Coregonus hoyi
bloater

Coregonus hoyi is endemic to the Great Lakes basin of North America where it inhabited all lakes except Lake Erie. This species was probably extirpated from Lakes Ontario and Nipigon and is threatened in Lake Michigan and declining in Lakes Superior and Huron. (Page and Burr, 1991)

Bloaters exist in both pelagic and benthic regions of deep, freshwater lakes. During the day Coregonus hoyi is found on or near the bottom, but it moves upwards in the water column at night. Bloaters are most abundant between 40 and 110 m, but have rarely been reported as shallow as 10 m and as deep as 180 m. (TeWinkel and Fleischer, 1999)

Coloration is silvery with some pink and purple iridescence, with a greenish tinge above lateral line and a silvery white ventral surface (Downs et al, 1998). These fish are distinctly laterally compressed with greatest body depth in front of dorsal fin. Coregonus hoyi has a long head and small eye, snout short or pointed with terminal mouth, lower jaw protruding past upper jaw with distinct tubercle, and a maxillary extending posteriorly to below the anterior portion of the eye. Gill raker number 37-50. A small dorsal adipose fin is present. The following is a characterizatin of the fins: dorsal 1, rays 9-11; caudal fin distinctly forked; anal rays 11-12; pelvic rays 11; pectoral rays 15-16. The scales are cycloid. It has 55-57 vertebrae. Females reach a slightly larger size than males (Crossman and Scott, 1973). (Crossman and Scott, 1973; Downs et al., 1998)

Coregonus hoyi remains in the larval state for aproximately 24 days with a length range of 8.6-14.9 mm. Larvae are most frequently located near the bottom at depths between 90 and 110 m in water with temperatures not exceeding 4.7 degrees C. Mature C. hoyi reach lengths of 200-250 cm, with size and growth rate variable between lakes.

Females have a lifespan of 10 to 11 years, and reach a slightly larger size than males, which live to an age of 9 years. (Crossman and Scott, 1973)

Coregonus hoyi spends daylight hours on or near the bottom, and rise into the water column to feed at night. Nighttime densities in the water column have been observed at up to 6.61 fish/m^3. These nocturnal migrations are variable with water temperature, light levels, and individual size. Migration patterns allow C. hoyi to maximize growth through increased consumption of Mysis relicta, which also migrates. (TeWinkel and Fleischer, 1999)

Coregonus hoyi is primarily a pelagic feeder, with occasional bottom feeding, but habits and diet appear to be variable between bodies of water. Analysis of stomach contents revealed primarily Mysis relicta and Pontoporeia affinis, both near bottom dwelling plankton. Copepods dominated some stomachs, indicating pelagic feeding. Fish eggs and fingernail clams were also found in some stomachs. Vertical migration on an individual basis is hypothesised to be driven by migration of invertebrates in the water column (TeWinkel and Fleischer, 1999). High potential feeding rates at cold temperatures allow bloaters to feed and grow during the winter when many competitors are inactive. Increased activity at cold temperatures also allows the bloater to take advantage of high early spring zooplankton abundance which its competitors cannot do.

Foods eaten: Mysis relicta, Pontoporeia affinis, Cyclops bicuspidatus, Diaptomu, copepods, fish eggs, fingernail clams (Rudstam, Binkowski, and Miller, 1994)

Historically, the main predator of Coregonus hoyi has been the lake trout (Salvelinus namaycush). After parasitic sea lampreys decimated the lake trout populations in the Great Lakes, introduced Pacific salmonids have become the dominant predators of C. hoyi. Populations can also be limited by competition and predation from alewives.

Coregonus hoyi is heavily parasitized, with common parasites including: Botriochocephalidae, Nematoda, Trematoda, Cestoda, Acanthocephala (Crossman and Scott, 1973)

Coregonus hoyi is a deep water planktivore that provides an important trophic link between zooplankton and top predators. After initial population declines due to competition with and predation by introduced alewives and rainbow smelt, C. hoyi populations increased greatly in response to alewife and smelt predation by introduced Pacific salmonids. This change in fish community resulted in a shift in dominance from epi- and metalimnetic planktivores (alewife) to a hypolimnetic benthivore (bloater). This can affect the abundance of prey for pescivores, prey abundance for the forage fish, and possibly water clarity and primary production. (Rudstam, Binkowski, and Miller, 1994)

Bloaters have no known negative impact on people.

Coregonus hoyi is an important component in the diet of larger salmonid species. Many of these salmonids are very valuable as gamefish, bringing economic benefit to the waters they inhabit. As the larger ciscoes have been depleted through overfishing, the smaller Coregonus hoyi has become an increasingly important component of commercial catches. Commercial catches of Coregonus hoyi, like other ciscoes are primarily smoked and distributed for human consumption. (Crossman and Scott, 1973)

Overfishing of other species of ciscoes has reduced populations, and shifted more pressure to Coregonus hoyi. Unchecked, this additional fishing effort could devastate Coregonus hoyi populations, as it has done to other ciscoe species. The bloater is not currently given special status under the Endangered Species Act. The IUCN classifies Coregonus hoyi as vulnerable. Fluctuations in the populations of bloaters have been inversely related to the introduced alewive, which competes for resources. Stocking of pacific salmonids in order to control alewive populations has allowed the Lake Michigan Coregonus hoyi population to rise from an estimated 4,000 metric tons in 1974 to 300,000 metric tons in 1987. (Rudstam, Binkowski, and Miller, 1994)