Natural reefs are generally good fishing grounds. To use fish resources in a sustainable manner, it is important to estimate and monitor fish biomass. However, fish biomass survey using trawl is not applicable to natural reefs because the rugged bottom breaks or tangles trawl nets. In such grounds, it is effective to use a quantitative echosounder for estimating fish biomass. Nevertheless, the rugged bottom produces an acoustic blind zone above the bottom, where volume back-scatter from fish under beam is shaded by echo from the rugged bottom within the beam at a depth shallower than the bottom on the beam axis. It must be taken into consideration to estimate fish biomass more accurately. We conducted fish biomass survey in a natural reef, Hachirigase, southwest of the Sea of Japan by using a quantitative echosounder in June 2000. This study aims to develop a statistical method to identify acoustic bottoms, upper boundary of blind zone, with a discriminant analysis by analyzing integral layers of each ping with the minimum thickness from volume back-scatter of echograms obtained by the survey. We examine an effect of blind zone thicknesses determined with different methods by extrapolating volume back-scatter of fish above the acoustic bottom to the blind zones to estimate fish biomass in the reef expressed as volume back-scatter.

We examined the sexual maturation, spawning period and batch fecundity of Japanese sardine Sardinops melanostictus collected from coastal waters off Kochi Prefecture (KP), northwestern Kyushu (NWK), and the San-in district (SD) during the 2008-2010 period of low stock abundance to understand the reproductive characteristics of the current stock biomass. All male specimens were mature in all age classes examined in the three locations. Similarly, most females in these locations were mature although inter-annual variation in the percentage of mature 1-year-old females was found in SD in association with differences in the body length and relative condition factor of individuals. Monthly changes in ovarian condition showed differences in the peak spawning period among sampling locations. The relative condition factor of females in the three locations decreased and reached a minimum at the peak spawning period, thereafter it increased from the late spawning period. Spatial variation in the relative condition factor of mature females was evident as females in SD had a better condition than those in KP and NWK. There were no significant differences in the relative batch fecundity between specimens at age 1 and 2+ years and among the three locations. These findings indicated that the majority of 1-year-old specimens off western Japan matured and spawned in contrast to the evidence found during the past time of higher and lower stock abundance.

To evaluate the impact of jellyfish and comb jellies on the fishery stock, the trophic positions of these animals as well as epipelagic fish species in marine ecosystems are to be clarified. Trophic positions of copepods, jellyfish and comb jellies, and epipelagic fish spices collected from the Hiuchi-nada, the central Seto Inland Sea, Japan, were examined based on carbon and nitrogen stable isotope distributions. The carbon and nitrogen stable isotope ratio (δ¹³C, δ¹⁵N) for (1) copepods (Acartia omorii, Corycaeus affinis, Oithona similis, Paracalanus parvus), (2) jellyfish and comb jellies (Aequorea coerulescens, Aurelia aurita, Chrysaora pacifica, Bolinopsis mikado), and (3) Japanese anchovy(Engraulis japonicus) was (-21.9 to -18.0‰, 7.6 to 11.9‰), (-21.1 to -15.7‰, 10.1 to 16.4‰), and (-21.0 to -15.2‰, 10.9 to 18.7‰), respectively. We assumed that an average δ¹⁵N value of 13.5‰ for 10–20 mm SL larval anchovy corresponds to trophic level 3 for calculation of trophic level for copepods, jellyfish and comb jellies, and other fishes. The 10–20 mm SL larval anchovy-based trophic position (TP) for copepods, and jellyfish and comb jellies was 1.7–2.0, and 2.6–3.0, respectively. The TP for Japanese anchovy, Japanese jack mackerel Trachurus japonicus, white croaker Pennahia argentata, largehead hairtail Trichiurus lepturus, Japanese Spanish mackerel Scomberomorus niphonius, and chub mackerel Scomber japonicus was 2.7–3.7, 4.2, 4.3, 4.1, 3.9, and 3.6, respectively. Overlaps in feeding habits between jellyfish+comb jellies, and larval anchovy were detected. Regarding the impact of jellyfish and comb jellies on anchovy stock in Hiuchi-nada, it is suggested that the effect of competing for the prey was stronger than that of the direct predation by jellyfish and comb jellies on larval Japanese anchovy.

In order to examine the effect of the Tsushima Warm Current on the swordtip squid Photololigo edulis fishery in the Sea of Genkai, we analyzed the operation record by squid fishing boats and satellite sea surface temperature (SST) in April and November from 2000 to 2005. The swordtip squid was fished in the area of limited range of SST during each month; however, the SST of the fishing ground in April was quite different from that in November. The distribution of fishing positions weighted-averaged by daily catch was classified into concentrated or dispersed patterns in April. On the other hand, the distribution of the fishing positions in November only had a concentrated pattern. Inflow of the Tsushima Warm Current into the Sea of Genkai in April was also classified into offshore or inshore patterns, judging from monthly satellite SST; however, it was not clearly consistent with the distribution patterns of the fishing positions. It was revealed that because the above temperature range in April was suitable for spawning of the swordtip squid, the fishing ground was concentrated into a narrow zone in the years when the area with the suitable water temperature was limited to the narrow zone, depending on the inflow of the Tsushima Warm Current. In the years when the area suitable for spawning was widely distributed, the fishing ground was dispersed. The fishing ground in November was mainly formed off Okinoshima and not related to the temperature suitable for spawning. These results indicated that inflow of the Tsushima Warm Current to the Sea of Genkai could have strong influence on the fishing ground formation of the swordtip squid through the temperature condition, particularly in its spawning season.