10-Year Trend: Unchanging
Long-term Trend (1973-2017): Undetermined
Rationale: Great Lakes prey fish community status remains ”Fair” based on diversity and percent native species, but individual lake status varied. Both diversity and percent native metrics were classified as “Good” in Lake Superior, but “Poor” in Lake Ontario (Table 1). Lakes Huron and Michigan were both “Fair” (Table 1). In Lake Erie, diversity remained “Fair,” but the proportion native species shifted to “Poor,” resulting in an overall conservative classification of “Poor” (Table 1). Four of the five lakes had the same status as the previous reporting period, but Lake Erie shifted from “Fair” to “Poor.”
At the ten-year timescale, lake-specific trends were “Unchanging” in three lakes and “Deteriorating” in two lakes (Table 2). The trend for all lakes was therefore categorized as “Unchanging.” It is important to recognize six of the ten individual prey fish metrics did not trend up or down over the past ten years (Table 2). In Lake Erie, diversity and percent native were both “Deteriorating” and the Lake Michigan diversity was noted as “Deteriorating.” The only “Improving” trend was observed in the Lake Ontario where the percent of native species significantly increased from two to four percent of the total catch due to increased relative abundance of native Deepwater Sculpin (Weidel et al., 2017b).
Long-term prey fish trends varied substantially with categorizations of “Improving,” “Deteriorating,” and
“Undetermined,” and two lakes were “Unchanging” (Table 2). Because many of the long-term trends were in opposite directions, the overall classification for the long-term trend was “Undetermined.” In Lake Superior, both metrics have “Improving,” in Lake Michigan diversity is “Improving,” and in Lake Huron the percent native metric is “Improving” as non-native Alewife declined, and the relative importance of native Bloater increased. Alternatively, Lake Ontario long-term trends are “Deteriorating” as the proportion of Alewife in catches has increased. No long-term trends were detected in either of the Lake Erie metrics.
Prey fish community changes are driven by changing ecosystem conditions including productivity changes, fluctuating predator composition and density, increasing water clarity, increasing water temperatures, and non-native species effects. While these driving factors are changing in similar directions across the region, because lakes are unique in their nutrient concentrations, morphometry, hydrology, and fish communities, the prey fish communities in each lake respond differently to changes in ecosystem drivers (Figure 1).