We used end-of-summer temperature profiles to examine the thermal structure of 86 small (Ͻ500 ha) lakes in Killarney Park, Ontario, Canada, during one cool (1997) and two extremely warm years (1998 and 1999). The main effect of the warm years, which had unusually high air temperatures during the spring, relative to the cool year was to create warmer surface waters, shallower mixing depths, and stronger metalimnetic thermal gradients in nearly all lakes. Changes in deep water temperatures differed between clear (DOC Ͻ 2 mg L Ϫ1 ) and colored (DOC Ͼ 4 mg L Ϫ1 ) lakes. During warm years, the volume of cold water (Ͻ10ЊC) was reduced in clear lakes. In colored lakes, deep water temperatures were more stable, and cold water volume actually increased during one warm year. We suggest that clear lakes will be more sensitive than colored lakes to the warming effects of climate change. Because clear lakes exhibit large thermal changes in response to small differences in DOC, they will also be more sensitive to changes in DOC levels associated with altered hydrological inputs, climate change, or acidification.
In lake trout (Salvelinus namaycush) lakes of northeastern Ontario, Canada, aerial surveys of fishing activity on individual lakes (N = 589) and quantitative gillnet surveys (N = 65) were used to assess the effects of road access on angling effort and the presence of introduced smallmouth bass (Micropterus dolomieu). Angling effort, particularly during the open-water season, was highest and often exceeded estimated sustainable levels on lakes with good road access. Approximately 25% of the remote lakes also received excessive pressure during the winter season. Angler numerical responses to lake trout abundance were detected in remote lakes, but not in road-accessible lakes. Smallmouth bass were more prevalent in lakes with road access and human settlement (either cottages or lodges), supporting the theory that they were introduced into these lakes. Lake trout populations were depleted throughout much of the study range. Even without road access or smallmouth bass, lake trout abundance was still 47% lower than in unexploited reference lakes. When bass and (or) road access were present, lake trout abundance decreased by 77%. Remote lake trout populations in this area are clearly vulnerable to the negative impacts of improved access, a vector for both overexploitation and species introductions.Résumé : Des inventaires aériens de l'activité de pêche sur des lacs individus (N = 589) et des inventaires quantitatifs au filet maillant (N = 65) nous ont servi à évaluer les effets de l'accès routier sur l'effort de pêche sportive et sur la présence d'achigans à petite bouche (Micropterus dolomieu) introduits dans des lacs à touladis (Salvelinus namaycush) du nord-est de l'Ontario, Canada. L'effort de pêche, particulièrement durant la période d'eau libre, est le plus élevé dans les lacs facilement accessibles par la route et il dépasse souvent le niveau admissible estimé. Environ 25 % des lacs éloignés subissent aussi une pression excessive durant la saison d'hiver. On remarque une relation entre le nombre de pêcheurs et l'abondance des touladis dans les lacs éloignés, mais pas dans les lacs accessibles par la route. Les achigans à petite bouche sont plus présents dans les lacs à accès routier et à présence humaine (chalets ou auberges), ce qui laisse croire qu'ils ont été introduits dans ces lacs. Les populations de touladis sont réduites dans presque toute la région d'étude. Même en l'absence d'accès routier et d'achigans à petite bouche, l'abondance des touladis est tout de même de 47 % inférieure à celle des lacs témoins non exploités. Quand il y un accès routier et (ou) présence d'achigans, l'abondance des touladis est réduite de 77 %. Les populations éloignées de touladis sont visiblement vulnérables aux impacts négatifs de l'amélioration de l'accès routier, un vecteur représentatif à la fois de la surexploitation et de l'introduction d'espèces.[Traduit par la Rédaction]
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