Growth,dispersal, and identification of planktonic labrid and pomacentrid reef-fish larvae in the eastern Pacific Ocean

Planktonic larvae of six genera of labrid and pomacentrid reef fishes were captured in march 1985 in the eastern Pacific Ocean several hundred kilometers from the nearest reefs. The larvae were identified to genus by fin-ray counts as well as by comparison of their larval otolith morphology with that of known species. The larval otolith morphologies of known species were derived from measurements of the larval otolith embedded within the otoliths of settled juveniles (as delineated by the daily otolith-increment marks corresponding to the late larval period). The body morphology and melanophore patterns of the eastern Pacific labird and pomacentrid larvae closely matched those of congeneric larvae described from other oceans. Growth rates of larvae less than about 70 d old were similar between taxa (from 0.13 to 0.19 mm d^-1). After about 70 d in the plankton, labrid larvae grew much more slowly (0.06 mm d^-1 in Xyrichtys sp.). Labrid larvae had long larval durations (up to 131 d in Xyrichtys sp.), while the larval lives of the pomacentrids appeared to be shorter and much less variable. Larvae of many different ages occurred within the same water mass, and young cohorts of larvae appeared continuously over the sampling period. Some larvae were as young as 21 d, indicating that reef-fish larvae are capable of rapid long-distance dispersal (at least 18 km d^-1).     

Dietary effects on multi-element composition of European eel (<Emphasis Type="Italic">Anguilla anguilla</Emphasis>) otoliths

Otolith microchemistry is widely used as a tool to track individual migration pathways of diadromous fish under the assumption that the elemental composition of fish otoliths is directly influenced by the physicochemical properties of the surrounding water. Nevertheless, several endogenous factors are reported to affect element incorporation into fish otoliths and might lead to misinterpretations of migration studies. This study experimentally examined the influence of eight different diets on the microchemical composition of European eel (Anguilla anguilla) otoliths using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Seven natural prey types and one artificial diet were fed during 8 weeks in freshwater circuits. Results show for the first time that food has no significant influence on the incorporation of Na, Sr, Ba, Mg, Mn, Cu and Y into European eel otoliths. This indicates that the incorporation of elements usually chosen for migration studies is not affected by diet and that individual feeding behaviour of A. anguilla will not lead to any misinterpretation of migration pathways.     

Larval ecology of the great barracuda, <Emphasis Type="Italic">Sphyraena barracuda</Emphasis>, and other sphyraenids in the Straits of Florida

The great barracuda (Sphyraena barracuda) is a widespread, ecologically and socioeconomically important coastal fish, yet very little is known about its larvae. We examined spawning and larval ecology of Western Atlantic sphyraenids using monthly ichthyoplankton samples collected over 2 years along a transect spanning the east–west axis of the Straits of Florida (SOF). Samples were dominated by the great barracuda (92.8%) and sennets (Sphyraena borealis and Sphyraena picudilla; 6.6%). While larval sennets and S. barracuda displayed similar vertical distributions (majority in upper 25 m), horizontal and temporal patterns of abundance suggested a spatial and temporal species replacement between larval S. barracuda and sennets that tracks adult ecology. The diet of both taxa consisted largely of copepods, with inclusion of fish larvae at 8 mm SL, and in S. barracuda alone, a switch in the wet season to exclusive piscivory by 12 mm SL (18 days post-hatch). A lack of piscivory in S. barracuda larvae captured in the dry season corresponded to slower larval growth than in the wet season. Larval growth was also related to size-at-hatch and larval age such that larvae that were larger at hatch or larger (older) at capture grew faster at earlier ages, suggesting faster larval growth, and indirectly larger hatch size, conveys a survival advantage. Unlike larval growth, instantaneous mortality rate did not differ with season, and no lunar cyclic patterns in spawning output were identified. Our results provide insight into the pelagic phase of sphyraenids and highlight the importance of both diet and hatch size to the growth and survival of fish larvae in low latitude oceanic environments.     

Swimming ability and its rapid decrease at settlement in wrasse larvae (Teleostei: Labridae)

Wrasses are abundant reef fishes and the second most speciose marine fish family, yet little is known of their larval swimming abilities. In August 2010 at Moorea, Society Islands, we measured swimming ability (critical speed, Ucrit) of 80 settlement-stage larvae (11–17 mm) of 5 labrid species (Thalassoma quinquevittatum [n = 67], Novaculichthys taeniourus [n = 6], Coris aygula [n = 5], Halichoeres trimaculatus [n = 1] and H. hortulanus [n = 1]) and 33 new recruits of T. quinquevittatum. Median (mdn) larval Ucrit was 7.6–12.5 cm s⁻¹. In T. quinquevittatum (n = 67), larvae of 12.5–14.5 mm swam faster (mdn 16.9 cm s⁻¹) than smaller or larger larvae (mdn 3.9 and 3.2 cm s⁻¹, respectively). Labrid larvae Ucrit is similar to that of other similar-sized tropical larvae, so labrids and species with comparable settlement sizes should have similar abilities to influence dispersal. Ucrit of T. quinquevittatum recruits decreased to 47–56% of larval Ucrit in 2 days, implying rapid physiological changes at settlement.     

Relationship of pheasant occurrence to barium in Illinois soils

Distribution of the ring-necked pheasant (Phasianus colchicus) is nearly co-extensive with the boundary of Wisconsinan glaciation; this is an area of base-rich soils that are mostly developed in calcareous tills. Anderson and Stewart (1973) speculated that barium in the diet might be a factor limiting the pheasant establishment in habitats adjacent to areas of long-term sustaining populations in Illinois. Total Ba was calculated for soils of 74 conterminous counties for which there were pheasant population data. Barium over the range of about 480 to 700 mg kg^–1 soil was unrelated to pheasant occurrence or to population indeces. The selective eating of iron-manganese concretions that are enriched in Ba might present a unique pathway for Ba loading.     

Variation in concentrations of trace elements in otoliths and eye lenses of a temperate reef fish, Parma microlepis, as a function of depth, spatial scale, and age

 Territorial Parma microlepis (Günther) (Pomacentridae) were collected at different depths, at three sites in each of four estuaries near Sydney, Australia. Element concentrations were measured by induc- tively-coupled–plasma mass spectrometry. Significant differences in concentrations of Mn and Ba were found in the otoliths of fish sampled in different depth strata, with concentrations generally greatest in fish found in water <4 m deep. Depth-related differences varied among estuaries (e.g. 0 to 1.2 μg Ba g⁻¹ otolith). In most estuaries there was a negative linear relationship between concentrations of Mn and Ba in otoliths and actual depth. Great variation was found within an estuary among sites separated by 0.5 to 3 km. In the eye lenses, concentrations of Rb differed according to depth of capture of fish. The age of fish (1+ to 34 yr) had no influence on the concentrations of elements in otoliths or lenses. Multivariate comparisons of elemental composition (= fingerprints) detected significant differences among depth strata. Fish collected from shallow water had the clearest multivariate classification according to depth. There was a close match between our shallow strata and the average depths reached by low-salinity/high-temperature estuarine plumes. The element composition of whole otoliths and lenses represents average concentrations experienced by the fish. The temporal resolution of differences in ambient conditions is likely to be coarse in the fish (i.e. months to years). Received: 9 April 1999 / Accepted: 29 February 2000     

Lipid and protein utilisation during early development of yellowtail kingfish (<Emphasis Type="Italic">Seriola lalandi</Emphasis>)

The pelagic yellowtail kingfish Seriola lalandi has become a target species for aquaculture in Asia and Australasia. Australasian production is reliant on larviculture from eggs of captive brood stock; however, knowledge regarding the nutritional requirements of larvae of this species is still scarce, particularly in relation to lipids. As a first step in establishing these requirements, eggs and larvae from captive S. lalandi brood stock were examined for differences in total protein, total lipid and lipid classes between individual spawning events, over the spawning season, and during larval development from fertilisation to 15 days post hatch. Results indicate that total protein egg⁻¹ varied significantly between individual spawning events within a season, but neither total lipid nor total protein egg⁻¹ varied significantly across the spawning season. Brood stock egg lipids were made up of approximately 60% phospholipid, 25% wax and/or sterol esters (WE), 15% triacylglycerol (TAG), and small amounts of sterols and free fatty acids. During the early larval period, both WE and TAG were utilised concurrently for energy. The larvae experienced very high mortality around 5–7 days post hatch, which coincided with very low levels of all neutral lipid classes. Although many other factors may also influence larval mortality, these results indicate that lipid provisioning may be an important factor in larval survival during the critical period around first-feeding in this species. Examination of ratios of TAG:ST, often used as a condition index in fish larvae, suggested that some of the larvae were suffering from starvation. However, as egg-derived WE appears to provide a significant source of energy during the early larval period in S. lalandi, it is suggested that WE should be included in any index of larval nutritional state.     

Acute toxicity of ammonia to a freshwater teleost,Labeo bata larvae

Ammonia toxicity tests were performed with Labeo bata (bata) larvae of three different size groups. One hundred percent survival of larvae (500.0?±?4.0?mg) was recorded when exposed to ammonia concentrations of 1.0–13.56?mg?L^?1 at 96?h of exposure. Bata larvae exposed to ammonia concentrations of 15.8–25?mg?L^?1 showed 10–74% mortalities. The 96?h LC₅₀ value for 200 (±5), 250 (±2) and 500 (±4) mg bata larvae were 11.5, 16.8 and 22.5?mg?L^?1 un-ionised ammonia concentrations, respectively. When fish were exposed to different doses of ammonia, behavioural changes immediately occurred even at the lowest dose. At first, the fish became hyperexcitable, the skin darkened and they showed an increased ventilation frequency, fish behaviour became normal, 24?h after exposure. A 96?h LC₅₀ value of un-ionised ammonia showed direct relationship with the increasing size of bata larvae.     

Underlying causes of habitat-associated differences in size of age-0 walleye pollock (Theragra chalcogramma) in the Gulf of Alaska

Age-0 walleye pollock (Theragra chalcogramma) caught in September in the Gulf of Alaska display habitat-associated differences in standard length (SL). Age-0 fish collected in the region around Sutwik Island and 375?km farther downstream near the Shumagin Islands most likely originate from the Shelikof Strait spawning aggregation. However, age-0 fish resulting from the same spawning aggregation differ in mean size up to 20?mm between areas by September. We examined the otoliths of the larval and age-0 stages of walleye pollock from these two areas in 2000 and 2001 to determine whether growth rate, hatch date, and/or temperature influenced fish size. Circulation models were used to determine whether transport of larvae from an upstream spawning group into the study areas could have occurred. Mean in situ temperature during sampling periods was not defined as a significant factor in altering growth rates. Overlapping hatch date distributions of the larval and age-0 fish in the Shumagin Island area confirmed that the fish were from the Shelikof Strait spawning group. Comparison of hatch date distributions in the upstream Sutwik Island area revealed larger/older larvae from an upstream spawning group mixed with larvae from the Shelikof Strait spawning group. Our results suggest that the offset of 20?mm SL between the groups of age-0 pollock was the result of a combination of enhanced survivorship of early-hatched larvae in the Sutwik area and the introduction and retention of the progeny of another spawning group originating upstream of Shelikof Strait.     

In situ experiment of ontogenetic variability in the otolith chemistry of Stegastes partitus

Otolith chemistry can be used to assess pelagic larval fish connectivity by comparing spatially variable otolith edge chemistry (corresponding to the site of collection) to otolith core chemistry (corresponding to the site of hatching). However, because the otolith’s edge and core represent different life stages, the deposition of elements may differ, thus complicating direct comparisons of edge and core chemistry to investigate connectivity. Here we present data from a field experiment in which otoliths from embryos (3 days post-fertilization) and juveniles of Stegastes partitus were collected at the same site and time, and chemically analyzed to assess whether elemental concentrations of otoliths vary ontogenetically. Separate multivariate analyses, each investigating the spatial/temporal variability in the chemistry of either embryo otoliths or the edges of juvenile otoliths, revealed significant differences, suggesting an environmental influence to the chemical signals of otoliths. A nested multivariate analysis assessing whether otolith chemistry varied with life history stage (i.e., ontogenetic variability) indicated that elemental concentrations of embryo otoliths were significantly greater than that of juvenile otolith edges. Specifically, embryo elemental concentrations of Mn, Zn, Sn, Ba, Ce, and Pb were between 2 and 163 times greater than those of the corresponding juvenile otoliths, and thus the environment was not the primary determinant of embryo otolith chemistry. Consequently, caution is warranted when interpreting environmental patterns of otolith cores, particularly when using them as a proxy for natal signatures.     

Flux by fin: fish-mediated carbon and nutrient flux in the northeastern Gulf of Mexico

Seagrass meadows are among the most productive ecosystems in the marine environment. It has been speculated that much of this production is exported to adjacent ecosystems via the movements of organisms. Our study utilized stable isotopes to track seagrass-derived production into offshore food webs in the northeastern Gulf of Mexico. We found that gag grouper (Myctereoperca microlepis) on reefs as far as 90 km from the seagrass beds incorporate a significant portion of seagrass-derived biomass. The muscle tissue of gag grouper, a major fisheries species, was composed on average of 18.5–25% seagrass habitat-derived biomass. The timing of this annual seagrass subsidy appears to be important in fueling gag grouper egg production. The δ³⁴S values of gag grouper gonad tissues varied seasonally and were δ³⁴S depleted during the spawning season indicating that gag utilize the seagrass-derived biomass to support reproduction. If such large scale trophic subsidies are typical of temperate seagrass systems, then loss of seagrass production or habitat would result in a direct loss of offshore fisheries productivity.     

Comparative foraging ecology and ecological niche of a superabundant tropical seabird: the sooty tern Sterna fuscata in the southwest Indian Ocean

Over 6-million pairs of sooty terns Sterna fuscata breed once a year in the southwest Indian Ocean, mostly on three islands of the Mozambique Channel (Europa, Juan de Nova and Glorieuses) and in the Seychelles region. Seasonal reproduction in either winter or summer is the dominant strategy in the area, but non-seasonal reproduction also occurred in some places like at Glorieuses Archipelago. The feeding ecology of the sooty tern was investigated during the breeding seasons to determine whether terns showed significant differences in their trophic ecology between locations. Regurgitations were analyzed to describe the diet of individuals when breeding, and stable isotopes and mercury concentrations were used to temporally integrate over the medium-term of the trophic ecology of both adults and chicks. Overall, the diet was composed of fish, flying squid and fish larvae in different proportions. At Europa and Aride in the Seychelles, where winter reproduction occurs, large epipelagic prey like flying fish or squid dominated the diet. At Juan de Nova, sooty terns reproduce in summer and rely mostly on fish larvae. At Glorieuses (non-seasonal breeding), the diet was intermediate with fish larvae and flying squid being important prey items. The stable-carbon and nitrogen isotope values in blood confirm the differences observed in dietary analysis, and demonstrate different feeding strategies between colonies. δ¹³C values of feathers showed spatial segregation between birds from the Mozambique Channel and the Seychelles region. Terns from the Seychelles had also higher δ¹⁵N values. Feather δ¹³C values also suggest a significant shift from summer to wintering habitat for birds from Juan de Nova. This study emphasizes the high phenotypic plasticity of the species, which may explain its numerical dominance in all tropical waters of the World’s Ocean.     

Relationship between growth rate and age at recruitment ofAnguilla japonica elvers in a Taiwan estuary as inferred from otolith growth increments

The growth history and recruitment dynamics of eel (Anguilla japonica) elvers were studied. Observations were based on growth increments in sagittal otoliths of elvers collected from Shuang-Chi River estuary off northeastern Taiwan, from November 1985 to February 1986. Total lengths of elvers upon arrival at the estuary were similar in most case; mean total lengths were from 55.99 to 59.06 mm. Daily ages of elvers at arrival ranged from 112.8±9.4 (±SD) to 156.5±13.5 d, indicating that migration of eel larvae from their oceanic spawning ground to the estuary requires 4 to 5 mo. Elver hatching dates, back-calculated from estimated daily ages, indicated that the spawning season lasted 5 mo (from late June to early October). Furthermore, the earlier eels spawned, the earlier elvers reached the estuary. The transition in growth history during the larval stage was obvious, as indicated from the change in increment width in elver otoliths. The inverse correlation between daily age and mean daily growth rates of fish length and otolith indicated that the age of elvers upon arrival at the estuary was susceptible to larval growth rate. In other words, the time taken on migration from oceanic spawning ground to the estuary was shorter for fast-growing larvae than for slowgrowing ones.     

Exploring the potential of otolith microchemistry to enhance diet analysis in pinnipeds

Increasing the scope and accuracy of information about pinniped diets obtainable from non-invasive techniques is increasingly important, particularly in cases where pinniped species are threatened or endangered. This study is the first to explore the potential for using elemental analysis of the otoliths found in scat to enhance the information available for diet analyses. We investigated the effects of pinniped digestion on otolith microchemistry using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). We compared the elements contained in the edges (adult stage) and cores (larval/juvenile stage) of otoliths from Pacific herring (Clupea pallasii), Atka mackerel (Pleurogrammus monopterygius), and Walleye pollock (Theragra chalcogramma) recovered from the scat of captive Steller sea lions (Eumetopias jubatus) to elements in a sample of pristine (undigested) otoliths. We found that digestion had a significant effect on four of the six sampled combinations of species and otolith region (herring edges and cores, mackerel edges, and pollock cores), and that Rb most frequently showed significant differences in concentration after digestion. We could significantly discriminate among species of both pristine and digested otoliths using either otolith edges or cores with the elements Ba, Rb, Sr, Y, and Mg. When compared to previously identified digested otoliths, unknown samples of the three species of digested otoliths could be discriminated with 55–100% accuracy depending on species and otolith region. When compared to a library of previously identified pristine and digested otoliths, unknown samples of digested otoliths could be discriminated to species with 65–88% accuracy. When the group of unknown digested otoliths was compared to known pristine otoliths, discrimination ranged from 45 to 65%. These results indicate that elemental analysis could be used to supplement visual identification of otoliths from scat. However, further research is required to determine whether elemental analysis of digested otoliths could be useful for prey fish population studies.     

Reproduction of the Antarctic fish Nototheniops nudifrons

The reproductive biology of the Antarctic fish Nototheniops nudifrons (Lönnberg, 1905) was analyzed by examination of the gonads of fish collected in March and April 1985 in trawls near Low Island, Antarctic Peninsula, and compared to direct observations of reproduction and early larval development in the laboratory from March to October of the same year. Males and females reached sexual maturity at an age of 4 to 5 yr. Mature males differed in coloration from females and immature males. Ovaries of sexually mature females contained two distinct size modes of vitellogenic oocytes, representing two separate clutches of developing eggs. Females spawned 100 to 3 500 demersal eggs, which were laid in a nest in crevices or under rocks, and guarded by the male for about 4 mo. Females did not assist in nest defense or egg care. Most spawning in the field and in the laboratory occurred in late fall and early winter (May to June). A second clutch may be spawned in spring (November and December). Eggs hatched after 124 d, and larvae were raised for 38 d. Otoliths of larvae contain internal microincrements, which are deposited in a daily fashion, and are visible by light microscopy. The otoliths of 32 adult fish were examined by scanning electron microscope, and counts of microincrements in these otoliths allowed the backcalculation of hatching dates. Estimated hatching dates were between September and May.     

Oxygen and carbon stable isotopes in the otoliths of wild and laboratory-reared Australian salmon (Arripis trutta)

Australian salmon,Arripis trutta, collected from the east coast of Tasmania, Australia, in 1987, were weighed and measured and their otoliths marked by immersing fish in an oxytetracycline hydrochloride/seawater solution before placement in constant-temperature aquaria. Individual somatic and otolith growth rates were determined for input into mass balance models. Mass balance models were used to determine the oxygen and carbon isotopic composition of otolith material produced during captivity. There was a significant relationship between ¹⁸O measured in the otolith aragonite and ambient temperature (r ² = 0.77). The linear relationship between these data, where ¹⁸O = 6.69 – 0.326 (T, °C), was not significantly different from a relationship indicative of equilibrium deposition of oxygen isotopes in aragonite. Otolith carbon was significantly depleted in¹³C relative to equilibrium deposition, with depletions >6.0 at all temperatures. There was no relationship between ¹³C and temperature. It was estimated that >30% of the otolith carbon was from metabolically derived sources. Significant differences in otolith carbon isotopes among wild juvenile Australian salmon were hypothesised to be attributable to differences in diet. Levels of variability for both oxygen and carbon isotopes in laboratory-maintained and wild fish were similar to that found by other researchers for foraminifera and these results highlight the importance of large sample sizes when estimating environmental temperatures from oxygen isotopes measured in fish otoliths.     

Behavioral response of fish larvae to low dissolved oxygen concentrations in a stratified water column

Density stratification and respiration lead to vertical gradients in dissolved oxygen in many aquatic habitats. The behavioral responses of fish larvae to low dissolved oxygen in a stratified water column were examined during 1990–1991 with the goal of understanding how vertical gradients in dissolved oxygen may directly affect the distribution and survival of fish larvae in Chesapeake Bay, USA. In addition, the effects of low oxygen on 24-h survival rates were tested so that results of behavior experiments could be interpreted in the context of risk to the larve. Naked goby [Gobiosoma bosc (Lacépède)] and bay anchovy [Anchoa mitchilli (Valenciennes)] larvae strongly avoided dissolved oxygen concentrations <1 mg 1^-1, which were lethal within 24 h at 25 to 27°C. In addition, naked goby larvae, whose behavior was tested at a wider range of dissolved oxygen concentrations, also showed a reduced preference for an oxygen concentration of 2 mg 1^-1, which leads to reduced survival during long-term exposures and to reduced feeding rates. There were no major differences in behavior or survival between the two species, or between the two age classes of naked gobies tested. Results suggest that behavioral responses to oxygen gradients will play a large role in producing marked vertical changes in abundance of feeding-stage larvae in Chesapeake Bay; mortality from direct exposure to low oxygen will likely be much less important in producing vertical patterns of larval abundance.     

RNA-DNA ratio: an index of larval fish growth in the sea

Data on water temperature, RNA-DNA ratio, and growth of eight species of temperate marine fish larvae reared in the laboratory were fit to the equation: $$G_{pi} = 0.93{\text{ }}\operatorname{T} + 4.75{\text{ RNA - DNA}} - 18.18$$ where G_pi is the protein growth rate in % d^-1 and T is the water temperature. Water temperature and larval RNA-DNA ratio explained 92% of the variability in growth rate of laboratory-reared larvae. The model is useful over the entire range of feeding levels (starvation to excess), temperatures (2° to 20°C) and fish species studied. Estimates of recent growth of larval cod, haddock, and sand lance caught at sea based on water temperature and RNA-DNA ratio ranged from negative to 26% d^-1. These data demonstrate the importance of food availability in larval fish mortality and suggest that short-term growth under favorable conditions may be considerably higher than expected from long-term indicators. RNA-DNA ratio analysis offers new possibilities for understanding larval growth and mortality, and their relation to environmental variability.     

Molecular structure and composition of fish otoliths

Recent and fossil otoliths from 25 different fishes have been studied for their amino acid content and for their C¹³/C¹² and O¹⁸/O¹⁶ distribution in the carbonate fraction. The selection includes specimens from a wide phylogenetic range as well as from various freshwater and marine habitats. All otoliths are composed of aragonite, and their total organic matter ranges from 0.2 to 10%. The organic matter is a protein (MW>150,000), which is characterized by a high abundance of acidic amino acids. In comparison to molluscs that exhibit a wide variety of different mineralized tissues which are species specific, the proteinaceous matter of all otoliths is chemically rather uniform. The high abundance of oxygen-rich amino acids accounts for the ease of mineralization of the organic template. Namely, oxygen supplied by carboxyl grops is used for the coordination of Ca⁺⁺ ions, resulting in the formation of metal ion coordination polyhedra. Carbonate groups linked via hydrogen bridges to the template will exchange their oxygen with that of the metal polyhedra to stabilize the structure; Ca⁺⁺O₉ polyhedra are the consequence. Subsequent nucleation and crystal growth will lead to aragonite. Oxygen and carbon isotope data indicate that the aragonite is formed close to isotopic equilibrium with the sea. This is surprising, because seawater has no direct access to the inner ear where the otolith orginates. Isotope data may serve a threefold purpose: (1) to determine the mean water temperature where the fish lived, (2) to distinguish between fresh water and marine fish in ancient deposits, and (3) to reveal information on migrtory tendencies of fish.Contribution No. 2214 from the Woods Hole Oceanographic Institution.     

Effect of low dissolved oxygen concentrations on behavior and predation rates on red sea bream Pagrus major larvae by the jellyfish Aurelia aurita and by juvenile Spanish mackerel Scomberomorus niphonius

A shift in outcomes of predator-prey interactions in plankton community may occur at sublethal dissolved oxygen concentrations that commonly occur in coastal waters. Laboratory experiments were conducted to investigate how a decline in dissolved oxygen concentration alters the predation rate on fish larvae by two estuarine predators. Behavior and consumption of larval fish by moon jellyfish Aurelia aurita (103.1±12.4 mm in bell diameter) and by a juvenile piscivore, Spanish mackerel Scomberomorus niphonius (30.1±2.1 mm in standard length: SL), were observed under four oxygen concentration treatments (1, 2 and 4 mg l^–1 and air-saturated: 5.8 mg l^–1). Larvae of a coastal marine fish species, red sea bream Pagrus major (7.21±0.52 mm SL), were used as prey for the experiment. Bell contraction rate of the jellyfish did not vary among the oxygen concentrations tested, indicating a tolerance to low oxygen concentration. Gill ventilation rate of the Spanish mackerel increased and swimming speed decreased as the oxygen concentration decreased, indicating that oxygen concentrations 4 mg l^–1 are physiologically stressful for this species. The number of larvae consumed in 15 min. by jellyfish increased whereas those consumed by Spanish mackerel decreased with the decrease in oxygen concentration. Low oxygen concentrations that are commonly observed in coastal waters of Japan during summer have the potential to increase the relative importance of jellyfish as predator of fish larvae and to change the importance of alternative trophic pathways in estuarine ecosystems.Communicated by T. Ikeda, Hakodate