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Final Report
Major Objectives and Goals: To determine whether round
gobies interfere with mottled sculpin spawning. To compare
spawning shelter requirements of the two species in a effort to
find methods of minimizing round goby spawning success while
maximizing mottled sculpin spawning success.
Summary of Progress: The purpose of our study is to
determine whether the local extinction of mottled sculpin could
be caused by round gobies interfering with spawning and
determine whether habitat modification could minimize round goby
reproduction while maximizing mottled sculpin reproduction. The
mottled sculpin is an important food item for yellow perch in
the Great Lakes. We were able to establish that round gobies can
interfere with mottled sculpin reproduction when male round
gobies seize the spawning shelter and eat the eggs. We were also
able to show that round gobies use larger spawning shelters than
mottled sculpins use. Hence, covering areas of intense round
goby spawning with rock too small for their nesting may be a
useful method for controlling round goby numbers and minimizing
their impact on native species.
Accomplishments: (1) Demonstration of
spawning interference by round gobies on mottled sculpins, (2)
demonstration of differences in nest site selection and
requirements in round gobies and mottled sculpins.
Narrative Report: Since its establishment in
southwestern Lake Michigan in the early 1990's, the round goby
has caused the local extinction of the mottled sculpin in areas
where the round goby has become abundant. The mottled sculpin is
particularly vulnerable because both mottled sculpins and round
gobies prefer similar rocky habitat, have similar diets, and
both species nest in rock cavities. Mottled sculpin are an
important component in the diet of yellow perch in rocky areas
of Lake Michigan and their loss may have a negative impact on
yellow perch. Our work prior to this study suggested that the
cause of mottled sculpin extinctions was due to nesting
interference. In the field we could find male and female mottled
sculpins in reproductive condition and post-reproductive
condition, but could find neither nests nor juveniles. This
project has as its main objectives the determination of whether
round gobies interfere with mottled sculpin nesting and the
determination of whether there are differences in the spawning
requirements for round gobies and mottled sculpins.
To establish whether round
gobies interfered with mottled sculpin nesting, we established
12 pairs of mottled sculpins in sections of an artificial
stream. In ten of the pairs there was successful nesting; two
pairs had one individual die before spawning. In five of the
successful nest site sections a large male round goby was added
and within a few days the mottled sculpin eggs mass had been
eaten by the round goby. In each case the male round goby
changed to its spawning coloration and began to defend the nest
site.
Our measurements of natural
spawning shelters of round gobies and mottled sculpins showed
that round gobies used larger rocks and required a larger
surface area for egg deposition. Nests were sampled from Lake
Michigan (mottled sculpins and round gobies) and Lake Erie, and
the Flint and Shiawassee Rivers, Michigan (round gobies). The
volumes of individual eggs of round gobies and mottled sculpins
are nearly identical, but round gobies lay their eggs in a
single layer while mottled sculpins lay their eggs in a
multilayered mass. Hence an equivalent weight, volume, or number
of mottled sculpin eggs requires a smaller surface area, cavity,
and rock. Importantly, there was virtually no overlap in egg
mass surface areas and very little overlap in rock size with
round gobies using larger rocks than mottled sculpin use. This
suggests that habitats might be modified to discourage round
goby spawning while enhancing mottled sculpin spawning.
Field experiments with
artificial shelters also indicate that it may be feasible to
either modify habitat to discourage round goby spawning and
enhance mottled sculpin spawning. Round gobies deposited eggs in
tile lean-to shelters that were 30 cm square, but usually not in
smaller shelters. Mottled sculpins deposited eggs in shelters
that were either 30 cm square or 15 cm square with about equal
percentages of each type being used. Hence smaller shelters
could potentially be provided for mottled sculpins to spawn in
and these shelters would not be particularly attractive to round
gobies.
Another factor that might be
useful in designing mottled sculpin specific shelters is where
eggs are placed. In lean-to shelters the mottled sculpins always
laid their eggs in corners between the roof and a supporting
wall. Round gobies avoided these locations and deposited their
eggs only on flat surfaces. This difference may be due to a
difference in female position during egg-laying. Female mottled
sculpins lie on their backs while spawning and it may be that
having an adjacent wall helps maintain the posture. Mottled
sculpins never lay their eggs on a ceiling higher than a bit
more than their body depth. Round gobies frequently deposit eggs
on ceilings higher than their body depth. We suspect that female
round gobies use their fused "suction-cup" pelvic fins
to attach to vertical walls and high ceilings during egg
deposition. Because it is difficult to attach to corners it is
difficult to lay eggs there. An artificial mottled sculpin
spawning shelter would probably be too small for round gobies to
use and the internal architecture could include corners that
would discourage use by round gobies while encouraging mottled
sculpin spawning.
At this point in the project we
think it is feasible to enhance mottled sculpin spawning while
inhibiting round goby spawning by (1) habitat modification to
cover round goby spawning areas with rocks too small for
spawning and (2) adding artificial spawning shelters to such
sites for mottled sculpin spawning.
We also began work in two
ancillary areas. During the project we found one site with
extremely dense round goby spawning. We mapped the site and its
nests to determine whether the round gobies tended to aggregate.
We have also been videotaping activity around the nests to
increase our knowledge of round goby spawning behavior. This
will be part of a Master’s thesis by Kirby Wolfe.
We also have been working on
designing round goby spawning shelters that could act as male
and egg traps. In some locales it may be difficult to modify the
habitat, but it might be feasible to attract spawning fish away
from the natural substrates and into shelters that can be
collected at appropriate time intervals during the spawning
season. We had some success with plastic drink containers which
could be cheaply modified into traps. This was the project of a
summer intern, Grace Kilbane.
Brief Summary: Our project has demonstrated
that mottled sculpins are severely impacted by round gobies and
the mechanism is likely to be nesting interference. Male round
gobies may evict defending mottled sculpins to take over a
spawning shelter. The round gobies also consume the mottled
sculpin eggs. It may be feasible to anticipate where round
gobies are most likely to nest and so become a nuisance. It may
also be possible to alter habitat to minimize round goby nesting
or design artificial shelters for mottled sculpins that round
gobies are unlikely to utilize. Is some places it may be
feasible to design "hyper-attractive" round goby nests
that can be harvested for egg destruction.
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