Fisheries biologists have greatly altered the natural distribution of the crustacean Mysis relicta by introducing it into many lakes in both North America and Scandinavia to serve as a supplementary food source for fish. The original concept of Mysis as a deepwater benthic organism which remains isolated in the profundal zone of lakes needs to be modified to include its downstream dispersal capabilities. After introduction to a lake, Mysis will probably eventually reach all lakes in the downstream watershed. Recent studies indicate that rates of increase of introduced Mysis populations probably depend on available food and temperature. As well, introduced populations have been shown to modify benthic, phytoplankton, zooplankton, and fish communities. Most Mysis introductions have taken place without consideration of general criteria suggested for introduced species.

Mysis relicta and planktivorous fish in Lakes Ontario and Michigan both feed on crustacean zooplankton in the metalimnion. Are these zooplankton sufficient to meet the energy requirements of mysids? Could mysids remove a significant proportion of zooplankton production? Are the energy requirements in the two lakes similar? Comparisons in Lake Ontario of zooplankton consumption based on clearance rates, with energy requirements based on bioenergetic modelling, revealed that individual mysids, particularly larger ones, required additional energy sources. The denser mysid populations beyond 100 m depth in Lake Ontario could exert high mortality rates on metalimnetic zooplankton, remove a significant proportion of daily zooplankton production, and consequently, potentially compete with planktivorous fish. At depths < 100 m, the mysid population was smaller and could remove only 6–19% of zooplankton production per day in summer when competition with planktivorous fish would be maximal. Generation time is shorter in Lake Michigan because winter growth rates remain high. Consequently, less energy is required to complete a generation in Lake Michigan than in Lake Ontario.

Feeding habits and assimilation efficiencies wcrc studied for Mysis relicta from Char Lake, N.W.T., and Stony Lake, Ontario.Stony Lake animals were rather voracious predators, feeding on Daphnia and other cladocerans during their vertical migration at night. Char Lake mysids fed primarily on diatoms and inorganic particles on a moss substrate. The gravimetric method showed an assimilation efficiency of 85% for Stony Lake mysids feeding on Daphnia pzhlex; the Conover ash-ratio method showed 52%. Results of the ashratio method corrected for ash assimilation, using the gravimetric data, gave an assimilation efficiency of 87%, indicating that ash assimilation can account for the difference between the two methods. A negative assimilation efficiency was obtained by the ash-ratio method for Char Lake mysids feeding on moss washings, suggesting that mysids may bc able to select organic material from the substrate.

Mysis relicta from Char Lake (N.W.T.) take 2 years to reach maturity, retaining 36 cal as net production; those from Stony Lake (southern Ontario) mature in 1 year, retaining 53 cal as net production. Growth rates were highest during summer months for individuals in both lakes. Results of respiration experiments at 0, 2, 4, 6, and 8 C for Char Lake mysids and at 2, 4, 7, 10, and 13 C for Stony Lake mysids were not significantly different, indicating that Mysis relicta demonstrates no metabolic compensation over its environmental temperature range. Generalized energy budgets were constructed for individuals from each population for their entire life-span. A female uses approximately 209 cal for growth and respiration to become a reproducing adult in one year in Stony Lake and approximately 206 cal to become a reproducing adult in 2 years in Char Lake.

Areal hypolimnetic oxygen deficits in 14 southern Ontario lakes devclopcd linearly throughout summer in 13 of the lakes and decreased, usually linearly, as the upper limit of the hypolimnion was chosen at successively greater depths. Areal hypolimnetic oxygen deficit ranged from 0.006 to 0.041 mg cm-' day-'; it was not correlated with average dry weight of seston in summer but was correlated with average Secchi depth during summer.

Eriksson et al (1980) hypothesized that the abundance of certain macroinvertebrate predators, such as larvae of the phantom midge Chaoborus, should increase in acidified lakes because of the elimination of fish. To examine the influence of pH and presence of fish on Chaoborus abundance, we surveyed Chaoborus populations in 33 lakes in Ontario, Canada which ranged in pH from 4.5 to 7.4. Chaoborus larvae were not more abundant in the acidified lakes that were devoid of fish than in the remaining lakes. Therefore, we concluded that pH and presence of fish are not prime determinants of total Chaoborus abundance in Canadian Shield lakes. We hypothesized that significant increases in Chaoborus abundance should only be anticipated when fish populations are eliminated by acidification of relatively nutrient rich lakes.

Seasonal changes in the percent total lipid content of Mysis relicta were investigated in four lakes in southern Ontario. Stony and Twelve Mile lakes contain populations of Mysis with a 1-yr life cycle, while Crystal and Boshkung lakes have populations with a 2-yr life cycle. Mature female and male mysids have a lipid content between 15 and 33% and between 14 and 25%, respectively, while juvenile mysids have approximately 10% less total lipids than adults. The relationship between total lipid and lipid-free dry weight is best described by the power curve Y = aXb and is the same (ANCOVA, p < 0.05) in all four mysid populations. Lipid content accumulates steadily with time as the mysid grows, and lipid reserves do not appear to influence mysid time to maturity. For studies on the transfer of energy and lipophilic organic contaminants through the food web, the results imply that seasonal and body size differences of organisms are important considerations.

Kootenay Lake, B.C. (395 km2), has experienced a series of major perturbations in the past 50 years, resulting in the 1980s’ collapse of South Arm kokanee salmon stocks and a dramatic decrease in the abundance of North Arm stocks. Historical data indicate the collapse was due to reservoir construction together with subsequent impoundment and nutrient retention on both main inflow tributaries (Kootenai and Duncan rivers) to Kootenay Lake, combined with the introduction of an exotic mysid shrimp which is an efficient competitor with kokanee for zooplankton. Nutrients (47.1 t of P and 206.7 t of N) are now being added annually to the 174 km2 North Arm of Kootenay Lake (271.3 mg-m-2 P and 1,190.5 mg-m-2 N) in a 5-year (1992-96) experiment with the goal of restoring historical kokanee salmon biomass and productivity. The concentration of total phosphorus in the North Arm currently ranges from 4–10 µg-L-1 P, which indicates oligotrophic to mesotrdphic conditions, whereas the concentration of dissolved inorganic nitrogen remains in the oligotrophic range (i.e., &lt;200 mg L-1 N) throughout the year. The seasonal mean zooplankton density observed in 1994 was higher than in 1992 and 1993, and similar to the highest densities observed from 1972 to 1984. Cladocerans comprised about 7.5% of the zooplankton in the North Arm in 1994, 12% in 1993 and 7.5% in 1992 as compared with less than 5% between 1949 and 1991. Combined North Arm kokanee escapement (Lardeau River and Meadow Creek) was 1.25 million fish in 1994 and was the highest observed since 1986, but lower than some escapements observed in the 1970s. Kokanee spawner size and fecundity have also increased. No significant trends have been observed in the Gerrard rainbow trout population. To date, the fertilization experiment has been successful at increasing the abundance of clado-ceran zooplankton and restoring kokanee populations in the North Arm of Kootenay Lake.