Life history, population dynamics, and sustainable use of Icefish Salangichthys microdon in Lake Abashiri, Hokkaido (Doctoral thesis)

 

 The Japanese icefish Salangichthys microdon of Lake Abashiri has high commercial value as a typical fisheries resource in inland water fisheries in Hokkaido. However, the　informations available on this species such as population dynamics and life history is very poor, and the annual catches of icefish are unstable. This study clarifies the development of fishery resource management for sustainable use of the icefish in Lake Abashiri. Therefore, the fluctuation mechanism of population size, spawning habitat and the life history of icefish in Lake Abashiri were elucidated. The life history was validated by field observations based on the hypothesis by the trace element analysis of otoliths and analysis data of icefish fisheries such as CPUE. In addition, the estimation method for population size of icefish in Lake Abashiri was developed in this study based on the features of their life history. And furthermore, an appropriate management strategy of icefish resources in Lake Abashiri was proposed based on Ricker's stock-recruitment curve. The results are summarized as below. The timing of start of upstream migration of adult icefish to Lake Abashiri from Port of Abashiri was observed from late April to early May, when the water temperature of Abashiri River reached 5℃. The numbers of upstream migration changed with the tidal period, and increased at the time of the spring tide when there was frequent flow of the seawater from Port of Abashiri. Icefish seem to migrate efficiently from Okhotsk Sea to Abashiri Lake using the saltwater intrusion. Icefish adults, which migrated to Lake Abashiri from Port of Abashiri, were distributed throughout Lake Abashiri but mainly around the mouth of the feeder river. No icefish migration to the upstream section from Lake Abashiri was observed. From the temporal changes in gonad somatic index (GSI) of female adult icefish collected in Abashiri River system, the spawning season was estimated to be from late May to mid-June. In addition, from the frequency distribution for the egg diameter of mature female icefish collected in Abashiri River system, the spawning behavior seems to be performed a plurality of times during the spawning season. The sex ratio greatly changed spatiotemporally. The migration history of 20 individuals of adult female icefish was examined by the otolith Sr:Ca ratio. The findings suggested that the life history pattern of icefish in Lake Abashiri consisted of two types: i.e., 3 individuals belonged to the lake resident group, 17 individuals belonged to the anadromous migration group. In the anadromous migration group, the estimated body lengths at the beginning of the downstream migration were approximately 30 mm (4 individuals) and approximately 50 mm (13 individuals). Icefish larvae emerged in Lake Abashiri in late May. The findings suggested that icefish larvae that hatched on the coast of Lake Abashiri were dispersed throughout the lake by diffusion of the lake flow. Vertical distribution of icefish larvae was observed in all water depths of the aerobic layer, but many individuals of icefish larvae tended to be distributed in the middle layer of the aerobic layer. In fast-growing individuals of icefish larvae, the body length was more than 25 mm in mid-July, and larvae were transferred to the juvenile stage. Icefish juveniles were distributed unevenly or extensively in Lake Abashiri. The growth curve of icefish in Lake Abashiri was fitted by using the exponential curve between July-November 1999-2006. The specific growth coefficient was 0.0065 on average. The specific growth coefficients of 2000 (0.0092) and 2005 (0.0081) were greater than those of other years. Seine net fishing that targets icefish juveniles in Lake Abashiri was started in September every year. It was estimated that icefish juveniles migrated to Okhotsk Sea from Lake Abashiri in September-October during the fishing season. The timing of downstream migration was synchronized with the tidal rhythm. Individuals of downstream migration increased at all times when the flow direction of Abashiri River current was favorable. The peak of downstream migration was observed at a time when the river temperature was below the seawater temperature. Icefish seemed to move toward the sea efficiently by using the water flow of Abashiri River. The growth curve of ice fish in Lake Abashiri was fitted by using von Bertalanffy’s growth curves based on the mean body length and variance by year class. The growth coefficient (k) of females ranged from 1.9 to 5.1, and males ranged from 2.0 to 4.5. The growth coefficient of the 2005 year class was maximum in both males and females. Icefish eggs were distributed in the sediment comprised of sand and gravel in the shallow coastal waters. The density of icefish eggs tended to be higher in medium sand-gravel where the grain size was larger. Furthermore, there were few icefish eggs in the sediment where silt clay was predominant. Icefish have lived in Lake Abashiri since the early 1930s because of climate change and topographical features, and have been caught by commercial fisheries since 1936. Their annual catches in 1936-2007 except 2005 fluctuated between 1 to 94 tons. The periodicity of one-year intervals relating to the life history of icefish was observed in annual catches. Thus, it seems that the population size of icefish is influenced by the parental numbers. When juvenile icefish began to emigrate from Lake Abashiri to Okhotsk Sea, the density of juvenile icefish declined rapidly. Additionally, fisherman in Lake Abashiri changed the target from icefish to Japanese smelt Hypomesus nipponensis. Therefore, changes in the pattern of CPUE decrease were caused by these two factors. In addition, the CPUE of icefish fishery was reduced by rising water followed by heavy rain. In 1992 and 1998, the CPUE of the first day of the fishing season showed an extremely low value by rising water just before the beginning of the fishing season. In 2001 and 2006, rising water followed by heavy rain occurred during the fishing season. The CPUE after rising water was much lower than that before rising water. The icefish seem to be stimulated into seaward migration by rising water. Blue tide occurred during the fishing season in Lake Abashiri in 2004, leading to high mortality rates of a large number of juvenile icefish. Furthermore, the population characteristics of icefish in the 2005 year class showed the lowest value in all developmental stages as exemplified by the significantly small number of adult fish. As a result, icefish fishery was suspended in 2005. Population sizes at the beginning of each fishing season from 1985 to 2006 were estimated to be 36,763×103-487,590×103 individuals based on DeLury's second model and the density of juveniles. The relationship between the number of adults and recruitment fitted well with Ricker’s reproduction model. Carrying capacity (K) and maximum sustainable yield (NMSY) estimated by the Ricker stock-recruitment curve were 31.345×106 and 106.597×106 individuals, respectively. The initial population size of icefish in Lake Abashiri increased with an increase in the number of previous generation adult fish. However, the initial population size was reduced to reverse when the number of previous generation adult fish exceeded NMSY. In 1992 and 2006, the initial population size of next generation fish was significantly reduced because the number of adult fish in both exceeded K. Icefish catch in Lake Abashiri fluctuated, reflecting the population size in principle, but the catch was unexpectedly poor when there was blue tide and rising water caused by heavy rain. The remaining population size (approximately equal to or the image of the number of adults) excessively impacted the initial population size of the next generation. For more sustainable and greater rational use of icefish resources in Lake Abashiri, leaving behind about 31 million uncaught individuals every year is necessary. In particular, in the case of missed opportunities of catch caused by rising water, it is necessary to catch fish that exceed NMSY after the migration from Okhotsk Sea to Lake Abashiri. As a result, it is considered possible to maintain appropriate adult fish numbers. .

 Recirculating aquaculture system (RAS)（Review）Ⅰ.　System structure and technological components

 

Many studies concerning recirculating aquaculture systems (RAS) have been conducted worldwide over the past four decades. RAS have the advantage of using much smaller quantities of water for fish production than flow-through systems. Using these systems, makes it is easier to maintain an optimum temperature for rearing species and also to disinfect the rearing water. Further, they have the advantage of having less direct impact on the aquatic environment than open systems when the waste materials and discharge water from the system are managed properly. The principal components of an RAS include a rearing tank; mechanical filter (form fractionator); biological filter, which removes ammonia, nitrite, and nitrate (nitrification and denitrification); CO2 stripping unit; DO control (oxygenator); sterilization (UV light); and temperature control (heating-cooling unit). The biological filter is the most important component of the system. In this review, many types of biological filters and the current status of RAS development in Europe, the USA, and other countries, including Japan, are described.

 Recirculating aquaculture system (RAS)（Review）Ⅱ.　Applications of RAS to seed production

 

Relationships between the specific growth rate (SGR) and biochemical constituents in the trunk of  juvenile barfin flounder (Verasper moseri) were investigated. The juveniles were reared for 25 days under 2 dietary conditions (0.5% and 2% body weight day^-1, respectively), to clarify the  biochemical constituents, for use as the substitute indicators for SGR. Specifically, we analyzed  moisture, crude fat, triglyceride, phospholipid, glycogen, protein, RNA, DNA, and free amino acid levels after feeding. Crude fat, moisture, RNA/DNA ratio, phospholipid/DNA ratio, and protein/DNA ratio exhibited positive linear regressions with SGR. Our results show that these  biochemical indicators may be potentially useful as a substitute index for SGR when  evaluating the growth rate of released barfin flounders in the field.
 

 Distribution of the walleye pollock, Gadus chalcogrammus (Theragra chalcogramma), and oceanographic conditions in the Nemuro Strait, Japan - II Distribution of spawning adults in the late 1990’s

 

The winter distribution of adult walleye pollock during the spawning season and oceanographic conditions in the Nemuro Strait were investigated based on integrated surveys, applying an echosounder and the STD from 1997 and 1999. The majority of walleye pollock were distributed in the modified Soya Warm Current water in this area. Their depth ranged from 140 to 440 m at the beginning of the spawning season, then decreased to more than 300 m from late February to early March during peak spawning. In late March, walleye pollock moved to shallower water. On the basis this vertical migration pattern, the thermal conditions where they were found changed from 2 to 5°C until mid-January and from 0 to 2°C after February. The observed fish movements and their interactions with the environment differed from those in other spawning grounds around Hokkaido. Walleye pollock in the Nemuro Strait may have adapted to the unique oceanographic conditions in this area.

 Identification of oysters in Okushiri Island by mitochondrial 16S rRNA analysis

 

Oysters around Okushiri Island are difficult to classify based on appearance. Therefore, we had identified their species using DNA analysis. After obtaining the results, we explored the relationship between species and habitat. Oysters were collected by scuba divers at 1－9 m depth in the sea at 7 different areas around Okushiri Island from November 2011 to the next November. It was difficult to identify the 297 oysters based on their morphological characters. Therefore, we analyzed the nucleotide sequences of mtDNA 16S rRNA genes. Polymerase chain reaction with a template of total DNA from muscle tissues of oysters successfully amplified DNA fragments of 464 bp in the 16S rRNA gene, which were identified as Crassostrea gigas, C. nippona, Ostrea circumpicta, and Saccostrea sp. All of the areas had at least two different species, and species matched their characteristic shell size and depth of habitation, but these were insufficient for certain classification. Therefore, the DNA fingerprinting technique were shown to be beneficial for the classification of oysters.

 Upstream and downstream migration history of lacustrine sockeye salmon captured in Lake Kussharo estimated from otolith microchemistry (Short Paper)

 

Extremely large lacustrine sockeye salmon, Oncorhynchus nerka (fork length: 63.6 cm), was captured in Lake Kussharo, eastern Hokkaido, Japan. Initially, the average otolith strontium:calcium (Sr:Ca) ratio of large fish was 1.10 (range: 0 – 4.22), which then elevated to 4, after which fluctuated about between 6 – 8 (mean 6.54, range 2.84 – 9.25), and dropped to 4 before the fish was captured in the lake. This fluctuation in the Sr:Ca ratio demonstrated that the large sockeye salmon that was captured in Lake Kussharo had migrated to the ocean.

 Instance of Small Pink Salmon, Oncorhynchus gorbuscha in rivers at the Nemuro Strait, Eastern Hokkaido, Japan (Short Paper)

 

I examined the body size, maturity and age of “small pink salmon” caught in the rivers at Nemuro Strait, Eastern Hokkaido, Japan. On September 28, 2009 a male pink salmon with a fork length of 31.5 cm and weight of 381.4 g was caught in Shibetsu River. The Gonad Somatic Index (GSI) was 7.86. This mature male had scales indicating 1-year old fish (i.e., “jack”). A female pink salmon caught in Furen River on 20 November 2010 had a fork length of 26.5 cm and weight of 156.8 g, indicating it was an immature (GSI 0.77) 1-year old fish. An immature female (28.5 cm in fork length) caught in Kunbetsu River on 10 September 2013 had otoliths marked by the Alizarin Complexone (ALC). Marked pink salmon were released from the Shibetsu Hatchery during May in 2012. Therefore this fish was a 2-year-old fish. “Small pink salmon” are likely of contain mixed age, sex and maturity, and the ocean conditions (e.g.,water temperature and nutritional conditions) affecting the growth of juvenile pink salmon during their first year at sea may have some influence on the appearance of small pink salmon.