Bad News for Ontario’s Loons

In the early 1980s, scientists and other loon-lovers first began to worry about how loons were faring. Was the Common Loon becoming less common? Was the acidification of lakes affecting loons and other lake-dependent wildlife?

The Ontario component of the Canadian Lakes Loon Survey (CLLS) was initiated in 1981 by Bird Studies Canada, the Canadian Wildlife Service – Ontario Region, the Ontario Ministry of Natural Resources and other co-operators. Using a large network of volunteer surveyors, the program’s objectives were to monitor Common Loon breeding productivity, and to provide information on the effects of acid precipitation, human disturbance and other factors on breeding loons. Working closely with Environment Canada’s lake biomonitoring program, the CLLS provided a unique regional picture of how loon breeding success and lake acidity were related.

Among the more crucial findings was that loons were less likely to nest on acidic (pH less than 5.5) lakes and tended to be less successful rearing two-chick broods on acidic lakes. These findings, along with associated toxicological impacts (e.g. mercury poisoning) documented by other researchers, illustrated that loons (and probably other lake-dependent wildlife) were at serious risk on acidic lakes.

Have things been getting better or worse over the last two decades? To answer this important question, analyses of the Ontario portion of the CLLS database, spanning 17 years from 1981 to 1997, were recently conducted using the following two measures of loon breeding success: 1) proportion of successful pairs with at least one large young; and 2) proportion of successful pairs with at least two large young.

Observations from 1047 lakes were used in the analysis. The majority of lakes had pH levels approximately neutral and most lakes were between 55 and 400 hectares in size (Table 1).

The mean proportion of loon pairs observed with at least one large young varied significantly from year to year and ranged from 0.22 to 0.77 (Figure 1). When a straight line (the most simple definition of a trend) was fit to the mean values, there was a significant decline in the proportion of successful pairs from 1981-1997. The decline in breeding success was even more dramatic over the 1987-1997 period. The proportion of successful pairs with at least two large young did not differ significantly among years and did not show a trend over time (Figure 2).

Having documented that the most basic level of loon breeding success (i.e. pairs with at least one large young) had declined over time, our next question was whether the pattern of decline differed among lake acidity types. A summary of breeding success, broken down by lake pH class (Figure 3), provided two important insights. First, when the period 1987-1997 was examined closely, it was clear that the difference in breeding success between lake pH classes was small in some years and large in others. This pattern suggested that some years were particularly bad for loons breeding on acid lakes. Second, and most importantly, many of these particularly bad years occurred in the latter half of the 1987-1997 period. This last observation suggested that the breeding success of loons on acid lakes diverged from that of loons on alkaline lakes over this 10 year period. When described as simple linear trends, loon breeding success also appeared to become progressively worse on acid lakes (Figure 4).

These analyses documented two alarming patterns in the reproductive success of loons on Ontario lakes. First, the proportion of successful loon pairs on surveyed lakes declined during the last 10 years, suggesting that while adult loons may still be present on surveyed lakes, progressively fewer of them have been raising young to six weeks. Second, the rate of this decline was more extreme on acid than on alkaline lakes, suggesting that lakes below pH 6 became progressively poorer in their ability to sustain chicks through six weeks of age. In short, the results suggest that the acidification problem appears to be getting worse, not better.

The ravages of acid precipitation was headline news in the early 1980s when the CLLS began, but the issue has faded to the background in recent years. Significant progress has been made in controlling many of the emissions that cause lakes and streams to acidify (see BirdWatch Canada; Winter 1998, No. 6). However, the problem of acid precipitation is far from being solved. If acid lakes become progressively worse for breeding loons, even a stabilization of the number of acid lakes on eastern Canadian landscapes is likely to have detrimental effects on loon populations. These effects may become more extensive if, as scientists predict, the number of acid lakes in eastern Canada increases over the next couple decades.

We hope our findings, while indicating distressing trends for breeding loons, will help strengthen efforts to control pollution and monitor the health of lakes and their wildlife throughout Canada.

— Russ Weeber

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