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Description
Ecological theory predicts that larger, faster-growing individuals should have higher survival and fitness, and these traits should impart a selective advantage leading to rates of growth nearing their physiological maxima. Growth rates, however, are often submaximal and vary considerably within and among populations. A growing number of studies have revealed relationships between locomotor abilities and traits such as growth, size, survival, and mating success. In fishes, swimming performance (U[lower case crit]) is a common currency for measuring and comparing physiological fitness among individuals. However, swimming performance and its ecological implications have received little attention relative to other processes that influence the population ecology of fishes. Swimming abilities may be especially important in marine fish larvae because they can spend weeks to months in a pelagic environment before and after completing metamorphosis. The 'growth-mortality hypothesis' predicts that larger, faster-growing marine fish larvae should have higher survival during their early life stages. However, there also may be trade-offs between rapid growth and physiological traits such as swimming performance. In Chapter 1, I conducted studies of swimming performance (U[lower case crit]) in recent settlers of three temperate marine reef fishes: blacksmith, Chromis punctipinnis, kelp bass, Paralabrax clathratus, and señorita, Oxyjulis californica and examined their performance relative to larval growth, size at settlement, and pelagic larval duration. The first two species showed a positive relationship between swimming performance and pre-settlement growth and negative relationships between growth and both size at settlement and pelagic larval duration, indicative of minimizing the amount of time spent as pelagic larvae ('stage duration mechanism). The third species showed opposite results, however, in a negative relationship between swimming performance and pre-settlement growth and a positive relationship between growth and size at settlement, with no relationship between growth and pelagic larval duration, suggesting that maximizing larval size before settlement is important ('bigger-is better' mechanism). I suggest that differences in post-settlement habitat associations, behavior, and physiological requirements (swimming) among settling species may explain how rapid growth translates into these two different mechanisms that both support the growth-mortality hypothesis. In Chapter 2, swimming abilities (burst swimming speed and critical swimming speed) were evaluated for the larvae of two temperate coastal fishes (yellowtail, Seriola lalandi, and white seabass, Atractoscion nobilis). Sustained swimming did not begin until larvae reached flexion (yellowtail: 17 days post-hatch (dph); white seabass: 20 dph). Expectedly, critical swimming speed and burst swimming speed increased with body size (mm total length). Interestingly, both measures of swimming performance were positively correlated for each species. Consequently, critical swimming speed and burst swimming speed may be valuable indicators for the survival potential of marine fish larvae. In Chapter 3, I used temperature to manipulate the growth trajectories of the early life stages of a marine fish, the white seabass (Atractoscion nobilis) while measuring their critical swimming speeds. Slower growing fish at colder temperatures exhibited higher swimming performance than their faster growing counterparts at warmer temperatures. In subsequent predation experiments, faster growing but slower swimming larvae reared at warmer temperatures experienced higher mortality at the same size as slower growing but faster swimming larvae reared at colder temperatures. Matched by predicted swimming speed, smaller fish reared at colder temperatures experienced higher mortality than larger fish reared at higher temperatures in one set of trials, but in a second set of trials with nearly twice the replication, mortality was similar between these two treatments. Because critical swimming speed and burst speed are positively correlated in this species, the mechanism by which mortality occurred is unclear, although burst swimming speed should be most important as an immediate response to predator attack whereas critical swimming speed may allow prey to maintain distance from predators. My results demonstrate that predation of the early life stages of marine fish may not be attributed solely or primarily to size in the context of the 'bigger is better' hypothesis. Indeed, physiological trade-offs with somatic growth, such as swimming performance, can play an important role in predator-induced mortality that have important population consequences.
