the rounded gobies coming that is possible, and
I have no idea how great or small this possibility may be, is a great decrease
in the fish population in the Lake as a whole. This only has a chance of
happening if the rounded gobies have a high population density on the other
reefs of the Lake and that they eat fish eggs. From a human point of view
such a decrease in the numbers of certain species of fish would be akin
to a disaster. From the perspective of a great number of creatures living
in the Lake though a decrease in the fish population could either a good
thing or one of complete indifference. The crustacean zooplankton, which
is preyed on by fish, would certainly not bemoan the loss of the fishery
and the bryozoans would not even notice their passing. How the changes
in the Lake is viewed is in many cases all a matter of what type of life
form you are. Whether the gobies have a long-term effect on this area of
the Lake is going to depend on their maintaining this dense population.
It is very possible that the gobies population will collapse within the
next year or two. This collapsing of population density occurred with the
zebra mussels back in the winter of 1991- 1992, a population collapse from
which the mollusks have never been able to recover.
I have wanted for some years to end this general
study of the reefs ecosystem and concentrate on studying a very limited
number of species or habitats. But whenever I think I've got a handle on
the overall cycles of the reef's ecosystem nature has thrown me a wicked
curve ball that I can only foul off leaving me at the plate waiting for
the next pitch. This is both frustrating and joyous at the same time. Frustrating
because I would like to eventually get to first base and advance on to
other things. Joyous because I'm still at the plate waiting to see what
pitch nature throws me next. I can't win the game but I sure can have fun
playing until death calls me out.
(Below is a photo of gobies
gathering around me.)
