Intrapopulation Variation in Egg Lipid and Fatty Acid Composition and Embryo Viability in a Naturally Spawning Walleye Population from an Inland Reservoir

—The objective of the study was to evaluate the variation in embryo viability within a population of walleye Sander vitreus from an inland reservoir throughout the spawning season. Egg size, egg lipid content, and fatty acid composition were used as criteria to evaluate egg quality. Additionally, we sought to verify whether any particular size-class of females produces superiorquality eggs or whether the time of spawning (early, middle, or late) has an effect on egg quality. Seventy-seven ovulating walleye females (total length, 465–885 mm) were captured in Salt Fork Reservoir, Ohio, throughout the spawning season. Although egg diameter after water hardening varied among females (1.85–2.38 mm), egg size did not correlate with female length (P . 0.05). Average egg lipid content was 12.0 6 1.3% (mean 6 SD) of wet weight and was unrelated to egg diameter (P . 0.05). Neutral and phospholipid classes in eggs comprised 77.5 6 4.7% and 22.5 6 4.7% of total lipids, respectively. Egg diameter was not significantly related to any of the specific fatty acids from neutral or phospholipid fractions (P . 0.05). Moreover, egg fatty acid compositions from both neutral lipids and phospholipids did not change during the spawning season. High survival of embryos (90.0 6 8.7%) from females across the observed size range was recorded regardless of the spawning period. We concluded that the quality of walleye eggs was consistently high and thus that the timing of gamete collection would not compromise hatchery programs. The walleye Sander vitreus is a highly valued game and food species in the north-central region of the United States and in central Canada. Public hatcheries annually produce over 1 3 109 walleye fingerlings for stocking into lakes and rivers to enhance natural resources (Fenton et al. 1996; Summerfelt 1996). Although many aspects of walleye culture have been significantly improved (for a review, see Summerfelt 1996), the majority of walleye juveniles used for stocking are currently produced from eggs and sperm collected from mature, wild fish (Summerfelt 1996; Mauk and Brown 2001). Therefore, the quality of eggs collected from natural populations may be an initial bottleneck in the production of stocking material. A combination of physical (e.g., water discharge from the river, turbidity, and temperature) and eco* Corresponding author: [email protected] 1 Present address: Illinois Natural History Survey, Lake Michigan Biological Station, Zion, Illinois 60099, USA. 2 In this fatty acid notation, the number to the left of the colon is the number of carbon atoms in the compound, the number immediately to the right of the colon is the number of double bonds, and the number after the hyphen indicates the position of the first double bond from the methyl end. Received October 9, 2003; accepted July 26, 2004 Published online February 28, 2005 logical (e.g., prey density and predation) factors may have profound impacts on the year-class strength of walleyes (Mion et al. 1998). All of these factors influence the age structure and size distribution of walleye populations, and consequently might affect the quantity and quality of eggs spawned every year. Egg quality is usually reflected in egg size and composition. Johnston and Leggett (2002) reported that within walleye populations, egg size increases with maternal size and age. These authors postulated that larger females produce larger eggs because (1) they breed earlier and consequently their offspring are exposed to less favorable growth conditions (e.g., cooler temperatures and lower food resources), and (2) they can acquire better spawning sites. Larger walleye eggs are usually characterized by larger yolks and oil globules, and therefore possess higher nutritional reserves than smaller eggs. Whether this variation reflects differences among females in the spawning population is unknown (Moodie et al. 1989). Egg size, as well as egg composition (especially lipids and fatty acids), can have a significant impact on the early life history of fish. Walleye egg size varies among populations with respect to both latitude and water body productivity (Johnston and Leggett 2002). The smallest walleye eggs contained low levels of n-3 polyunsaturated fatty acids D ow nl oa de d by [ SU N Y B ro ck po rt ] at 1 2: 12 1 9 D ec em be r 20 11 123 WALLEYE EGG QUALITY (PUFAs), and larvae from these eggs survived for only 10 d after hatching (Moodie et al. 1989). The eicosapentaenoic acid (EPA; 20:5[n-3]2) and docosahexaenoic acid (DHA; 22:6[n-3]) contents of neutral lipids in walleye eggs have been correlated positively with embryo survival (Czesny and Dabrowski 1998). A negative correlation was also observed between survival and the ratio of EPA to arachidonic acid (AA; 20:4[n-6]) in whole-body phospholipids of juvenile walleyes (Czesny et al. 1999). In our previous work (Czesny and Dabrowski 1998), walleye eggs from three distinct populations were compared in terms of lipid mass and fatty acid composition. We found that egg lipid content and fatty acid composition differed significantly among populations. However, within individual populations, egg composition on a yearto-year basis was relatively constant. In a recent study, Wiegand et al. (2004) reported that the lipid mass composition of eggs was very consistent among 10 walleye populations. In contrast, the fatty acid composition of the egg lipids differed significantly among walleye populations. In the present study, we quantified the degree of intrapopulation variation in walleye female size, egg lipid composition, and fatty acid composition and examined correlations between these traits and embryo survival or timing of spawning. Our objectives were to determine whether any particular size-class of females produces superior-quality eggs or whether egg quality changes as the spawning season progresses. Such information could benefit fishery management operations by indicating the specific ‘‘target’’ females that could be used to optimize fingerling production for stocking programs.