The Dynamic Nature of Diadromy Preamble
Roger A. Rulifson
Diadromy is a behavior that involves physiological changes required to accommodate the extremes in habitat shifts between freshwaters and seawaters at a number of different life cycle stages, depending on the species. The physiological responses to changing environmental conditions such as temperature and day length trigger or are otherwise modulated by important behavioral changes that influence migration, environmental preferences, and other factors.
What makes diadromous behavior so interesting and difficult to understand is the inconsistency of diadromy among related species and within one species. Closely related species within a family may or may not exhibit diadromy. For one species, there may be latitudinal differences in populations exhibiting diadromy rather than residency. For some species, the diadromy-residency life histories differ between watersheds, and for others, both residency and diadromy occur within the same watershed. There also can be great individual variability within a species as to when diadromy begins and also whether diadromy continues through the life of the fish or ceases temporarily or permanently at some point during the fish’s life. Thus, there is great variability in the general concept of diadromy, and a diadromous life history itself can be extremely plastic or dynamic.
At the first diadromy symposium held in 1986 (Dadswell et al. 1987), the prevailing paradigm for diadromy was the concept of fish populations exhibiting a uniform behavior. Now, researchers are beginning to identify migratory contingents within a single watershed and even differences in migratory behaviors between individuals within a contingent. Individuals may become anadromous or resident or “change their minds” and their lifestyles later in life (e.g., Secor 1999; Gemperline et al. 2001; Paramore and Rulifson 2001).
This session addresses these inconsistencies to bring further understanding to the causative mechanisms involved in facilitating the life histories of diadromous fishes. At the first diadromy symposium, Gross (1987) proposed a paradigm for how anadromy and catadromy might have evolved. In the current symposium, this paradigm is challenged by Dodson et al. (2009, this volume) and by Tsukamoto et al. (2009, this volume). Dodson et al. (2009) use a comparison of families in the Salmoniformes and Osmeriformes to hypothesize the existence of contrasting evolutionary pathways that explain the current observed patterns of diadromy in these groups. Tsukamoto et al. (2009) offer an explanation of how upstream migration of fish might be initiated and how it prevails through generations, through positive responses of individual fish in schools to fish density, endocrinological condition, and hunger level.
Patterns of diadromy that do not hold across species within families or subpopulations within a single watershed continue to be discovered. Bradbury et al. (2009, this volume) examine how anadromous smelt populations in postglacial temperate latitudes may have poor gene flow, resulting in different migration patterns and morphological phenotypes. Howland et al. (2009, this volume) examine how one coregonid species in northern Canada, the inconnu Stenodus leucichthys, exhibits residency, anadromy, and potadromy (e.g., estuarine anadromy) within a single watershed. Finally, Grothues et al. (2009, this volume) review how nonnatal habitats are used by migrating species.