Variations in otolith patterns, sizes and body morphometrics of jack mackerel Trachurus japonicus juveniles

Variations in otolith patterns, sizes and body morphometrics of jack mackerel Trachurus japonicus juveniles were investigated. Under transmitted light, translucent (W(t)) and opaque otoliths (W(o)) were detected in juveniles collected from Wakasa Bay between July 2005 and April 2006, whereas only opaque otoliths (G(o)) were detected in Goto-nada Sea individuals between May and June 2006. Three groups of juveniles were distinguished based on differences in hatch season, otolith size and growth history, and body morphometrics. As T. japonicus has different spawning seasons according to spawning grounds, each group was estimated to hatch in different waters. Juveniles with W(t) otoliths were considered to have stayed in coastal habitat longer, as the hatch area was estimated to be near Wakasa Bay. Juveniles with W(o) and G(o) otoliths appear to recruit to coastal waters at larger size, since their hatch areas were estimated to be far from each collection area. Larger otoliths of W(t) were attributed to otolith accretion after the second growth flexion, which was observed only for W(t) . Standard length of W(t) fish at the second otolith growth flexion was estimated to correspond to recruitment size to coastal rocky reefs in Wakasa Bay. Body morphometrics were correlated with otolith size after removing body size effect, suggesting that morphological variations of T. japonicus juveniles were also associated with the timing of recruitment to coastal habitat.     

Evidence of genetic and environmental influences on meristic variation in the rainbow trout,Salmo gairdneri Richardson

Synopsis An investigation of the effect of parentage and temperature on the expression of selected meristic characters of rainbow trout, Salmo gairdneri, reveals statistical differences in the number of vertebrae, dorsal fin rays and anal fin rays among test batches of progeny of different parentage. Batches of fertilized ova of the same stock or parentage, when incubated at 9.5 °C and 14.8 °C, produced lower mean dorsal and anal fin ray counts and a higher mean pectoral fin ray count at the higher temperature. Incubation temperature had no significant effect on the number of vertebrae.Significant statistical differences occurring in various characters among groups of rainbow trout of different parental origin provide convincing evidence of genetic inputs into meristic expression. However, the existence of both environmental and genetic inputs into meristic expression in rainbow trout necessitates further investigation before the meaning of observed meristic plasticity in rainbow trout can be resolved.     

Developmental change in RNA: DNA ratios of fed and starved laboratory-reared Japanese flounder larvae and juveniles, and its application to assessment of nutritional condition for wild fish

Starved Japanese flounder Paralichthys olivaceus larvae were characterized by relatively lower levels of RNA content throughout their early life stages. Significant differences in the RNA: DNA ratios were found between fed and starved fish, and appeared to increase as starvation proceeded. Ontogenetic changes in RNA: DNA ratios were clearly observed during metamorphosis, especially decreasing during the period from the late-metamorphic to postmetamorphic stages. The criteria established from these laboratory experiments, were applied to the nutritional condition of wild larvae and juveniles collected in Wakasa Bay, Sea of Japan in 1994 and 1995 by measuring RNA and DNA content. Starved fish were mainly found in stage I (settling stage) fish during the late season of settlement in 1995. This suggests that starvation could be associated with settlement in Japanese flounder.     

Disappearance of a spring cohort in a population of the dragonet,Repomucenus valenciennei, with spring and autumn spawning peaks in Tokyo Bay, Japan

The occurrence of eggs, pelagic larvae and juveniles, and settled juveniles of the dragonetRepomucenus valenciennei in Tokyo Bay, Japan, were investigated by plankton net and bottom trawl surveys between September 1990 and September 1991. Eggs, and pelagic larvae and juveniles appeared from April to November (spring to autumn), peaking in both spring and autumn. From the temporal pattern of egg and pelagic fish occurrence, and pelagic duration reported elsewhere (ca. one month), settlement could be predicted as occurring from late spring to autumn. However, settled juveniles appeared from August to December, with an abrupt peak in November. Aging from daily increments in the otoliths of settled recruits in 1990 indicated that the latter comprised individuals which had hatched between mid-September and early November (i.e. autumn cohort), implying that individuals which had hatched in spring to summer (April to August) were not recruited. Benthic hypoxia occurs widely in Tokyo Bay, from June to October each year, particularly in the northern part, which is the main nursery area ofR. valenciennei. The timing of dissolved oxygen recovery, and appearance of settled fish coincided closely (i.e. November), indicating that summer hypoxic conditions prevented the settlement of the spring cohort and hence recruitment to the population.     

A new gobiid fish,Acanthogobius insularis,from the Ryukyu Islands,Japan

A new gobiid species,Acanthogobius insularis, is described from 88 specimens collected from Amami-oshima Island and Okinawa Island, Ryukyu Islands, southern Japan. The species is distinguished from its congeners by the combination of dorsal and anal fin ray counts, vertebral counts, cephalic sensory system patterns and coloration.     

Influence of mass release of hatchery-reared Japanese flounder on the feeding and growth of wild juveniles in a nursery ground in the Japan Sea

The feeding and growth of wild juvenile Japanese flounder Paralichthys olivaceus (Temminck et Shlegel), collected after experimental mass releases of marked hatchery-reared juveniles of the same species, was examined at Wada Beach, Wakasa Bay, the Japan Sea, in 1997 and 1998. The feeding and growth of wild juveniles in the eastern area where the hatchery-reared juveniles were released and recaptured were compared with those in the western area (control area) where no hatchery-reared juveniles were recaptured. The stomach contents of both hatchery-reared and wild juveniles collected in the eastern and western areas consisted mainly of mysids in the early season (late-May to mid-June), and fish larvae in the late season (late-June to early July) in both 1997 and 1998. The stomach content index (SI) of juvenile flounder collected in the eastern and western areas showed that the feeding conditions followed similar patterns. However, the percent of empty stomachs in the wild flounder juveniles collected within 4 days after the mass release in the eastern area was as high as 10-30% in 1997 and 40% in 1998, whereas it was 0% in the western area in both years. Otolith microstructure analysis showed that the growth rate in the eastern area was not significantly different before and after release in both years. These results demonstrate that the mass release of hatchery-reared juvenile flounder temporarily affects the feeding efficiency of the cohabiting wild juveniles, but not to an extent that significantly reduces the growth rate.     

Geographical variations in freshwater populations of the three-spined stickleback,Gasterosteus aculeatus,in Japan

This paper describes an investigation of morphological variation among six freshwater populations (dominantly low-plated morph) of the three-spined stickleback,Gasterosteus aculeatus, in Japan. Such populations are mainly distributed in restricted water areas within a band from Ise to Wakasa Bays, the most constricted part of Honshu Island. According to some differences in morphological variation, i.e. plate morph, number of lateral plates, body size, body shape and body colour, they were classified into two types corresponding to occurrence in the Ibi River and Lake Biwa water systems. The former type was monomorphic dominated by lowplated morphs in spring-fed water, whereas the latter was a dimorphic population consisting of low- and partially-plated morphs. My results suggest that the plate morph type is not correlated with climate nor predation but is related to geological isolation patterns during the course of the landlocking process. Conversely, variations in meristic (number of dorsal and anal fin rays and gill rakers) and morphometric (body shape and body colour) characteristics may have been related to different environmental conditions. This study also provides supporting evidence that the freshwater three-spined stickleback is a distinct species from the anadromous stickleback,G. aculeatus.     

Relationships between release season and feeding performance of hatchery-reared Japanese flounder Paralichthys olivaceus: In situ release experiment in coastal area of Wakasa Bay, Sea of Japan

Two groups of hatchery-reared juveniles of Japanese flounder Paralichthys olivaceus (Temminck and Shlegel) were experimentally released in Wada Beach, Fukui Prefecture, Japan and their feeding state examined in comparison to wild juveniles to investigate the optimal release season and the relationship between food availability and feeding performance of juveniles. We released two groups of 40,000 juveniles of ca. 50 mm size, one group in the early (29 May) and the other in late (2 July) seasons, periods with predicted high and low densities of prey mysids, respectively, although the abundance of mysids fluctuated markedly during the season. Food availability was higher for the early release group than the late release group. Although the main stomach contents of released and wild juveniles were mainly mysids and fish, the feeding state differed between the early and late release groups. The early release group had a stomach content index (SI) similar to that of the wild juveniles, but the SI of the late group was significantly less than that of the early release group and wild juveniles. The percentage of fish with empty stomachs was much higher in the late release group than in the other two groups. In addition, the density decrease rate was significantly more rapid for the late release group than the early release group. Clear correspondence between the food availability and feeding state of the released flounder juveniles indicates that the release of juveniles in an appropriate season when mysids are abundant is crucial for successful stocking.     

Development of juveniles of four flatheads (Perciformes, Platycephalidae) occurring in southern Japan

The juvenile development of four Japanese flatheads (Platycephalidae),Onigocia macrolepis. O. spinosa, Inegocia japonica andRogadius asper, is described and illustrated from specimens trawled in Tosa Bay, southern Japan. Juveniles of twoOnigocia species are diagnosed by an ocular flap and absence (O. macrolepis) or presence (O. spinosa) of distinct dark bars on the body.Inegocia japonica juveniles are distinctive in having a heavily pigmented body and 12 dorsal and anal fin soft rays.Rogadius asper has a unique antrorse preopercular spine prior to 15 mm SL. During the early juvenile stage, strongly negative allometric growth occurs in the interorbital region, accompanied by migration of the eyes from the lateral to dorsal surface of the head.     

Postlarvae and juveniles of silver sea bream,Sparus sarba occurring in the surf zones of Tosa Bay,Japan

A total of 515 larval and juvenileSparus sarba (8.2–17.8 mm TL) was collected at semimonthly intervals with a small seine in surf zones of Tosa Bay during the period of May 1981 to May 1982. They had morphological characteristics common to the larvae and juvenile of Sparinae, but were distinguished from the others by 24 dorsal and 14 anal fin rays, the first soft ray of pelvic fin not elongated and small melanophores along the dorsal margin occurring first on the caudal peduncle. They occurred in surf zones of Tosa Bay in two separate seasons, from late March to late May, and from late November to late January, being most abundant in April and May. In past studies using traditional larval nets or minnow-nets in coastal or shallow waters of Tosa Bay, larval and juvenileS. sarba were not reported. It seems that their distribution is limited in extremely shallow waters such as surf zones.     

Field evaluation by RNA/DNA ratios on post-release nutritional status of released and wild Japanese flounder Paralichthys olivaceus juveniles

A large-scale juvenile Japanese flounder (Paralichthys olivaceus) release-recapture experiment was undertaken to find the optimal release season by evaluating the nutritional status of released fish at different seasons during which food abundance was significantly different. Forty thousand fish were released at depths of 1.5 m for early-release (May 29, 1997) and 2 m for late-release (July 2, 1997) (42.1±3.5 and 40.9±4.2 mm body length, respectively) in an experimental field, Wakasa Bay, the Sea of Japan. Samples were taken, after the releases, at Wada beach at intervals of 1, 2, 3, 6, 10, 16 and 30 days after release (DAR), including pre-surveys before each release. Released fish recaptured from the two different release groups totaled 764; 467 from the early-release group (ER) and 297 from the late-release group (LR). A total of 1956 wild flounder juveniles were simultaneously collected (1041 ER, 915 LR). ER fish were subject to higher food availability and were exposed to less pressure from predation by smaller wild juvenile flounder. RNA/DNA ratios in ER juveniles were significantly higher than those of LR fish during all samples. Especially, RNA/DNA ratios in ER juveniles were higher than in wild juveniles from 3 to 50 DAR. In the LR group, the nutritional status of juveniles was relatively low in shallower water. These findings corresponded well with feeding incidence examined by coworkers. Mass release of hatchery-reared juveniles apparently reduced RNA/DNA ratio of the wild juveniles right after releasing. The present study showed that earlier release of hatchery-reared juvenile Japanese flounder with higher RNA/DNA ratio could increase the possibilities of survival right after release in the nursery ground, and that RNA/DNA ratio appeared to be a good tool in evaluating nutritional status of released juveniles as well as wild juveniles in Japanese flounder.     

Discovery of fossil Ayu (<Emphasis Type="Italic">Plecoglossus altivelis</Emphasis>) from the Upper Miocene of Shimane Prefecture,Japan

The Ayu, Plecoglossus altivelis, comprising three subspecies, is the only species in the genus Plecoglossus in the family Osmeridae. Here, we describe for the first time the discovery of fossil specimens of P. altivelis from the Upper Miocene sediment of Matsue, Shimane Prefecture, Japan. The fossil individuals are subadult and have comb teeth in the second and third generations. These fossil specimens are clearly similar to the recent P. altivelis, the former’s meristic characters being included in the three recent subspecies. In terms of numbers of pectoral fin rays and vertebrae, the fossil specimens are more similar to the subspecies Plecoglossus altivelis ryukyuensis than the other subspecies, but they also show different dorsal fin shapes and anal fin ray counts. This discovery of fossil P. altivelis indicates the persistence of the species for at least 10 million years, from the Late Miocene to the present.     

A new gobiid fish,Fusigobius signipinnis,from the western tropical Pacific

An undescribed goby has been collected from several localities in the western troplcal Pacific, from Japan to Australia. This species is described as a new species,Fusigobius signiplnnis. It differs from other species ofFusigobius in pectoral ray counts, colouration, largely separate pelvic fins, and shape of the first dorsal fin. The species is found in groups on sand, frequently flicking its first dorsal fin. Males are larger than females in mean size. Sex ratios are uneven, and females are twice as numerous as males.     

Morphological differences among three color morphotypes of Sebastes inermis (Scorpaenidae)

 Morphological differences among three color morphotypes occurring almost sympatrically (A, reddish type; B, blackish type; C, brownish type) of Sebastes inermis are clarified. Twenty-six morphometric characters and seven meristic characters were examined on about 20 specimens of each morphotype, 15 of the morphometric characters differing significantly. Principal component analysis (PCA) of the morphometric characters showed morphotype C to differ from the other two morphotypes. Canonical discriminant analysis (CDA) of the morphometric characters showed clear separation of each morphotype. Four meristic characters also differed significantly. Modes of pectoral fin ray counts of morphotypes A, B, and C were 15, 16, and 17, respectively, and the modes of anal fin ray counts of morphotypes B and C were 7 and 8, respectively. Many individuals of morphotype A had seven or eight anal fin rays. These differences suggested that the three morphotypes may be reproductively isolated from each other. Received: October 15, 2001 / Revised: March 14, 2002 / Accepted: April 9, 2002     

Early morphological development ofOmobranchus fasciolatoceps andO. Punctatus (Blenniidae: Omobranchini) reared in an aquarium

Larval and juvenile development of two blenniids,Omobranchus fasciolatocepts andO. punctatus, is described using eggs collected from natural waters in Tokyo Bay and incubated in an aquarium. These larvae and juveniles are compared with those of two otherOmobranchus species,O. elegans andO. loxozonus, distributed widely in Japan.Onobranchus punctatus is characterized by a unique, pointed snout in preflexion larvae, no melanophores proximally on the lower part of the pectoral fins in flexion and postflexion larvae, and pterygiophores projecting externally as blades between the dorsal and anal fin-rays in postflexion larvae and juveniles.Omobranchus fasciolatoceps has the following characteristics: a few melanophores on the fore-and mid-brain, but none on the hind-brain in preflexion larvae; no melanophores on the cleithral symphysis in flexion and postflexion larvae; no external pterygiophore blades in postflexion larvae and juveniles; and a unique dorsal skin flap on the head in juveniles. Ontogenetic developement of dorsal and anal pterygiophores is described forO. fasciolatoceps andO. punctatus. InO. punctatus, the postero-distal part of each proximal radial projects remarkably to form the external blades between the soft fin-rays, whereas the external blades between the fin spines are formed by fusion of a dermal bone developed from the antero-distal part of each proximal radial with the adjacent distal radial.     

Early life history of a seahorse, Hippocampus mohnikei, in Tokyo Bay, Japan

The early morphological development, seasonal and spatial occurrence patterns, and food habits of a seahorse, Hippocampus mohnikei, in offshore waters of Tokyo Bay, central Japan, were studied on the basis of 206 juvenile and young specimens (6.0–65.3 mm TL) collected between August 1995 and January 1999. All the specimens were collected within the period from May to January, inclusive, each year. In the least developed specimen (6.0 mm TL), the number of dorsal, anal, and pectoral fin rays had attained the adult complement, whereas the minute caudal fin, consisting of two rays, was present in juveniles of 6.0–26.4 mm TL. Hippocampus mohnikei≧35 mm TL, being larger than settlement size (ca. 30 mm TL), had very low gut fullness index values (GFI = 0, >70% of specimens), whereas those of 15–29 mm TL had higher values (GFI = 2–4, >80% of specimens). In addition, larger individuals selectively fed on larger planktonic animals (species of Brachyura), which occurred naturally at low densities, although smaller food items, such as Oithona davisae and Penilia avirostris, occurred abundantly, being consumed by smaller H. mohnikei individuals (15–34 mm TL). These results indicated that food availability for H. mohnikei in offshore waters of Tokyo Bay is significant for individuals larger than settlement size, because their food preference would shift from smaller food items to larger food items, which would be scarce in their environments. Received: January 12, 2001 / Revised: May 13, 2001 / Accepted: June 14, 2001     

Meristic and morphometric variation in the silver perch, Leiopotherapon plumbeus (Kner, 1864), from three lakes in the Philippines

A number of researchers have applied multivariate methods to elucidate the population structure of fishes. In this study, we also used multivariate techniques to examine meristic and morphological variations in the silver perch, Leiopotherapon plumbeus (Kner, 1864), from three Philippine lakes, namely, Laguna de Bay, Sampaloc Lake, and Taal Lake. We also aimed to determine whether or not there are meristic and morphological differences among the populations of silver perch from the three lakes, considering that this fish species was introduced from Laguna de Bay into Sampaloc Lake and Taal Lake in the late 1950s and early 1970s, respectively. A total of 710 specimens from four different sites were used in the study: 155 each from Binangonan and Tanay areas of Laguna de Bay and 200 each from Sampaloc Lake and Taal Lake. Based on analysis of variance, eight meristic and 26 transformed morphometric characters were selected for subsequent analyses. Nineteen (19) principal components extracted from the 34 significant variables accounted for 82.3% of the variation in the original variables. Factor analysis using varimax rotation produced four factors: factor 1 was dominated by fin measurements while the highest loadings for factor 2 were gill raker counts. Factors 3 and 4 were dominated by various body and head measurements. Cluster analysis showed specimens from Sampaloc Lake and Taal Lake in one group, while majority of the specimens from Binangonan and Tanay are in another cluster. This suggests a closer morphological similarity between specimens from Sampaloc and Taal. Discriminant analysis gave relatively high correct classification rates (76.13–95.50%). Lower gill raker count was the most discriminating variable. Since both the silver perch from Sampaloc Lake and Taal Lake were introduced from Laguna de Bay, the observed clustering and morphological variation could be attributed to similarities and differences in the lake environments. Laguna de Bay is a shallow eutrophic lake, while Sampaloc Lake and Taal Lake are deep lakes. Further studies, however, are needed to determine which of the myriad of biological and/or physico–chemical factors might have the greatest influence on the observed morphological divergence between the source population and transplanted populations that we found in our study.     

Timing of Parturition, Planktonic Duration, and Settlement Patterns of the Black Rockfish, Sebastes inermis

The microstructure of otoliths was used to examine the relationship among timing of parturition, duration of planktonic stage, and temporal patterns of settlement in young-of-the-year (YOY) Sebastes inermis collected during the settlement seasons of 1998–2001 in a seagrass bed at the northwestern coast of Japan. Parturition occurred in winter, predominantly around the new and full moons, and parturition dates were distributed mainly from early January to late February. In the 4 years YOY S. inermis settled into distinctive groups at early spring, synchronized mainly to a semi-lunar cycle at new and full moon phases; except for 1998, in which settlement occurred around the first and third quarter moons. In all years settlers extruded earlier in the parturition season encompassed the first settlement groups that appeared in the bed and vice versa. Planktonic period ranged from 53 to 113 days, and its mean values varied significantly among settlement groups and years. Mean size at settlement showed less variability among settled groups and between years (range: 18–26mm total length), which suggested an ideal size for settling and active migration. On the basis of our findings we discuss the extent to which the 'tidal control', 'settlement linkage', and 'competent size' hypotheses explain the temporal pattern of settlement.