To assess potential effects of climate change on Great Lakes fish populations, we evaluated trends in the reproductive phenology of Yellow Perch Perca flavescens (spring spawner) and Lake Trout Salvelinus namaycush (autumn spawner). For Yellow Perch in Lake Michigan, the estimated reproductive midpoint date (50% of mature females ripe or spent, 50% not yet spawned) took place 6.2 d/decade earlier in the spring near Milwaukee from 1988 to 2012 and 1.8 d/decade earlier in Green Bay from 1980 to 2012. At both locations water temperatures at the spawning sites on the midpoint date showed no trends, but mean water temperatures during the spring at the spawning site and midlake increased over the study period. This suggests that Yellow Perch spawning areas were warming sooner in the spring and that Yellow Perch were spawning earlier to maintain a consistent spawning temperature. Lake Trout phenological patterns were more complex. For Lake Trout in Lake Michigan near Milwaukee, there was a marginally significant trend for spawning to take place 2.1 d/decade later in the autumn from 1983 to 2006. However, water temperatures at the spawning site at the midpoint date did not change and autumn temperatures at the site and at midlake did not show a warming trend. For Lake Trout in Lake Superior near the Apostle Islands, the midpoint date did not change from 1988 to 2012. Water temperatures at the spawning site on the midpoint date and during the autumn also showed no trends, but midlake summer and autumn water temperatures increased significantly. Overall, Yellow Perch in Lake Michigan have shifted reproductive timing in a manner consistent with a warming climate, but the relationship of climate change to reproductive phenology remains unclear for Lake Trout in Lake Michigan and Lake Superior.The earth's climate is warming, resulting in major changes in the thermal conditions of many lakes (Magnuson et al. 2000;Schneider and Hook 2010). Over the last century, the Laurentian Great Lakes have experienced well-documented trends towards less ice and shorter winters (Assel and Robertson 1995;Assel et al. 2003), warmer summer temperatures

Unraveling the diverse forces controlling the abundance and distribution of fish across a landscape has been challenging, in part because few techniques exist to address multiple factors simultaneously in a single analysis. We used spatially explicit, varying‐coefficient generalized additive models to relate the summertime abundance and distribution of yellow perch Perca flavescens to a variety of predatory, water quality, landscape, and density‐dependent factors. The model used 25 years of fishery‐independent trawling data from southern Green Bay, Lake Michigan, an area that supported major fisheries for yellow perch until their decline in the 1990s. Local catch per unit effort (CPUE) of both age‐0 and age‐1 and older yellow perch was affected by the abundance of double‐crested cormorants Phalacrocorax auritus, dissolved oxygen, water clarity, and bottom depth, but not water temperature. In addition, the local response of age‐0 yellow perch CPUE to most predictor variables, including their own global density, had a unique spatial structure. For instance, increased cormorant abundance was related to declines in local yellow perch CPUE, especially near cormorant nesting islands, while increased dissolved oxygen levels were correlated with increased local yellow perch CPUE in shallow nearshore areas of southern Green Bay. In addition, local yellow perch CPUE increased near the mouth of the Fox River during years with higher water clarity, suggesting that water quality may be limiting to yellow perch in this region. Our results suggest that it is important to explicitly account for the different ways in which predictor variables influence fish abundance across the aquatic landscape.

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