All prey items were identified to the lowest possible taxon. Multiple fragments of individual organisms were counted as single individuals, unless the number could be more accurately estimated from the fragments by counting structures such as eyes or appendages. Colonial organisms e. Cephalopod beaks were sorted into Octopodiformes and Decapodiformes. When only otoliths were found in a gut, the species was identified if voucher otoliths were available for comparison.
Voucher specimens in the Grice Marine Laboratory collections were consulted to confirm identifications of fish and invertebrates. Wreckfish collections before rarely had accurate stomach content data.
In many cases only prey items of particular interest were saved. Wreckfish prey items that were regurgitated on deck and kept in jars without pertinent collection data were not used in the detailed diet analyses, but were included in overall prey lists see Goldman, To quantify feeding habits, the relative contribution of food items to the total diet of each predator was determined using three methods: percentage frequency of occurrence F , percentage composition by number N , and percentage composition by weight W.
In addition, an index of relative importance IRI; modified from Pinkas et al. These measures were also calculated for stomachs grouped by standard length SL size classes and by season. To examine differences among predator lengths SL or seasons, only those samples with no interactions between those two factors were compared. Stomach fullness was not estimated to illustrate the seasonal changes in feeding activity because of the frequent regurgitation or stomach eversion of these large piscivorous predators.
Hydrographic seasons were used to derive seasonal breakdowns: winter consisted of January through March, spring was April through June, summer was July through September, and autumn was October through December. Samples were broken down by season because there are known ecological shifts, such as squid migrations Vecchione, ; Carpenter, , that could lead to changes in diet.
Stomach contents from fish representing three species Table 1 were collected. Combining all samples, approximately 46 prey items were identified. A total of 33 prey taxa from wreckfish stomachs was identified Table 2.
Teleosts were by far the most important food item, and Beryx splendens was the most common teleost prey. Wreckfish consumed midwater fish in low frequencies and numbers. The most common cephalopod consumed was Illex illecebrosus , the northern shortfin squid.
Wreckfish also preyed on Octopus sp. Sharks were the third most important prey category. Many species of benthic and pelagic crustaceans were consumed by wreckfish, but these taxa had low percentage IRIs. Cephalopods were consumed in all seasons, but were most frequent in autumn. Decapods were consumed more frequently in winter, but different decapods were consumed in each season: in autumn and spring samples decapod prey consisted mostly of benthic species, while pelagic shrimps were consumed more often in winter and summer.
Sharks were eaten at all times of the year. Teleosts were eaten in lower numbers and frequencies in autumn, while the highest numbers of teleosts were consumed in spring. Chi-squared analysis did not reveal significant differences in diet among size classes for any prey item. The analysis of dietary shifts by size showed cephalopods and teleosts present in all size classes, with similar frequency values.
Octopus sp. Few species of benthic decapod were present in the smallest size class. Sharks were not present in the smallest size class. Stomach contents included fish remains, northern shortfin squid, unidentified squid, a crab claw, and a sea star. A total of 22 prey taxa was identified Table 3.
The teleost prey were almost exclusively midwater fish. Cephalopods consisted mostly of northern shortfin squid, but barrelfish also consumed Octopus sp. Crustacean prey included pelagic shrimps, crabs, and phronimid amphipods. The largest fish were caught in autumn, and once this season was removed, no interaction was detected.
Therefore, autumn was removed from the seasonal analysis so as to examine the effect of season alone. As in the seasonal analysis, autumn was removed from the length analysis to examine the effect of length alone. Chi-squared analysis revealed no significant differences in prey frequency between size classes. Barrelfish fed on cephalopods at all sizes, with frequencies increasing with fish length. Decapods were absent from the smallest size class.
Phronima sp. Pyrosoma atlanticum and teleosts were present in all size classes. Most red bream gut contents were in an advanced state of digestion, making it impossible to identify prey further than higher taxonomic groupings.
Crustaceans included benthic and pelagic shrimps, galatheids, and isopods. Although teleosts were the most common prey item, Beryx splendens was the only identifiable species. Several guts contained Pyrosoma atlanticum. Percentage IRI for all prey categories varied greatly by season. Pyrosoma atlanticum and splendid alfonsino were present only in the stomachs of autumn samples. Chi-squared analysis showed significant differences in frequency for two prey categories: teleosts and decapods.
Chi-squared analysis did not indicate significant differences in prey frequencies among size classes. The length analysis showed that red bream consumed teleosts at all sizes.
Only the largest fish preyed on splendid alfonsino. Fish in the smallest size class consumed decapods more frequently than fish in the larger size classes. The isopod Aega antillensis was only seen in the two largest size classes. Although wreckfish gut contents have been examined in other studies Deudero and Morales-Nin, ; Weaver and Sedberry, ; Peres and Haimovici, , a detailed quantitative analysis has not been performed.
Likewise, little insight has been given into seasonal changes and onotogenetic shifts. Weaver and Sedberry found squid to be the most frequent and numerically dominant prey of wreckfish, followed by teleost fish. Conversely, the present study found fish to be the most frequent and numerically dominant prey, followed by squid.
Peres and Haimovici reported that adult wreckfish preyed on crabs, squid, and fish. In a study of wreckfish associating with floating objects in the Mediterranean, juvenile fish and isopods were the main diet components Deudero and Morales-Nin, Because wreckfish are generalized predators, it is probable that much of the observed variation between studies is caused by random variation and differences in prey abundance. In the present study, wreckfish consumed prey items both at the bottom and in the water column, and appeared to be opportunistic feeders.
Wreckfish showed the greatest diversity of prey items, but this is probably in part a result of the high sample size compared with the other predators. One rare cephalopod Galiteuthis armata , the armed cranch squid was regurgitated by a wreckfish on the deck of a wreckfish boat. Deep-sea squid are believed to be mostly planktonic micronektonic predators Boyle and Rodhouse, The exception was the sea star, which may have been forced into the mouth of the fish during the trawl.
The abundance of squid in the diet of wreckfish in autumn is probably the result of the migration patterns of northern shortfin squid, which is the most common and abundant species on the continental slope Vecchione, Northern shortfin squid inhabit inshore waters in summer and move to deeper offshore waters of the slope in autumn and winter.
This probably results in opportunistic prey-switching by wreckfish as squid become abundant in autumn. Barrelfish fed primarily in the water column on mesopelagic organisms; however, the presence of Octopus sp. There have been few studies on Hyperoglyphe sp. Pyrosoma atlanticum was the primary prey of barrelfish in the present study.
Pyrosoma atlanticum form dense colonies Stocker, and is probably important in many foodwebs around the world. The importance of P. In eastern Australia, Hyperoglyphe antarctica , the blue-eye trevella, was found to prey primarily on P. Cowper reported trevella around Tasmania to consume primarily P. Barrelfish may feed heavily on P. In addition, there may be evolutionary reasons that barrelfish eat these jelly-like animals.
Barrelfish are in the family Centrolophidae medusafish. Juvenile and young adult fish in this family often associate with pelagic medusae or floating objects and feed on jellyfish Haedrich, On both natural hard bottom and oil platforms in the Gulf of Mexico, barrelfish have been sighted in large schools within a few metres of the bottom.
They are also seen up to m above the seabed on petroleum platforms, and actively forage in the open water next to the structures D. Weaver, pers. Based on their dietary patterns, barrelfish appear to migrate vertically to forage in the water column. The diet of red bream has been studied in the Canary Islands, where it is a secondary target species in a local fishery for splendid alfonsino Durr and Gonzalez, Consistent with the present study, red bream prey on fish, crustaceans, and cephalopods; few prey species could be identified because of the advanced stage of digestion Durr and Gonzalez, Red bream seem to feed mostly in the water column on mesopelagic fish, pelagic shrimp, and squid.
The presence of benthic shrimp and Munida sp. Red bream consumed Aega antillensis , an isopod of the family Aegidae, which is a parasite that has been known to show host preferences in fish Ross et al. For all predators, the changes in seasonal prey frequency are probably caused by opportunistic feeding behaviour, seasonal abundance or availability of prey, and the migration patterns of northern shortfin squid.
Many of the predators are probably opportunistic feeders, particularly in an environment that may have limited resources. Any seasonal changes in diet are probably caused by the changes in prey availability and by predators eating what is obtainable. For all species, seasonal differences could easily be a result of random variation or local habitat differences in prey availability.
Significant differences among length categories were not seen in the present study. This is probably because of gear selectivity and the limited size range of predatory fish at the Charleston Bump. We relied predominantly on commercial fishers to collect samples, and the gear used in the wreckfish fishery is selective for certain species and sizes. Perhaps smaller fish are present at the Charleston Bump, but the gear does not select for these individuals.
Both large fish wreckfish and barrelfish and smaller fish species splendid alfonsino, small sharks , however, are taken as bycatch. This may indicate that there is a limited size range of fish found at the Charleston Bump. Only larger wreckfish and barrelfish have been seen at the Charleston Bump Sedberry et al. A recent age-growth study on barrelfish collected around the Charleston Bump found the youngest specimen was 5 years old, and the smallest specimen was mm FL, indicating that barrelfish may not recruit into the fishery around the Charleston Bump until around the age of 5 Filer and Sedberry, Competition for identical resources is only likely if resources are in short supply Pianka, Overall, wreckfish and red bream have similar feeding habits, which in a food-poor environment would lead to competition.
Barrelfish was the only predator that exhibited selective feeding on pelagic tunicates, and there would be little to no competition between barrelfish and other slope predators for this resource. However, as major currents collide with the Charleston Bump, productivity is enhanced, which may lower competition.
The present study does not provide evidence of food supply scarcity. Few studies have examined stomach contents of juvenile wreckfish or examined ontogenetic shifts in diet.
Peres and Haimovici found juvenile wreckfish to consume mostly fish. Except the echinoderms, these smaller wreckfish consumed similar prey to their larger Charleston Bump counterparts. On the continental slope, daytime depths for vertically migrating organisms coincide with the bottom depth of the slope Weaver and Sedberry, Therefore, these animals become an important food source for demersal fish Sedberry and Musick, Vertically migrating fish, squid, and crustaceans provide an active transport mechanism of energy to the benthic predators on the Charleston Bump.
Of the approximately 37 prey species identified, 12 are known to perform daily vertical migrations. The cephalopods Enoploteuthis sp. Northern shortfin squid is one of the most important prey items for predatory fish on the Charleston Bump.
It is available to both pelagic and benthic predators Weaver and Sedberry, Several crustaceans found as prey Oplophoridae, Sergia sp. Micronektonic crustaceans, including the species listed above, are a major component of pelagic ecosystems, and play an important role in trophic dynamics at intermediate levels in the foodweb Hopkins et al.
Beryx splendens , Gonostoma elongatum , gempylids, Promethichthys prometheus , and Stomias sp. Pyrosoma atlanticum , the primary food item for barrelfish, makes extensive diel vertical migrations Anderson et al. One of the predators in this study—red bream—is known to make vertical migrations following vertically migrating prey Gomes et al. Based on the data in the current study, we suspect that barrelfish too are vertical migrators.
Weaver and Sedberry also found the foodweb model of the middle slope hard-bottom community to be based primarily on pelagic pathways driven by the migratory behaviour of midwater fish and squid. Similarly, communities on the continental slope off the Mid-Atlantic coast were dependent on vertically migrating food chains Sedberry and Musick, The importance of a trophic connection between deep-sea demersal fish and pelagic foodwebs was also pointed out by Mauchline and Gordon The wreckfish fishery impacts not only the target species, wreckfish, but many other Charleston Bump residents.
These residents include bycatch fish, as well as other organisms in the ecosystem. Removing predators from any ecosystem will change the prey populations, including pelagic and benthic shrimps, fish, cephalopods, and other crustaceans.
Other linkages at the Charleston Bump include those to highly migratory species. Swordfish Xiphias gladius are abundant on the Charleston Bump, and there is an active longline fishery that also catches non-targeted species, such as marlin Makaira sp.
These billfish live in midwater or at the surface, where they probably have some prey overlap with benthic predators. As they are highly migratory, energy is potentially transferred to areas away from the Charleston Bump.
In addition, the cyclonic Charleston Gyre creates a large upwelling of deep water Sedberry et al. It is likely that the enhanced productivity at the Charleston Bump supports a benthic population from the bottom up instead of from the top down. Overall, the predator and prey organisms in this study are probably an adequate representation of common members of two trophic levels on the Charleston Bump.
Not all predators on the Blake Plateau are caught in the wreckfish fishery, so prey items of other Bump species sixgill shark, Hexanchus griseus ; Laemonema spp. Weaver and Sedberry, may not have been seen in the current study.
Ross and Quattrini used a submersible and otter trawls to document fish associated with deep coral banks from North Carolina to Florida and identified 99 fish species. Of those species, only nine were seen as either predator or prey in the current study. Likewise, Ross and Quattrini documented 42 demersal reef fish on deep reefs along the southeastern US slope, 6 of which were found in the current study. Many of the same predators in this study wreckfish, barrelfish, conger eels are found on Lophelia reefs in the Gulf of Mexico Sulak et al.
Although the species in the current study may be the most abundant fishery species, they are probably not the most numerically abundant fish in the community. All predator diets had high values of unidentified taxa. This affects the results and interpretation of data because identifiable taxa are probably underrepresented in the analyses. Back-calculated body mass and length were not calculated for individual prey e. In addition, future studies of the Charleston Bump ecosystem should include examination of the stomach contents of prey items to further understand other trophic levels in the ecosystem and to explore links to areas outside the Charleston Bump.
This could provide a more complete understanding of the trophic links from the surface waters to the benthos. To confirm foraging patterns of predators, submersible surveys or hydroacoustic methods could be employed.
This study described the diet of three demersal fish on the Charleston Bump and provided a quantitative description of diet for wreckfish, barrelfish, and red bream using standard stomach content analysis procedures. By penetrating the first 5-cm sediment In total, 69 sediment cores 18 cores in May, 22 cores in July, 9 cores in October and 20 cores in January were taken.
The centrifugation was repeated three times at 4, rpm for 6 min, respectively. Stained organisms were identified to lowest possible taxonomic level and counted separately. Harpacticoids similar in morphology were separated and fixed in glycerol on a prepared glass slide. Identification to species level was conducted using a Leitz Dialux 22 microscope. To obtain meiofaunal biomass data, all specimens except harpacticoids and nematodes were weighed wet to 0.
Biomass of harpacticoid copepods and nematodes was determined from volume calculations. For the nematodes, a common factor of was applied Warwick and Price For the copepods, each species was categorized visually into one of eight body forms, and conversion factors were applied which were derived from scale models in plasticene see McIntyre and Warwick The proportion of total biomass attributable to each species was calculated by multiplying the total numbers present by the adult body volume, assuming that the size distribution relative to the size of the adult is the same for each species, as is the conversion factor from volume to biomass.
The net opening was ca. Mesh size gradually decreased from to 50 mm and a codend liner of mm mesh opening for details on the rigging see ICES The GOV trawl was towed 30 min with a constant speed of four knots over ground.
Towing time started with bottom contact and vertical stabilization of the net opening. The 2-m beam trawl carried a chain matt to prevent catching boulders and to enhance catch efficiency. It was fitted with a mm stretched mesh and a codend liner of 4-mm knotless mesh. A detailed description of the beam trawl construction is given in Jennings et al. A SCANMAR depth sensor was attached to its top just behind the steel beam to determine the exact time and position of contact with the seabed.
From the moment of contact with the seabed, the beam trawl was towed with a speed of 1. From each haul, all individuals of B. Because digestive enzymes in the stomachs still work after the death of a fish, a min limit for preparation and weighing was applied after having the nets on board. In the laboratory, these fish were thawed, measured L T and weighed M again before stomachs were removed.
In the laboratory, the contents of each stomach were rinsed in fresh water, and the prey items contained were divided into macro- and meiofaunal prey. For the macrofauna, each prey item was identified to lowest taxonomic level possible and counted. Reliable biomass data for each macrofaunal prey species were obtained from infauna samples taken simultaneously in the field during the same surveys.
For the meiofauna, each prey item was identified to lowest taxonomic level possible and counted. All harpacticoid copepods found in the stomachs were fixed in glycerol on a prepared glass slide and identified to the lowest taxonomic level possible and counted. Meiofaunal prey in fish stomachs are often strongly degraded, and reliable biomass data can be difficult to obtain. Therefore, the mean individual weights obtained for each species from the simultaneously taken sediment samples in the field were used to convert prey abundance into prey biomass of the stomach content after visual categorization into prey size classes.
The stomach contents of A. Only the relative proportion of macrofaunal relative to meiofaunal prey was determined for each fish species. Individuals of L. Empty stomachs were not included in the diet analysis Table 1. The relative importance of each meiofaunal prey item in the sediment cores and in the stomach contents was expressed by 1 frequency of occurrence I O , 2 percentage of numerical abundance I N and 3 percentage of biomass I W for each season Hyslop To study the role of harpacticoids as prey source, the index of relative importance R I , which combines the relative contribution of a food item by number I N and biomass I W , as well as by the percentage of frequency of occurrence I O , was calculated for each harpacticoid species in the sediment and in the stomachs, respectively, according to the following formula:.
A Bray—Curtis coefficient similarity matrix was calculated for both data sets consisting of non-transformed numerical abundances of each harpacticoid species recorded in the sediment cores and in the stomachs of each fish species. Using a similarity of percentage analysis SIMPER , characteristic harpacticoid species in the sediment and in the stomachs were identified for each season.
To assess the relationship between the abundance of harpacticoid prey species in the sediment and the harpacticoid prey in the stomachs, the Ivlev selection index E was calculated per season:. Negative values indicate avoidance or inaccessibility of prey, and positive values indicate selection for a prey species.
Non-selective feeding is indicated by values around zero. Diet differences in harpacticoid prey selection between the studied fish species were tested using a one-way ANOVA based on numerical abundance data.
For examining size-related diet variations in harpacticoid prey selection, the percentage of frequency of occurrence and abundance of harpacticoid prey was determined per L T for each fish species.
In terms of biomass, harpacticoids and juvenile polychaetes were most dominant in each season Table 2. In terms of numerical abundance, meiofauna dominated the diets of B. In contrast, there was a complete absence of meiofaunal prey in the stomachs of P. Concerning prey biomass in the fish diets, macrofaunal prey generally dominated. A dominance in biomass of meiofaunal prey was only found for B.
Percentage of a numerical abundance and b biomass of meiofaunal and macrofaunal prey in the diet of A. The most important meiofaunal prey group of B. Over the course of the season, meiofauna dominated the diet of B. Among different meiofaunal groups, almost exclusively harpacticoids were found numerically, gravimetrically as well as in terms of occurrence in each season Table 3.
For P. Although the diversity of meiofaunal prey per season was generally higher in the diet of P. According to the three indices, the second most important prey group in most months May, July and January were nematodes.
Ostracods were found regularly in the stomach contents in all seasons, but only in low numbers. Clear seasonal differences in both the frequency of occurrence and abundance of meiofaunal prey were found in the diet of A. Meiofauna dominated the diet of A. Among the meiofaunal prey groups, harpacticoids dominated the diets of both fish species in each season according to the frequency of occurrence, abundance and biomass Table 3.
Juvenile bivalves became relevant in abundance for A. Ostracods were remarkably frequent in the diet of L. Pleuronectes platessa did not utilize meiofauna prey during the entire study period Fig. Therefore, P. In total, 19 different harpacticoid species belonging to 9 families were found in the study area.
The most important family in the harpacticoid community was Ectinosomatidae, almost all belonging to the two species H. Other important species that mainly contributed to the harpacticoid assemblage were Pseudameira crassicornis Sars, G. A similar seasonal pattern for these two harpacticoid species was found using the importance index R I , which combines beside percentage of abundance also frequency of occurrence and biomass Table 4.
In contrast, an opposite trend with the highest abundances as well as importance values in summer and lowest in winter was found for L. Particularly in summer, the harpacticoid community was mainly characterized by L. Beatricella aemula Scott T. No general seasonal trends according to the R I index were found for P.
In total, eight different harpacticoid species belonging to six families were found in the stomach contents. The most important families in the diet of all the four fish species were Ectinosomatidae, belonging almost entirely to the species Pseudobradya spp. Table 6. Families such as Ameiridae, Idyanthidae and Miraciidae were generally of low importance in the fish diets, although they occurred in the sediment cores.
Significant differences in the abundance of harpacticoid species were found in all studied fish diets Table 7. For B. This was attributed to Pseudobradya spp. A high positive selection was found for Pseudobradya spp. High E values of H. A similar seasonal trend of diet differences in harpacticoids was found in the stomachs of P. Pseudobradya spp. Apart from E. Only two harpacticoid species, namely Pseudobradya spp.
There were significant seasonal differences in the diets in both fish species Table 7. According to R I values, Pseudobradya spp. In the diet of L. This contrasting prey selection between A. A decreasing frequency of occurrence of harpacticoid prey with increasing fish length L T was found in the diet of all studied fish species Fig.
A similar trend in harpacticoid abundances was found for B. Highest abundances of harpacticoids were found in the diet of B. Percentage of a frequency of occurrence and b abundance of harpacticoid prey per fish length L T of the studied fish species. On the basis of these results, it should be noted that no individuals of P.
However, meiofauna was previously found to be an important prey in small-sized plaice between fish lengths of 4 and 10 cm L T Gee Numerically, meiofaunal prey dominated the diet of B. Thus, although sharing the same habitat, seasonal differences in meiofauna prey resources do exist between these small-sized demersal fish species. In terms of prey biomass, macrofaunal prey dominated in all fish diets reflecting the higher weight of macrofaunal compared to meiofaunal prey.
Among different meiofaunal prey groups, harpacticoids were always of primary importance in the diet of each of the studied fish species during all seasons, whereas nematodes dominated the meiofauna community in the study area.
Nematodes were always the dominant meiofauna group in sediment samples in terms of abundance, while harpacticoids occurred most frequently.
Seasonal differences were negligible for meiofaunal abundance but significant for biomass, due to a marked increase in harpacticoids in summer. Their densities ranged from 61 to 4, ind. Harpacticoids ranked second in abundance, whereas other groups such as polychaetes, kinorhynchs, gastrotrichs, bivalves and ostracods were far less abundant Juario ; Govaere et al.
Seasonally, meiofauna abundance generally peaks in spring and summer following an increase in food supply after the spring phytoplankton bloom, whereas abundance is low during autumn and winter, when most meiofauna groups live deeper in the sediment Olafsson and Elmgren Only harpacticoids are known to occur most of the year, concentrated in the upper six centimetre of the sediment Huys et al.
Only an increase in copepodite occurrence as well as of harpacticoid biomass was found during summer, probably triggered by reproductive activities of harpacticoid copepods and sufficient food supply due to phytoplankton sedimentation after the spring bloom Rudnick et al.
The pelophilic species P. In contrast, interstitial species were completely absent in the present study, although interstitial species belonging to the family Leptastacidae mainly Leptastacus and Paraleptastacus were described as characteristic species in previous studies Heip et al. Leptastacus and Paraleptastacus are both known as interstitial sliders Huys et al.
Seasonally, abundances of Ectinosomatidae and Longipediidae differed significantly, being highest for P. Such seasonal changes of harpacticoid densities are mostly observed in vertical distribution patterns, which are caused by migrations in response to seasonal fluctuations in environmental parameters e.
More details in terms of species-specific migration patterns in harpacticoids will be given in the section below directly related to its function as potential prey source for the studied demersal fish. Diets of B. Such a dominance of harpacticoid prey throughout the seasons has also been reported for B. Other meiofaunal prey groups juvenile bivalves, ostracods and nematodes became important in their diets only in May, indicating a seasonal change in prey preferences during spring.
Constrained by their small mouth gapes and a more sediment surface-orientated feeding strategy, B. Consequently, the relatively low costs of capturing harpacticoids and their relatively high caloric content turn them into a more energy-efficient prey. However, the seasonal trend to meet energy requirements in spring also by eating other meiofauna prey groups e.
Meiofauna was more important as a seasonal prey source in the diet of A. Meiofauna mainly characterized the diet of A.
In contrast, harpacticoids dominated the diet of L. Similar to both fish species discussed before, juvenile bivalves became an important prey group in spring. On the basis of their mouth morphology, it can be assumed that A. Consequently, mainly macrofaunal prey e. Confirming this, preferential feeding on macrofauna was also found in previous diet studies for both fish species e. Gibson and Ezzi ; Bayan et al. However, a reduced feeding activity of A.
Assuming for dab a rather weak condition in spring after the winter feeding pause and the spawning period Knust ; Hinz et al. Increasing abundances of juvenile bivalves in all fish diets in May indicated a match with spawning periods of bivalves in this area Beukema et al. The subsequent decrease of juvenile bivalves in occurrence, abundance and biomass in the field during summer might be due to predation pressure of the studied fish species. Thus, seasonal changes of meiofauna in the diet composition of small demersal fish could be the result of seasonal availability of suitable meiofaunal prey see Tables 2 , 3.
This agrees well with the literature showing that harpacticoids are usually the most abundant prey group in the fish diets, whereas nematodes dominate the sediment e. Gee and references therein. Food selection of demersal fish depends on the availability of the prey, which is mainly determined by its density, visibility, accessibility and mobility Nelson and Coull Feeding of the studied fish on meiofauna was mainly focused on harpacticoids living on or near the sediment surface, whereas nematodes have a deeper vertical distribution Aarnio and Bonsdorff ; Aarnio Another factor that may explain the absence of nematodes in the stomachs could be differences in digestion rates for these two taxa.
Harpacticoids have an exoskeleton that is slowly digested and remains in the gut for several hours after ingestion, while nematodes are soft-bodied and are digested rapidly Alheit and Scheibel ; Scholz et al. A third explanation implies that physical disturbance caused by searching fish in the sediment may have suspended nematodes from the uppermost sediment layer or swept them away Fitzhugh and Fleeger ; Gee This last explanation, regarding our study, also indicates best why, on the one hand, nematodes are absent in the flatfish diets, but on the other hand, are a dominant prey item in the goby diet.
Similar findings of nematodes in gobiid stomachs were also reported for P. The latter assumed that gobies graze sediments more or less indiscriminately in addition to sight feeding for larger prey. This rather passive feeding strategy in searching prey may contradict with a more active visual feeding strategy of flatfish remaining motionless on the bottom at first, and then periodically lunging rapidly forward, causing the upper sediment layers to float in suspension de Groot ; Hoghue and Carey Also morphological differences e.
Fish predation on harpacticoids was highly selective for the two species Pseudobradya spp. Selective feeding on harpacticoid species seems to be common in many fish species. For instance, Alheit and Scheibel showed exclusive feeding on L. Hicks , in his study of flatfish feeding on intertidal sandflats in New Zealand, also found exclusive feeding on one harpacticoid species P. Firstly, by comparing the meiofauna community in the sediment with that in the fish stomachs, it becomes clear that the studied fish species fed almost exclusively on the most abundant harpacticoids in the sediment.
Thus, fish predation on Pseudobradya spp. Secondly, harpacticoids differ in vertical distribution within the sediment. By dwelling in the uppermost sediment layers, Pseudobradya spp. Consequently, interstitial species, such as B.
Only the upward migration in the sediment of B. Moreover, Pseudobradya spp. Thus, Pseudobradya is classified as a moving water emerger during all seasons, whereas Longipedia moves in the water column mainly in summer Thistle , leading to a greater susceptibility of Longipedia sp.
Diets of the studied fish species changed towards an intensively feeding on Longipedia spp. In this context, prey selection on Longipedia spp. Longipedia spp. Such a prey size selection was also found by McCall for juvenile flounder, mainly feeding on the largest available harpacticoids.
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