We used a field experiment, population modeling, and an analysis of 30 years of data from walleye (Sander vitreus; a freshwater fish) in Lake Erie to show that maternal influences on offspring survival can affect population dynamics. We first demonstrate experimentally that the survival of juvenile walleye increases with egg size (and, to a lesser degree, female energy reserves). Because egg size in this species tends to increase with maternal age, we then model these maternal influences on offspring survival as a function of maternal age to show that adult age structure can affect the maximum rate at which a population can produce new adults. Consistent with this hypothesis, we present empirical evidence that the maximum reproductive rate of an exploited population of walleye was approximately twice as high when older females were abundant as compared to when they were relatively scarce. Taken together, these results indicate that age- or size-based maternal influences on offspring survival can be an important mechanism driving population dynamics and that exploited populations could benefit from management strategies that protect, rather than target, reproductively valuable individuals.
A subsample of eggs from 109 walleye (Stizostedion vitreum) from Crean Lake, Saskatchewan, was sieved soon after fertilization to yield four groups of eggs ranging in size from 4.86 to 5.62 mm3. Overall egg size tended to correspond to yolk and oil volumes. At hatching, larvae from the eggs with the least yolk and oil were significantly shorter than larvae from all other egg groups. At the end of the experiment, 13 d after hatching, the larvae from the eggs with the most yolk were significantly larger than larvae from the other egg groups. Differences in the mean amount of food ingested by larvae were related to initial egg size. The yolk of the smallest eggs was deficient in the (n-3) series of polyunsaturated fatty acids. Larvae from these small eggs had a high level of major body deformities and a mortality rate which reached 100% by day 10. Small egg size in itself did not appear disadvantageous; larvae from a different stock (Lake of the Prairies, Manitoba) which produced eggs with a mean size less than that of the smallest sieved eggs, had survival and growth rates comparable with those of the larger Crean Lake eggs.
We examined variation in the mass and composition of egg lipids within and among 10 walleye (Sander vitreus) populations. Larger eggs contained more lipid, but neither absolute lipid content (mg·egg1) nor relative lipid content (percent of egg mass as lipid) were related to maternal age or size. Among populations, the proportion of egg mass as neutral lipid varied significantly, whereas the proportion as polar lipid did not. Egg fatty acid composition did vary with respect to maternal traits. In the Lake Ontario population, larger females appear to produce a qualitatively superior egg; relative abundances of docosahexaenoic acid (22:6(n3), DHA) and arachidonic acid (20:4(n6), AA) increased, and the relative abundance of eicosapentaenoic acid (20:5(n3), EPA) decreased in egg fatty acids with increasing maternal length. We predicted that fatty acid profiles would be less variable in the polar than in the neutral lipid fraction. Relative abundances of DHA, AA, the sum of AA and EPA, and several other fatty acid classes were all significantly less variable in the polar than in the neutral fraction both among females and among populations. Our results indicate a high level of conservatism of egg lipid mass and polar lipid composition in this species.
In each of two separate experiments, eggs from a single female goldfish were fertilized, incubated at 22~ and sampled regularly up to day 6 when the larvae could be expected to commence feeding. Hatching normally occurred on Day 4. Lipids were extracted from the eggs and larvae and the neutral lipid and neutral phospholipids were isolated on aminopropyl columns. Fatty acid analysis of the eggs revealed the typical situation in fish where the phospholipids were rich in polyunsaturated fatty acids (PUFA) and the neutral lipids were rich in monounsaturated fatty acids (MUFA). Assay of lipid masses revealed that little depletion of lipid occurred until after hatch and that the neutral phospholipids were the principal fraction consumed. Although the neutral lipid mass did not change substantially during development, its fatty acid profile did. The proportions of several PUFA in the neutral lipids, especially 22:6(n-3), 20:5(n-3) and 20:4(n-6), increased substantially during development while proportions of MUFA and 18:2(n-6) declined. This appears to be a mechanism by which the larva can retain essential fatty acid released on hydrolysis of phospholipid while deriving the benefits of catabolism of phospholipid as fuel, namely the provision of phosphate and choline for intermediary metabolism and for the synthesis of macromolecules and neurotransmitter.
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