<p>Alternative reproductive tactics (ARTs) have been described across taxa numerous times, and are especially common in fishes. The advent of molecular techniques has enhanced our understanding of such mating systems, yet these techniques have only been applied to relatively few species. Furthermore, ecological variation has long since been recognized as an important factor influencing mating patterns and sexual selection, yet it is often ignored. In my thesis, I conducted field studies to investigate the reproductive ecology of a species with well characterized ARTs, the plainfin midshipman (<em>Porichthys notatus</em>), and to explore how these mating patterns varied across time and space. This species is characterized by two male tactics: guarders and sneakers. I show that both tactics are successful at gaining fertilization. Indeed, I found evidence for several behavioural adaptations by the guarder male tactic to gain fitness, including guarding, nest takeovers, and plastic cuckoldry behaviours. I also documented the lowest paternity for guarder males for a species with obligate male parental care. Paternity increased over the course of the breeding season, which was likely a result of a higher occurrence of nest takeovers early in the season. In a cross-population analysis of plainfin midshipman, I found significant differences in nest availability and density between two genetically distinct populations, but this did not translate into significant differences in mating patterns. Taken together, my thesis helps elucidate the reproductive ecology of a classic species with ARTs, and has important implications for our understanding of ecological influences on mating patterns and sexual selection.</p> / Doctor of Philosophy (PhD)