Keywords

anadromous, sea-run, lake-run, river-run, partial migration, life history dimorphism, latitudinal cline, cost-benefit, genetic variation, phenotypic plasticity, fitness, parasites

Study species

masu salmon, white-spotted charr, Dolly Varden, Japanese dace, freshwater leeches, parasitic copepods

To sea or not to sea

Partial migration is a phenomenon where both migratory and resident individuals coexist in a single population. This is reported in many different taxa, such as ungulates, birds, fishes, and insects, but salmonids are the most remarkable. Migratory individuals generally exploit abundant food resources, producing huge phenotypic dimorphisms between migrants and residents in breeding areas. What types of individuals migrate or reside, if they are determined by genetics or the environment, which have a higher life time fitness, what are the population- and ecosystem-level consequences? We combine field observation, lab experiment, and next-generation sequencing to address these questions. This is one of the main projects in our lab.

Et tu, Dace?

The pattern of anadromy (i.e. ocean migration) is well known in salmonids: for example, anadromous fish are larger than residents, females tend to migrate more than males, anadromy increases with latitude. However, it remains largely unknown for non-salmonid anadromous fishes. Japanese daces, Tribolodon spp. are anadromous cyprinids, which are very rare among the > 2,000 cyprinid species. Tribolodon hakonensis is widely distributed along the Japanese archipelago, consisting of both resident and anadromous forms. This species is so common that many people treat it as “small fish (Zako in Japanese)”, however they are ecologically important. We’ve done a preliminary analysis on their anadromy, such as growth rate, sex-biased migration, and latitudinal cline. However, it seems that this species shows quite different patterns compared to salmonids. To better understand the evolution of anadromy, we should pay more attentions to examples in various species.

Ocean, lake, and large stream: what differs?

As mentioned above, anadromy (sea-run) are well understood in salmonids, as well as lake-run. It is also reported that some stream-dwelling salmonids undergo seasonal migration between small tributaries and the larger mainstem, but detailed information has been lacking. We found in Dolly Varden in the Sorachi River that migratory individuals grow bigger and females tend to migrate more than males (Koizumi et al. 2006). As far as we know, this female-biased migration and size dimorphism are the first evidence among stream-dwelling salmonids, and provide important insights into the migration of salmonids in general. On the other hand, we haven’t found clear smoltification so far and there should be some differences compared to sea-run and lake-run populations. We consider this system as a “microcosm” of anadromous systems where detail field observation and manipulation are possible.

• Koizumi I., Yamamoto S. & Maekawa K. (2006) Female-biased migration of stream-dwelling Dolly Varden in the Shiisorapuchi River, Hokkaido, Japan. Journal of Fish Biology, 68, 1513-1529.
• Koizumi I. & Maekawa K. (2003) Spawning migration of stream-dwelling Dolly Varden in spring-fed tributaries of the Shiisorapuchi River, Japan. Journal of Freshwater Ecology, 18, 321-331.