Environmental Constraints on Spawning Depth of Yellow Perch: The Roles of Low Temperature and High Solar Ultraviolet Radiation

—The roles of temperature and ultraviolet radiation (UVR) in determining the spawning success of yellow perch Perca flavescens were investigated in two Pennsylvania lakes with different dissolved organic carbon (DOC) concentrations. In situ incubation experiments were used to manipulate temperature and UVR and to examine hatching time and hatching success. Extensive scuba surveys were used to document actual spawning depths. Differences in the temperature and UVR profiles of the two lakes led to contrasting responses of incubated yellow perch eggs. Higher temperatures in the surface waters of the higher-DOC lake led to hatching times that were 10–26 d shorter than those in the surface waters of the low-DOC lake or in the deeper waters of the higher-DOC lake. The high levels of UVR in the surface waters of the low-DOC lake killed 100% of the eggs before hatching. Ultraviolet radiation had little effect on survival in the higher-DOC lake or in deeper waters of the low-DOC lake. Scuba surveys revealed that spawning in the lowDOC lake occurred at greater depths than previously recognized. Ninety-two percent of the eggs spawned in the low-DOC lake were located at depths greater than 3 m, while 76% of eggs in the higher-DOC lake were spawned in water less than 1 m deep. Temperature and UVR are both important in determining among-lake differences in spawning depths of yellow perch. Yellow perch are able to spawn at shallow depths in higher-DOC lakes, where warmer temperatures accelerate developmental rates and DOC blocks potentially damaging UVR. In low-DOC lakes, yellow perch must spawn at greater depths to avoid UVR damage. Spawning at greater depths may be costly due to the substantially slower developmental rates at lower temperatures. Our data suggest that the conflicting selective pressures of UVR and temperature create an optimal spawningdepth range for yellow perch that differs among lakes as a function of DOC concentration. Yellow perch Perca flavescens eggs are embedded in a gelatinous, accordion-like matrix, and spawners prefer to entangle egg strands on substrates such as woody debris, submerged macrophytes, and other structures. The duration of the spawning period is variable and can last from 1 to 4 weeks (Hokanson 1977; Thorpe 1977). Magnuson (1991) describes yellow perch as coolwater fish with a preferred temperature range of 20–258C as adults. They are annual spawners with synchronous oocyte growth during fall through winter, and spawning occurs in spring, when thermal stratification is common in temperate lakes (Hokanson 1977; Reyes et al. 1992; Sandström et al. 1995). In large lakes (.500 ha; Fee et al. 1996), winddriven mixing reduces temperature gradients, but in smaller lakes, pronounced vertical gradients in temperature and solar ultraviolet radiation (UVR) * Corresponding author: [email protected] 1 Present address: Oregon Department of Environmental Quality, Laboratory Division—Northwest Region, Watershed Assessment Section, 2020 SW 4th Avenue, Suite 400, Portland, Oregon 97201, USA. Received February 25, 2003; accepted November 20, 2003 can lead to variation in quality of spawning habitat with depth. Seasonal variations in temperature have been demonstrated repeatedly to be important in the spawning success of yellow perch (Hokanson and Kleiner 1974; Hokanson 1977; Clady 1976; Gillett and Dubois 1995), and cold temperatures are known to contribute both to longer development time and to decreased hatching success (Hokanson and Kleiner 1974). However, little attention has been given to the importance of vertical temperature gradients to yellow perch spawning success either within or among lakes. Similarly, there is evidence that UVR can lead to high mortality rates of yellow perch eggs incubated in the surface waters of lakes with low dissolved organic carbon (DOC) levels (Williamson et al. 1997), but there is little information about how the potential for UVR damage in surface waters may interact with vertical thermal gradients to alter the spawning success of yellow perch. Dissolved organic carbon can strongly influence the thermal structure of small lakes (,500 ha; Fee et al. 1996) by decreasing the mixing depth as DOC increases. Dissolved organic carbon also has a strong influence on potentially damaging UVR 719 YELLOW PERCH SPAWNING DEPTH (Morris et al. 1995) by attenuating UVR penetration, particularly that of the more damaging shorter wavelengths. This suggests that the vertical gradients in temperature and UVR within a lake where yellow perch spawn may be influenced by the DOC concentration of that lake. Knowledge is scarce about the possible constraints or benefits that DOC variations confer on fish spawning depths in either highor low-DOC lakes. Though the avoidance of high-UVR strata in the surface waters of low-DOC lakes by spawning yellow perch is understood, the tendency of yellow perch in higher-DOC systems to avoid spawning at greater depths, where UVR causes no damage (Williamson et al. 1997), is less clearly understood. One possible explanation is that warmer temperatures in the surface waters create a beneficial environment that accelerates developmental rates and thus shortens hatching