Abstract

  Intensive long-term index sampling of the fish communities of eastern Lake Ontario (outlet basin-gill netting, 1958-98, average annual effort 16.3 km of netting; bottom trawling, 1972-98, 23.0 km; Bay of Quinte-bottom trawling, 1972-98, 12.8 km) provided extensive summer catch data that we used to calculate indices to assess annual relative abundance, variability, and a combination of the two to reflect stability, or status. One of the unique and valuable attributes of these long-term data sets is that they use a community-assessment approach, which includes ecologically important small species and values the data on all species equally. These valuable series confirm that during the past decade, most major species in both the lake and bay have undergone a dramatic, somewhat synchronous change, unprecedented in almost three decades, including the catastrophic winterkills of the late 1970s. Abundance increased to record-high levels in the late l980s and early l990s, then decreased abruptly, two years earlier in small species (rainbow smelt, alewife and slimy scalpia) than large ones (lake flout, lake whitefish and walleye). The period of pivotal change was 1991-1993, coinciding with a significant decrease in water temperature from a period of six abnormally warm years, 1986-1991 (Apr.-Sept.-18.7°C) to three abnormally cold years, 1992- 1994 (17.3°C). Temperature decrease from 1991 to 1992 (2. 1°C) was the most extreme in four decades. Intensive rehabilitative stocking of lake trout, commencing in the mid-I 970s, has helped restructure and mature the cold-water community to a point where large species were at record-high levels in the late 1980s and early 1990s, creating increasing prey demand that required maximum prey production; this was possible an the recent high-temperature regime (late 1980s) but was not sustainable in the low-temperature regime that followed. After 1992, large species lost condition, more dead fish were observed, and abundance decreased. This low-temperature perturbation induced a succesional setback (enjuvenation event) that should be only temporary in the overall maturity of the ecosystem. However, recent colonization by dreissenids, which are inducing biological oligotrophication, could compound this. © 1999 Elsevier Science Ltd and AEHMS. All rights reserved.

Keywords: Indices; Trends; Abundance; Variability; Stability; Small and large fish species                 assemblages; Lake Ontario; Long-team community sampling; Gill netting; Trawling;                 Temperature