An age-structured model of stock dynamics without stock and recruitment model was applied to data of the Pacific stock of deep-sea smelt in order to estimate stock sizes from 1980 to 2018. The data sets used for estimations were series of catch over the years, that of abundance index with variance estimate, an estimate of the average fishing coefficient over 1970-2009 and some biological parameters. The age-structure consists of only two cohorts because the lifespan of the smelt is three years. Using the given fishing coefficient and age-composition in the first year of data and the abundance index, the annual stock size by age in subsequent years is computed by those values, biological parameters and deterministic equations in the model. Unknown parameters are estimated using the estimate of the average fishing coefficient, and the parametric bootstrap method is used to estimate the variance of parameter estimates. The estimation results show that point estimates of spawning stock biomass ranged from 965 (SE: 161) tonnes to 3,069 (SE: 515) tonnes, the biomass fluctuated in a cycle of about 20 years and those of fishing coefficient gradually decreased in proportion to effective fishing effort. The features of the estimation method and modification of the estimations in the future are discussed.

　In the Yodo River system, where various ayu stocks are released, it is not clear which forms of ayu, amphidromous or landlocked, are established and contribute to reproduction. Microsatellite DNA polymorphism analysis was used to determine the phylogeny of spawning stocks, estuarine and coastal larvae and juveniles, and upstream migrants in the Yodo River system to elucidate their reproductive contribution and presence. In all the living seasons, we confirmed that there were mainly individuals belonging to the amphidromous form, but we also confirmed individuals from the landlocked form, and it became clear that there was a high possibility that ayu from the landlocked form also reproduced.

　Ecklonia radicosa (Laminariales), a canopy-forming brown alga, was reported to have disappeared since 2016 at a site surveyed under a nationwide long-term monitoring survey for the natural ecosystem (Monitoring Site 1000, Japanese Ministry of Environment) in Nagashima Island, Kagoshima Prefecture, Japan. The distribution of seaweed assemblages along the coast of the island was surveyed in 2019 to compare with that in 2005. In 2005, E. radicosa and the temperate species of Sargassum (Fucales) were widely distributed, and comprised the large seaweed assemblages along the coast facing both the East China Sea and Yatsushiro Bay. However, in 2019, these species, including an edible alga, Sargassum fusiforme, were not found off the coast of the former, suggesting the degradation of kelp and fucoid assemblages. These algae were still found at the sites in Yatsushiro Bay as large seaweed assemblages; however, some tropical species of Sargassum that were uncommon in 2005 were also observed in 2019. The reason for the disappearance of these canopy-forming algae has not been elucidated yet; however, the herbivory impacts of fish on the algae may have caused the elimination of these algae from the coast facing the East China Sea in relation to climate change.

　Seaweeds belonging to the family Lessoniaceae are one of the major components of marine forests in the warm temperate region of Japan. Over the last few decades, marine kelp forests of Lessoniaceae have been decreasing along the western coast of Kyushu, presumably due to increasing seawater temperatures. We monitored the seawater temperature and distribution of the kelp off Waka on Ikishima Island, northwestern Kyushu from 2004 to 2018. From 2004 to 2007, the summer seawater temperatures were relatively low, and the distribution and density of the kelp increased. However, in 2010, 2013 and 2016, when the summer seawater temperatures were higher than those measured in the other years, kelp thalli were observed in higher proportions without any blades. The density of adult kelp plants drastically decreased throughout the following year. After the decline in adult density, many recruits grew in 2011 and 2014, and the kelp forests recovered. However, few recruits appeared in 2017, leading to the absence of the kelp at the study site in 2018. The deterioration of the adult community accompanied by higher seawater temperature and subsequent failure of recruitment would be responsible for the disappearance of the kelp forests.

　In order to clarify predatory impacts on fyke-net catches by invasive channel catfish, the stomach contents of 176 specimens (4.6-57.6 cm in standard length, SL), collected with a large fyke-net in Lake Kasumigaura, Ibaraki Prefecture, Japan between afternoon and the next morning once a month from June to August 2016 and from March to August 2017, were examined. Major food items of the catfish comprised undigested important fisheries species, such as Hypomesus nipponensis, Salangichthys microdon, Gymnogobius urotaenia and Macrobrachium nipponense, frequently occurring also in the fyke-net catches. Mean stomach fullness index in each size class larger than 20 cm SL was extremely high (6.7-8.8) compared to those in the natural environment. It is therefore highly possible that individuals larger than 20 cm SL ate small fishes and shrimp in the fyke-net. The estimated proportion of predation on each species by the catfish in the fyke-net throughout the study period was 73.7%, 80.1%, 36.6% and 33.3% for H. nipponensis, S. microdon, G. urotaenia and M. nipponense entering the net, respectively. Thus, predation by the catfish on important fisheries species inside the fyke-net was considered to be a major problem in local lake fisheries.

　Harmful algal blooms (HABs) of unarmored dinoflagellate Karenia mikimotoi have often been recorded first in Sasebo Bay, Nagasaki, Kyushu in each year since 2000. For better forecasting of HABs in Sasebo Bay, the cell density of HAB species and other dominant microalgae was regularly monitored and compared with temperature and salinity for 7 years (2013-2019). Presence and density of resting cysts were also estimated from the cells germinated from the seabed sediments collected in 2013 and 2018. Blooms of three HAB species, Heterosigma akashiwo, K. mikimotoi and Chattonella spp., were observed in this study. Blooms of H. akashiwo often formed earlier than the other two, in the range of 20-23℃ and in the years with long sunshine hours from early to mid May. Blooms of K. mikimotoi occurred every year, proliferating from May or June, and denser blooms were recorded in the years with higher total water temperature from January to April and total air temperature from December to March. Chattonella blooms were recorded from 2013 to 2015, and mean Chattonella cyst density was 6.7 cysts cm⁻³ in 2013, while no cyst was detected in 2018.

　Fish farming in coastal waters generates large amounts of particulate organic wastes, such as waste feed (unconsumed feed) and fecal matter. We collected sediment core samples in a small bay where fish farming has been actively conducted and determined the vertical profiles of total organic carbon (TOC), total nitrogen (TN) and phosphorous (TP). Fish farm TOC and TN sediment contents showed similar profiles with those of a non-fish farming area (reference area), but TP content was higher. It is thought that the TP content in fish feed is high due to the bones of small fish; is hard to be assimilated by cultured fish; and is hard to be decomposed in the sediment. Our results suggest that the environmental impacts of fish farming can be evaluated from the TP content in core samples, as an indicator of organic waste produced by fish farming.

　In order to investigate the actual state of contamination of fisheries feeds by mycotoxins, we examined a method for simultaneously analyzing 10 species of trichothecenes in fisheries feeds. A pretreatment method with a high purification effect was constructed using a multifunctional column and graphite carbon, and a high-speed and highly robust analytical method was established by using a C18 column for UHPLC. When this analysis method was used to analyze commercially available fisheries feeds, several types of fungal toxins such as deoxynivalenol and its modified mycotoxins were detected in many feeds. All of the detected feeds contained wheat flour, suggesting the possibility of contamination of the raw material flour.