Yuko Hiraoka

Feeding habits of stone flounder Platichthys bicoloratus larvae in Mutsu Bay, Japan

To clarify the feeding strategy of pelagic larvae of stone flounder in Mutsu Bay, the dietary composition and pray size was investigated from February to April during 1989–1999. Diets were compared with the numerical and volumetric compositions and frequency of occurrence of each prey species. Mensuration formulae were applied to estimate individual prey volume in the diet, while the length of planktonic species was measured from net and water samples. Prey shapes were assumed as sphere, cylinder, ellipsoid, pyramid, two elliptical cones, or a combination of ellipsoid and cylinder. Prey-size range increased as the larvae grew. Preflexion larvae fed mainly on copepod nauplii. Flexion and postflexion larvae ingested primarily appendicularians, with a suggestion that these larvae might depend on some parts of the microbial food web. Low frequencies of flexion and postflexion larvae with empty guts (1.7 and 1.4%, respectively) might be derived from feeding on slow-swimming appendicularians. From a size comparison between ‘house’-like organ length and trunk length of the appendicularian Oikopleura sp., almost all house-like organs with trunks in the larval diet were nonexpanded ‘house rudiments’, not expanded ‘houses’. Thus, stone flounder larvae may not chew the houses, but swallow the house rudiments with trunks.

Relative abundance trend of the blue shark Prionace glauca based on Japanese distant-water and offshore longliner activity in the North Pacific

The Kuroshio–Oyashio transition zone is one of the most important fishing grounds for pelagic fish, including blue shark and swordfish, which are primarily targeted by shallower Japanese longliners. We evaluated the fishing behavior of Japanese longliners based at the Kesennuma fishing port to estimate the standardized catch per unit of effort (CPUE) of blue shark. In our analysis, we first used data that had been filtered for blue shark sets and found that annual target shifts occurred seasonally and geographically; the greatest change in target species, from swordfish to blue shark, occurred in spring (April–June). Beginning in the early 1990s, the fishing grounds shifted from the southwest to the northeast of the North Pacific, and the number of sets targeting blue shark gradually increased. To incorporate this variable target behavior into the abundance index, the 10th percentile of the swordfish CPUE values was applied as the target indicator in the CPUE standardization model. The results indicate an upward population trend that has been caused by a decrease in fishing pressure from the early 1990s due to the prohibition of drift nets in 1992 and the subsequent decrease in longline vessels.

Feeding habits of bigeye tuna (Thunnus obesus) in the North Pacific from 2011 to 2013

In the present study, we analysed the stomach contents of 585 bigeye tuna (BET; Thunnus obesus) caught in the Kuroshio–Oyashio transition zone, a high-productivity region in the western North Pacific Ocean, to describe feeding habits and prey size. We identified 46 prey species belonging to 40 genera. Fish otoliths and squid beaks found in stomachs were used to calculate prey body length and weight from allometric relationships. The percentage index of relative importance (%IRI) was calculated from the mean percentage of occurrence, number and weight of each prey species. Squid and fish were the main prey of BET in the sampling area and the highest %IRI prey species during the survey was Eucleoteuthis luminosa (luminous flying squid; 7.6%), followed by Gonatopsis makko (mako armhook squid; 4.8%) and Magnisudis atlantica (duckbill barracudina; 2.3%). The %IRI of E. luminosa decreased and that of M. atlantica increased with increasing BET body length. The size of prey fish increased with increasing BET size, whereas the size of prey squid was similar across BET size. The results indicate ontogenetic shifts in the feeding habits of BET in the study area. These data provide fundamental information that will improve our understanding of oceanic food webs in the Kuroshio–Oyashio transition zone, an important foraging area for many pelagic species.