Midwater Fishes Research Articles

Mesopelagic fishes of the North-West African Upwelling from the Discovery Collections.

Mesopelagic fish specimens from two stations in the NW African Upwelling were identified and catalogued to produce a Darwin Core-aligned dataset. A total of 9655 individual fishes were identified, with 9017 specimens identified at least to genus level and 3124 specimens identified to species level. This dataset comprises specimens collected from the 1990 RRS Discovery (III) Cruise D195 and was used to investigate depth-related trends in diversity and community composition alongside species-specific migratory behaviour. The finalised dataset was published on OBIS through the Deep-Sea node. This dataset contains occurrence and abundance data for midwater fishes caught between the Mauritanian coast and Cape Verde, published for the first time. The dataset records 146 different fish taxa. Twenty-three taxa in the dataset are not present in any prior OBIS datasets that cover the area. These novel taxa are: Bathylagusandriashevi, Bolinichthysindicus, Bolinichthyssupralateralis, Cyclothoneparapallida, Dolichopteroidesbinocularis, Gigantactis indet. Gymnoscopelus stet., Howellaatlantica, Hygophumproximum, Hygophumtaaningi, Ichthyococcuspolli, Lampadenaanomala, Lampanyctuscuprarius, Lampanyctusisaacsi, Lampanyctuslineatus, Lampanyctusmacdonaldi, Lampanyctusnobilis, Lestidiopsmirabilis, Loweinarara, Macroparalepisbrevis, Melamphaesmicrops and Melanonusgracilis. An anglerfish specimen belonging to Linophrynidae was also found, the first in the leftvent family to be logged in the area on OBIS; however, the specimen was too damaged to identify beyond this level.

Microplastic Contamination in Different Marine Species of Bintaro Fish Market, Indonesia

Indonesia is among the largest fish-producing countries. The West Nusa Tenggara (WNT) region is one of the highest producers of fish in Indonesia. Hence, the levels of MP contamination in commercial fish should be assessed to ensure food safety, food security, and socio-economic sustainability. This study investigates MP contamination in commercial fish in one of the largest fish markets in the WNT region, the Bintaro fish market. Three commercial fish species were evaluated for MP contamination in this study, Nasso thynnoides, Auxis rochei, and Caesio teres. The highest number of MPs was detected in A. rochei (21.60 ± 8.70 MPs/100 g). The other pelagic fish species, N. thynnoides, also shown considerably high MP contamination (18.17 ± 7.93 MPs/100 g). On the other hand, the midwater fish Caesio teres showed the least MP contamination (7.07 MPs/100 g). In addition, most of the MPs detected in all three fish species were fiber MPs of small sizes (100–500 µm). Based on FTIR analyses, the MP samples from all three fish species mainly consisted of polyamide (PA), which is the polymer used to form fiber for textiles. These results potentially reveal the degree of microplastic pollution in not only coastal areas of WNT, but also Sulawesi and also East Nusa Tenggara (ENT) since the fish distributed at the Bintaro fish market came from these regions. Better solid waste management in Indonesia is needed to reduce plastic waste management, particularly household waste, which is the potential major source of fiber MPs.

In situ approach to characterize effects of pile-driving on black sea bass behavior, Centroprisits striata

With the threat of climate change and global energy demands rapidly growing, offshore wind farms are quickly becoming a contender in renewable energy strategies in countries around the world, including the United States. In the US, numerous offshore wind turbines are in the planning process for coastal Mid-Atlantic waters, spurring investigation into potential effects of the high-intensity construction noise on local species. Black sea bass, Centropristis striata, is a commercially important mid-water fish that inhabits these same waters and are reported to experience behavioral changes in response to pile-driving, although only demonstrated in the lab. Verification and expansion on these results requires an in situ approach to minimize tank effects on sound propagation and animal behavior. This study, conducted in coastal waters of Woods Hole, MA, used video observation to characterize behavior of 40 caged black sea bass before and during 15-min bouts of actual impact and vibratory hammer pile driving, repeated 5 times per day. Pressure and particle motion were measured. We examined immediate reactions to the onset of noise, prolonged behavioral changes, and habituation. Initial results show increased hiding or sheltering behaviors, as well as bursts of escape-motivated movements in response to the noise. While preliminary, the results suggest that pile driving can induce a range of behavioral impacts on this important fisheries species, and some concern is warranted with respect to offshore wind development.

Modifying a pelagic trawl to better retain small Arctic fishes

Pelagic trawls are one of the primary methods of sampling midwater fishes. However, these trawls are species- and size-selective, and small fish can escape through trawl meshes. This can introduce uncertainty and bias into survey abundance estimates if not accounted for. The small, abundant pelagic fishes of the Alaska Arctic are challenging to sample with trawls as they are sufficiently motile to avoid small fine-mesh trawls but are also small enough to escape through the meshes of trawls designed to capture larger fishes. A pelagic herring trawl equipped with a fine-mesh codend liner was used to quantify the size and species composition of pelagic fishes during a baseline acoustic-trawl survey of the Chukchi Shelf. Subsequent experiments with recapture nets attached to the outside of the trawl netting suggested that escapement of small fishes was substantial, particularly in the aft net section. Thus, the trawl was further modified by reducing the taper in the aft net section and adding a small-mesh section in front of the codend to potentially reduce escapement. Further use of recapture nets during two subsequent acoustic-trawl surveys confirmed that this trawl modification substantially increased retention of small fishes and resulted in less size selectivity. These improvements will reduce biases in estimates of abundance, size, and species composition of pelagic Arctic fishes. This work highlights the importance of quantifying escapement from survey trawls and demonstrates that escapement estimates can guide successful trawl modifications.

Hydroacoustic surveys reveal the distribution of mid-water fish around two artificial reef designs in temperate Australia

Artificial reef designs have evolved rapidly over the past two decades, but further work is needed to inform how specific design features affect reef function. In south-eastern Australia, most artificial reefs consist of either a field of concrete modules, or large steel structures which are deployed to enhance recreational fisheries. To evaluate how these two designs might influence the densities and distribution of fish, hydroacoustic surveys were completed across multiple concrete module and steel reefs located in the temperate inshore environment of New South Wales, Australia. There were no differences in the densities of fish between the two reef types, however there was an interactive effect between reef design and distance-from-structure. Fish associated with steel reefs at greater distances from the structure, with this effect occurring throughout the water column. Steel reefs had higher relief and may therefore provide more structure to support mid-water fish at greater distances up in the water column. Assemblages were dominated by small pelagic species that likely benefit from such high relief structure. In contrast, the concrete module reef fields provide numerous structures for mid-water fish to congregate, which may therefore reduce competition for space and the need to disperse large distances from the reef. At both reef types, fish densities were greatest in the direction of prevailing currents, which may reflect foraging behaviour by small zooplanktivores which were the most abundant type of species observed. Regardless of the area occupied by artificial reefs or the type of material, the inclusion of high vertical relief structures appears an important design feature in establishing mid-water fish assemblages.

Extraoral Taste Buds on the Paired Fins of Damselfishes.

SynopsisSome fish species have taste buds on the surface of their bodies and fins, as well as in the oral cavity. The extraoral taste system of fish has traditionally been studied in species that inhabit environments and/or employ feeding strategies where vision is limited. Here we examined taste sensation in a new ecological context by investigating the paired fins of damselfish (Pomacentridae), a group of diurnal midwater fishes that inhabit the light-rich waters of coral reefs. Immunohistochemistry demonstrated the presence of taste buds on the paired fins of Chromis viridis, including on the distal tips of elongate leading-edge pelvic fin rays, where they are particularly densely packed, suggesting specialization for chemosensation. Similar anatomical results were also recorded from two other species, Pomacentrus amboinensis and Pomacentrus coelestis. We found that afferent pectoral fin nerves of C. viridis responded to a food-derived stimulus. By investigating the extraoral taste system in a new phylogenetic and ecological context, these results show that taste buds on fins are more widespread amongst fish than previously known and are present even in highly visual environments.

Chinook salmon prey upon large midwater fish in offshore Pacific Ocean

Chinook salmon prey upon large midwater fish in offshore Pacific Ocean

Uncrewed surface vehicle survey of walleye pollock, Gadus chalcogrammus, in response to the cancellation of ship-based surveys

In 2020, the developing COVID-19 pandemic disrupted fisheries surveys to an unprecedented extent. Many surveys were cancelled, including those for walleye pollock (Gadus chalcogrammus) in the eastern Bering Sea (EBS), the largest fishery in the United States. To partially mitigate the loss of survey information, we deployed three uncrewed surface vehicles (USVs) equipped with echosounders to extend the ship-based acoustic-trawl time series of pollock abundance. Trawling was not possible from USVs, so an empirical relationship between pollock backscatter and biomass established from previous surveys was developed to convert USV backscatter observations to pollock abundance. The EBS is well-suited for this approach since pollock dominate midwater fishes in the survey area. Acoustic data from the USVs were combined with historical surveys to provide a consistent fishery-independent index in 2020. This application demonstrates the unique capabilities of USVs and how they could be rapidly deployed to collect information on pollock abundance and distribution when a ship-based survey was not feasible. We note the limitations of this approach (e.g., higher uncertainty relative to previous ship-based surveys), but found the USV survey to be useful in informing the stock assessment in a situation where ship-based surveys were not possible.

Uncrewed surface vehicle (USV) survey of walleye pollock, Gadus chalcogrammus, in response to the cancellation of ship-based surveys

In 2020, the developing COVID-19 pandemic disrupted fisheries surveys to an unprecedented extent. Many surveys were cancelled, including those for walleye pollock (Gadus chalcogrammus) in the eastern Bering Sea (EBS), the largest fishery in the United States. To partially mitigate the loss of survey information, we deployed three uncrewed surface vehicles (USVs) equipped with echosounders to extend the ship-based acoustic-trawl time series of pollock abundance. Trawling was not possible from USVs, so an empirical relationship between pollock backscatter and biomass established from previous surveys was developed to convert USV backscatter observations into pollock abundance. The EBS is well suited for this approach since pollock dominate midwater fishes in the survey area. Acoustic data from the USVs were combined with historical surveys to provide a consistent fishery-independent index in 2020. This application demonstrates the unique capabilities of USVs and how they could be rapidly deployed to collect information on pollock abundance and distribution when a ship-based survey was not feasible. We note the limitations of this approach (e.g. higher uncertainty relative to previous ship-based surveys), but found the USV survey to be useful in informing the stock assessment in a situation where ship-based surveys were not possible.

Twilight Zone Observation Network: A Distributed Observation Network for Sustained, Real-Time Interrogation of the Ocean's Twilight Zone

Abstract The ocean's twilight zone (TZ) is a vast, globe-spanning region of the ocean. Home to myriad fishes and invertebrates, mid-water fishes alone may constitute 10 times more biomass than all current ocean wild-caught fisheries combined. Life in the TZ supports ocean food webs and plays a critical role in carbon capture and sequestration. Yet the ecological roles that mesopelagic animals play in the ocean remain enigmatic. This knowledge gap has stymied efforts to determine the effects that extraction of mesopelagic biomass by industrial fisheries, or alterations due to climate shifts, may have on ecosystem services provided by the open ocean. We propose to develop a scalable, distributed observation network to provide sustained interrogation of the TZ in the northwest Atlantic. The network will leverage a “tool-chest” of emerging and enabling technologies including autonomous, unmanned surface and underwater vehicles and swarms of low-cost “smart” floats. Connectivity among in-water assets will allow rapid assimilation of data streams to inform adaptive sampling efforts. The TZ observation network will demonstrate a bold new step towards the goal of continuously observing vast regions of the deep ocean, significantly improving TZ biomass estimates and understanding of the TZ's role in supporting ocean food webs and sequestering carbon.

Analysis of organic carbon sources in tropical seagrass fish: a case study of the east coast of Hainan Province

ABSTRACTWe analysed the carbon stable isotope (δ13C) of various organic carbon sources, including particulate organic matter (POM), sediment organic matter (SOM), seagrass and its epiphytes, and compared these results with that of 36 fishes, including 27 types of demersal fishes, 6 types of midwater fishes, and 3 types of surface fishes. Our results indicated that there were significant differences in the characteristics of carbon stable isotopes of different organic carbon sources, with δ13C values ranging from −23.35 (POM) to −9.56‰ (seagrass). The range of the δ13C value of the fishes was broad, from –21.84‰ (Dendrophysa russelii) to –10.57‰ (Dasyatis akajei), indicating that fishes had a variety of food sources. For 30 of the 36 fish species, including Plotosus lineatus, Gerres erythrourus, Hypodytes rubripinnis, etc, seagrass and its epiphytes were the main contributors. For 23 of the 36 fish species, including Upeneus tragula, Dasyatis akajei, Cheilodipterus quinquelineatus, their recorded body lengths were less than respective lengths of maturity. These findings indicate that seagrass and its epiphytes are important organic carbon sources for most fishes and play a key role in the energy flow of food web in seagrass ecosystems in this study.

Diversification Patterns of Lanternfishes Reveal Multiple Rate Shifts in a Critical Mesopelagic Clade Targeted for Human Exploitation

The mesopelagic (midwater) and deep-sea environments together comprise over 90% of the volume of the world ocean [1] and provide services that are only recently becoming recognized [2]. One of the most significant of these services relates to midwater fish biomass, recently estimated to be two orders of magnitude larger than the current worldwide fisheries catch [3, 4]. Calls to exploit midwater fish biomass have increased despite warnings about the unknown recovery potential of such organisms [2] and despite existing data suggesting that deep-sea fishes could be classified as endangered [5]. Here, to provide a null model for the respondability of midwater fishes, I use lanternfishes-which comprise themajority of worldwide midwater fish biomass [6]-to examine the diversification response of a critical midwater clade to oceanic changes over evolutionary timescales, including several extinction and turnover events. Using a time-calibrated molecular phylogeny based on seven autosomal protein-coding loci, with over 50% species sampling and three ingroup node calibrations, I show that lanternfishes exhibit a continuously increasing diversification rate, consistent with nonequilibrium speciation dynamics, and three major evolutionary rate shift locations with timing that is similar to those of marine clades in more well-known environments. These results suggest that lanternfish diversification patterns overlapped with major events in the physical partitioning ofthe ocean volume and that the clade has responded positively to a range of pre-Anthropocene extinction drivers [7]. However, lanternfish respondability to modern extinction drivers-habitat loss and overexploitation-is best addressed with populational and ecological data and remains largely unknown.

Return of warm conditions in the southeastern Bering Sea: Phytoplankton - Fish.

In 2014, the Bering Sea shifted back to warmer ocean temperatures (+2 oC above average), bringing concern for the potential for a new warm stanza and broad biological and ecological cascading effects. In 2015 and 2016 dedicated surveys were executed to study the progression of ocean heating and ecosystem response. We describe ecosystem response to multiple, consecutive years of ocean warming and offer perspective on the broader impacts. Ecosystem changes observed include reduced spring phytoplankton biomass over the southeast Bering Sea shelf relative to the north, lower abundances of large-bodied crustacean zooplankton taxa, and degraded feeding and body condition of age-0 walleye pollock. This suggests poor ecosystem conditions for young pollock production and the risk of significant decline in the number of pollock available to the pollock fishery in 2–3 years. However, we also noted that high quality prey, large copepods and euphausiids, and lower temperatures in the north may have provided a refuge from poor conditions over the southern shelf, potentially buffering the impact of a sequential-year warm stanza on the Bering Sea pollock population. We offer the hypothesis that juvenile (age-0, age-1) pollock may buffer deleterious warm stanza effects by either utilizing high productivity waters associated with the strong, northerly Cold Pool, as a refuge from the warm, low production areas of the southern shelf, or by exploiting alternative prey over the southern shelf. We show that in 2015, the ocean waters influenced by spring sea ice (the Cold Pool) supported robust phytoplankton biomass (spring) comprised of centric diatom chains, a crustacean copepod community comprised of large-bodied taxa (spring, summer), and a large aggregation of midwater fishes, potentially young pollock. In this manner, the Cold Pool may have acted as a trophic refuge in that year. The few age-0 pollock occurring over the southeast shelf consumed high numbers of euphausiids which may have provided a high quality alternate prey. In 2016 a retracted Cold Pool precluded significant refuging in the north, though pollock foraging on available euphausiids over the southern shelf may have mitigated the effect of warm waters and reduced large availability of large copepods. This work presents the hypothesis that, in the short term, juvenile pollock can mitigate the drastic impacts of sustained warming. This short-term buffering, combined with recent observations (2017) of renewed sea ice presence over southeast Bering Sea shelf and a potential return to average or at least cooler ecosystem conditions, suggests that recent warm year stanza (2014–2016) effects to the pollock population and fishery may be mitigated.

Potential for broadband acoustics to improve stock assessment surveys of midwater fishes

Acoustic-trawl surveys rely on a combination of backscatter measured with echosounders and species composition data from trawls to apportion the backscatter to different species and size classes. Narrowband echosounders have been widely used in this context for decades. Multi-frequency analysis of narrowband echosounder data has been shown to be effective for discriminating between diverse taxa (e.g., euphausiids vs. swimbladdered fishes) but distinguishing morphologically similar species (e.g., swimbladdered fishes) remains a major challenge. Previous work indicates that broadband backscatter techniques have the potential to improve such acoustic target characterizations by exploiting nearly continuous frequency spectra, but these methods have not been widely applied in fisheries surveys. The recent commercial availability of broadband transceivers is accelerating the evaluation of this technology. We present operational data from two broadband acoustic scattering systems: (1) 14-160 kHz and (2) 3-10 kHz...

Species Distribution Modeling of Deep Pelagic Eels.

The ocean's midwaters (the mesopelagic and bathypelagic zones) make up the largest living space on the planet, but are undersampled and relatively poorly understood. The true distribution of many midwater species, let alone the abiotic factors most important in determining that distribution, is not well known. Because collecting specimens and data from the deep ocean is expensive and logistically difficult, it would be useful to be able to predict where species of interest are likely to occur so that sampling effort can be concentrated in appropriate areas. The distribution of two representative midwater fishes, the gulper eel Eurypharynx pelecanoides and the bobtail eel Cyema atrum (Teleostei: Saccopharyngiformes), were modeled with MaxEnt software to examine the viability of species distribution modeling (SDM) for globally distributed midwater fishes using currently available environmental data from the ocean surface and bottom. These species were chosen because they are relatively abundant, easily recognized, and unlikely to have been misidentified in database records, and are true midwater fishes, not known to undertake significant vertical diurnal migration. Models for both species show a generally worldwide distribution with some exceptions, including the Southern Ocean and Bering Sea. Variable contributions show that surface and bottom environmental variables correlate with species presence. Both species are more likely to be found in areas with low levels of silicate. SDM is a promising method for better understanding the ecology of midwater organisms.

Trimethylamine oxide accumulation as a function of depth in Hawaiian mid-water fishes

Trimethylamine oxide (TMAO) is a common osmolyte and counteracting solute. It is believed to combat the denaturation induced by hydrostatic pressure as some deep-sea animals contain higher TMAO levels than their shallow water counterparts. It has also been proposed that TMAO may accumulate passively during lipid storage resulting in a correlation between lipid content and TMAO levels in some groups. Previous research showed that lipid content decreased with depth in species of Hawaiian fishes presenting a novel test of these competing hypotheses. TMAO ranged from 20.4 to 92.8mmol/kg. Lipid content ranged from 0.50 to 4.7% WW. After completing a comprehensive search for depths available in the literature, provided here, we analyzed TMAO and lipid as a function of average, minimum and maximum depth of occurrence for 27 species of fishes from nine orders. We found that TMAO is positively correlated with all measures of habitat depth (hydrostatic pressure) but the relationship is strongest with average depth. We further showed using phylogenetic independent contrasts that this relationship was not influenced by the evolutionary relatedness of these species. Interestingly, we found that lipid content increased with depth, in direct contrast to previous studies. TMAO is thus also positively correlated with lipid content. While we are unable to distinguish between these hypotheses, we show that TMAO is strongly correlated with depth in mid-water fishes.

Abundance and distribution of Arctic cod (Boreogadus saida) and other pelagic fishes over the U.S. Continental Shelf of the Northern Bering and Chukchi Seas

We conducted acoustic-trawl (AT) surveys of the northern Bering and Chukchi Seas during ice-free periods in 2012 and 2013. The mixed species assemblages in the study area required refinement of standard AT survey methods, and adjustment of trawl catches for the effects of trawl selectivity. Sensitivity analyses indicate that the AT abundance estimates are relatively robust to the assumptions of the analysis. These surveys indicate that midwater fishes are dominated by age-0 Arctic cod (Boreogadus saida), age-0 saffron cod (Eleginus gracilis), capelin (Mallotus villosus), and Pacific herring (Clupea pallasii). In both years, age-0 Arctic cod were distributed principally ≥69.5°N, age-0 saffron cod were abundant in coastal areas between 66.5 and 69.5°N, and Pacific herring were distributed south of 67°N. These three fishes exhibited consistent associations with temperature, salinity and bottom depth: e.g., age-0 Arctic cod were abundant at lower mean water column temperatures than saffron cod. In contrast, capelin were distributed throughout the study area, and were not consistently associated with environmental measures. There was a geographic trend in body length, with smaller Arctic cod, saffron cod and capelin in northern areas, but smaller herring in the south. Arctic cod, saffron cod, herring and capelin were all >2 times more abundant in 2013 than 2012. Sizeable populations of age-0 Arctic cod were observed in the northern Chukchi Sea, which suggests that this area is an important nursery ground. However, relatively few older Arctic cod were observed in this and other surveys of the area, which suggests that either overwinter mortality of age-0 Arctic cod is high, and/or these fish are not retained on the Chukchi shelf.

Drifting baited stereo‐videography: a novel sampling tool for surveying pelagic wildlife in offshore marine reserves

We present a novel system of drifting pelagic baited stereo‐video cameras that operate in deep‐water, topographically complex environments typically considered inaccessible for sampling. The instruments are portable, semi‐autonomous and inexpensive, allowing the recording of high‐definition video footage in near‐real time and over broad stretches of ocean space. We illustrate their benefits and potential as non‐extractive monitoring tools for offshore marine reserves with a pilot study conducted within the newly established Perth Canyon Commonwealth Marine Reserve, southwestern Australia (32° S, 115° E). Using occupancy and maximum entropy models, we predict the distribution of midwater fishes and sharks and show that their most suitable habitat encompasses a wider fraction of the canyon head than is covered by park boundaries. Our proof‐of‐concept study demonstrates that drifting pelagic stereo‐video cameras can serve as appropriate field platforms for the construction of species distribution models with implications for ocean zoning and conservation planning efforts.

Diet and feeding strategies of mesopelagic fishes in the western Mediterranean

Myctophids, gonostomatids and sternoptychids are the most abundant teleosteans worldwide and constitute an important assemblage of the mesopelagic ecosystem, functioning as vehicles of energy and matter through trophic webs. This study concentrates on the trophic ecology of the most abundant mesopelagic fishes of the western Mediterranean (WM) based on stomach content analysis. The myctophids (in this study: Benthosema glaciale, Ceratoscopelus maderensis, Lobianchia dofleini, Myctophum punctatum, Hygophum benoiti, Hygophum hygomii, Lampanyctus crocodilus, Lampanyctus pusillus and Notoscopelus elongatus) perform extensive diel migrations across the water column, between the surface to as deep as 1000m, interacting with plankton and micronekton at multiple depths, and generally feeding in the epipelagic layers at night. In contrast, the gonostomatids Cyclothone braueri, Cyclothone pygmaea, and the sternoptychid Argyropelecus hemigymnus remain below epipelagic layers, feeding at different times throughout the day and night. The diet composition, trophic niche breadth and prey selectivity of 11 of these fish species were determined for juvenile and adult individuals from two surveys performed in December 2009 and July 2010 in the western Mediterranean Sea. The number of prey items varied among species, e.g. Myctophum punctatum was the species with the highest feeding intensity, reaching ca. 700 prey items in a stomach, whereas the mean number of prey in Cyclothone braueri was low (usually 1 or 2 prey per stomach). A dietary shift towards larger prey was evident from juveniles to the largest and oldest adult individuals, despite trophic niche breadths did not increase with body length for any of these mesopelagic species. The diets of the small gonostomatids, sternoptychid and early juveniles of myctophids were dominated by non-calanoid copepods, ostracods, and other small zooplankton, whereas medium-sized myctophids, e.g. L. dofleini or H. benoiti, preyed mainly on calanoids. The oldest stages of L. crocodilus and N. elongatus fed mostly on macrozooplankton and micronekton. There was high diet overlap among mesopelagic fish species and strong heterogeneity in diet composition at intraspecific level. Nevertheless, some species showed certain degree of segregation of food resources, determined by the developmental stage or spatial distribution, and positive selection towards particular prey items. The Chesson’s electivity index showed that L. dofleini, N. elongatus, L. crocodilus and L. pusillus preyed selectively on euphausiids; B. glaciale was selective on the calanoid genus Pleuromamma, and C. maderensis preferred to feed on larvaceans in autumn. The two congeneric species of Hygophum consumed a high number of food items, but H. hygomii showed positive selection for euphausiids, whilst H. benoiti preferred small corycaeid copepods. Overall, the main trophic difference among mesopelagic fishes in the WM was observed between the small non-migratory species that do not evidence a diel rhythm, feeding during both day- and night-time on small zooplankton, and the largest-sized myctophids, which fed on meso- and macrozooplankton and, more occasionally, on small fishes. Mediterranean midwater fishes can be characterised by the adoption of mixed feeding strategies, with varying degrees of specialisation on different prey types that allow flexibility in a changeable environment.

Temporal variation in environmental conditions and the structure of fish assemblages around an offshore oil platform in the North Sea

This study reports temporal variations in the environmental conditions and the structure of fish assemblages observed in the vicinity of an offshore oil platform and the surrounding seafloor in the North Sea. Multi-seasonal sampling was conducted at a typical large steel jacketed facility, using mid-water fish traps at three different depths (i.e., 10, 50 & 100 m). Commercially important gadoids such as saithe Pollachius virens, haddock Melanogrammus aeglefinus and cod Gadus morhua were the most abundant species, however, the species composition and the relative abundances of the species varied with depth, season and between years. Comparisons with a large-scale bottom trawl survey data suggested highly dynamic and species-specific interactions between fish movements, changing environmental conditions and the physical presence of an offshore platform. Given the number of platforms currently installed across the North Sea, there is a need to identify biological mechanisms behind such dynamic interactions.