Dominance hierarchies (“peck orders”) are either linear or despotic. In linear hierarchies, an alpha animal dominates all others, a beta animal is subordinate to the alpha but dominates lower ranked individuals, etc., down to the last, or omega, individual. Such a hierarchy exists in harems of the sex-changing Cleaner Wrasse, Labroides dimidiatus. A single male dominates up to six females, which in turn have their own linear hierarchy. Linear hierarchies also exist in salmonids, several livebearers, and centrarchid sunfishes (Gorlick 1976). In despotic situations, a single individual, the despot, is dominant over all other individuals, while subordinate animals have approximately equal ranks. In captive anguillid eels, a single large individual can monopolize 95% of a 300 L aquarium, relegating 25 other individuals to the remaining area where they mass together in continual contact. Despotic hierarchies have also been observed in Coho Salmon, Oncorhynchus
kisutch, and bullhead catfishes, Ameiurus spp. (Paszkowski & Olla 1985).
Dominance can be determined by size, sex, age, prior residency, and previous experience. In general, large fish dominate over smaller, older over younger, and residents over intruders. In many species, males usually dominate females, whereas in others, such as Guppies, females dominate males. Previous experience, in terms of recent wins and losses, often determines the outcome of future interactions; victorious fish tend to be aggressive and defeated fish submissive. Dominant fish typically occupy the most favorable microhabitats, relegating subordinates to suboptimal sites with respect to cover availability, current velocity, or prey densities. As a consequence, dominant individuals will have higher feeding rates, which ultimately lead to faster growth, better condition, and higher fitness (e.g., salmonids; Bachman 1984; Gotceitas & Godin 1992). Dominance hierarchies have also been observed in requiem and hammerhead sharks, minnows, ictalurid catfishes, amblyopsid cavefishes, cods, ricefishes (Oryziidae), topminnows, livebearers, centrarchid sunfishes, cichlids, labrids, blennies, and boxfishes (Ostraciidae).