Background
In eastern North America, white suckers are a fish that range from Newfoundland south to the Tennessee River. White suckers primarily occupy cool water lakes and ponds, but migrate into tributaries to spawn in the spring ¹¹. They are a cool water adapted species, which means they require water temperatures of 15 to 25 °C (59 to 77 °F) ⁶. They have a maximum tolerance of about 27.4 °C (around 81 °F) ³. White suckers inhabit shallow water at night where they feed on invertebrates, mayfly nymphs, chironomid larvae and detritus (waste and debris), and move into cooler, deeper sections of water during the day ⁹. In northern temperate fish communities, white suckers have a significant influence on the growth and production of other fishes because they are strong competitors ⁹.

Climate Impacts
Climate change is likely to affect white suckers through habitat loss and distributional changes. Under a projected doubling of atmospheric carbon dioxide levels, the number of water bodies in the contiguous US with suitable cool water fish habitat is expected to decrease by 30%, with the largest impact on cool water habitats occurring in shallow lakes less than 4m deep ¹⁶. Cold-adapted species are also predicted to shift north and move deeper in the water column ¹⁰. For instance, lake whitefish (Coregonus clupeaformis) that are also adapted to cool temperatures and lower levels of oxygen in the winter ¹⁵ closely track temperature in their lake habitats in May, indicating that the species’ distribution may be affected by climate change ⁵.

Increased water temperature is also expected to have a number of impacts on the physiology and demography of cool water fishes like white suckers. Temperature increases are expected to cause greater surface water temperatures and longer periods of thermal stratification (different temperatures at different depths) and ice-free conditions in lakes and ponds ¹⁴. Cool water fish have preferred temperature ranges centered around 24° C that define the range at which these fish achieve maximum growth rates, activity levels, and swimming performance ¹⁴. In turn, changes in growth rates due to temperature shifts can also lead to demographic differences such as age of maturity of adults and survival rate of juveniles ¹⁴. For instance, white suckers that inhabit deep lakes with cooler temperatures tend to grow faster and be larger-bodied ⁹. An even more cold-adapted species, the burbot (Lota lota), showed significant decrease in hatchling and larval success with increasing temperatures and reduced access to prey resulting in a 50 year decline in Lake Oneida, New York ⁸. In addition, increased summerkill (death resulting from high temperature and low dissolved oxygen content in the water) is expected to have significant adverse effects on cool water habitats ¹⁶.

Changes in the timing of life history events induced by climate change may also affect white suckers. Shifting the timing of morphological development required for feeding and other life history traits may disrupt the overlap in timing between predators and prey ¹⁹. This phenomenon has been seen for related species. For instance, larval yellow perch (Perca flavescens) in Oneida Lake, New York, got larger earlier in the year, correlated with above average May water temperatures ⁷. Warming water temperatures also advance hatching in lake whitefish ¹³. In addition to these temperature limitations, habitat fragmentation and land conversion are negatively impacting some fish populations ^1,12. White suckers make long migrations to access spawning habitats; thus, habitat connectivity is important to consider in their conservation ¹¹.

Finally, changes in community structure can also be an important factor. For example, invasion by the parasitic sea lamprey (Petromyzon marinus) has already contributed to major declines in many Great Lakes fish populations and even higher rates of mortality are expected as warmer waters lead to larger lamprey, higher feeding rates, and higher mortality of host fishes ¹⁷. Changes in community structure can also be caused by extreme events, stemming from or worsened by climate change ^2,18. For instance, a population of slimy sculpin (Cottus cognatus), a cool-adapted species, declined significantly as a result of a mid-winter ice break-up and the associated flood and ice scour disturbance it caused ⁴.