Deepwater Sculpin Research Articles

Toxaphene congeners in the Canadian Great Lakes basin: temporal and spatial food web dynamics

Samples of a top predator fish species, lake trout (Salvelinus namaycush) and predominant forage species including smelt (Osmerus mordax), alewife (Alosa pseudoharengus), slimy sculpin (Cottus cognatus), deepwater sculpin (Myoxocephalus quadricornis) and lake herring (Coregonus artedii) were, collected from throughout 4 of the 5 Great Lakes (Superior, Huron, Erie and Ontario) (Fig. 1) . Lake trout were also collected from three isolated lake systems (Lakes Nipigon, Simcoe and Opeongo), all located within the basin. All the samples were analysed for body burdens of total toxaphene and 22 toxaphene congeners. In addition, from each of the Great Lakes sites samples of major invertebrate dietary items, which included Mysis relicta, Diporeia hoyi and plankton, were similarly analysed. Whole lake trout samples, archived at −80°C, were reanalysed to determine historical trends of toxaphene congeners plus carbon and nitrogen stable isotope signatures. The Lake Superior food web consistently had the highest levels of total toxaphene of all the Great Lakes monitored. The primary source of toxaphene to Lake Superior has been identified as atmospheric transport and deposition from sites in the southern US, Mexico and Central America (Hoff, R.M., Strachan, W.M.J., Sweet, C.W., Chan, C.H., Shackelton, M., Bidleman, T.F., Brice, K.A., Burnison, D.A., Cussion, S., Gatz, D.F., Harlin, K., Schroeder, W.H., 1996. Atmospheric deposition of toxic chemicals to the Great Lakes: A review of data through 1994. Atmospheric Environ. 30, 3505–3527). Of the offsystem lakes surveyed, Lake Nipigon, situated due north of Lake Superior and with a Lake Basin area of about 6% of Lake Superior (Hendendorf, C.E., 1982. J. Great Lakes Res. 8(3), 379–412) had total toxaphene levels in lake trout equivalent to about 50% of those found in lake trout from Lake Superior. Temporal trend toxaphene congener analysis was conducted on archived whole fish samples collected over the period 1980 through to the 1990’s. Initially a nonachlorobornane congener (Parlar #50) was predominant, with congeners #40, #62 and #21 being the next most prominent in the 1980 samples. Samples from the 1990’s showed a significant decline in the presence of lower chlorinated congeners #40 and #21. Analysis of total toxaphene in food webs, indicated elevated levels in lower trophic level species such as Diporeia and Cottus sp. which have a benthic association. The stable isotope temporal trend 13C signature identified a significant shift in the lake trout diet over the period 1993 to 1996.

Using linear models with correlated errors to analyze changes in abundance of Lake Michigan fishes: 1973-1992

We examined annual changes in relative abundance of Lake Michigan fishes using linear models with correlated errors in space and time. Abundance of bloater (Coregonus hoyi), deepwater sculpin (Myoxocephalus thompsoni), slimy sculpin (Cottus cognatus), alewife (Alosa pseudoharengus), and rainbow smelt (Osmerus mordax) was monitored with bottom trawls at 10 discrete depths (between 18 and 110 m) off eight fixed ports from 1973 to 1992. The model describing abundance included fixed effects of year, port, depth, and interaction terms as well as quadratic and cubic effects of year and depth because changes in abundance were not strictly linear. Observed temporal trends in abundance varied with species and depth. Additionally, trends in alewife and slimy sculpin abundances depended on port. Cubic trends in the abundance of bloater and quadratic trends in deepwater sculpin and rainbow smelt abundances were similar among ports, permitting lakewide inferences for these species. Mean bloater abundance was low throughout the 1970s, increased during the 1980s, and reached high levels by 1990. Mean abundances of deepwater sculpin and rainbow smelt increased from 1973 to the mid-1980s and declined thereafter. The linear model with correlated errors can be readily applied to repeated-measures data from other fixed-station fishery surveys and is appropriate for data exhibiting spatial and temporal autocorrelations.

Using linear models with correlated errors to analyze changes in abundance of Lake Michigan fishes: 1973-1992

We examined annual changes in relative abundance of Lake Michigan fishes using linear models with correlated errors in space and time. Abundance of bloater (Coregonus hoyi), deepwater sculpin (Myoxocephalus thompsoni), slimy sculpin (Cottus cognatus), alewife (Alosa pseudoharengus), and rainbow smelt (Osmerus mordax) was monitored with bottom trawls at 10 discrete depths (between 18 and 110 m) off eight fixed ports from 1973 to 1992. The model describing abundance included fixed effects of year, port, depth, and interaction terms as well as quadratic and cubic effects of year and depth because changes in abundance were not strictly linear. Observed temporal trends in abundance varied with species and depth. Additionally, trends in alewife and slimy sculpin abundances depended on port. Cubic trends in the abundance of bloater and quadratic trends in deepwater sculpin and rainbow smelt abundances were similar among ports, permitting lakewide inferences for these species. Mean bloater abundance was low throughout the 1970s, increased during the 1980s, and reached high levels by 1990. Mean abundances of deepwater sculpin and rainbow smelt increased from 1973 to the mid-1980s and declined thereafter. The linear model with correlated errors can be readily applied to repeated-measures data from other fixed-station fishery surveys and is appropriate for data exhibiting spatial and temporal autocorrelations.

Lipid Concentrations in Lake Michigan Fishes: Seasonal, Spatial, Ontogenetic, and Long-Term Trends

Lipid concentrations were measured in seven species of fish from several locations in Lake Michigan during spring, summer, and fall in 1994 to 1995. Adult alewife ( Alosa pseudoharengus) and age-2 coho salmon ( Oncorhynchus kisutch) exhibited pronounced seasonal changes in lipid content. Adult alewives averaged 7.4% lipid, on a wet weight basis, during spring (May), 2.6% in summer (July), and 12.2% in fall (late September through October). Spring lipid concentration was low in age-2 coho salmon, averaging only 1.9%, then increased to 7.8% during summer and decreased to 4.5% by fall. In contrast, lipid content in adult bloater ( Coregonus hoyi) was relatively constant with respect to season, ranging between 10.6% and 12.4% during the year. Lipid concentration increased with fish size for all species except rainbow smelt ( Osmerus mordax). Although deepwater sculpin ( Myoxocephalus thompsoni) were considerably larger than slimy sculpin ( Cottus cognatus) (mean total length of 117 mm vs 68 mm), mean lipid content of deepwater sculpin (7.6%) was only slightly higher than that for slimy sculpin (6.6%). Comparison of lipid concentrations from this study with previous studies indicated that lipid concentration in lake trout ( Salvelinus namaycush) and alewives in Lake Michigan did not change significantly from 1969–1971 to 1994–1995. Lipid concentration in large (about 250 mm total length) adult bloaters near Saugatuck (along the southeastern shore of the lake) decreased from 23.3% in 1980 to 11.9% in 1986, but showed no significant change between 1986 and 1994–1995.

Spatial Patterns in PCB Concentrations of Lake Michigan Lake Trout

Most of the PCB body burden in lake trout ( Salvelinus namaycush) of the Great Lakes is from their food. PCB concentrations were determined in lake trout from three different locations in Lake Michigan during 1994–1995, and lake trout diets were analyzed at all three locations. The PCB concentrations were also determined in alewife ( Alosa pseudoharengus), rainbow smelt ( Osmerus mordax), bloater ( Coregonus hoyi), slimy sculpin ( Cottus cognatus), and deepwater sculpin ( Myoxocephalus thompsoni), five species of prey fish eaten by lake trout in Lake Michigan, at three nearshore sites in the lake. Despite the lack of significant differences in the PCB concentrations of alewife, rainbow smelt, bloater, slimy sculpin, and deepwater sculpin from the southeastern nearshore site near Saugatuck (Michigan) compared with the corresponding PCB concentrations from the northwestern nearshore site near Sturgeon Bay (Wisconsin), PCB concentrations in lake trout at Saugatuck were significantly higher than those at Sturgeon Bay. The difference in the lake trout PCB concentrations between Saugatuck and Sturgeon Bay could be explained by diet differences. The diet of lake trout at Saugatuck was more concentrated in PCBs than the diet of Sturgeon Bay lake trout, and therefore lake trout at Saugatuck were more contaminated in PCBs than Sturgeon Bay lake trout. These findings were useful in interpreting the long-term monitoring series for contaminants in lake trout at both Saugatuck and the Wisconsin side of the lake.

Occurrence of the Deepwater Sculpin ( Myoxocephalus thompsoni) in Western Lake Erie

Reported here is the collection of two specimens of deepwater sculpin ( Myoxocephalus thompsoni) from Ohio waters of western Lake Erie in 1995. Both specimens were collected while sampling for pelagic walleye ( Stizostedion vitreum) larvae. A 15.0-mm TL deepwater sculpin larva was collected over Toussaint Reef on 29 April 1995 and a 17.0-mm TL juvenile was collected west of South Bass Island State Park on 12 May 1995. It appears that there are no references to collections of deepwater sculpins from western Lake Erie in the literature or from communications with local management agency personnel. While these young deepwater sculpin may have come from ballast water or from a reproducing population in Lake Erie, the collection of 21 deepwater sculpin (12 to 19 mm TL) in the St. Clair River in May 1990 provides evidence of downstream transport from Lake Huron where indigenous populations exist.

Parasites of the Deepwater Sculpin, Myoxocephalus thompsoni (Cottidae), from Lake Michigan and Lake Huron

A total of 190 deepwater sculpins, Myoxocephalus thompsoni, collected in 1995 from Michigan waters of Lake Michigan and Lake Huron was examined for parasites. Five parasite species occurred in sculpins from Lake Michigan with Echinorhynchus salmonis being most common. Six parasite species infected sculpins from Lake Huron, with Haplonema sp. the most common. Haplonema sp. is the only gravid helminth species reported from deepwater sculpins. Pleistophora sp. and Trichodina sp. infected sculpins from Lake Huron and Lake Michigan. Parasite species richness for sculpins at the 3 locations ranged from 5 to 6; mean values ranged from 1.18 to 1.39 for examined fish. The restricted diet of deepwater sculpin, which may be related to the depth of its habitat, appears to determine its helminth fauna. Deepwater sculpin may be an important transport host for E. salmonis, Cyathocephalus truncatus, and Eubothrium salvelini to lake trout and burbot that commonly feed on them.

Diet of Burbot in Green Bay and Western Lake Michigan With Comparison to Other Waters

We determined foods eaten and factors influencing the diet of burbot (Lota lota) and compared our results with those of other studies to help assess the role of burbot in the fish community of Lake Michigan. We examined stomachs of 3,570 burbot of 215–845 mm total length caught by 10 methods from 5 areas of Green Bay and western Lake Michigan in depths of 1 to 160 m during September 1986 to November 1988. Fish composed 94% of the volume of the diet; percent volumes of fish identifiable to genus or species were alewife (Alosa pseudoharengus), 31; rainbow smelt (Osmerus mordax), 25; bloater (Coregonus hoyi), 13; sculpins of the genus, Cottus, 11; deepwater sculpin (Myoxocephalus thompsoni), 8; yellow perch (Perca flavescens), 8; other fish, 5. Cottus spp. were more important than their volume indicated because they were the most frequently occurring fish. Important invertebrates were Mysis relicta, found in 26% of the burbot containing food, and Pontoporeia hoyi. The diet varied with size of burbot, i.e., invertebrates were more important for the smaller burbot, and with area and depth of capture. Alewives were more important at depths of 20–39 m than at shallower and deeper depths. In waters exceeding 40 m depth the diet of burbot in Lake Michigan seemed to have changed little since the 1930s, and it was similar to the diet of burbot in Lake Superior in the 1960s. Foods commonly important for burbot in many waters in North America are sculpins, coregonids other than lake whitefish (Coregonus clupeaformis), Pontoporeia spp., M. relicta, and yellow perch, and crayfish outside of the Great Lakes. Trout and salmon ( Salmo spp., Salvelinus spp., Oncorhynchus spp.) and suckers (Catostomus spp .) were inconsequential in the diet of burbot in Lake Michigan, and they have not been found to be important in the diet of burbot in other studies.

Predation on Mysis relicta by Slimy Sculpins ( Cottus cognatus) in Southern Lake Ontario

Mysis relicta and Diporeia dominated the diet of slimy sculpins ( Cottus cognatus) caught at depths of 55, 75, and 95 m at two locations in southern Lake Ontario. Length of prey was positively correlated with length of slimy sculpin. At a given depth, mean length of slimy sculpins was greater at Rochester than at Nine Mile Point, and mean length increased with depth at both locations. Frequency of occurrence of Mysis in the stomachs of slimy sculpins increased with depth, whereas frequency of Diporeia generally decreased with depth. Diporeia dominated the slimy sculpin diet at 55 m, whereas Mysis dominated the diet at 95 m. Slimy sculpins, some as small as 30 mm long, fed successfully on Mysis in Lake Ontario. We suggest that extirpation of the deepwater sculpin ( Myoxocephalus thompsoni) and perhaps the coregonines ( Coregonus spp.) in Lake Ontario allowed slimy sculpins to expand into deeper areas where they could exploit Mysis. We also suggest that at depths ≥75 m in Lake Ontario, the current Mysis-benthivore link with slimy sculpins may be similar to the past when deepwater sculpins were present.

Mio-pliocene seasonality on the snake river plain: comparison of faunal and oxygen isotopic evidence

Analysis of fish faunas and oxygen isotopic composition of a fish otolith from lacustrine deposits of southwestern Idaho provide a means of evaluating regional Miocene and Pliocene climates. A disharmonious assemblage consisting of coldwater salmon and trout and warmwater sunfish and catfish from the Chalk Hills Formation of the Snake River Plain indicates that the climate of the late Miocene was warm and moist with cool summers and mild winters. Colonization of the lake by deepwater sculpins and whitefish in the Pliocence indicates that the climate was moist and equable, but with summers cooler than either the Miocene or Quaternary. Oxygen isotopic variation among seasonal growth rings in an aragonitic otolith of a Pliocene littoral sunfish suggests a seasonal range of temperatures locally more equable than at present. Extremely depleted values of δ 18O in carbonates suggest that the lake was maintained by tributaries from high-elevation watersheds, with locally low evaporation, rather than high precipitation.

The life‐history strategy of deepwater sculpin, Myoxocephalus thompsoni (Girard), in Lake Michigan: dispersal and settlement patterns during the first year of life

Deepwater sculpin, Myoxocephalus thompsoni (Girard), were sampled from six stations from the 15–100 m depth contours in Lake Michigan between April 1983 and July 1984. In south‐eastern Lake Michigan M. thompsoni lay benthic eggs in offshore waters, which hatch between November and August, with peak hatching in March. Abundance of larvae in pelagic samples was higher offshore than inshore, but larval size was greater and development more advanced at inshore stations, indicating an inshore movement after hatching. Larvae reached metamorphosis at 20 mm and settled to the bottom beginning in July. Pelagic larvae 20–40 mm were found in the lower water column at all stations, but newly settled individuals were only captured with bottom trawls at inshore locations (≤60 m depth). Data from ichthyoplankton and bottom trawl samples in 1983 and 1984 indicated that locations for successful settlement of larvae to the bottom extended only as deep as the shallowest fringe of the adult population (> 50 m in 1983). In 1983, maximum density of larvae reached 0.4 individuals m−3 by June. Survival from the pelagic larval stage to the demersal young‐of‐year stage in 1983/1984 was c. 0.1–0.4%. The specific mechanism of mortality at the time of transition to a demersal habit has not been determined.

Spatial and Temporal Changes in the Offshore Larval Fish Assemblage in Southeastern Lake Michigan

The larval fish assemblage in southeastern Lake Michigan was sampled monthly (May to September 1983) between 1.7 and 23.9 km offshore. Larval densities were calculated for the neuston, epilimnion, metalimnion, and hypolimnion strata of the water column. Larval densities were generally less than 0.4 m −3 with the exception of bloater ( Coregonus hoyi) which reached 15 m −3 in the offshore neuston in August. The abundance of larvae was estimated for a 1-km-wide (N–S) by 26.2-km-long (E–W) strip of the lake and discussed with respect to inshore (0.7–7.1 km), intermediate (7.1–11.1 km), and offshore (11.1–26.9 km) zones. The inshore was dominated by larval rainbow smelt ( Osmerus mordax) and alewife ( Alosa pseudoharengus), and the offshore zone was dominated by bloater and deepwater sculpin ( Myoxocephalus thompsoni). The changes in abundance and distribution of larvae reflected differences in spawning and hatching seasons and behavior of the different species. Species richness ( number of species) increased and subsequently decreased in a progression from inshore to offshore. Maximum species richess occurred in June and July. Abundance was always higher in offshore waters. Maximum larval abundance occurred early in the season ( June) and declined steadily through to September. Deepwater sculpin was the most abundant larval species in the areas sampled. Together, deepwater sculpin and bloater constituted 70–99% of the total abundance of larvae. In general, there was little overlap in the distributions of the various species over temporal and depth scales. Bloater and deepwater sculpin were separated from each other by their depth distributions; alewives were separated temporally from all other species.

The biomagnification of polychlorinated biphenyls, toxaphene, and DDT compounds in a Lake Michigan offshore food web

The biomagnification of polychlorinated biphenyls (PCB), toxaphene, and the DDT family of metabolites was investigated in the epibenthic Mysis relicta (mysid), the benthic Pontoporeia hoyi (amphipod), plankton, particulate flux, surficial sediments, and Myoxocephalus thompsoni (deepwater sculpin) in southeastern Lake Michigan. DDE was the most strongly biomagnified compound, increasing 28.7 times in average concentration from plankton to fish. PCB increased 12.9 times in average concentration from plankton to fish while toxaphene increased by an average factor of 4.7. Particle flux was comprised of lower chlorinated PCB homologues (average chlorine number = 3.8) than the biota (4.5-5.0) and sediments (4.6), possibly reflecting strong influences from atmospheric deposition and/or zooplankton egestion. The percent of higher chlorinated PCB homologues (5 and 6 chlorine atoms per PCB molecule) increased from 54-56% of the total PCB in plankton and M. relicta, to 61% in P. hoyi, to 74% in sculpins. Amphipods contained greater concentrations than mysids of PCB, DDT residues, and toxaphene, possibly reflecting differences in habitat (benthic vs epibenthic) and diet (detritus vs plankton). Based on estimates of average areal biomass and contaminant concentration, offshore Lake Michigan P. hoyi populations contain approximately 15.0 times as much toxaphene, 9.5 times as much total DDT, and 12.0 times as much PCB as the offshore M. relicta populations. Thus, amphipods may represent a greater reservoir than mysids for contaminant storage and subsequent recycling in offshore Lake Michigan.

Comparative Ecology of Pontoporeia Hoyi Populations in Southern Lake Michigan: The Profundal Region Versus the Slope and Shelf Regions

This study investigates Pontoporeia hoyi ecology in the profundal region (approximated by a 97-m station) of southern Lake Michigan and compares these results with previous investigations conducted in the slope and shelf regions of the lake. Pontoporeia typically attains its maximum abundance in the slope region, suggesting that this region of the lake is the most favorable for amphipod growth and survival. Profundal and slope P. hoyi exhibited little seasonal variation in mean size while shelf-region populations exhibited strong seasonal variation in mean size. Deepwater sculpins, a major predator on profundal populations of P. hoyi, selectively consumed the largest amphipods: mean size consumed was 6–7 mm. The relative sparsity of larger (> 5 mm) P. hoyi in the profundal, slope, and shelf-regions of the lake may arise from intense size-selective fish predation on this size class of amphipods. Gut content studies revealed that profundal P. hoyi populations feed intermittently. Such feeding behavior was subtly different from that previously observed for slope P. hoyi populations: profundal populations feed more continuously but less intensively than slope populations. Regional differences in feeding behavior may be related to differences in food regime and to predation avoidance strategies. Pontoporeia hoyi apparently is capable of inhabiting a broad range of depth regimes by modifying its physiology (reproductive cycles, generation time) and behavior (feeding, motility) to adjust to spatial variations in temperature, food level, and predation.

Proposed Biological Management ofMysis relictain Colorado Lakes and Reservoirs

The opossum shrimp Mysis relicta has been introduced into many lakes and reservoirs outside its native range to supplement the prey bases for fish. Colorado has received more introductions of the mysid than any other state or province in western North America, but not all of these introductions have been beneficial or innocuous. Mysid predation on zooplankton, particularly cladocerans, has resulted in diminished food for some fishes, especially for planktivores such as the kokanee Oncorhynchus nerka and small trout Oncorhynchus spp. Few fishes forage on the often abundant populations of introduced mysids. Management options are limited in lakes where M. relicta competes for, or has eliminated, the large zooplankters eaten by fish. Furthermore, the mysid has shown enough vagility to expand its nonnative range and may harm fisheries in waters that it enters. Because its eradication is not now feasible, the development of strategies to control or benefit from established mysid populations is warranted. Biological controls appear most feasible. Amphipods and fishes that have the potential to control or benefit from nuisance mysids through competition or predation are candidates for introduction. Of the 13 species that we examined, the deepwater sculpin Myoxocephalus thompsoni was our first choice for introduction into waters containing established mysids. We based this choice on its potential to either control mysid numbers or channel mysid production to predators with little or no adverse effects on the rest of the community.

In Situ Studies of Organism-Sediment Relationships in the Caribou Island Basin, Lake Superior

In 1986, using the DSR V Johnson Sea-Link II, we established a permanent bottom station at 350 m in the Caribou Island basin, Lake Superior, in order to examine physical and biological contributions to general disturbance at the sediment-water interface, particle mixing, and to begin to quantify feeding rates of the macro-infaunal invertebrates. To accomplish these longrange goals we needed background information on this specific study site. Therefore, we collected eight 225 cm 2 box cores for photographic and faunal analyses, and 19 subcores for X-radiography to examine and record epifaunal and infaunal biogenic structure and physical sedimentary features. These data are critical as no detailed in-situ studies have been conducted to date on these deep profundal environments. The infaunal community at the site was dominated by amphipods, oligo-chaetes, nematodes, and ostracods. Average infaunal density approximated 4,700m –2, with 90% of the organisms appearing within 4 cm of the sediment-water interface (SWI). Core-section photography revealed layered, fine-grained sediments overlying post-glacial clays. Some cores contained laminae of manganese and iron oxides. Several subcores contained distinct “dish-like” laminae, which were most likely relict biogenic features created by the deepwater sculpin ( Myoxocephalus thompsoni). Biogenic structures produced by infaunal burrowers were small (approximately 1 mm diameter), and confined to the upper 3 cm of the sediment. Burrow frequency varied widely from core to core. However, subcores containing “dish-shaped,” high-density horizons had the fewest number of infaunal burrows. Our observations suggest that much of the near-surface sediment-mixing activity at the Caribou basin site is likely due to demersal fish; during our dives only indirect evidence of physical disturbance of the sediment-interface was observed.

Comparative Biology of the Sculpins of Lake Superior

The slimy sculpin ( Cottus cognatus), spoonhead sculpin ( Cottus ricei), and deepwater sculpin ( Myoxocephalus thompsoni) are abundant fishes in Lake Superior. Slimy and spoonhead sculpins occupy a zone from near shore to depths of 210 m but are generally most abundant in waters 50 to 90 m deep. Deepwater sculpins are found in waters from 15 to 407 m deep and are most abundant at depths greater than 70 m. All three species of sculpins eat mainly burrowing amphipods ( Pontoporeia affinis) although deepwater sculpins also eat substantial quantities of opossum shrimp ( Mysis relicta). The three sculpins grow at similar rates. Among the fishes aged, the maximum age of slimy sculpins was 5 years, compared with 6 years for spoonhead sculpins, and 7 years for deepwater sculpins. Indirect evidence indicates that slimy and spoonhead sculpins spawn in the spring, and deepwater sculpins spawn in midwinter.

Status of the Deepwater Sculpin, Myoxocephalus thompsoni, in Canada

Status of the Deepwater Sculpin, Myoxocephalus thompsoni, in Canada

Bythotrephes Cederstroemi: its New Appearance in Lake Michigan

Bythotrephes cederstroemi was newly found in Lake Michigan in September 1986. This predaceous cladoceran was previously detected in Lake Huron (December 1984) and subsequently dispersed into Lakes Ontario and Erie (autumn 1985). It was previously known only from Europe. Entry into Lake Michigan may have occurred through the exchange of Lakes Huron and Michigan water masses in the Straits of Mackinac. Bythotrephes cederstroemi was consumed by Lake Michigan deepwater sculpins in October 1986. Since Bythotrephes is believed to be a surface-dwelling species, the most probable route by which it entered the sculpin diet was through the rapid sinking of dying (and dead) animals and their subsequent ingestion by sculpins at the sediment-water interface. Bythotrephes cederstroemi exhibited allometric and cyclomorphic growth. The caudal process was relatively straight in immature specimens and became progressively more kinked in larger (older) animals. Furthermore, the kink was more strongly developed in autumn (October) than summer. (July) B. cederstroemi populations.

Recent Changes in Lake Michigan's Fish Community and Their Probable Causes, With Emphasis on the Role of the Alewife (Alosa pseudoharengus)

Major changes in fish populations occurred in Lake Michigan between the early 1970s and 1984. The abundance of lake trout (Salvelinus namaycush) and several nonnative species of salmonines increased greatly as a result of intensive stocking. The exotic alewife (Alosa pseudoharengus), which had proliferated to extremely high levels of abundance in the mid-1960s, declined, particularly in the early 1980s. We believe that the sharp decline in alewives in the 1980s was caused primarily by poor recruitment during the colder than normal years of 1976–82. Several of Lake Michigan's endemic species of fish appeared to be adversely affected by alewives: bloater (Coregonus hoyi), lake herring (C. artedii), emerald shiner (Notropis atherinoides), yellow perch (Perca flavescens), and deepwater sculpin (Myoxocephalus thompsoni), and possibly spoonhead sculpin (Cottus ricei). All declined when alewives were abundant, and those that did not become rare, i.e. the bloater, perch, and deepwater sculpin recovered when alewives declined. We present evidence suggesting that the mechanism by which alewives affect native species is not by competition for food, as has often been hypothesized, and discuss the possibility that it is predation on early life stages. Despite the decreased availability of alewives in the early 1980s, salmonines continued to eat mainly alewives. The highly abundant alternate prey species were eaten only sparingly, but alewives still may have been abundant enough to meet the forage requirements of salmonines. Two new exotics, the pink salmon (Oncorhynchus gorbuscha) and threespine stickleback (Gasterosteus aculeatus), increased in abundance in the 1980s, and could become detrimental (particularly the salmon) to other species.