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.     

Use of dual-beam acoustic technique for detecting young whitefish, Coregonus sp., juveniles: first experiments in an enclosure

We tested the possibility of using a high-frequency, BioSonics, dual-beam, acoustic system with an upwards-looking transducer to detect very young whitefish, Coregonus sp., juveniles. These first experiments were conducted in an enclosure with 100 fish (20–30 mm total length).
Observations over 24-h periods showed wide variations of the fish vertical distribution (> 5 m) and two typical aggregation patterns in near-surface waters at dawn and dusk. No direct relationship was found between light intensity and distribution of the fish during the day.
The mean target strength of young whitefish juveniles was found to be – 56 dB when fish were staying near the surface and – 63 dB as they were moving up- or downwards.     

The early life history of kurosoi,Sebastes schlegeli (Scorpaenidae), in the Sea of Japan

Larval and juvenile stages of kurosoi,Sebastes schlegeli, are described and illustrated from wild specimens. Some ecological aspects of larvae and juveniles are also described. Notochord flexion occurred between 5.6–7.5 mm SL. Transformation occurred between 13–20 mm SL. Preflexion and flexion larvae ofS. schlegeli can be distinguished from similar larvae by the pigmentation of the dorsal and ventral midlines of the tail and absence of pigmentation on the ventral portion of the rectum. After notochord flexion, the dorsal and lateral regions in both larvae and pelagic juveniles were heavily pigmented, suggesting adaptation for neustonic life style. Larvae and juveniles were caught at many coastal stations, but did not occur in cooler offshore waters. Larvae smaller than 20 mm SL inhabited surface waters. Until ca. 40 mm SL, juveniles inhabited mainly surface waters (without drifting seaweed), but also used other habitats, such as the drifting seaweed, and near the sea bed. Small larvae (<7 mm SL) fed mainly on copepod nauplii. Larger larvae fed on calanoid copepodites andEvadne nordmanni. Pelagic juveniles fed mainly on fish eggs, with fish larvae also being important food items for some individuals. Most food items taken by juveniles that were associated with drifting seaweed were eggs with attaching filaments (Cololabis saira andHyporhamphus sajori), suggesting that the high density of such food items both attracts and keeps juveniles around drifting seaweed.     

太湖新银鱼移植对(鳖)早期摄食和生长的影响

研究选择长江中游西洞庭湖水系太湖新银鱼移植水体(黄石水库)和未移植水体(蒙泉水库),研究太湖新银鱼(Neosalanx taihuensis Chen)移植对浮游动物食性鱼类(Hemiculter leucisculus Basilewsky)早期生长和摄食的影响。2009年7月下旬和8月中旬共采集稚鱼157尾,其中7月下旬采集稚鱼在14—23日龄之间,两水体间生长差异不显著;8月中旬采集稚鱼在20—49日龄之间,黄石水库生长率显著小于蒙泉水库。对样品耳石日轮分析发现25日龄之前两水体稚鱼生长率相似,之后黄石水库稚鱼较蒙泉水库生长慢。食性分析发现25日龄前两水体稚鱼食物组成相似,主要摄食轮虫、小型枝角类和桡足类;25日龄后黄石水库稚鱼食性没有显著变化,而蒙泉水库稚鱼则转食大型枝角类、昆虫及鱼卵和仔鱼。两水体气候条件、营养状况、鱼类区系组成上基本相同,是否有太湖新银鱼移植是两水体间的主要差别。太湖新银鱼春群在1—5月间繁殖,而的繁殖在6月下旬之后。因此在早期生活史阶段与太湖新银鱼的食物竞争会主要发生在转食大型浮游动物之后。太湖新银鱼摄食使黄石水库大型浮游动物饵料资源短缺,稚鱼在25日龄后不能转食,是导致黄石水库幼鱼在25日龄后生长减慢的重要因素。     

Species composition and distribution of ichthyoplankton in the Sea of Japan in Autumn 1995 and 2001

It was determined on the basis of data on species composition and distribution that ichthyoplankton in the Sea of Japan within the exclusive economic zone of Russia was represented in October 1995 and September 2001 by 13 species. Anchovy Engraulis japonicus and saury Cololabis saira were the most abundant and widely distributed. The area of distribution of anchovy larvae and juveniles was great, but was restricted mostly to subtropical waters. The larvae and juveniles of saury were captured to the west of the frontal interface in the waters of subarctic structure. The character of the distribution of early progeny suggests that the spawning of saury in summer ran in the economic zone of Russia. The early progeny of saury was absent in the zone of Japan filled with subtropical waters. Because of peculiarities of spawning strategy of the spawners, the regions of distribution and maximum concentrations of their larvae and juveniles did not overlap and were associated with different water masses. The species composition of larvae and young fishes collected with the Maruchi-Ami fry net was more diverse in comparison to the fish-egg net IKS-80. The parameters of the catches of both nets on conversion to the filtered volume of water were comparable only in the regions with increased concentrations of larvae and juveniles. The fry net was not very efficient for calculating recruitment in the dispersed autumn aggregations.     

Ontogenetic habitat preferences of the narrownose smooth-hound shark, Mustelus schmitti, in two Southwestern Atlantic coastal areas

The aim of this study was to determine the ontogenetic habitat preferences of the narrownose smooth-hound shark, Mustelus schmitti, in Río de la Plata and El Rincón coastal areas. There, canonical correspondence analyses of fishery research survey data showed that M. schmitti ontogenetic stages were differentially affected by depth, temperature and salinity. Neonates and juveniles were more abundant and remain in nearshore waters, suggesting the presence of nursery areas in which food availability and environmental conditions allow a faster growth. Adults M. schmitti presented different environmental associations mainly associated with depth and temperature. During non-reproductive season they were associated with deeper coastal waters. With the arrival of the reproductive season, adults migrate from deep to nearshore waters and show sexual segregation associated with pre- or post-mating behaviors. Hence, the year-round presence of neonates and juveniles and the seasonal occurrence of adults in nearshore waters make this habitat essential to M. schmitti population dynamics since key ontogenetic stages will be vulnerable if a direct impact occurs there.     

Shifts in the microhabitat of larval and juvenile roach, Rutilus rutilus (L.), in a floodplain channel

The microhabitat of roach larvae (stages L1–L6) and 0+ juveniles was examined in a braided channel (Ilon) of the Upper Rhone River, France, from weekly samples (spring-autumn 1985) using Point Abundance Sampling by electrofishing. During early development, microhabitat exploitation was characterized by two transitions: the first at larval step L3, when morphological and physiological attributes permit the active choice of microhabitat; the second at the end of the larval and start of the juvenile period. At larval step L3, and up to L5, roach were strongly associated with lentic waters of medium depth (0.5–1.0m) and with either macrophytes or ligneous debris. The L6 and 0+ juveniles were found more often than expected in shallower waters (< 0.2m, 0.2–0.5 m) and were no longer associated with dense vegetation or ligneous debris. However, both larval (L3–L6) and 0+ juvenile roach preferred silty sand and/or silted gravel, and a lack of current. This shift, from moderately deep waters with vegetation and/or branches as larvae to open shallow waters as juveniles, demonstrates the young roach's perception and response to environmental change, with shallows exploited as a refuge from predation when protective structures become scarce or unavailable.     

Rapid adaptive responses of rosette‐type macrophyte Vallisneria natans juveniles to varying water depths: The role of leaf trait plasticity

Rosette‐type submerged macrophytes are widely distributed across a range of water depths in shallow lakes and play a key role in maintaining ecosystem structures and functions. However, little is known about the rapid adaptive responses of such macrophytes to variations in water depth, especially at the juvenile stage. Here, we conducted a short‐term in situ mesocosm experiment, in which the juveniles of Vallisneria natans were exposed to a water depth gradient ranging from 20 to 360 cm. Twenty‐two leaf‐related traits were examined after 4 weeks of growth in a shallow lake. Most (18) traits of V. natans generally showed high plasticity in relation to water depth. Specifically, juveniles allocated more biomass to leaves and had higher specific leaf area, leaf length‐to‐width ratio, chlorophyll content, and carotenoids content in deep waters, displaying trait syndrome associated with high resource acquisition. In contrast, V. natans juveniles in shallow waters had higher leaf dry matter content, leaf soluble carbohydrate content, carotenoids per unit chlorophyll, and peroxidase activity, pertaining to resource conservation. Notably, underwater light intensity was found to be the key factor explaining the trait plasticity along the water depth gradient, and 1.30 mol photons m⁻² d⁻¹ (at 270 cm) could be the optimal irradiance level based on the total biomass of V. natans juveniles. The present study highlights the significance of leaf trait plasticity for rosette‐type macrophytes in response to variations in water depth and sheds new light on the differences between trade‐offs in deep‐ and shallow‐water areas.     

ONTOGENY OF MOVEMENTS AND FORAGING RANGES IN THE AUSTRALIAN SEA LION

This study tracked the movements of Australian sea lion ( Neophoca cinerea ) pups, juveniles, and adult females to identify home ranges and determine if young sea lions accompanied their mothers at sea. Satellite tags were deployed on nine 15-mo-old pups, nine 23-mo-old juveniles, and twenty-nine adult female Australian sea lions at Seal Bay Conservation Park, Kangaroo Island, South Australia. Females did not travel with their offspring at sea, suggesting young Australian sea lions learn foraging behaviors independently. Although home ranges increased with age, 23-mo-old juveniles had not developed adult movement capacity and their range was only 40.6% of the adult range. Juveniles traveled shorter distances (34.8 ± 5.5 km) at slower speeds (2.0 ± 0.3 km/h) than adults (67.9 ± 3.5 km and 3.9 ± 0.3 km/h). Young sea lions also stayed in shallower waters; sea floor depths of mean locations were 48 ± 7 m for juveniles and 74 ± 2 m for females. Restricted to shallow coastal waters, pups and juveniles are more likely to be disproportionately impacted by human activities. With limited available foraging habitat, young Australian sea lions appear particularly vulnerable to environmental alterations resulting from fisheries or climate change.     

Origin of juvenile chum salmon <Emphasis Type="Italic">Oncorhynchus keta</Emphasis> (Salmonidae) in the Sea of Okhotsk coastal waters off South Sakhalin

Based on the data of the analysis of distribution of juvenile chum salmon Oncorhynchus keta that were marked at salmon hatcheries in the southern part of the Sakhalin Island and Japan it has been established that their stocks in the Sea of Okhotsk coastal waters off South Sakhalin and Japan are of mixed origin. One part of the stock is composed of juveniles from hatcheries located in the southern part of Sakhalin, and the other part is composed of migrants from the waters adjacent to Japan. The first part is formed by juveniles both from hatcheries on the Sea of Okhotsk and Sea of Japan coasts of South Sakhalin. The second part is formed by juveniles reared at hatcheries in all regions of Japan: the Pacific and Sea of Okhotsk coasts of Honshu and Hokkaido, and from the Sea of Okhotsk coast of Hokkaido and Izmena (Nemuro) Strait. In July, in the Sea of Okhotsk coastal waters off South Sakhalin, the length and weight of most juvenile chum salmon of Japanese origin exceed 10 cm and 10 g, and juveniles of Sakhalin origin are always smaller than 10 cm and 10 g, which makes it possible to differentiate juveniles in trawl catches.     

The importance of light and food upon the early growth of the reef coral favia fragum (esper)

Planulae larvae of the reef coral Favia fragum (Esper) were allowed to settle and metamorphose to juveniles on glass microscope slides. The growth of the juveniles under conditions of light and darkness, with and without food, was measured for a period of about three months. Only those juveniles which were given food and incubated under a daily programme of 12 hours light continued to grow throughout the whole period of the experiment. Juveniles deprived of either light or food did not increase in size after the first week, but continued to survive for periods of up to three months with tissue atrophy.     

Segregation and foraging ecology of whale sharks, Rhincodon typus, in the southwestern Gulf of California

Sharks segregate by sex and size, but few studies have attempted to explain such behaviors. To address this, we examined aggregations and the foraging ecology of whale sharks in Bahía de La Paz (BLP) with aerial and ship surveys and direct observation. Zooplankton abundance and composition, and hydrographic conditions were analyzed in relation to whale shark occurrence to explore underlying factors causing segregations. We observed large aggregations of juveniles (<9 m total length, TL) inshore, comprised by 60 % male individuals, and small aggregations of adults (>9 m TL) offshore, composed of 84 % females. Juvenile sharks were associated to turbid shallow waters in BLP, where they performed stationary and dynamic suction feeding on dense copepod swarms. Adults occurred in oceanic waters and fed by ram-filtering on diffuse patches of euphausiids, with no association to oceanographic conditions. Such segregation may be advantageous to juvenile R. typus utilizing shallow coastal waters to find abundant preferred prey needed for their fast growth rates. Our studies suggest that the main driving forces of whale shark segregation by sex and size in BLP may be diet preference for juveniles and habitat preference for adult sharks.     

Intraspecific geographic and seasonal physiological variability in an intertidal fish, Girella laevifrons, along a climatic gradient

Metabolic rate and condition factor ( K ) of juvenile Girella laevifrons were highest in southern populations along the Chilean coast. Since adults of G. laevifrons complete their life cycle in subtidal waters, the results suggest a metabolic cold adaptation in juveniles of this species and physiological compensation that enable them to move early to subtidal environments for reproduction     

Ontogeny of turbiditaxis in hatchery‐reared Japanese anchovy Engraulis japonicus

The ontogenetic change in turbiditaxis (i.e. attraction to turbid waters) was examined in the larvae and juveniles of Japanese anchovy Engraulis japonicus by testing three levels of turbidity (0, 20 and 100 ppm of kaolin). Larvae of 12, 20 and 30 mm standard length (L_S) exhibited turbiditaxis to both 20 and 100 ppm of kaolin, whereas 6 mm L_S larvae and 45 mm L_S juveniles did not exhibit any turbiditaxis. Turbiditaxis might explain the ontogenetic habitat shift from coastal to offshore waters reported for this species.     

Spawning of the kissing loach (Leptobotia curta) is limited to periods following the formation of temporary waters

The kissing loach, an endangered species surviving only in a few Japanese rivers, spawns in the rice-field areas after migration from rivers in early June. To characterize the environmental conditions required for spawning of the kissing loach, spawning was assessed for two years both by direct observation of spawning behavior and by the appearance of eggs, larvae, and juveniles from June to October. All spawning of the kissing loach was limited to within a couple of days after the formation of temporary waters by remarkable rises in water level. Water temperature and daily rainfall fluctuated during the investigation periods, and no clear relationships with spawning were detected. Furthermore, all spawning was observed only in temporary waters with terrestrial grasses. Thus, spawning of the kissing loach is rigidly limited spatio-temporally to after the formation of temporary waters over terrestrial vegetation. Appropriate management of temporary waters will be crucial for the continued existence of this species.     

Seasonal distribution of juvenile Atlantic cod in the southern Gulf of St Lawrence

Seasonal changes in the geographic distribution of juvenile (ages 1–4) Atlantic cod Gadus morhua were determined from research surveys conducted in the southern Gulf of St Lawrence during 1986–87, 1990–91, and 1994. From late June to early October, juvenile cod were most abundant in shallow water (<50 m deep). By early November, all age groups (adults and juveniles) had migrated to deep (>100 m) offshore waters for the winter. For most juveniles, this represented a minimum migration of >225 km (each way). Age 3 and 4 cod migrated greater distances than age 1 and 2 fish. Some age 3 and 4 cod migrated as far as 650 km from their summer and early autumn areas. Although there was partial geographic segregation of juvenile and adult fish during summer and winter, all age groups were found together during the migration. No cod were present in the shallow waters (<80 m deep) of the south-western Gulf during April due, in part, to later than average ice-melt. Large numbers of all age-classes were present in most of the south-eastern Gulf by mid-May.     

Complexity and variation in loggerhead sea turtle life history

Juvenile loggerhead sea turtles spend more than a decade in the open ocean before returning to neritic waters to mature and reproduce. It has been assumed that this transition from an oceanic to neritic existence is a discrete ontogenetic niche shift. We tested this hypothesis by tracking the movements of large juveniles collected in a neritic foraging ground in North Carolina, USA. Our work shows that the shift from the oceanic to neritic waters is both complex and reversible; some individuals move back into coastal waters and then return to the open ocean for reasons that are still unclear, sometimes for multiple years. These findings have important consequences for efforts to protect these threatened marine reptiles from mortality in both coastal and open-ocean fisheries.     

Schooling behaviour of juvenile Pacific bluefin tuna Thunnus orientalis depends on their vision development

The effects of vision development and light intensity on schooling behaviour during growth in juvenile Pacific bluefin tuna Thunnus orientalis were investigated using both behavioural and histological approaches. The schooling behaviour of three age groups [25, 40 and 55 days post hatching (dph)] of juvenile T. orinetalis were examined under various light intensities. Subsequently, schooling variables, such as the nearest neighbour distance (D(NN) ) and the separation swimming index (I(SS) ), were also measured under different light intensities. Furthermore, retinal indices of light adaptation in juvenile fish at each experimental light intensity and visual acuities in six stages (25-55 dph) of juveniles were examined histologically. During growth, the light intensity thresholds of I(SS) decreased from 5 to 0·05 lx, and D(NN) under light conditions (>300 lx) also decreased from 9·2 times the standard length (L(S) ) to 1·2 times L(S) . The thresholds of light intensities for the light adaptation of retinas in juveniles (25-55 dph) similarly decreased from 5 to 0·05 lx with growth. In addition, the visual acuities of juveniles developed from 0·04 to 0·17 with decreasing D(NN) . These data clearly indicate that the characteristics of schooling behaviour strongly correspond to the degree of vision development. Juvenile T. orinetalis also appear to be more dependent on cone rather than rod cells under low light intensity conditions, resulting in a relatively high light intensity threshold for schooling. These results suggest that juveniles can adapt to darker conditions during growth by developing improved visual capabilities.     

Ecology of pelagic larvae and juveniles of the genusSebastes

Synopsis Pelagic larvae and juveniles of the genusSebastes are widely distributed in the continental shelf and slope waters of subarctic to temperate oceans, with greatest abundance in the Northern Hemisphere. We review the ecology and distribution of these planktonic and micronektonic life stages in relation to oceanographic conditions. Special attention is paid to the west coast of North America, where abundance of larvae from samples collected during 1951–1981 is described. After transformation, the pelagic juveniles are widely distributed, often at great distances from benthic adult habitats. These stages are most frequently distributed in either midwater or near-surface habitats; this dichotomy may require different strategies for successful recruitment.