Mass movements of fish from one place to another on a regular basis are common and such migratory behavior can range in occurrence from seasonal to daily. Studies of fish migrations are generally of two kinds: those that set out to describe the migrations and those that attempt to explain them. Descriptive work deals with the overall pattern of migratory history as determined using mark-recapture, biotelemetry, micro data recording tags, and elemental analyses of hard tissues such as the otoliths, scales and spines of fish. Mark-recapture studies of fish started on a large scale between 1890 and 1900, and these studies have been one of the best ways of following the migrations of fish by analyzing the distribution of the recoveries of tagged or marked individuals. However, the track taken by the fish during migration is unlikely to be a straight line, and therefore it is difficult to determine the exact migration route. Recently, technological developments in biotelemetry and micro data recording tags have made it possible to trace the migratory history of fishes, including external and internal information recorded simultaneously as digital data. However, those methods are restricted by several limitations. For biotelemetry, there is the difficulty of tracking in rough seas or bad weather, the difficulty of tracking over long periods of time, and the frequent loss of the transmitter signal. For mark-recapture methods and micro data recording tags, recovery of the fish after tagging must be made. With smaller individuals such as larvae and juveniles, the elemental composition of hard tissues, especially in otoliths, has been used to provide valuable information on the temperature and salinity of the migratory environment experienced by each fish throughout its life history. All these methods have certain limitations that are clear when they are examined in detail and the advantages and disadvantages of each can then be evaluated and the best methods can be selected for studying the migration of each life history stage of fishes.
In order to clarify the effects of dams on the life cycle of anadromous white-spotted charr Salvelinus leucomaenis, their life-history traits were studied in both below- and above-dam sections of two rivers in northern Japan. In the below-dam sections, the frequency of smolts was over 55.9%, male mature parr only being captured. In the above-dam sections, however, the frequency of smolts was under 15.2%, the maturity rate of parr being over 54.9% in both sexes. This suggests that the charr life cycle type in the below-dam sections was typically anadromous, compared with a land-locked type in the above-dam sections. The latter sections had lower parr densities compared with the below-dam sections, the fork length of young parr (aged 0+ and 1+) in the above-dam sections being greater than those in the latter, suggesting that a higher growth rate as a result of low density had lead to landlocking of the above-dam population. There was no difference between above- and below-dam sections in the frequency of over threshold size for smoltification (10 cm F.L. in autumn). However, the frequencies of smolts in the below-dam sections were higher than those in the above-dam sectins, suggesting that the inhibition of smoltification had promoted maturation as parr in the above-dam sections. The mechanisms of landlocking based on the threshold sizes of maturation and smoltification are discussed.
Life history and population size of an endemic Japanese spinous loach, Cobitis takatsuensis, an endangered species, were surveyed in the Hijikawa and Iwamatsugawa river systems, Shikoku Island. The loach was visible on the river bed when water temperatures rose above 13-15°C and throughout this period inhabiting pebble and boulder areas under shallow banks of pools. No significant migration was observed. Hibernation, reproduction and early development were believed to occur deep in the pebble/boulder interstices. Gravid females were observed when water temperatures reached 16-20°C, from May to July, the sex ratio becoming more female-biased during the reproductive season. Adult fish size and minimum female size at maturity were both larger in the Iwamatsugawa River. The mean diameter of ovarian large size eggs which will actually be produced is 1.3 to 1.4mm in the Hijikawa River population, rather smaller than (1.4 to 1.5 mm) in the Iwamatsugawa River, the size and number of eggs of the latter being equivalent to other populations surveyed in the Chugoku district. A mark and recapture exercises indicated each population to comprise less than 400 adults, the high recapture rates, 15-25% of adult fish, suggesting that the populations could easily decrease in size due to overcatching. In addition, limited habitat resources also increase the possibility of the population's accidental extinction. The need for active conservation measures, considering the characteristics and geographical variations in the life history of C. takatsuensis, was also discussed.
A series of investigations to study the relationship between the habit of stream fishes and physical environmental factors was conducted during 1999-2000 in the upper reach of the Katsuura River, Tokushima Prefecture. A semi-quantitative method (applied line-transect) was applied, and the occurrence/non-occurrence of each fish species recorded. Logistic regression models were constructed using binary fish data as objective variables and 15 physical environmental factors as predictor variables. The models were evaluated from the aspect of predictability involving both occurrence and non-occurrence of fishes. Although the models obtained for 18 of the 20 species in the area were significant, the models for only seven species were appropriate for predictability. For other species, the occurrence ratios indicated either extremely low or extremely high values. A few models, such as that for Oncorhynchus masou ishikawae, however, indicated sufficient predictability regardless of a low occurrence ratio.
Twenty-one specimens of the Pacific lamprey, Entosphenus tridentatus, were collected from the Hoki (8 specimens), Yusaka (11 specimens) and Arakawa Rivers (1 specimen), all tributaries of the Naka River, and the mainstream of the Naka River (1 specimen), Tochigi Prefecture, central Japan, between August 1999 and April 2001. Seven spawning redds, possibly constructed by Entosphenus tridentatus, were found in the Yusaka River on May 2 and 11, 2000, eggs being observed in one of them. Two specimens, collected from the Yusaka River and reared in an aquarium with a gravel substrate, subsequently spawned about 1000 eggs being observed in the redd. Forty-one of the eggs were reared at 15°C in an electric incubator, hatching starting 13-17 days after spawning. The hatching percentage was 92.7%, the accumulative temperature to hatching being between 195 and 255°C.