J. Great Lakes Res. 19(3):630-633
Internat. Assoc. Great Lakes Res., 1993
Note
Growth-Temperature Relation
for Young-Of-The-Year Ruffe
Thomas A. Edsall1, James H. Selgeby2,
Timothy J. DeSorcie1, and John R. P. French III1
Abstract.
The ruffe (Gymnocephalus cernuus) was accidentally introduced into
the Great Lakes basin from Eurasia and has established a breeding population in the St.
Louis River, a major tributary to western Lake Superior. We captured young-of-the-year
ruffe in the St. Louis River; acclimated groups of 90-91 fish to test temperatures of 7, 10,
15, 20, and 25°C; and fed them ad libitum for 42 days at those temperatures. Ruffe grew
at all five temperatures, but the optimum temperature for growth was about 21°C.
Because the optimum temperature for growth of walleye (Stizostedion vitreum), sauger
(Stizoste-dion canadense), and yellow perch (Perca flavescens) is about 22°C, ruffe will
probably attempt to share their thermal habitat. A recent survey of the St. Louis River
revealed that yellow perch and small forage fish declined sharply as ruffe abundance
increased. A similar decline in yellow perch abundance in Lakes Michigan, Huron, and
Erie would seriously affect the fisheries in these lakes.
INDEX WORDS: Ruffe, temperature optimum, fish, growth.
Introduction
The ruffe (Gymnocephalus cernuus), a small Eurasian percid, was recently introduced
into the Great Lakes basin and has established a breeding population in the St. Louis
River at the western end of Lake Superior (Simon and Vondruska 1991, French and
Edsall 1992, GLFC 1992, Pratt et al. 1992). In Europe, the ruffe contributes little to the
fishery and is generally considered a predatory and competetive liability in the fish
community. It competes with other more desirable fishes for food, preys on their eggs and
larvae, and is not a preferred prey of the larger predator species (Fedorova and Vetkasov
1974, Sterligova and Pavloviskiy 1984, Pratt et al. 1992). The introduction of ruffe was
blamed for a decline of 50% in whitefish populations (Coregonus spp.) in Russia
(Pavlovsky and Sterligova 1987) and for an 88% decrease in the abundance of Eurasian
perch (Perca fluviatilis) in Loch Lomond, Scotland (Maitland et al. 1983, Maitland and
East 1989).
1
U.S. Fish and Wildlife Service, National Fisheries Research Center-Great
Lakes, 1451 Green Road, Ann Arbor, Michigan 48105, USA
2
U.S. Fish and Wildlife Service, Ashland Biological Station,
2800 Lake Shore Drive, East, Ashland, Wisconsin 54806, USA
There is considerable concern that the ruffe will spread throughout the Great Lakes and
into the connecting Hudson and Mississippi river drainages, and efforts are underway to
reduce the abundance of ruffe and prevent its dispersal from the St. Louis River (GLFC
1992). We undertook this study of the growth-temperature response of the ruffe to
provide a basis for defining thermally suitable habitats in the Great Lakes that ruffe might
colonize with adverse effects on desirable native or introduced fishes.
Methods
Young-of-the-year ruffe were collected with bottom trawls in the St. Louis River at the
western end of Lake Superior on 5 August 1992 and transported immediately to the
laboratory (National Fisheries Research Center-Great Lakes, Ann Arbor, Michigan). We
held the ruffe in the laboratory in aerated, flowing water at 15°C and fed them
macroinverte-brates (mainly Mysis relicta and Diaporeia affinis) that we had collected
with a sled trawl in Lake Michigan near Saugatuck, Michigan, on 1-2 July 1992. The
trawl was fitted with a 1-m-diameter, conical plankton net constructed of 656-µm-mesh
Nitex. Trawl tows lasted for 30 min and the catch from each tow was immediately frozen
in water in 0.5-kg lots and stored at -10°C until used.
Mortality of ruffe in the laboratory ceased and they were feeding regularly by mid- to late
August. On 26 August we apportioned the ruffe among five 190-L fiberglass tanks and
began acclimating them to the test temperatures at rates of about -0.5 or +l°C/day (Table
1). Acclimation schedules were designed to ensure all fish completed their acclimation on
8 September. Each tank was equipped with a thermostatically controlled heater to
maintain the acclimation and test temperatures. Temperature was measured daily to the
nearest 0.1°C with an electronic thermometer, and tank thermostats were adjusted as
needed to maintain the desired temperature. A water flow of 2 L/min was maintained in
each tank, and continuous aeration was provided with a compressed air bubbler in each
tank. Dissolved oxygen concentration was measured biweekly throughout the study with
a YSI Model 54-A oxygen meter3 to ensure oxygen concentrations remained at 80-90% of
saturation. Photoperiod was controlled with a timer set to provide 15 hours of light and 9
hours of darkness. Tanks were covered with lids to create a dimly lit environment for the
ruffe during daylight hours. We also placed short pieces of PVC pipe in each tank to
provide structure and cover; the ruffe used them extensively during daylight hours. Fish
were fed macroinvertebrates, ad libitum, twice daily (up to 20% body weight/day), and
the feeding rate was adjusted daily to ensure that uneaten food remained in each tank at
the end of the day. Uneaten food was left in the tank overnight, and tanks were cleaned
each morning before new food was presented. Mortality was recorded daily.
On 9 September (day 1 of the growth study), we removed each test group from its tank
and placed it in a beaker containing water and MS 222. When the ruffe were
immobilized, we poured them into a fine-meshed net to remove excess water, placed
them in a small plastic bag, and weighed them as a group to the nearest 0.1 g. We
immediately returned the ruffe to their test tank and then reweighed the bag (and any
water that had accumulated in it from the fish) and subtracted that weight from the
previous weight to give the corrected fish weight. The ruffe recovered from the anesthetic
in less than 5 min and fed normally later in the day when food was offered. We repeated
this weighing proceedure on days 21 and 42; the test ended on day 42. All weighings
were done in the morning before the fish were fed. On day 1, the 7 and 10°C tanks each
contained 91 ruffe, and the 15, 20, and 25°C tanks each held 92. On day 43, the ruffe
were anesthetized and measured (total length) to the nearest 1.0 mm.
3
Mention of trade names or manufacturers does not imply U.S.
Government endorsement of a commercial product.
Results and Discussion
Ruffe adapted quickly to conditions in the test tanks and readily ate the macroinvertebrate
diet we provided. Mortality during the 42 days of the growth study was 0.7% (Table 1).
One fish died on day 3 and another on day 19 in the 15°C tank, one fish died on day 3 in
the 20°C tank, and no fish died in the other tanks.
Ruffe increased in weight throughout the study at all of the temperatures tested (Table 1).
After 42 days of ad libitum feeding, ruffe were heaviest and longest at 20°C, slightly
heavier and shorter at 25°C than at 15°C, and progressively smaller at 15, 10, and 7°C.
We did not develop a length-weight relation for the ruffe that we tested, but the weightto-length ratios on day 43 were highest at 20°C and progressively lower at 25, 15, 10, and
7°C. Thus, after 42 days of ad libitum feeding, ruffe at test temperatures near the
optimum temperature for growth were not only heavier and longer, but were also heavier
for their length than were ruffe held at the other test temperatures. Specific growth rate
was highest at 20°C and progressively lower at 25, 15, 10, and 7°C (Table 1). A curve
fitted by spline function (SlideWrite for Windows V1.0) to the specific growth rate data
for days 1-42 indicated that the optimum temperature for growth in weight was about
21°C (Fig. 1).
A growth optimum at about 21°C indicates that the ruffe is a temperate mesotherm
(Hokanson 1977), as are the larger native North American percids—walleye (Stizostedion
vitreum), sauger (Stizostedion canadense), and yellow perch (Perca flavescens)—each of
which has a growth optimum at 22°C (Koenst and Smith 1976, Huh et al. 1976). Thus,
the fundamental thermal niche (defined by Magnuson et al. 1979 as the optimum
temperature for growth in weight ±2°C; redefined as +1, -3°C by Christie and Regier
1988) is 19-23°C for walleye, sauger, and yellow perch and 18-22°C for ruffe. This
thermal niche correspondence among the four species indicates that they are likely to
share the same habitat in the Great Lakes.
Christie and Regier (1988) developed estimates of the thermal habitat area (THA)
available to walleye in each of the Great Lakes, and these estimates provide a measure of
the THA available to ruffe, as well as to sauger and yellow perch. The THA is a
cumulative measure (hectares, summed at 10 day intervals) of the amount of lake bottom
within the fundamental thermal niche of the species during the growing season. Christie
and Regier (1988) calculated the THA for each lake from isotherm representations of the
summer (5 June-2 September) thermal structure in the lake and the hypsograph
(cumulative area versus depth curve) for the lake.
Walleye THA in the Great Lakes totaled 6.6 million hectares (Christie and Regier 1988).
Lake Erie, the shallowest and warmest lake, had 57.6% of the total; Lake Huron, strongly
influenced by the North Channel, Georgian Bay, and Saginaw Bay, 21.1%; Lake
Michigan and Green Bay, 11.9%; Lake Ontario, 7.4%; and Lake Superior, the deepest
and coldest lake, 1.9%. Thus, if ruffe spread throughout the Great Lakes, and if walleye
and ruffe have similar realized niches (Magnuson et al. 1979), Lakes Erie, Huron, and
Michigan might develop the largest populations of ruffe.
The effects of large populations of ruffe on the fish communities of Lakes Erie, Huron,
and Michigan could be substantial. The ruffe population of the 4,400-hectare St. Louis
River estuary increased sharply following their discovery in 1987, to more than 2 million
fish in 1992, and the abundance of yellow perch and small forage fish declined sharply
during the same period (GLFC 1992). The yellow perch is an important recreational and
commercial species in Lakes Michigan, Huron, and Erie. A decline in yellow perch
abundance similar to that seen in the St. Louis River estuary would seriously affect the
fishery for yellow perch in the Great Lakes, which is presently valued at US $101 million
in Lake Erie alone (GLFC 1992).
Acknowledgments
We thank G. Christie, J. Gannon, and J. Savino for constructive reviews of the manuscript. This is contribution 831 of the National Fisheries Research Center-Great Lakes,
U.S. Fish and Wildlife Service, 1451 Green Road, Ann Arbor, Michigan 48105 USA.