Tuna longline fishing around West and Central Pacific seamounts

Background

Seamounts have been identified as aggregating locations for pelagic biodiversity including tuna; however the topography and prevailing oceanography differ between seamounts and not all are important for tuna. Although a relatively common feature in oceanic ecosystems, little information is available that identifies those that are biologically important. Improved knowledge offers opportunities for unique management of these areas, which may advance the sustainable management of oceanic resources. In this study, we evaluate the existence of an association between seamounts and tuna longline fisheries at the ocean basin scale, identify significant seamounts for tuna in the western and central Pacific Ocean, and quantify the seamount contribution to the tuna longline catch.

Methodology/Principal Findings

We use data collected for the Western and Central Pacific Ocean for bigeye, yellowfin, and albacore tuna at the ocean basin scale. GLMs were applied to a coupled dataset of longline fisheries catch and effort, and seamount location information. The analyses show that seamounts may be associated with an annual longline combined catch of 35 thousand tonnes, with higher catch apparent for yellowfin, bigeye, and albacore tuna on 17%, 14%, and 14% of seamounts respectively. In contrast 14%, 18%, and 20% of seamounts had significantly lower catches for yellowfin, bigeye and albacore tuna respectively. Studying catch data in relation to seamount positions presents several challenges such as bias in location of seamounts, or lack of spatial resolution of fisheries data. Whilst we recognize these limitations the criteria used for detecting significant seamounts were conservative and the error in identification is likely to be low albeit unknown.

Conclusions/Significance

Seamounts throughout the study area were found to either enhance or reduce tuna catch. This indicates that management of seamounts is important Pacific-wide, but management approaches must take account of local conditions. Management of tuna and biodiversity resources in the region would benefit from considering such effects.     

Differences in regional oceanography and prey biomass influence the presence of foraging odontocetes at two Atlantic seamounts

The importance of seamounts as foraging hotspots for cetaceans depends on interactions between ocean flow and topographical features that concentrate prey. However, the oceanographic processes driving these aggregations are still unclear. Here, we analyzed two months of passive acoustic recordings from two remote seamounts in the Northeast Atlantic, Atlantis and Irving, in relation to regional oceanography and estimates of prey biomass. Delphinids and sperm whales were detected in both seamounts with higher foraging activity at night, indicating feeding on diel migrating prey. There were more detections of delphinids and sperm whales at Atlantis than at Irving. These two seamounts lie in different oceanographic settings created by the Azores Current that separates colder and less saline water masses in the north (Atlantis seamount) from warmer and more saline waters in the south (Irving seamount). Irving seamount is only affected by transient features like eddies that enhance productivity for short time periods. These conditions translate into more productive waters at Atlantis seamount than at Irving, as shown by predicted prey biomasses that ultimately attract top predators. Comparative studies such as this one can help to explain the main drivers of presence of top predators at seamounts.     

Seamounts: centres of endemism or species richness for ophiuroids?

Aim To test the hypotheses that seamounts exhibit high rates of endemism and/or species richness compared to surrounding areas of the continental slope and oceanic ridges. Location The south‐west Pacific Ocean from 19–57° S to 143–171° E. Methods Presence/absence museum data were compiled for seamount and non‐seamount areas at depths between 100 and 1500 m for the Ophiuroidea (brittle‐stars), an abundant and speciose group of benthic invertebrates. Large‐scale biogeographical gradients were examined through multivariate analyses at two spatial scales, at the scale of seamounts (< 1° of latitude/longitude) and regions (5–9°). The robustness of these patterns to spatially inconsistent sampling effort was tested using Monte Carlo‐style simulations. Levels of local endemism and species richness over numbers of samples were compared for seamount and non‐seamount areas using linear regressions. Non‐seamount populations were randomly generated from areas and depth ranges that reflected the typical sampling profile of seamounts. Results Seamount ophiuroid assemblages did not exhibit elevated levels of species richness or narrow‐range endemism compared with non‐seamount areas. Seamounts can exhibit high overall species richness for low numbers of samples, particularly on seamounts supporting a dense coral matrix, but this does not increase with additional sampling at the rates found in non‐seamount areas. There were relatively few identifiable seamount specialists. In general, seamount faunas reflected those found at similar depths in surrounding areas, including the continental slope. Seamount and non‐seamount faunas within the study area exhibited congruent latitudinal and bathymetric species turnover. Main conclusions Seamount faunas were variable for ophiuroid faunal composition, species richness and narrow‐range endemism, reflecting their environmental diversity and complex history. The continental slope was also variable, with some areas being particularly species rich. Broad geomorphological habitat categories such as ‘seamounts’ or ‘continental slope’ may be at the wrong scale to be useful for conservation planning.     

Comparative characteristics of food supply of Pacific salmon (<Emphasis Type="Italic">Oncorhynchus</Emphasis> spp.) in the Bering Sea from 2002 to 2006

Based on complex epipelagic surveys in the western Bering Sea, a comparative analysis of food supply of Pacific salmon (Oncorhynchus spp.) was conducted in summer and fall from 2002 to 2006. Nine indirect indices of food supply used in the study were as follows: feeding similarity, width of the feeding spectrum, diet feeding ration, diet feeding rhythms, fraction of accessory food in the ration, growth rate of the fish, abundance of food resources, and abundance of salmon. The food supply of salmon is lower in summer 2003 and fall 2006 in comparison to the food supply in other years of the study. However, well expressed feeding selectivity, consumption of prey items of certain type, and small proportion of accessory food (copepods and chaetognaths) prevailed in plankton, suggests the presence of sufficient food resources for Pacific salmon in the western Bering Sea.     

Relationships between Na+, K+-ATPase, sialic acid content and fluidity in rainbow trout density-separated erythrocytes

Smolt of anadromous Arctic charr Sahelinus alpinus (L.) migrating from Storvatn, northern Norway, had a similar feeding intensity as resident Arctic charr from the benthic- and pelagic zone in early summer in fresh water. While smolts fed mostly on surface insects, however, resident Arctic charr had a more broader diet consisting mostly of chironomids, Bosmina sp., benthic prey and surface insects. This indicates that Arctic charr smolts have adopted a surface oriented feeding behaviour prior to their sea migration.     

Feeding habits and gill raker morphology of three planktivorous pelagic fish species off the coast of northern and western Kyushu in summer

Feeding habits and gill raker morphology were examined for the three major planktivorous pelagic fishes, Japanese anchovy Engraulis japonicus , Pacific round herring Etrumeus teres and Japanese jack mackerel Trachurus japonicus , off the northern and western coasts of Kyushu, in the north‐eastern part of the East China Sea in the summer months of 2001. Using fishes in the same size range (80–140 mm, standard length), the stomach contents of the three fish species were compared. The diet of the Japanese anchovy mainly consisted of Oncaeidae copepods, while the diets of the Pacific round herring and Japanese jack mackerel were dominated by calanoid copepods at all stations. Comparisons between prey size in the stomach, zooplankton size in the water and gill raker morphology suggested that the stomach contents of the three species were characterized mainly by the difference in the feeding behaviour between Japanese anchovy (filter‐feeding) and the other two species (particulate‐feeding), rather than by the difference in the morphology of feeding apparatus only. It was concluded that behavioural adaptations in the feeding of these pelagic fishes brought about trophic partitioning to some degree in this pelagic ecosystem in summer. Although the diets of these three species overlapped to some extent, there was still little likelihood of competition between the Japanese anchovy and the other two species. The potential for competition between the Pacific round herring and the Japanese jack mackerel is discussed.     

Seamounts, New Moon and eel Spawning: The Search for the Spawning Site of the Japanese eel

After analyzing all the collection data for larvae of the Japanese eel, Anguilla japonica, in the western North Pacific, we found that the spawning site of this species appears to be near three seamounts in the West Mariana Ridge, 2000–3000km away from their freshwater habitats. These seamounts are located in the westward flow of the North Equatorial Current and are hypothesized to provide cues for migrating silver eels and to serve as possible aggregation sites for spawning. Back-calculated birth dates based on otolith microstructure of leptocephali indicate that the Japanese eel does not spawn continuously during the long spawning season from April to November, but is synchronized to spawn periodically once a month during new moon. This lunar periodicity of spawning and the seamount spawning hypothesis are new developments in the millennium-old mystery of eel spawning that has fascinated naturalists since the time of Aristotle.     

Endemicity,Biogeography, Composition,and Community Structure On a Northeast Pacific Seamount

The deep ocean greater than 1 km covers the majority of the earth''s surface. Interspersed on the abyssal plains and continental slope are an estimated 14000 seamounts, topographic features extending 1000 m off the seafloor. A variety of hypotheses are posited that suggest the ecological, evolutionary, and oceanographic processes on seamounts differ from those governing the surrounding deep sea. The most prominent and oldest of these hypotheses, the seamount endemicity hypothesis (SMEH), states that seamounts possess a set of isolating mechanisms that produce highly endemic faunas. Here, we constructed a faunal inventory for Davidson Seamount, the first bathymetric feature to be characterized as a ‘seamount’, residing 120 km off the central California coast in approximately 3600 m of water (Fig 1). We find little support for the SMEH among megafauna of a Northeast Pacific seamount; instead, finding an assemblage of species that also occurs on adjacent continental margins. A large percentage of these species are also cosmopolitan with ranges extending over much of the Pacific Ocean Basin. Despite the similarity in composition between the seamount and non-seamount communities, we provide preliminary evidence that seamount communities may be structured differently and potentially serve as source of larvae for suboptimal, non-seamount habitats.Open in a separate windowFigure 1Bathymetric map of the Central California Coast with Monterey Canyon and Davidson Seamount.     

Is There a Seamount Effect on Microbial Community Structure and Biomass? The Case Study of Seine and Sedlo Seamounts (Northeast Atlantic)

Seamounts are considered to be "hotspots" of marine life but, their role in oceans primary productivity is still under discussion. We have studied the microbial community structure and biomass of the epipelagic zone (0-150 m) at two northeast Atlantic seamounts (Seine and Sedlo) and compared those with the surrounding ocean. Results from two cruises to Sedlo and three to Seine are presented. Main results show large temporal and spatial microbial community variability on both seamounts. Both Seine and Sedlo heterotrophic community (abundance and biomass) dominate during winter and summer months, representing 75% (Sedlo, July) to 86% (Seine, November) of the total plankton biomass. In Seine, during springtime the contribution to total plankton biomass is similar (47% autotrophic and 53% heterotrophic). Both seamounts present an autotrophic community structure dominated by small cells (nano and picophytoplankton). It is also during spring that a relatively important contribution (26%) of large cells to total autotrophic biomass is found. In some cases, a "seamount effect" is observed on Seine and Sedlo microbial community structure and biomass. In Seine this is only observed during spring through enhancement of large autotrophic cells at the summit and seamount stations. In Sedlo, and despite the observed low biomasses, some clear peaks of picoplankton at the summit or at stations within the seamount area are also observed during summer. Our results suggest that the dominance of heterotrophs is presumably related to the trapping effect of organic matter by seamounts. Nevertheless, the complex circulation around both seamounts with the presence of different sources of mesoscale variability (e.g. presence of meddies, intrusion of African upwelling water) may have contributed to the different patterns of distribution, abundances and also changes observed in the microbial community.     

Feeding Ecology of Northeast Atlantic Mackerel,Norwegian Spring-Spawning Herring and Blue Whiting in the Norwegian Sea

The Norwegian spring-spawning (NSS) herring (Clupea harengus), blue whiting (Micromesistius poutassou) and Northeast Atlantic (NEA) mackerel (Scomber scombrus) are extremely abundant pelagic planktivores that feed in the Norwegian Sea (NS) during spring and summer. This study investigated the feeding ecology and diet composition of these commercially important fish stocks on the basis of biological data, including an extensive set of stomach samples in combination with hydrographical data, zooplankton samples and acoustic abundance data from 12 stock monitoring surveys carried out in 2005–2010. Mackerel were absent during the spring, but had generally high feeding overlap with herring in the summer, with a diet mainly based on calanoid copepods, especially Calanus finmarchicus, as well as a similar diet width. Stomach fullness in herring diminished from spring to summer and feeding incidence was lower than that of mackerel in summer. However, stomach fullness did not differ between the two species, indicating that herring maintain an equally efficient pattern of feeding as mackerel in summer, but on a diet that is less dominated by copepods and is more reliant on larger prey. Blue whiting tended to have a low dietary overlap with mackerel and herring, with larger prey such as euphausiids and amphipods dominating, and stomach fullness and feeding incidence increasing with length. For all the species, feeding incidence increased with decreasing temperature, and for mackerel so did stomach fullness, indicating that feeding activity is highest in areas associated with colder water masses. Significant annual effects on diet composition and feeding-related variables suggested that the three species are able to adapt to different food and environmental conditions. These annual effects are likely to have an important impact on the predation pressure on different plankton groups and the carrying capacity of individual systems, and emphasise the importance of regular monitoring of pelagic fish diets.     

The food supply of the pacific salmon of the genus Oncorhynchus in the northwestern pacific ocean. 1. The dynamics of the food spectra and feeding intensity

Based on the data of 28 surveys that were carried out by the Pacific Fisheries Research Center in the Sea of Okhotsk, Bering Sea, and Pacific waters during 2001–2010, we analyzed the interannual variability of indirect indices of the food supply of the Pacific salmon (Oncorhynchus): the daily food ration, daily consumption rate, diel feeding chronology, diet overlap, trophic niche breadth, number of prey items, and the share of minor food. The years of the most pronounced changes in the diet composition and consumption rate of Pacific salmon were revealed. The variability of different trophic characteristics as indicators of the salmon food supply is discussed. Despite a significant increase in salmon abundance in the 2000s compared to previous years, no marked changes occurred in their feeding spectra and consumption rates.     

Humpback Whale Song on the Southern Ocean Feeding Grounds: Implications for Cultural Transmission

Male humpback whales produce a long, complex, and stereotyped song on low-latitude breeding grounds; they also sing while migrating to and from these locations, and occasionally in high-latitude summer feeding areas. All males in a population sing the current version of the constantly evolving display and, within an ocean basin, populations sing similar songs; however, this sharing can be complex. In the western and central South Pacific region there is repeated cultural transmission of song types from eastern Australia to other populations eastward. Song sharing is hypothesized to occur through several possible mechanisms. Here, we present the first example of feeding ground song from the Southern Ocean Antarctic Area V and compare it to song from the two closest breeding populations. The early 2010 song contained at least four distinct themes; these matched four themes from the eastern Australian 2009 song, and the same four themes from the New Caledonian 2010 song recorded later in the year. This provides evidence for at least one of the hypothesized mechanisms of song transmission between these two populations, singing while on shared summer feeding grounds. In addition, the feeding grounds may provide a point of acoustic contact to allow the rapid horizontal cultural transmission of song within the western and central South Pacific region and the wider Southern Ocean.     

Changes in Nematode Communities in Different Physiographic Sites of the Condor Seamount (North-East Atlantic Ocean) and Adjacent Sediments

Several seamounts are known as ‘oases’ of high abundances and biomass and hotspots of biodiversity in contrast to the surrounding deep-sea environments. Recent studies have indicated that each single seamount can exhibit a high intricate habitat turnover. Information on alpha and beta diversity of single seamount is needed in order to fully understand seamounts contribution to regional and global biodiversity. However, while most of the seamount research has been focused on summits, studies considering the whole seamount structure are still rather poor. In the present study we analysed abundance, biomass and diversity of nematodes collected in distinct physiographic sites and surrounding sediments of the Condor Seamount (Azores, North-East Atlantic Ocean). Our study revealed higher nematode biomass in the seamount bases and values 10 times higher in the Condor sediments than in the far-field site. Although biodiversity indices did not showed significant differences comparing seamount sites and far-field sites, significant differences were observed in term of nematode composition. The Condor summit harboured a completely different nematode community when compared to the other seamount sites, with a high number of exclusive species and important differences in term of nematode trophic diversity. The oceanographic conditions observed around the Condor Seamount and the associated sediment mixing, together with the high quality of food resources available in seamount base could explain the observed patterns. Our results support the hypothesis that seamounts maintain high biodiversity through heightened beta diversity and showed that not only summits but also seamount bases can support rich benthic community in terms of standing stocks and diversity. Furthermore functional diversity of nematodes strongly depends on environmental conditions link to the local setting and seamount structure. This finding should be considered in future studies on seamounts, especially in view of the potential impacts due to current and future anthropogenic threats.     

Diel movements of scalloped hammerhead sharks, Sphyrna lewini Griffith and Smith, to and from a seamount in the Gulf of California

Occurrences of 18 scalloped hammerhead sharks tagged with coded ultrasonic transmitters were recorded at seamount El Bajo Espiritu Santo in the Gulf of California over a 10-day period by automated, data-logging monitors. These sharks remained grouped at the seamount during the day and moved separately into the surrounding pelagic environment at night. The temporal pattern of shark occurrences was related to the light-dark cycle: individuals departed prior to dusk and returned near dawn next morning. However, arrivals and departures of more than one shark within short daytime periods indicated group movements to and from the seamount. Sharks returned repeatedly to the tagging site and not to another site centred less than 240 m from the former site.     

Population Dynamics of the Pink Salmon Oncorhynchus gorbuschain the Basin of the Sea of Okhotsk in the 1990s

In the 1990s, an extensive body of data was gathered on the size of the Oncorhynchus gorbuschapink salmon populations of the Sea of Okhotsk at all the main developmental stages. A significant increase in numbers was found for juvenile pink salmon migrating into the offshore regions of the Sea of Okhotsk and the Pacific waters around the Kurils: from 250–450 million in 1990–1991 to 807–1570 million fish in 1993–1999. The overall number of migrating pink salmon in even years sharply increased in 1994 up to 215 million fish. After 1994, this estimate exceeded the number of migrating pink salmon in preceding odd years. Ocean survival of juvenile pink salmon gradually declined. This may be related to changes in the North Pacific pelagic ecosystems.     

An ecosystem evaluation framework for global seamount conservation and management

In the last twenty years, several global targets for protection of marine biodiversity have been adopted but have failed. The Convention on Biological Diversity (CBD) aims at preserving 10% of all the marine biomes by 2020. For achieving this goal, ecologically or biologically significant areas (EBSA) have to be identified in all biogeographic regions. However, the methodologies for identifying the best suitable areas are still to be agreed. Here, we propose a framework for applying the CBD criteria to locate potential ecologically or biologically significant seamount areas based on the best information currently available. The framework combines the likelihood of a seamount constituting an EBSA and its level of human impact and can be used at global, regional and local scales. This methodology allows the classification of individual seamounts into four major portfolio conservation categories which can help optimize management efforts toward the protection of the most suitable areas. The framework was tested against 1000 dummy seamounts and satisfactorily assigned seamounts to proper EBSA and threats categories. Additionally, the framework was applied to eight case study seamounts that were included in three out of four portfolio categories: areas highly likely to be identified as EBSA with high degree of threat; areas highly likely to be EBSA with low degree of threat; and areas with a low likelihood of being EBSA with high degree of threat. This framework will allow managers to identify seamount EBSAs and to prioritize their policies in terms of protecting undisturbed areas, disturbed areas for recovery of habitats and species, or both based on their management objectives. It also identifies seamount EBSAs and threats considering different ecological groups in both pelagic and benthic communities. Therefore, this framework may represent an important tool to mitigate seamount biodiversity loss and to achieve the 2020 CBD goals.     

Daily feeding rhythm and rations of Pacific salmon of the genus Oncorhynchus in the period of marine prespawning migrations

The daily feeding rhythm and rations of the humpback salmon Oncorhynchus gorbuscha, the sockeyed salmon O. nerka, and the chum salmon O. keta during marine prespawning migrations is investigated with consideration of materials collected at daily stations in waters off eastern Kamchatka in June–July 1999 and 2001 (from catches of drift nets). The bulk of humpback salmon and sockeyed salmon food consists of euphausiids, hyperiids, large copepods, pteropods, and fish juveniles. In the food of chum salmon, pteropods dominated. In a 24-h period, salmon manifest a pronounced evening peak of stomach fullness, while at night feeding discontinues. Samples collected in the morning consisted of fish who had just started feeding after the night pause. In addition to nightly period of rest, all species manifested a daytime decrease in foraging activity, though less pronounced than in the night. The daily rhythm of Pacific salmons’s feeding depends on the vertical migrations of their food items (representatives of sound-scattering layers). During the marine feeding period, the most intensive feeding is recorded in the humpback salmon and chum salmon. The daily ration of the humpback salmon is lower than that of the chum salmon but includes animals of higher food value. Due to a high digestion rate in combination with a large stomach volume, the chum salmon can consume a large quantity of low-calorific food in a short time. The daily ration of the sockeyed salmon is considerably lower than that of other salmon species.     

The trophic ecology of Atlantic cod: insights from tri-monthly, localized scales of sampling

The meso-scale trophic dynamics of cod Gadus morhua were examined based upon tri-monthly stomach sample collections from a nearshore, localized ( c. 800 km²) region off Cape Cod, Massachusetts, U.S.A. The major objective for this work was to relate any changes in cod diet and amount of food eaten to seasonal variations in prey availability, water temperature and spawning at a spatial scale between broad-scale and laboratory studies. Results suggested that the type and amount of food eaten by cod was generally consistent throughout a year and repeatable across years. Cod feeding was marked by two periods of increased feeding, corresponding to the arrival of small pelagic fishes in the area. This pelagic migration and subsequent increased feeding by cod occurred during important periods in the life history of cod ( e.g . spawning and overwintering). Similar annual patterns in food consumption and diet composition were remarkably consistent over the 2·5 years of the project, suggesting important feeding periods for cod that correspond to environmental and biological events. The diet of cod was composed primarily of several species of forage fishes [ e.g. herrings (predominantly Atlantic herring Clupea harengus ), sand lance Ammodytes sp. and Atlantic mackerel Scomber scombrus ], ophiuroids, Cancer sp. crabs and other small crustaceans. It was inferred that cod exhibited a maintenance diet on local forage fishes and benthic macroinvertebrates, augmenting their diet by seasonally gorge feeding upon migrating pelagic species.     

Feeding dynamics,consumption rates and daily ration of wahoo Acanthocybium solandri in Indo‐Pacific waters

This study reports the diet composition of 363 wahoo Acanthocybium solandri captured from the Indo‐Pacific. The study also provides the first estimates of consumption and daily ration for the species worldwide, which are important parameters for ecosystem models and may improve ecosystem‐based fisheries management. Thirty‐four prey taxa were identified from A. solandri stomachs with Scombridae having the highest relative importance. Actinopterygii comprised 96% of the total prey wet mass, of which 29% were epipelagic fishes, with 22% alone from Scombridae. There was no significant relationship between fish size and the size of prey items consumed. Feeding intensity, as measured by stomach fullness, did not significantly differ either among seasons or reproductive activity. The mean daily consumption rate was estimated as 344 g day^?1, which corresponded to a mean daily ration of 2·44% body mass day^?1. The results from this study suggest A. solandri is an opportunistic predator similar to other pelagic piscivores, worldwide.