Quantitative composition,distribution, and feeding of large jellyfish (<Emphasis Type="Italic">Scyphozoa et Hydrozoa</Emphasis>) on the West Kamchatka shelf in summer

The total biomass of jellyfish on the shelf of the eastern Sea of Okhotsk in the summer is estimated as 1672700 tons according to the results of hydroacoustic measurements and 901000 tons by the method of squares. The use of hydroacoustic technologies makes evaluation of the actual stock and range of medusae more accurate, and the further enhancement and perfection of the hydroacoustic method based on multifrequency measurements enables one to obtain more reliable estimates. A significant increase (nearly 25 times) of the total jellyfish biomass takes place in the summer period. Cyanea prevailed in biomass in the spring and Chrysaora melanaster prevailed in the summer. Some species showed considerable expressed spatial differentiation of distribution and affinity to certain environmental conditions. The studied species were almost exclusively zoophages. Their algal diet consisted mainly of diatom algae. Scyphomedusa’s diet mainly included the so-called “peaceful” zooplankton, viz., euphausiids and copepods (as a rule, over 50% the mass), at the same time carnivorous zooplankton, saggits, amphipods, and small medusae also formed a substantial share of their diet. One individual of the predominant jellyfish species consumes a total of 6.1 to 70.5 kcal during its lifecycle, which corresponds to 79.1–513.0 g of raw organic material, assuming 70% assimilability. The relatively low demand for food of this sort can be explained by the low caloric value of the jellyfish body, 96–97% of which consists of water. The distribution and composition of the jellyfish prey show that scyphomedusae exert the greatest influence on the nekton community, as they concentrate in the shelf area of the eastern part of the sea, at walleye pollock spawning sites. There the larvae of bottom invertebrates, including commercially valuable organisms, such as crab and shrimp, are also consumed. In the summer, jellyfish eat nearly 100 billion eggs and 20 billion larvae of walleye pollock, as well as 130 billion decapod (mostly crab) larvae each day, which corresponds to 0.03% of the eggs and 0.003% of the larvae of walleye pollock and 0.003% of the decapod larvae in the estimated stock.     

Do shark declines create fear‐released systems?

We illustrate the theoretical plausibility that fishery removal of sharks can indirectly alter predation pressure on different fish species via the behavioural responses of mesoconsumers released from predator intimidation. Our dynamic state variable model of foraging decisions by harbour seals, a mesopredator, predicted indirect effects of the removal of Pacific sleeper sharks on two species consumed by seals, Pacific herring and walleye pollock, as mediated by seal behaviour. Herring, a fatty fish, form near‐surface aggregations that often are ephemeral and widely dispersed. Pollock are found in the deeper strata preferred by sharks and have lower energy density than herring, but also are larger and their more continuous distribution potentially makes them the more predictable resource for seals. During simulations, predation risk from sharks produced an asymmetric trophic cascade mediated by the seal's underutilisation of deeper, riskier strata. Risk management by seals reduced mortality on pollock, which required riskier access in deep strata, while increasing mortality on herring, which could be accessed with less risk in shallow strata. This effect, however, attenuated if herring were scarcer and seal energy state was poor. During shark removal scenarios, seals shifted to deeper strata, increasing pollock consumption and substantially decreasing use of herring; the proportional change was greatest if seals were in good energy state. Prior studies have addressed how shark declines might affect community structure through density responses by species consumed by sharks; earlier models incorporating antipredator behaviour of the mesopredator (i.e. Ecosim/Ecospace) allow for activity reduction but not for the spatial shifts that altered the asymmetric trophic cascade in our simulations. Our results suggest that shark declines might have stronger ecological consequences than previously recognised if we account for spatial and diet shifts by mesoconsumers released from shark intimidation.     

The effect of early and late hatching on the escape response of walleye pollock (Theragra chalcogramma) larvae

Hatching of fish eggs fertilized at the same time occurs overa period of several days. Differences in the escape responseof fish larvae during the hatching period have not hithertobeen studied. In this study, the escape response of walleyepollock (Theragra chalcogramma) larvae over the hatching periodwas examined. Escape speed, response to multiple touches witha fine probe, response to water currents generated by a predatorand predation by euphausiids (Thysanoessa inermis) and amphipods(Pleusirus secorrus) were measured in the laboratory. Otolithmeasurements of field-collected larvae support a broad hatchingperiod for walleye pollock eggs in the sea similar to that observedin the laboratory. The escape response of walleye pollock larvaewas affected by rank in the order of hatching, thus with respectto predation, hatching order may affect the survival of larvaein the sea. Early hatching larvae were smaller, less sensitiveto tactile stimulation, had a slower, weaker escape responseand higher laboratory rates of predation mortality than thosethat hatched later.     

The abundance of Anisakis simplex L3 in the body-cavity and flesh of marine teleosts

Smith J. W. 1984. The abundance of Anisakis simplex L3 in the body-cavity and flesh of marine teleosts. International Journal for Parasitology14: 491–495. In experiments conducted at sea, whole (ungutted) fish of three species were stored on ice at 3–5 °C for periods from 0 to 72 h. Some Anisakis simplex L3 migrated from the body-cavity into the flesh in mackerel Scomber scombrus, but not in either blue whiting Micromesistius poutassou or whiting Merlangius merlangus. Earlier work has shown that larvae migrate into the flesh of herring Clupea harengus post mortem but not of walleye pollock Theragra chalcogramma. It seems, therefore, that larvae migrate post mortem into the flesh of ‘fatty’ species (e.g. herring, mackerel) but not of ‘non-fatty’ species (e.g. blue whiting, whiting, walleye pollock). No significant larval excapsulation occurred in isolated mackerel or whiting viscera. In vivo, most encapsulated L3 occur in the body-cavity of euphausiid-feeding fish (herring, mackerel, blue whiting, walleye pollock) but are more widely distributed throughout the tissues of piscivorous fish (whiting, cod). These observations are discussed in relation to the apparent importance of euphausiids as intermediate hosts of A. simplex.     

Reproductive conditions of walleye pollock <Emphasis Type="Italic">Theragra chalcogramma</Emphasis> (Gadidae) off the northeastern coast of Sakhalin Island,Sea of Okhotsk

The spatial distribution of eggs and larvae of the walleye pollock Theragra chalcogramma is considered in respect to dynamics of oceanologic processes, nutrients, chlorophyll а and zooplankton off the northeastern coast of Sakhalin Island in spring 2012. It is shown that the effect of severe temperature regime in the near-bottom horizons in the western Sea of Okhotsk during the spawning period of walleye pollock becomes milder due to specific features of water dynamics. The egg distribution is determined by mesoscale eddies in the region. The species survival depends on the effect of such environmental factors as freshwater discharge from the Amur River, eddy structure in waters of the Sea of Okhotsk, and dynamics of phytoplankton and zooplankton development.     

Predation on fish eggs by white perch,Morone americana,in western Lake Erie

Synopsis The white perch,Morone americana, is an east coast estuarine species that invaded Lake Erie in the 1950's, but did not increase in abundance until the mid 1970's. We studied its distribution and feeding during spawning in the Sandusky River, Ohio in 1981–1983. White perch were present in the area from early April through May, but abundance was highest on bedrock riffles about 45 km upstream from Lake Erie. Spawning activity peaked in the last week of April when temperatures approached 18°C. White perch collected in early April had eaten walleye,Stizostedion vitreum vitreum, eggs. As spawning activity of white perch increased, feeding activity declined, and most fish collected during late April contained no food. Egg predation increased again in May, but the eggs eaten then were those of white bass,Morone chrysops, white perch, and possibly other species. We have no evidence that egg predation by white perch has affected walleye or white bass recruitment, but it could become a problem if white perch continue to increase in abundance.     

Pacific salmons in the nekton community of the upper epipelagic layer of the Northwest Pacific at Kuril Islands in the early summer of 2012

According to the data of trawl catches performed by the R/V “TINRO” in the upper epipelagic zone (0–50 m) of Pacific waters off the Kuril Islands from June 2 to July 8, 2012, the abundance of nekton was the lowest for the recent 9 years; thus, its density in 2012 (1.5 t/km²) was lower than the average value for the 2004–2012 period (2.40 ± 0.39 t/km²). The major portion of the nekton biomass was formed by pacific salmon (706 000 tons, or 43.6%); their role grows abruptly during pre-spawning migrations of pink salmon, whose proportion reached 29.8% (483 000 t). In 2012, the recorded biomass of pre-anadromous pink salmon was as high as those in the previous 3 years, and even higher than the estimate of 2010 (479 800 t), which is the largest value for even-numbered years. The value of 2012 also was comparable with the estimates for the previous odd-numbered year, 2011, when the total biomass of the species constituted 496 500 t. Chum salmon stands out among other salmon for its record-high biomass (192 700 t, 11.9%), which was twice as high as that in 2011. The biomass and the share of the mesopelagic fish complex was the lowest (286800 t, 17.7%) since 2004. Among other fish species, Kuril groups of walleye pollock also had a substantial biomass (381 200 t, 23.5%). The total biomass of squid (225 300 t, 13.9%) was lower than the value of 2011 (326 300 t, 14.8%) owing to the decline in the biomass of Boreopacific gonate squid by 90 000 t.     

Body mass and composition responses to short-term low energy intake are seasonally dependent in Steller sea lions (Eumetopias jubatus)

Steller sea lions (Eumetopias jubatus) were fed restricted iso-caloric amounts of Pacific herring (Clupea pallasi) or walleye pollock (Theragra chalcogramma) for 8–9 days, four times over the course of a year to investigate effects of season and prey composition on sea lion physiology. At these levels, the sea lions lost body mass at a significantly higher rate during winter (1.6 ± 0.14 kg day⁻¹), and at a lower rate during summer (1.2 ± 0.32 kg day⁻¹). Decreases in body fat mass and standard metabolic rates during the trials were similar throughout the seasons and for both diet types. The majority of the body mass that was lost when eating pollock derived from decreases in lipid mass, while a greater proportion of the mass lost when eating herring derived from decreases in lean tissue, except in the summer when the pattern was reversed. Metabolic depression was not observed during all trials despite the constant loss of body mass. Our study supports the hypothesis that restricted energy intake may be more critical to Steller sea lions in the winter months, and that the type of prey consumed (e.g., herring or pollock) may have seasonally specific effects on body mass and composition.     

The influence of light on egg buoyancy and hatching rate of the walleye pollock, Theragra chalcogramma

Changes in buoyancy in fertilized bathypelagic eggs of the walleye pollock, Theragra chalcogramma , collected from Shelikof Strait in the Gulf of Alaska were measured under controlled laboratory conditions in density gradient columns from 90 h post–fertilization through hatching. Eggs were incubated at 6° C and exposed to either diel light or constant dark. Eggs held under diel light conditions became more dense than eggs under constant dark beginning <10 h after exposure to light and remained so until 12 h before hatching. Eggs held under constant dark then became more dense than those under diel light. Hatching of eggs under both conditions began at the same time but eggs under diel light showed a delayed hatching rate. Light–induced changes in egg density indicate the ability of walleye pollock eggs to respond to external stimuli and thereby alter their position in the water column in an ecologically meaningful way.     

Jellyfish as Prey: Frequency of Predation and Selective Foraging of Boops boops (Vertebrata,Actinopterygii) on the Mauve Stinger Pelagia noctiluca (Cnidaria,Scyphozoa)

In recent years, jellyfish blooms have attracted considerable scientific interest for their potential impacts on human activities and ecosystem functioning, with much attention paid to jellyfish as predators and to gelatinous biomass as a carbon sink. Other than qualitative data and observations, few studies have quantified direct predation of fish on jellyfish to clarify whether they may represent a seasonally abundant food source. Here we estimate predation frequency by the commercially valuable Mediterranean bogue, Boops boops on the mauve stinger jellyfish, Pelagia noctiluca, in the Strait of Messina (NE Sicily). A total of 1054 jellyfish were sampled throughout one year to quantify predation by B. boops from bite marks on partially eaten jellyfish and energy density of the jellyfish. Predation by B. boops in summer was almost twice that in winter, and they selectively fed according to medusa gender and body part. Calorimetric analysis and biochemical composition showed that female jellyfish gonads had significantly higher energy content than male gonads due to more lipids and that gonads had six-fold higher energy content than the somatic tissues due to higher lipid and protein concentrations. Energetically, jellyfish gonads represent a highly rewarding food source, largely available to B. boops throughout spring and summer. During the remainder of the year, when gonads were not very evident, fish predation switched towards less-selective foraging on the somatic gelatinous biomass. P. noctiluca, the most abundant jellyfish species in the Mediterranean Sea and a key planktonic predator, may represent not only a nuisance for human leisure activities and a source of mortality for fish eggs and larvae, but also an important resource for fish species of commercial value, such as B. boops.     

The Seasonal Dynamics of the Abundance and Species Composition of Nekton in the Upper Epipelagic Layer of the Western Bering Sea

The western Bering Sea is an important region that is used by many nekton species for feeding. From the seasonal aspect, these waters are characterized by pronounced dynamics of the abundance and structure of the nekton community. The pattern of seasonal variations in the total biomass, composition, and structure of nekton in the upper epipelagic layer (0–50 m) of this region are considered based on the data of the complex studies conducted by the Pacific Research Fisheries Center (TINRO Center) in the deep-sea basins of the western Bering Sea and the Navarin area in June–October, 2003–2015. During June–October, the total nekton biomass changed by more than an order of magnitude: from 100 kg/km² in early June it increased to a maximum of 2700 kg/km² in the middle of August and then declined significantly, to 200 kg/km², in late October. The major contribution to the nekton biomass was made by Pacific salmon (Oncorhynchus spp.), mainly O. keta, as well as by the boreopacific gonate squid (Boreoteuthis borealis) and the shortarm gonate squid (Gonatus kamtschaticus). As well, walleye pollock (Theragra chalcogramma), Pacific herring (Clupea pallasii), and capelin (Mallotus villosus) were abundant in waters near the shelf. The dynamics of the species structure can be divided into three periods: (1) early summer, from June to the second 10 days of July, when pre-anadromous pink (O. gorbuscha) and chum salmon predominate and the species diversity is at a medium level (the polydominance index is 3.5–4.0); (2) summer, from the third 10 days of July to the second 10 days of September, when chum salmon becomes dominant (more than 70% of the biomass) and the species diversity is at a minimum (1.5–2.0); and (3) autumn, from the third 10 days of September to October, when common species such as chum salmon, sockeye salmon, and boreopacific gonate squid have relatively equal proportions, the proportion of pink salmon underyearlings is also high, and the species diversity is at a maximum (4.5). The pattern of the spatial distribution in the early summer period is characterized by active formation of the nekton community due to the large-scale migrations from the central and eastern Bering Sea and from the Pacific Ocean. In the summer period, the concentration of the nekton in the western Bering Sea, particularly in the Aleutian Basin, reaches the maximum level and the migratory activity decreases. Reverse migration processes are observed in the autumn period: a major portion of the nekton biomass redistributes to the southeastern Commander Basin for further movement to the ocean and the central Bering Sea.     

Caddisfly egg mass morphology mediates egg predation: potential costs to individuals and populations

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Potential predation pressure of littoral mysids on herring (Clupea harengus membras L.) eggs and yolk-sac larvae

The predation potential of littoral mysid shrimps (Mysidacea) on Baltic herring (Clupea harengus membras L.) eggs and yolk-sac larvae was studied experimentally. The results showed that littoral mysids feed actively on both eggs and yolk-sac larvae. It was shown that Neomysis integer preys on eggs, which are not attached to the substrate. Alternative food (yolk-sac larvae or zooplankton) did not decrease feeding rate on eggs. Only gravel as a bottom material lowered the ingestion rate to nearly zero. The largest of the mysid species Praunus flexuosus ate yolk-sac larvae more than other mysids and most efficiently. Mysids switched to feed on eggs when larvae and eggs were offered simultaneously, thus predation focused on eggs. It is possible that hydrodynamic signals of moving larvae induced mysids to prey and eggs were easier prey to catch as well as more numerous. In addition egg size is optimal and the nutritive value (measured as C:N ratio) is better compared with larvae. The results indicate that mysids may have local effects on populations of Baltic herring by eating the early life stages, mostly eggs. Especially when large swarms of N. integer shoal in the spawning areas. However, the effect on recruitment of herring is still hard to evaluate.     

Is the mass emergence of Themisto libellula in the northern Bering Sea an invasion or a bloom?

The mass occurrence of the large hyperiid Themisto libellula was recorded in both the western and the eastern Bering Sea within 2007–2011. Those were the years of a relatively long 6-year period of cold, which was caused mainly by the inflow of cold waters from the north; this is confirmed by the distribution of bottom and surface temperatures and also by the ice-cover values. This hyperiid became dominant in the diet of salmon, walleye pollock, herring, and several other nekton fish species. T. libellula periodically spreads southward with cold northern waters, finding favorable conditions in “new” areas. Being a rapidly growing species with a short life cycle, within 1 or 2 years it reaches a high abundance, which then gradually declines and remains at a mean or low level, as usually occurs with species that were introduced into a new habitat. After the environmental conditions deteriorate, as a “warm” period arrives with changes in the general circulation and a growing inflow of warmed Pacific waters, the southern boundary of the species range moves back far northward and it completely disappears in the areas where it prevailed in the plankton and was a main forage item in the diet of many fish species. Taking into account the durations of warm and cold periods from 1980 until 2010, an event like this in the Bering Sea can be expected within 1 or 2 years. In the eastern Bering Sea, the abundance and dominance of a number of zooplankton species may vary simultaneously. This effect is more pronounced in T. libellula and for this reason the species is considered as a biological indicator of the described climatic changes in the Bering Sea.     

Social enhancement of foraging on an ephemeral food source in juvenile walleye pollock,Theragra chalcogramma

Synopsis The foraging effectiveness of walleye pollock juveniles, Theragra chalcogramma, was determined experimentally to test the hypothesis that social cues may facilitate the ability of individuals to exploit ephemeral food patches. Fish were tested when isolated, paired with one other fish, and in a group of six fish. Test fish exploited more food patches while in a group of six than when they were isolated. Patch exploitation by paired fish was intermediate to but not statistically different than isolate or grouped treatments. The number of pellets eaten by test fish in a group and a pair was more than 3.5 times that of when they were isolated, although the overall relationship between the amount of food eaten and group size was not statistically significant. Results support the hypothesis that juvenile walleye pollock exploit ephemeral food patches more effectively in the presence of conspecifics. In planktivores such as walleye pollock, social cues may enhance foraging on transient food sources either by facilitating detection of food patches (local enhancement) or by stimulating foraging activity when a food patch is located (social facilitation).     

Inter-island variation in the diet of female northern fur seals (Callorhinus ursinus) in the Bering Sea

The diet of adult female northern fur seals ( Callorhinus ursinus ) is examined through the analysis of faecal material collected during the summer breeding season at three breeding locations in the Bering Sea: St. Paul Island (1988, 1990) and St. George Island (1988, 1990) of the Pribilof Islands Group (USA), and Medny Island (1990) of the Commander Islands Group (Russia). Prey consumption varies annually and accordingly with the physical and biological environment surrounding each island. Juvenile walleye pollock ( Theragra chalcogramma ) is the most common prey of northern fur seals from St. Paul Island; the island is surrounded by a broad neritic environment with widely separated frontal zones and is the greatest distance from the continental shelf-edge. Gonatid squid ( Gonatopsis borealis/Berryteuthis magister and Gonatus madokail Gonatus middendorffi ) were the most common prey of northern fur seals from Medny Island; the island is surrounded by a compressed neritic environment and is adjacent to the continental shelf-edge and the oceanic marine environment. A combination of walleye pollock and gonatid squid is consumed by northern fur seals from St. George Island; the island has a surrounding oceanographic environment intermediate between the other two islands.
Variability in predation on walleye pollock is consistent with fishery information concerning the relative abundance and availability of walleye pollock around St. George and St. Paul Islands. The abundance and availability of these prey resources during the summer breeding season are key factors which influence the health and growth of the northern fur seal populations in the Bering Sea.     

Oviposition site choice under conflicting risks demonstrates that aquatic predators drive terrestrial egg-laying

Laying eggs out of water was crucial to the transition to land and has evolved repeatedly in multiple animal phyla. However, testing hypotheses about this transition has been difficult because extant species only breed in one environment. The pantless treefrog, Dendropsophus ebraccatus, makes such tests possible because they lay both aquatic and arboreal eggs. Here, we test the oviposition site choices of D. ebraccatus under conflicting risks of arboreal egg desiccation and aquatic egg predation, thereby estimating the relative importance of each selective agent on reproduction. We also measured discrimination between habitats with and without predators and development of naturally laid aquatic and arboreal eggs. Aquatic embryos in nature developed faster than arboreal embryos, implying no cost to aquatic egg laying. In choice tests, D. ebraccatus avoided habitats with fish, showing that they can detect aquatic egg predators. Most importantly, D. ebraccatus laid most eggs in the water when faced with only desiccation risk, but switched to laying eggs arboreally when desiccation risk and aquatic predators were both present. This provides the first experimental evidence to our knowledge that aquatic predation risk influences non-aquatic oviposition and strongly supports the hypothesis that it was a driver of the evolution of terrestrial reproduction.     

Feeding habits the Pacific cod <Emphasis Type="Italic">Gadus macrocephalus</Emphasis> in oceanic waters of the Northern Kuril Islands and Southeast Kamchatka

This study examines the feeding habits of the Pacific cod Gadus macrocephalus in waters off the eastern coast of the northern Kuril Islands and southern Kamchatka. In November–December 1996, the cod primarily consumed fish, which made up 47.6% of the total food mass. The proportion of cephalopods, fishery offal discarded from fishing vessels, and decapods did not exceed 18.5, 17.4, and 12.2%, respectively. Among fishes, the main prey item of the cod was atka mackerel (15.4%); among cephalopods, octopus (16.8%); among fishery offal, heads of atka mackerel (14.2%); and among decapods, majid crabs (6.4%). The rather low percentage of walleye pollock (7.3%) in the cod diet was due to the decline of the east-Kamchatka walleye pollock stock.     

Sex-biased egg cannibalism in spawning walleye pollock: the role of reproductive behavior

We studied inadvertent egg cannibalism in spawning stocks of walleye pollock, Theragra chalcogramma, in the Gulf of Alaska and the Bering Sea between 1986 and 1996. Male pollock had on average 3 times more eggs in their stomachs than females. For both sexes adult fish of average body length had more eggs in their stomach than did smaller or larger fish. When maturity of fish was taken into account, actively spawning males had the highest numbers of eggs in their stomachs. We found weak evidence for a diel variation; the number of eggs per stomach was high in both sexes during the day and lower during the night. We suggest two possible explanations for this phenomenon. Sex-biased egg cannibalism may reflect the differential time spent in layers of high egg densities. Hydroacoustic and trawl catch data from both areas suggest that males aggregate deeper than females. Spawning takes place in the deeper layers of the fish aggregation, so males spend more time in high egg densities. Alternatively, males may be more active than females and increased gill ventilation and/or drinking rates may be responsible for the differences in egg cannibalism.     

Biodiversity Loss following the Introduction of Exotic Competitors: Does Intraguild Predation Explain the Decline of Native Lady Beetles?

Exotic species are widely accepted as a leading cause of biodiversity decline. Lady beetles (Coccinellidae) provide an important model to study how competitor introductions impact native communities since several native coccinellids have experienced declines that coincide with the establishment and spread of exotic coccinellids. This study tested the central hypothesis that intraguild predation by exotic species has caused these declines. Using sentinel egg experiments, we quantified the extent of predation on previously-common (Hippodamia convergens) and common (Coleomegilla maculata) native coccinellid eggs versus exotic coccinellid (Harmonia axyridis) eggs in three habitats: semi-natural grassland, alfalfa, and soybean. Following the experiments quantifying egg predation, we used video surveillance to determine the composition of the predator community attacking the eggs. The extent of predation varied across habitats, and egg species. Native coccinellids often sustained greater egg predation than H. axyridis. We found no evidence that exotic coccinellids consumed coccinellid eggs in the field. Harvestmen and slugs were responsible for the greatest proportion of attacks. This research challenges the widely-accepted hypothesis that intraguild predation by exotic competitors explains the loss of native coccinellids. Although exotic coccinellids may not be a direct competitor, reduced egg predation could indirectly confer a competitive advantage to these species. A lower proportion of H. axyridis eggs removed by predators may have aided its expansion and population increase and could indirectly affect native species via exploitative or apparent competition. These results do not support the intraguild predation hypothesis for native coccinellid decline, but do bring to light the existence of complex interactions between coccinellids and the guild of generalist predators in coccinellid foraging habitats.