Title A study on the population dynamics of yellowstriped butterfish in the waters around the northern part of the Izu Islands.
Authers Shingo WATARI
Keywords yellowstriped butterfish, age, growth, otolith, abundance estimation
Citation Bull. Fish. Res. Agen. supplement No. 18,167-242, 2006
The yellowstriped butterfish, Labracoglossa argentiventris, inhabit shallow waters over rocky bottoms along the Pacific coast from Kyusyu Island to the Boso Peninsula. This stock is important to coastal commercial fisheries in the northern part of the Izu Islands, which use bulk net, gill net, seine and set net to catch this resource. Assessment and management of the stock are necessary for its effective utilization. This thesis focuses on the population dynamics and stock assessment of yellowstriped butterfish, such as age and growth, standardization of catch per unit effort (CPUE), and abundance estimation, to provide information for fisheries resource management.
In Chapter 2, a method to determine the age of yellowstriped butterfish using from the otolith was studied. The edge of the opaque zones as a ring mark on sectioned sagittal otoliths was used. Monthly changes in the marginal growth rate and proportion of appearance of ring marks on the edge of the otolith were examined. Formation of the first ring was observed during spring or summer, corresponding to one and a half years after hatching. Thereafter, one ring was formed each year in the same season as the previous year.
In Chapter 3, age, growth and maturity were examined using samples collected in the waters around the northern part of the Izu Islands from 1994 to 2003. Age distribution was estimated from the otoliths and scales. Age distribution determined from the scales resulted in an underestimation compared to the values from the otolith analysis. The comparison of multiple age determination indicated that within reader bias of otolith was smaller than that of scale.
Growth was investigated by fitting the von Bertalanffy growth model to the age-length data and the allometric growth model to the length-weight data using maximum likelihood methods. Because the number of small sized fish was insufficient, all parameters could not be estimated by areas (Izu Oshima Island, Toshima Island~Shikinejima Island, Kouzushima Island). The selected von Bertalanffy growth model, based on the Akaike Information Criterion (AIC), was the differential asymptotic length and variance by area and sex, and growth coefficient by sex. The growth of yellowstriped butterfish was fast until 2 years of age. The estimated asymptotic length of females was larger than that of males. The estimated asymptotic length in the southern part of the study area was larger than that in the northern part of the study area. The effect of the difference of ageing character, otolith or scale, on the growth curve was investigated. Age before 1 year old fish was determined using the large differences in fork length between the older fish, so ageing character was not used. Because the growth speed of large sized fish was slow, age-length relationship was almost same after 4 years of age. For these reasons, difference between the two growth curves was much smaller. The selected allometric growth model, based on the AIC, was multiplicative error structure of differential sex in parameters. The monthly change of the value of Gonado-somatic Index was high from September to December, spawning season, and the peak was October. The maturity rate of 1 year old was 15% and that of after 2 years old was 100%.
In Chapter 4, standardization of CPUE was carried out to improve the reliability of the abundance index. To estimate a year effect, any other factors that may influence CPUE were removed from index by using the generalized linear model. Landing in weight by each operation of gill net of Izu Oshima Island, seine from Toshima Island to Shikinejima Island, and bulk net of Kouzushima Island were obtained from 1991 to 2003 by landing slips. Data of sea surface temperature and current pattern of Kuroshio were also obtained for the same period. Because there was little information about movement in the study area, two hypotheses were considered about the movement after recruitment, the case to move from one island to other islands, and the case that there is no movement among islands. Year, month, area, fishing methods, sea surface temperature, and current pattern of Kuroshio were incorporated as main effects and two-way interactions. For each area, the best model was statistically selected by the AIC from all candidate models. The year effect of standardized CPUE was calculated by least squared mean. The trend of relative abundance of yellowstriped butterfish has remained at the same level after 1991.
In Chapter 5, stock size of yellowstriped butterfish was estimated by the tuning virtual population analysis based on two hypotheses about the movement among islands using the catch at age and CPUE data. The previous study of rearing experiment showed that probability of agreement of true age by using data from otoliths was higher than that from scales. In addition, previous chapter also indicated that within reader bias for otoliths was lower than that for scales. For these reasons, the otolith ageing was considered to be true for catch at age estimation, and the bias in scale ageing was corrected for the basis for otoliths. The standardized CPUE was used as an abundance index for the tuning. Parameters were estimated by the least squares method and standard error was also estimated by the bootstrap method using resampling data of CPUE and catch at age. The yearly change of stock size of yellowstriped butterfish in the northern part of the Izu Islands after 1998 ranged from 600 to 700 metric tons. The catch rate of this stock ranged from 20 to 30%. The estimated standard error of the stock size became larger in recent years. Coefficient of variation of stock size in 2003 was 0.2. The selectivity of older fish was higher than that of younger fish. This result suggests that the target of this fishery is large sized fish which are high priced, and fishing intensity of small sized fish is low. The estimated stock size, fishing mortality, and selectivity, using two hypotheses of the movement shows the same trends. A similar conclusion of the stock management plan was possible. The percent spawning per recruitment was about 50% in 2003 in each area. The results showed that if the fishing effort is increased from the present level, the amount of catch of large sized fish would decrease, although the total amount of catch would
The present level of fishing mortality is close to F0.1. Yield per recruit and spawning per recruitment analysis also suggested that the current level of exploitation is not overfishing. For the sustainable use of yellowstriped butterfish stock, both the current level of fishing effort and low fishing intensity on younger fish should be maintained.
URI http://www.fra.affrc.go.jp/bulletin/bull/bull18/watari.pdf