Kazumi Sakuramoto

Does the Ricker or Beverton and Holt type of stock-recruitment relationship truly exist?

This study investigated the main causes of population abundance fluctuations. Particularly, attention was paid to whether a density-dependant factor, such as a stock-recruitment relationship (S-R relationship) or a density-independent factor such as an environmental factor, is more important. Using data pertaining to the number of eggs of the Pacific stock mackerel and information about regime shifts and sea surface temperature, the shape of the S-R relationship was discussed and these shapes with the results of simulation trails were compared. Further other historical S-R relationship data were analyzed. The results are as follows: (i) a new mechanism that causes population fluctuations could be proposed, that is, (a) the recruitment is proportional to the spawning stock biomass (SSB) and the relation is expressed by several lines with the same slope, and (b) the shift between the lines occurs due to environmental conditions; and (ii) the density-dependent S-R relationship, which suggests that recruitment decreased due to high density in SSB, proposed by Ricker or Beverton and Holt, would not exist.

Effects of process and/or observation errors on the stock–recruitment curve and the validity of the proportional model as a stock–recruitment relationship

We examined the effects of process and observation errors on the selection of the stock–recruitment relationship (SRR) curve using simulations. When the process and observation errors were added to both spawning stock biomass and recruitment, the results were as follows: (1) When the proportional model was set as the true SRR model, there was a high probability that the Ricker or Beverton and Holt model was selected in response to the errors; (2) When the Ricker or Beverton and Holt model was set as the true SRR model, the proportional model was seldom selected in response to the errors; (3) The proportional SRR model should be accepted as the optimum SRR model for the Pacific stock of Japanese sardine; (4) We should use an SRR model that is constructed from more than two independent variables (spawning stock biomass, environmental factors, etc.) when we discuss management of fisheries resources.

Estimating and monitoring the stock size of sandfish Arctoscopus japonicus in the northern Sea of Japan

The stock size of sandfish in the northern Sea of Japan was estimated by a virtual population analysis (VPA) and sensitivity analyses were attempted on the VPA estimate. The stock size estimates were approximately 600–900 million until 1975, but since 1976 they have rapidly decreased. In the sensitivity analyses, the estimates of absolute stock size were not sensitive against the changes in the fishing mortality coefficient for terminal age and the measurement error in catch-at-age. This suggested that the relative stock size remains almost unaffected by the error in the data used in the VPA, if the degree of catch-at-age error and the natural mortality coefficient is correct. The relationships between the biomass estimated by the VPA and the density index from Danish seine fisheries, and between the biomass and the catch per unit effort (CPUE) from the experimental survey using Danish seine nets, were also examined. The density index and the CPUE indicated significant relations with the biomass. Consequently, the CPUE is useful to monitor the relative stock size in a timely manner, and the VPA estimate and the CPUE should be utilized for adjusting the total allowable catch in the multiseasons.

Proposal for stock-recruitment relationship for Japanese sardine Sardinops melanostictus in North-Western Pacific

This study assessed the stock-recruitment relationship (SRR) for the Japanese sardine Sardinops melanostictus in the North-western Pacific. Of the 20 SRR models investigated, the Akaike information criterion (AIC) was the minimum (AIC=551.2) when the data were separated into two groups (A and B) and the log-normal distribution was applied as the error term. Group A was constructed with SRR data from 1976–1987 and 1992–2004. Group B consisted of data from 1988–1991. The AIC minimum model was R=22.8 S × eε for Group A, where R, S, and ε denote the recruitment of sardine (individual number of 0-year old fish), spawning stock biomass (SSB), and error term, respectively. This model indicated that recruitment was proportional to the SSB and that no density-dependent effect operated over the range of SSB investigated (51000–11.3 million t). Recruitment was markedly higher (lower) when the sea surface temperature (SST) of the Kuroshio Extension area in February was low (high). The essential SRR can simply be expressed as R=22.8 S × eε with the level of recruitment deviating from the model to a greater or lesser degree depending on the environmental conditions.

A Common Concept of Population Dynamics Applicable to Both Thrips imaginis (Thysanoptera) and the Pacific Stock of the Japanese Sardine(Sardinops melanostictus)

The aim of this study was to discuss a common concept of population dynamics applicable to both Thrips imaginis (Thysanoptera) and the Pacific stock of the Japanese sardine (Sardinops melanostictus). First, I elucidate the mechanism that produces the false density-dependent effect on population changes of Thrips imaginis, I conducted simple deterministic simulations to discuss the qualitative viewpoints. Second, I conducted Monte Carlo simulations by using the average population size and standard deviation of the thrip data used by Davidson and Andrewartha for the quantitative discussion. In simple deterministic simulations, the resultant plots of population change against population size showed a decreasing trend for which the slope was statistically significant even though the true relationship between the population change and population size had no density-dependent effect. The results of the Monte Carlo simulations indicated that nearly 70% of the trials showed false density-dependent effects. The provability of the false decision, which was to recognize the existence of density-dependent effects, increased as the standard deviation in population size in a month i-1 increased. When the number of samples increased, the probability of the false decision greatly increased. The conclusion from these simulations was that the density-dependent effect that emerged in the population change of T. imaginis was artificially produced and invalid. Further, the thrip population size in month i was determined in proportion to that in month i-1; and environmental conditions in month i. This mechanism was completely same of that shown in the Japanese sardine (Sardinops melanostictus). Therefore, the fluctuations in populations of T. imaginis and Japanese sardine could be explained with the same concept of population dynamics shown here.

Dynamics of two sailfin sandfish (Arctoscopus japonicus) stocks in the Sea of Japan, and their management

The dynamics of the northern and western Sea of Japan sailfin sandfish (Arctoscopus japonicus) stocks (NSJ and WSJ, respectively) varied simultaneously until the mid-1970s but differently thereafter. To identify the mechanisms underlying these dynamics, we evaluated the impact of environment and harvesting on NSJ by developing a recruitment model and simulating NSJ dynamics. The model was constructed based on egg production, density effect, and water temperatures in winter and spring, which correlated significantly and positively with the estimated surface mixed-layer depth. Winter water temperatures and egg production were variables common to the two stocks. Our results suggest that simultaneous dynamics resulted from similarities in variations in winter water temperatures for each stock and the difference in dynamics resulted from NSJ depletion caused by overfishing. We suggest management strategies to prevent NSJ depletion caused by unfavorable environmental conditions and overfishing, as well as to al...

Relationship between sea-surface temperature and catch fluctuations in the Pacific stock of walleye pollock in Japan

This paper investigates the relationship between sea-surface temperature (SST) and catch fluctuations in the Pacific stock of walleye pollock Theragra chalcogramma in Japan. Incorporating time lags between years of birth and harvest, the correlation coefficients between the catch and SST in two regions off the east coast of Hokkaido were calculated. The catch in year t had a high negative correlation with the SST during January-April and November-December of the years t-2 and t-3 in the spawning area. These results coincided well with the correlation observed in the northern ‘Sea of Japan’ stock. Both analyses suggested that the long-term catch fluctuations of the two stocks could be explained by the same mechanism, that is, the fluctuations would be explained by the SST in their spawning are a during the spawning season using 2–3 or 3–5 years time lags, which corresponded to the dominant age of the catch within these two stocks.

A New Concept of the Stock-Recruitment Relationship for the Japanese Sardine, Sardinops Melanostictus

The aim of this paper is to discuss the validity of the results of Wada and Jacobson (Can. J. Fish. Aquat. Sci. 55, 1998) and to propose a new concept of the stock-recruitment relationship for the Japanese sardine. Two regression analyses were used: simple regression and Deming regression. Reproductive success was defined as recruitment (R) di- vided by egg production. A false decreasing trend was produced in the regression line of ln(reproductive success) against ln(egg production) in response to observation error. When Deming regression was applied, the slope of the regression line of ln(R) against ln(egg production) did not statistically differ from unity. This indicates that a density-dependent effect cannot be detected on the stock-recruitment relationship in the data used in this study and that it is reasonable instead to accept a proportional model as the optimal stock-recruitment relationship model for the Japanese sardine. The results of this study are diametrically opposed to those of Wada and Jacobson (1998), who assumed that density-dependent effects on recruitment typically exist for Sardinops species.

Catch forecasting and relationship between water temperature and catch fluctuations in snow crab Chionoecetes opilio in the western Sea of Japan

Catch forecasting and the relationship between water temperature and catch in the snow crab Chionoecetes opilio in the western Sea of Japan were investigated. Catch was used as an index of abundance on the basis of high correlations between catch per unit effort for the period when the latter data were available. The pattern of fluctuations, in catches and index coincided well with each other. Therefore, catch data were regarded as an index of abundance and the correlation coefficient between catch and water temperature was calculated at several depths from 1964 to 1999. Catch forecasting models were composed using significantly correlated variables with the following results: (i) in April and September, the catch showed high positive correlation with water temperatures (depth 50, 100, and 200 m, time lag 4–6 years); and (ii) a model using water temperatures in April alone (depth 100 m, time lag 4–7 years) forecasted the catches with a coefficient of determination of 0.504, where models using more variables (water temperatures in the 2 months and catches) showed a coefficient of 0.587 at most. Environmental conditions during the early life stages of the snow crab are thought to deeply influence the fluctuations, in abundance.

Population size estimation of the pond smelt Hypomesus nipponensis in Lake Kasumigaura and Lake Kitaura, Japan

We estimated the population size of the pond smelt Hypomesus nipponensis at the beginning of the fishing season in Lake Kasumigaura and Lake Kitaura, Japan using two DeLury methods and cohort analysis. A growth curve was estimated on the basis of monthly standard length. The relationship between standard length and weight was utilized for calculating the mean weight on the survey day in a particular month. Total monthly catches in Lake Kasumigaura and Lake Kitaura were calculated using partial monthly pond smelt catch obtained from processing plants located near the lakes between July and December. The total monthly catch was calculated from the total monthly catch weight and the monthly mean weight. The number of boats operating each month, compiled by the Kasumigaura-Kitaura Fisheries Office of Ibaraki Prefecture, was also employed. The estimated initial population sizes were compared with the population level index (PLI) estimated from survey data before the start of the fishing season. No significant differences were detected among the initial population sizes estimated by DeLury method, cohort analysis and PLI. The estimates ranged from 7.4 million to 410 million in Lake Kasumigaura, and from 7.7 million to 44 million in Lake Kitaura.