Hiroshi Hakoyama

Behavioural individuality assessed from two strains of cloned fish

Abstract Individualistic behavioural traits have long been overlooked, dismissed as either noise or maladaptive deviation to atypical situations. We investigated individual variation in two strains of cloned red-spotted cherry salmon, Oncorhynchus masou macrostomus , in a series of experiments under laboratory conditions. Using principal component analysis, we found that their basic responses such as distance moved, space used and foraging mode grouped along five major dimensions; individuals could be discriminated from each other reliably and showed clear behavioural individuality. Interrelationships between behaviour patterns that differed between the two clonal strains suggested the potential for a genetic basis to behavioural individuality. One or two of the behaviour patterns explained intraclonal variation in individual growth histories. We conclude that behavioural individuality can have an adaptive significance.

Transition from a random to an ideal free to an ideal despotic distribution: the effect of individual difference in growth

Abstract Individual differences in growth can lead to a monopolistic form of food competition. We studied the long-term transition in the mode of competition and the distribution of individuals between food patches of the cloned salmonid fish, Oncorhynchus masou ishikawae, in the laboratory. This transition was accompanied by growth depensation, i.e., the increase over time in the variance of size between individuals resulting from the differences in individual growth rates. The 120-cm experimental tanks were divided into two compartments (patches) between which an opaque partition was placed. Fish were able to move freely between the patches and therefore were able to assess the patch quality using long-term memory, but they were not able to see the food input in the other patch directly. The distribution between the two food patches, the amount of food gained, and the growth and the agonistic behavior of four groups of six individuals were observed over 4 weeks. We found that (1) within-group variation in body weight increased with time; (2) on average, the better patch was used by more individuals than predicted by a random distribution but fewer individuals than predicted by an ideal free distribution, and (3) the distribution and pattern of resource use by the fish changed over the 4-week experimental period from a random distribution to an ideal free distribution and finally to an ideal despotic distribution. We suggest that growth depensation causes the long-term change in the spatial distribution and pattern of resource use by competitors.

Estimate of population extinction risk and its application to ecological risk management

Abstract Environmental threats, such as habitat size reduction or environmental pollution, may not cause immediate extinction of a population but may shorten the expected time to extinction. We developed a method to estimate the mean time to extinction for a density-dependent population with environmental fluctuation and to compare the impacts of different risk factors. We first derived a formula of the mean extinction time for a population with logistic growth and environmental and demographic stochasticities expressed as a stochastic differential equation model (canonical model). The relative importance of different risk factors is evaluated by the decrease in the mean extinction time. We studied an approximated formula for the reduction in habitat size that enhances extinction risk by the same magnitude as a given decrease in survivorship caused by toxic chemical exposure. In a large population (large K) or in a slowly growing population (small r), a small decrease in survivorship can cause the extinction risk to increase, corresponding to a significant reduction in the habitat size. Finally, we studied an approximate maximum likelihood estimate of three parameters (intrinsic growth rate r, carrying capacity K, and environmental stochasticity σ2e) from time series data. By Monte Carlo sampling, we can remove the bias very effectively and determine the confidence interval. We discuss here how the reliability of the estimate changes with the length of time series. If we know the intrinsic rate of population growth r, the mean extinction time is estimated quite accurately even when only a short time series is available for parameter estimation.

The information of food distribution realizes an ideal free distribution: Support of perceptual limitation

Previous tests of ideal free distribution (IFD) under continuous input conditions have demonstrated that more profitable patches tend to be relatively underused compared to that predicted by the theory. We tested the hypothesis that competitors’ perceptual constraints of resource distribution cause this deviation from the IFD. A laboratory experiment was conducted to determine whether additional information on food distribution by a light cue that indicates the food input point improves the IFD theory’s fit to the distribution of clone red-spotted masu salmons (Salmonids),Oncorhynchus masou ishikawai, that had been conditioned to the light as a cue indicating the site with a higher input rate. In the treatments without a light cue, the distribution of fish was closer to a random pattern than an IFD. In contrast, in the treatments with light cue, the distribution of fish was closer to the expected value of an IFD rather than to a random pattern, supporting the perception-limit hypothesis. The distribution and the pattern of resource use by fish in the treatments without the light cue were best explained by the perception-limit model. Our results suggest that it is perceptual constraints that cause deviation from the IFD.

Why is competition more intense if food is supplied more slowly

Abstract We investigated competition for food among two groups of six clone amagos (salmonids), Oncorhynchusmasoumacrostomus, in a laboratory experiment with different rates of food input. We examined the effect of temporal clumping of food resources on the inequality of food sharing between competitors. Monopolization of food by dominants was greater at a low input rate (one food item per 10 s) than at a high input rate (1 food item per 1 s). Aggressive behavior by dominants was more frequent at the low input rate than at the high input rate; its purpose was presumably to interfere with the feeding behavior of subordinates. We assessed the relative importance of three foraging factors (the number of approaches to food items, the chance per approach and the gain per chance) in enhancing inequality in food gain between individuals. Dominants had a disproportionately high chance per approach and gain per chance at the low input rate, but not at the high input rate. The chance of obtaining a food item per approach depended on how many competitors approached simultaneously. The gain per chance depended on the competitive ability of the approaching fish. There was an interaction between these components, such that the number of approaches affected the chance per approach and gain per chance. We evaluated the independent effect of the chance per approach, and showed that it was higher for dominants than for subordinates at the low input rate, but not at the high input rate. This implies that subordinates changed their behavior and became more likely to avoid approaching food at the same time as dominants at the low input rate.

Spawning Behavior of Hemibarbus barbus (Cyprinidae)

Hemibarbus barbus is an endemic Japanese cyprinid whose spawning behavior has not been described. The fish is the largest among endemic Japanese cyprinids, reaching a maximum total length of 60 cm. Spawning occurs during a short period after floods in the April to June rainy season (Okada and Nakamura, 1947; Kubo et al., 1953; Takeshita and Kimura, 1991). It is in the subfamily Gobioninae (Banarescu and Nalbant, 1973; Hosoya, 1986) whose breeding habits are not well known. In this paper, the breeding habits of H. barbus are described and compared with those of four other species of Gobioninae.

Female-biased operational sex ratio of sexual host fish in a gynogenetic complex of Carassius auratus

Abstract To study the coexistence of sexual and gynogenetic forms, we examined the population structure of a gynogenetic complex of the Japanese crucian carp, Carassius auratus Temminck et Schlegel, during the April–June reproductive season by collecting 1225 mature fish that migrated from Lake Suwa to a tributary river for spawning. There were more sexual fish (about 80%) than gynogenetic fish in this complex, and the operational sex ratio in the sexual form was female biased (males were about 20%). Mean standard length and body weight of sexual females were larger than those of sexual males. Sex ratio was male biased in smaller fish (standard length, <8.5 cm) but female biased in larger fish (standard length, ≥8.5 cm). We determined age by scale ring marks; the average age of sexual females was higher than that of males, but there was no significant difference in the average age between sexual and gynogenetic females. Sex ratio in the sexual form was more female biased for old than for young fish, and the mean size of sexual females was larger than that of males of the same age. The clear female-biased sex ratio and age difference between sexual females and males can be explained either by (1) higher mortality of males or by (2) female-biased sex allocation. The latter process reduces the disadvantage of sex and contributes to the coexistence of sexual and gynogenetic forms.

Compilation of Japanese fisheries statistics for the Japanese eel, Anguilla japonica, since 1894: a historical dataset for stock assessment

Fishery sustainability and the extinction risk of the Japanese eel, Anguilla japonica, are of global concern. The landings of the Japanese eel in Japan comprise a large part of the landings in East Asia. This study provides a compiled dataset of the annual fisheries statistics of the Japanese eel in Japan for stock assessment. The Japanese government has been recording Japanese eel statistics annually since 1984 in five series of annual reports by conducting systematic questionnaire surveys of fisheries managers and associations; however, most of these data are stored in analog format. The key variables in the dataset include the harvest weight of eels, the harvest weight and number of seeds for aquaculture, the number of eels stocked, and the number of management entities engaged in the eel fishery. The levels of spatial aggregation of the variables include the site (river and lake), prefecture, inland and coastal waters, and total in Japan. We also incorporated location data (latitude and longitude) of the site and prefecture into the dataset. Eel harvest includes primarily yellow eels (late juvenile stage) and silver eels (mature stage). Seed harvest in inland waters includes glass eels (intermediate stage between leptocephalus and elver) and elvers (early juvenile stage). Seed harvest from coastal waters comprises glass eels. This dataset provides information to assess long-term trends in the Japanese eel population.

Allometric analysis of a morphological anti-predator trait in geographic populations of Japanese crucian carp

Costly anti-predator traits tend to be expressed only in high-predation conditions. For the cyprinid fish genus Carassius, deeper body depth is more adaptive to avoid predation by gape-limited piscivorous fish, but it raises swimming costs. It is therefore predicted that the relative body depth will decrease when the prey fish has reached a size larger than the predator gape-size. This prediction was tested by allometric analysis of the relation between body depth and standard length of triploid asexual females of the Japanese crucian carp (Carassius auratus sspp.) sampled from 13 geographic populations. The overall allometric relation was not significantly different from isometry. The estimate of the common major-axis slope was close to 1 (near-isometry). The mean relative body depth differed significantly among populations. A significant positive correlation was found with the mean annual air temperature. The geographic variation suggests that local selection pressures vary. In conclusion, the hypothesis that larger fish will have lower body depth was not supported, perhaps indicating that deep body depth in large fish is adaptive for some reason other than defense against piscivorous fish.