The problem of marine litters for cultured teleost.

Here, characteristics of marine litter ingested by Pacific bluefin tuna (PBF, Thunnus orientalis) juveniles under captive conditions were investigated. Swimming speeds of PBF juveniles with pseud-ingested polystyrene chips were compared, and mortality due to polystyrene chip ingestion in cultured teleosts (red sea bream, greater amberjack, and white trevally) was examined in the laboratory. Marine litter ingested by the PBF juveniles included mainly microplastics. The body size of dead specimens with ingested marine litter was significantly smaller than that of other dead fish. We suggest that when the PBF juveniles ingested the marine litter, they died due to energy exhaustion within a few days. All the examined species ingested polystyrene chips, but no related mortality was confirmed. These results suggest that only the PBF could not vomit or excrete the ingested marine litter. This study indicates that the marine litter problem significantly affects the aquaculture industry, especially tuna aquaculture.

Muscular system in the pacific bluefin tuna Thunnus orientalis (Teleostei: Scombridae).

The muscular system in the Pacific bluefin tuna Thunnus orientalis is studied in detail. For the first time, a complete description of the muscular anatomy of a thunnid is provided here. Eighty-two elements including subdivisions of components of the muscular system are identified. This is less than found in a basal perciform and two other investigated scombrid species, owing mainly to the absence or fusion of pectoral, pelvic and caudal fin muscles. The absence of elements of the basal perciform pattern was most prominent in the caudal fin, which includes only the flexor dorsalis, flexor ventralis, hypochordal longitudinalis, and interradialis. In the caudal fin, the medial fan-shaped ray was identified as the first dorsal ray, judging from myological and neuroanatomical characters. The highly developed gill filament muscles in Thunnus orientalis and sheet-like rectus communis control gill ventilation. Long body muscle tendons reduce the metabolic energy needed during rapid and continuous swimming. These characters are interpreted as adaptations in the context of the oceanic life style of the species.

Visual spectral sensitivity of photopic juvenile Pacific bluefin tuna (Thunnus orientalis).

Although Pacific bluefin tuna is a species that relies on vision, its photopic visual function is not well known; we therefore recorded electroretinograms to investigate photopic spectral sensitivity in juveniles of this species (49-81 days post-hatch; standard length 74-223 mm). The peak spectral sensitivity wavelength was 505 nm. We estimated that two (λ(max) = 512-515 nm and 423-436 nm) or three (λ(max) = 512-515 nm, 423-436 nm, and 473 nm) types of cone visual pigments contribute to photopic vision; these spectral sensitivities are adapted to surface water habitats in clear ocean and coastal water.

Changes in the scotopic vision of juvenile Pacific bluefin tuna (Thunnus orientalis) with growth.

In cultured juvenile Pacific bluefin tuna (Thunnus orientalis), reducing the mass deaths caused by collision or contact with tank or net walls at night is a priority for seedling production. Pacific bluefin tuna is a visually dependant species, although its scotopic vision is poor. We recorded electroretinograms to investigate the visual function with growth in the dark-adapted eyes of juvenile Pacific bluefin tuna. Peak wavelengths of spectral sensitivity [38-62 days posthatch (dph), 77-167 mm standard length (SL)] were observed between 474 and 494 nm. Visual light sensitivity has a tendency to increase slightly with growth at 28-64 dph in individuals that measured 29-175 mm SL. However, visual temporal resolution did not significantly increase with growth at 38-62 days dph in individuals that measured 77-167 mm SL. These results suggest that the mass death continues between 28 and 64 dph because of low visual function and increasing swimming speed with growth.

Electroretinographic analysis of night vision in juvenile pacific bluefin tuna (Thunnus orientalis).

We used electroretinogram recordings to investigate visual function in the dark-adapted eyes of the juvenile scombrid fishes Pacific bluefin tuna (Thunnus orientalis) and chub mackerel (Scomber japonicus) and the carangid fish striped jack (Pseudocaranx dentex). Despite the fast swimming speed of the Pacific bluefin tuna, analysis of flicker electroretinograms showed that visual temporal resolution in this species was inferior to that in chub mackerel. Peak wavelengths of spectral sensitivity in Pacific bluefin tuna and striped jack were 479 and 512 nm, respectively. The light sensitivity of Pacific bluefin tuna was comparable to that of chub mackerel but lower than that of striped jack. The Pacific bluefin tuna may not need high-level visual function under dim light conditions in natural habitat because it is a diurnal fish. However, this low temporal resolution and light sensitivity probably explain the mass deaths from contact or collisions with net walls in cultured Pacific bluefin tuna.