Redd‐substrate composition, water velocity, depth, and other environmental variables associated with redd‐site selection and spawning by brook trout Salvelinus fontinalis and brown trout Salmo trutta in southwestern Ontario streams were examined. Sympatric and allopatric populations spawned in similar ranges of specific conductance (225–810 μmhos/cm), pH (7.0–8.2), dissolved oxygen (>83% saturation), and stream gradient (0.2–2.3%). Brook trout spawned exclusively in areas of groundwater seepage, typically near headwaters where streamflow did not exceed 177 litres/second. Brown trout spawned in a wider range of flows (21–600 liters/second), and utilized locations with and without groundwater seepage. Spawning by brook trout usually began by the second week of October, by brown trout a week later. Brook trout spawning periods lasted 3–5 weeks, those of brown trout, 2–4 weeks. In sympatric populations, an overlap in spawning time occurred for up to 3 weeks. Reuse of redds was mostly intraspecific, although interspecific reuse of brook trout redds by smaller brown trout did occur, particularly below barriers to upstream movement. Mean water depth over redds selected by brook (24.0 cm) and brown trout (25.5 cm) were similar (P > 0.05). However, mean stream velocities were significantly (P < 0.001) slower at brook trout (17.6 cm/second) than at brown trout redds (46.7 cm/second). Average geometric mean sediment size of brook trout redds was significantly smaller than that of brown trout redds (5.7 mm versus 6.9 mm; P < 0.02), but less well sorted. Redd‐site preference by brook trout for areas of groundwater seepage and by brown trout for faster water velocities and coarser substrates minimized species interactions during spawning. Larger body size of mature brown trout (18.0–54.5 cm fork length) than of mature brook trout (8.4–29.0 cm) was probably a factor in the brown troutˈs ability to utilize faster currents where coarser gravels were found. Received December 3, 1982 Accepted August 5, 1983

Acoustic abundance of fish depends directly on the target strength (TS) of the fish surveyed. We analyzed 70 and 120 kHz acoustic data from two lakes with abundant rainbow-smelt (Osmerus mordax) populations. Using repeated surveys through the summer growing season, we derived a relationship between TS (dB) and fish length L (cm) at 120 kHz (TS = 19.9 log10 L − 67.8). Values for 70 kHz were similar. In situ TS increased with fish density, indicating a bias from accepting multiple targets at high fish densities. Correcting for this bias increased estimates of smelt abundance by up to 18% in Lake Erie and up to 100% in Lake Champlain. Multiple modes in the TS distributions observed for older fish do not reflect different size groups, as the same modes can be observed from measurements from a single fish. Smelt released gas bubbles during the evening ascent, and these bubbles had TS (−60 to −58 dB) within the range of TS observed from the fish. Gas-bubble release occurred mostly during the migration. Conducting surveys after the ascent is completed will decrease bias associated with counting bubbles as fish.

Effects of three homogeneous gravels (2.7, 6.2 and 9.2 mm in diameter) and five heterogeneous gravel mixtures (with 0, 20, 40, 60 and 80% sand; geometric mean diameters = 14.2, 10.1, 7.2, 5.1, 3.7 mm, respectively) on embryo survival and subsequent emergence of brook charr (Salvelinus fontinalis) and brown trout (Salmo trutta) were examined in the laboratory using vertical flow incubators. Differences in survival to emergence, temporal components of emergence and developmental stage of emergents are significant [Formula: see text] among gravel types within and between species. Alevin survival, time interval to first and 50% emergence and duration of emergence period vary directly with gravel size and inversely with sand concentration. Survivals of 0 to 20% occurred in unigranular gravels 6.2 mm or finer and in multitextured gravels with 60% or more sand and rates of 60 to 96% were found in 9.2-mm gravel and gravels with 20% or less sand. Emergent survival increased from 14 to 79% in gravels of 6.2 to 9.2 mm and from 2 to 96% in sand concentrations between 60 and 20% because of reduced entrapment. Period of emergence was longest in 9.2-mm gravel (mean for charr, 381 day degrees; mean for trout, 423 day degrees) and in 0% sand–gravel mixture (mean for charr, 232 day degrees; mean for trout, 179 day degrees). Premature emergence of alevins over a shortened emergence period in finer gravels is identified as a stress response. Larger gravel and lower sand concentrations produced the largest and most advanced alevins at emergence. Ecological implications of the results are examined.

We coupled bioenergetics modeling with bottom trawl survey results to evaluate the capacity of piscivorous fish in eastern Lake Erie to exert predatory control of the invading population of round goby Neogobius melanostomus. In the offshore ([20 m deep) waters of eastern Lake Erie, burbot Lota lota is a native top predator, feeding on a suite of prey fishes. The round goby invaded eastern Lake Erie during the late 1990s, and round goby population size increased dramatically during 1999-2004. According to annual bottom trawl survey results, round goby abundance in offshore waters peaked in 2004, but then declined during 2004-2008. Coincidentally, round goby became an important component of burbot diet beginning in 2003. Using bottom trawling and gill netting, we estimated adult burbot abundance and age structure in eastern Lake Erie during 2007. Diet composition and energy density of eastern Lake Erie burbot were also determined during 2007. This information, along with estimates of burbot growth, burbot mortality, burbot water temperature regime, and energy densities of prey fish from the literature, were incorporated into a bioenergetics model application to estimate annual consumption of round goby by the adult burbot population. Results indicated that the adult burbot population in eastern Lake Erie annually consumed 1,361 metric tons of round goby. Based on the results of bottom trawling, we estimated the biomass of yearling and older round goby in offshore waters eastern Lake Erie during 2007-2008 to be 2,232 metric tons. Thus, the adult burbot population was feeding on round goby at an annual rate equal to 61% of the estimated round goby standing stock. We concluded that the burbot population had high potential to exert predatory control on round goby in offshore waters of eastern Lake Erie.