Michio Omori

Some ecological observations on fishes associated with drifting seaweed off Tohoku coast, Japan

Fishes associated with drifting seaweed were sampled in the Tohoku area, northwest Pacific, in late spring 1988, to analyse the community structure and ecological relationships of the dominant species.Seriola quinqueradiata Temminck et Schlegel was the dominant species and its escort was composed ofThamnaconus (Navodon)modestus, Sebastes inermis, Hyperoglyphe japonica andEnedrias nebulosus. E. nebulosus, which fed essentially on Gammarid crustaceans, was collected mainly in Sendai Bay; the other species, found in offshore water, were plankton feeders and did not exhibit any strong competition, indicating close cohabitation. The abundance of drifting seaweed in the Tohoku area peaked in May–June and decreased in July; they seemed to come from the nearest coast. JuvenileS. quinqueradiata (> 15 cm long) were collected with set nets in Sendai Bay during summer. They seemed to use drifting seaweed (along with the warm Kuroshio current) as a means of transportation during their migration from the southern spawning ground to northern Tohoku area. Drifting seaweed could be considered as a nursery, and the association of fishes with floating algae might have an influence on species recruitment.

Coexistence of Anadromous and Resident Life History Styles of Pond Smelt, Hypomesus nipponensis, in Lake Ogawara, Japan, as determined by Analyses of Otolith Structure and Strontium:Calcium Ratios

Anadromous and resident forms of pond smelt, Hypomesus nipponensis, were found to occur in sympatry in Lake Ogawara, Japan. Profiles of Sr: Ca ratios from individuals could be grouped to two patterns (1) a ‘resident’ pattern with low Sr: Ca ratios from core to edge and (2) an ‘anadromous’ pattern with relatively low Sr: Ca ratios near the core with abrupt increases in ratios at a location approximately 0.3 mm from the core. Spawners smaller than 60 mm standard length (SL) were resident, between 60 to 80 mm were mixed resident and anadromous, and larger than 80 mm were anadromous. Anadromous individuals first migrated after 40 to 82 days from hatching (mean±sd, 59.1±13.5 d) and 14.6 to 30.9 mm SL (22.2±5.3 mm). There was no difference in SL between resident and anadromous individuals during age at first migration, suggesting that size may not be the mechanism for divergence of alternative life history styles.

Morphogenesis in hatchery-reared larvae of the black rockfish,Sebastes schlegeli, and its relationship to the development of swimming and feeding functions

The developmental sequence of morphological characteristics related to swimming and feeding functions was investigated in hatchery-reared larvae and juveniles ofSebastes schlegeli, a viviparous scorpaenid. The fish were extruded at an early larval stage, when the mean body size was 6.23 mm TL.Fin-ray rudiments became visible at 9.0 mm TL in the dorsal and anal fins, at 8.0 mm TL in the pectoral and pelvic fins and 6.0 mm TL (size at extrusion) in the caudal fin. Completion of segmentation of soft rays in the dorsal and anal fins was attained by 14 mm TL and in all fins by 17 mm TL. Branching of soft rays in the respective fins started and was completed considerably later than the completion of segmentation, as well as ossification of the fin-supports. Morphological transformation from larva to juvenile was apparently completed by about 17 mm TL. Although the completion of basic juvenile structures was attained by transformation at that body size, succeeding morphological changes occurred between 17 mm and 32 mm TL.Newly-extruded larvae possessed one or two teeth on the lower pharyngeal and pharyngobranchials 3 and 4, but lacked premaxillary, dentary, palatine and prevomer teeth. The fish attained full development of gill rakers and gill teeth by 15 mm TL, the upper and lower pharyngeal teeth subsequently developing into a toothplate. Development of the premaxillary, dentary and palatine teeth was completed at about 30 mm TL, by which time loop formation of the digestive canal and the number of pyloric caeca had attained the adult condition.The developmental sequence of swimming and feeding functions during larval and early juvenile periods appeared to proceed from primitive functions to advanced or complex ones, from the ability to produce propulsive force to that of swimming with high maneuverability and from development of the irreducible minimum function of passing food into the stomach to the ability to actively capture prey via passive food acquisition with the gill rakers and gill teeth.The relationship of morphological development to the behavior and feeding activity of artificially-produced hatchlings is also discussed.

Maturation and spawning processes of anadromous and resident pond smelt in lake ogawara

Pond smelt,Hypomesus nipponensis McAllister, in Lake Ogawara demonstrate alternative life history strategies, as evidenced by the coexistence of anadromous and resident fish. However, it is unknown if anadromous and resident groups interbreed. In this study, maturation and spawning processes were examined and compared between anadromous and resident groups. Histological observations indicated negligible variation in the maturational stage composition of oocytes, the frequency of oocyte diameter being unimodal for all specimens at different maturational stages. Oocytes were absent in the ovaries of spent fish. Accordingly, the species can be considered a semelparous spawner with unimodal oocyte diameter distribution. Temporal changes in the proportion of spent fish were compared between anadromous and resident groups. Spawning of both groups began in late March and peaked over April 8–12. Although both groups did not differ significantly in the period of peak spawning, anadromous fish finished spawning earlier than resident ones. Anadromous fish were not able to spawn upon migration into Lake Ogawara, and quickly matured after immigration, contrasting with resident fish.

Predation by scavenging amphipods to injured hatchery-raised juvenile Japanese flounder Paralichthys olivaceus under laboratory conditions

The attacking potential of the scavenging amphipod Scopelocheirus onagawae on artificially injured hatchery-raised Japanese flounder Paralichthys olivaceus juveniles was investigated in relation to the degree of injury on the fish. All injured flounder juveniles were attacked by amphipods regardless of the degree of injury, while non-injured juveniles were not attacked. The attack by amphipods on the juveniles generally depended on the amount of glycine, a main feeding stimulant for the amphipod, released from the injury opening. The swimming ability of flounder juveniles was important to avoid the attack of amphipods. Furthermore, an area of injury allowing the amphipods to cling to the fish affects to the vulnerability of juveniles against the predation of amphipods. This study suggests that scavenging amphipods are potentially involved in the rapid reduction of the number of hatchery-raised juveniles.

Food intake and digestion in the scavenging lysianassid gammaridean Scopelocheirus onagawae

The lysianassid gammarid Scopelocheirus onagawae, which inhabits temperate shallow waters, was investigated for its digestion time and amount of food intake in feeding experiments with fish paste containing food dye as bait, being compared to those of cold-water species. The specimens were collected in Onagawa Bay, north-eastern Japan, from May through November 2001, using baited traps. The experiment on digestion time was run at 8°C, and the experiment on amount of food intake at 6, 8, 13, 18 and 21°C. Each individual that was fed fish paste was crushed in 3% ammoniaethanol solution to extract the food dye. After centrifuging, the fluorescence intensity of the supernatant was measured and converted to food intake. Individuals of 0-h non-feeding showed the relative food intake of approximately 1.0%, and those of 24-h non-feeding showed the value of 4.3%. Based on the finding that the satiation ratio ranged from 3.6 to 4.1% for all feeding individuals, this species were considered to digest food completely within 24 h. The satiation ratios were much less than those of cold-water species. Several tens of thousands of S. onagawae were inferred to congregate on carcasses and consume it in a short period after sunset.

Analyses of growth processes of pond smelt, Hypomesus nipponensis, in Lake Ogawara, Japan, through the use of daily otolith increments

It has been reported that pond smelt, Hypomesus nipponensis, in Lake Ogawara, Japan, appear in small and large size groups during spawning despite being an annual fish. It is hypothesized that pond smelt have a bimodal life history, anadromous for large size groups and resident in the lake for small size groups. We calculated the body length and growth rate of the small and large size groups through the use of daily otolith increments and compared growth parameters between groups. In addition, the growth processes of resident fish in Lake Ogawara and anadromous fish in the adjacent sea were examined and compared with small and large size groups, respectively. We found that the two size groups diverged after 40–50 days from hatching with significant size groups present after 50 days. Through otolith increment analysis the growth processes of the small and large size groups were correlated with resident and anadromous forms respectively. These results revealed the utility of otolith increment analyses to clarify population structure of this species.

Some notes on the maturation and spawning of the bramid fish,Brama dussumieri, in the southeastern waters of Japan

T he pomfret, Brama dussumieri, is distributed in tropical and subtropical waters of the Pacific, Indian and Atlantic Oceans (Mead, 1972). Mead (1972) considered the spawning area of the species to extend across the tropical and subtropical regions of all oceans, judging from larval and juvenile capture records, although adult B, dussumieri were not known from the abovenamed oceans. Ecological aspects were described for the first time for mature Brama dussumieri (17.8-22.5cm standard length [SL]) caught in April and October in the southeastern Pacific Ocean (12~176 79~ 89~ by Pavlov ( 1991). However, spawning of this species in the northwestern Pacific Ocean has not been verified to date. The present paper describes the likely spawning of B. dussumieri in waters off southeastern Japan, with reference to maturation and spawning inferred from a distribution curve of ovarian egg diameters, being the first such description for this species.