Fast response of freshwater consumers to a new trophic resource: Predation on the recently introduced Asian bivalve Limnoperna fortunei in the lower Paraná river,South America

Abstract Comparison of Limnoperna fortunei numbers and biomass in screened (5, 15 and 40 mm) and unscreened cages deployed for 18 months in the lower Paraná delta indicates that predators harvest 26–79% (numbers), or 20–85% (biomass) of the mussel population. Predation impact decreases with mussel size. On average, 6 kg of whole live mussel × m⁻² × year⁻¹ (0.36 g of dry mussel tissue × m⁻² × day⁻¹) were eliminated from the unscreened cages. Cages with 15 and 40 mm screens lost between 1 and 2 kg × m⁻² × year⁻¹. Aquatic mammals, birds, and especially fish, are probably the main consumers of large mussels. Small L. fortunei are most probably eaten by fish and also by several invertebrates, including crustaceans, leeches and gastropods. It is suggested that L. fortunei intercepts a significant fraction of the organic carbon that the Paraná‐Uruguay rivers flush into the ocean, locally boosting numbers of benthophagous animals, deposit feeders and, indirectly, higher level predators. Our results indicate that only 15 years after its first introduction in South America this invasive species is very actively consumed by local predators, but predatory suppression of the mussel seems very unlikely. Comparisons with the effects reported for the zebra mussel (Dreissena polymorpha) in Europe and North America suggest that L. fortunei is consumed more actively and that its negative impact on the local fauna is more restricted. These differences are attributed to the fact that while D. polymorpha feeds chiefly on plankton, a limited resource, L. fortunei feeds on detrital particulate organic matter, whose supply in these large South American rivers largely exceeds consumption.     

In situ experiments on predatory regulation of a bivalve mollusc (Dreissena polymorpha) in the Mississippi and Ohio Rivers

1. In situ exclosure experiments in the Mississippi and Ohio Rivers determined the importance of fish predation in regulating zebra mussels (Dreissena polymorpha), an increasingly important constituent of the benthic invertebrate assemblages in both rivers. 2. We evaluated the effects of predatory fish on the density, biomass and size distribution of zebra mussels in a floodplain reach of the upper Mississippi River and in a naturally constrained reach of the Ohio River. Fifty, six-sided, predator-exclusion cages and fifty ‘partial’ cages (mesh at the upstream end only) were deployed, with half the cages containing willow snags and half clay tiles suspended 12–16 cm above the bottom. A single snag or tile sample unit was removed from each cage at approximately monthly intervals from July to October 1994. Types and relative abundances of molluscivorous fish were evaluated by electrofishing near the cages in both rivers. Actual and potential recruitment of young zebra mussels on to the substrata were measured using benthic samples in both rivers and estimated (Ohio River only) from counts of planktonic veligers. 3. Zebra mussels were consumed by at least three fish species in the upper Mississippi River (mostly carp, Cyprinus carpio, and redhorse suckers, Moxostoma sp.) and five species in the Ohio River (primarily smallmouth buffalo, Ictiobus bubalus, and channel catfish, Ictalurus punctatus), but potential recruitment seemed adequate to replace consumed mussels, at least in the Ohio River. The number of juvenile benthic mussels showed no apparent link with the density of veligers soon after initiation of reproduction. Recruitment of juveniles on snags and tiles was not affected by cage type (thus eliminating a potentially confounding ‘cage effect’). 4. Fish significantly influenced mussel populations, but the impact was often greatest among low density populations in the upper Mississippi. Density and biomass differed in both rivers for cage type (higher inside cages), substratum (greater on tiles), and date (increased over time). Presumed size-selective predation was present in the Mississippi (greater on larger size classes) but was not evident in the Ohio. We hypothesize that fish in the Mississippi can more easily select larger prey from the low density populations; whereas size-selective predation on tightly packed zebra mussels in the Ohio would be difficult. 5. Although fish can reduce numbers of Dreissena polymorpha in the two rivers, current levels of fish predation seem insufficient to regulate zebra mussel densities because of its great reproductive capacity. The recent invasion of zebra mussels, however, could lead to larger fish populations while promoting greater carbon retention and overall ecosystem secondary production.     

Predator exclusion experiments in an intertidal mud flat

The intensity of predation pressure exerted on the macrofauna in muddy sediments was investigated in Königshafen (island of Sylt, eastern North Sea). Tests in aquaria revealed shrimpCrangon crangon, juveniles of the shore crabCarcinus maenas, and gobiid fishPomatoschistus microps as the most important local predators. Their high abundance from July to September led to the hypothesis, that predation pressure on the infauna will be most intense during that period. Cages were set up to protect the infauna against these predators. No protective effect was achieved with cages constructed of 20-mm mesh nylon net; however, cages with screen wire and gauze of 5-mm mesh and smaller resulted in a considerable increase of the infauna. In cages of 1-mm mesh gauze, set up from March to June, the macrofauna reached an abundance of four times the control density. In the period from July to October, the factor of increase was as high as 23. Species density was 28 400 cm^–2, as compared with only 7 in the control. Species which colonized the mud flat during spring and early summer established dense settlements inside the cages, but failed to do so in uncaged mud-flat areas. It is concluded that predation by young crabs, shrimp and gobies determines to a large extent the structure and dynamics in the local intertidal macrofauna.     

Vulnerability of microcrustaceans to predation by the invasive filter-feeding mussel Limnoperna fortunei (Dunker)

Limnoperna fortunei is an Asian mussel introduced to South America around 1990. One of the most important impacts of this invader is probably its grazing on the plankton. In this study we evaluate the vulnerability of several planktonic microcrustaceans from the Paraná River floodplain to predation by adult L. fortunei. We conducted 2-h laboratory feeding experiments where the bivalves were offered microcrustaceans differing in overall body shape, size, and locomotive abilities. Ingestion and clearance rates for each taxon were estimated. Results suggest that, in addition to detritus and phytoplankton, microcrustaceans may be a very important food item for this invasive mollusc. Limnoperna fortunei can prey on larger organisms (up to 1100?µm) than Dreissena polymorpha, the European and North American invasive mussel.     

Net cages enhance golden mussel (Limnoperna fortunei) larval density and condition factor

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Development of an immunological technique for identifying multiple predator–prey interactions in a complex arthropod assemblage

A simplified but highly effective approach for the post‐mortem evaluation of predation on several targeted members of an arthropod assemblage that does not require the development of pest‐specific enzyme‐linked immunosorbent assay (ELISA) (e.g. pest‐specific monoclonal antibodies) or PCR assays (DNA primers) is described. Laboratory feeding studies were conducted to determine if predation events could be detected from predators that consumed prey marked with foreign protein. I determined that large and small rabbit immunoglobulin G (IgG)‐marked prey can be detected by a rabbit‐IgG‐specific ELISA in the guts of chewing and piercing–sucking type predators. I then conducted multifaceted inclusion and exclusion field cage studies to qualify the degree of interguild and intraguild predation occurring among a complex arthropod assemblage during four separate light phase treatments. The field cages contained an arthropod assemblage consisting of 11 or 12 species of predaceous arthropods and three pest species. The three pests introduced into the cages included third instar Trichoplusia ni marked with rabbit IgG, third instar Lygus hesperus marked with chicken IgG and Pectinophora gossypiella sentinel egg masses. The inclusion cages allowed foraging fire ants, Solenopis xyloni, to freely enter the cages while the exclusion cages contained barriers that prevented ant entry. The results obtained using the conventional inclusion/exclusion field cage methodology revealed that there was substantial interguild and intraguild predation occurring on the majority of the arthropods in the assemblage, particularly in those cages that included ants. I then precisely identified which predators in the assemblage were feeding on the three targeted pests by conducting three post‐mortem gut content analyses on each individual predator (1503 individuals) in the assemblage. Specifically, P. gossypiella egg predation events were detected using an established P. gossypiella‐egg‐specific ELISA, and third instar T. ni and L. hesperus predation events were detected using rabbit‐IgG‐specific and chicken‐IgG‐specific ELISAs, respectively. Generally, the gut ELISAs revealed that Collops vittatus, Spanagonicus albofasciatus and Geocoris punctipes readily preyed on P. gossypiella eggs; Nabis alternatus, Zelus renardii and spiders (primarily Misumenops celer) readily preyed on marked L. hesperus nymphs, and spiders, S. albofasciatus and N. alternatus readily preyed on T. ni larvae. Furthermore, the cage methods and the post‐mortem predator gut ELISAs revealed very few distinctive patterns of predation with regard to the light cycle the assemblage was exposed to.     

Effects of microhabitat,time of day,and weather on predation of gypsy moth larvae

Summary Wire cages with different-sized meshes were placed on trunks and around leaves at different heights in oak trees and in forest litter. Gypsy moth, Lymantria dispar, instars II–V tethered with threads were placed in each cage (instars II–III only in leaf cages) as well as outside the cages. Predation of larvae decreased from near ground to mid-crown in trees and was highest in litter and very low on leaves. Predation in litter was not strongly related to cage type, suggesting that small, invertebrate predators were active there, but IV–V instars on trunks were mainly killed by relatively large predators, probably forest mice and shrews. Influences of the time of day and weather on predation were evaluated by observing tethered V–VI instars in litter and on tree trunks hourly. Ants, particularly carpenter ants (Camponotus ferrugineus) and Formica sp., and probably vertebrates, were conspicuous predators in the litter. Ants were most active at lower relative humidities, while other predators were apparently not influenced by humidity. No daily activity rhythms of predation were noted. Invertebrates appear to be important predators of larvae only in the litter whereas vertebrates, such as forest mice and shrews, also readily attack larvae on tree trunks.     

Reproduction and larval developmental stages of the freshwater mussel Limnoperna fortunei at three sites in a temperate reservoir

Longitudinal changes in larval densities and developmental stage compositions of the freshwater mussel Limnoperna fortunei were investigated to understand the patterns of reproduction and development at three sites in Lake Ohshio, a small reservoir in the temperate region of central Japan. Onset and termination of L. fortunei larval presence were coincident at the three sites in the reservoir. This may have been caused by environmental homogenization owing to artificial aeration for water quality improvement of the reservoir. The observations in this study support temperature as the trigger for onset of reproduction as suggested by previous studies. Occurrence of high turbidity that reduced food availability for mussels was probably the trigger for termination of reproduction in Lake Ohshio. During the period when L. fortunei larvae were present, the D-form larva of the earliest developmental stage was always observed and at much higher density than later developmental stages. The results indicate that spawning of L. fortunei continued in the reservoir throughout the period that larvae were observed and that predation and other factors reduced the proportion of later developmental stage larvae.     

A new method to measure substrate coherent strength of <Emphasis Type="Italic">Stenopsyche marmorata</Emphasis>

We developed a method for estimating the substrate coherent strength of a net-spinning caddis larva, Stenopsyche marmorata, in the field. Plastic experimental cages (prefabricated containers with 5-mm mesh; 0.1 m high, 0.12 m wide, 0.2 m long) that enclosed gravel substrate and an objective stone (5–6 cm in diameter) were prepared. We expected the caddisflies to build a retreat(s) between the objective stone and cage substrate when submerged in a riffle channel reach for 6 days. Ten final-instar larvae (4–5 cm long) were placed in the cage and allowed to form retreats. Two treatments (cages with and without larvae) with 15 replicates each were used in two experimental trials. The vertical lift-force of objective stones associated with a retreat (coherent strength) was measured using a spring weighing scale. In 87% of 30 cages of with-larvae treatment, retreats were formed between the objective stone and cage substrate. Coherent strength ranged from 0.0 to 1.6 kg. Our approach can be used under varying flow velocities, substrate conditions, and larval biomass, all of which are normally difficult to test in flume experiments. Findings based on our field method provide unique physical properties of the caddisfly retreats in stream ecosystems.     

Test of an antifouling treatment on tuna fish-cages in Boston Bay,Port Lincoln,South Australia

Abstract

A test of the antifouling properties of Wattyl NetClear® was conducted on fish farms containing southern bluefin tuna (Thunnus maccoyii) during a 6-month period (February-July 2002). Wattyl NetClear® is a water-based synthetic latex-based coating where the active antifouling agent is a mixture of two isothiazolinones likely to affect both biochemical conditioning and bacterial colonisation. The development of fouling was monitored using underwater stereo-photogrammetry on 3 treated and 3 untreated net cages with the main factors treatment, depth and cage nested within treatment. A significant treatment and depth effect was found but variable through time and in some instances with significant cage effects. By the end of the study, the difference in fouling load between treated and untreated nets was 14.7%. In total 72% of all free-space data points were on treated nets. The dominating fouling organisms were Enteromorpha sp. and sponges with low settlement of blue mussel and paper oysters. Passive deposition of tuna faeces contributed significantly to the cover. The largest cover of fouling was observed in April-May with a dominance of sponges in June-July. Enteromorpha sp. dominated shallow depths while sponges dominated at deeper levels. Tuna faeces were distributed independent of depth but varied with time. The results showed that fouling of fish cages consisted of both active settlement and passive deposition, the latter independent on antifouling treatment.     

Variation in the diet of the Patagonian toothfish with size, depth and season around the Falkland Islands

The ontogenetic and seasonal variations in the feeding spectrum were studied in 756 specimens of the Patagonian toothfish Dissostichus eleginoides (16–159 cm total length, L_T) collected on the shelf, continental slope and bathyal waters (67–1960 m, depth range) around the Falkland Islands between April 1999 and August 2002. On the shelf, small toothfish (<40 cm L_T) were active predators taking mostly one relatively large prey item at a time (mainly near‐bottom Patagonotothen ramsayi and Loligo gahi). Medium‐size toothfish (40–60 cm L_T) fed on the same prey, but the number of prey items increased to 1–2 items per fish. Large toothfish (>60 cm L_T) switched their diet to other large pelagic fishes occurring near the bottom (Macruronus magellanicus and Micromesistius australis australis), again taking mostly one prey item at a time. The diet of medium‐size D. eleginoides on the shelf varied seasonally depending on the abundance and migrations of the major prey species. Patagonotothen ramsayi was abundant in the diet throughout the year, whereas L. gahi appeared only from February to October during its offshore seasonal migrations to the depth range of D. eleginoides. During November to January, L. gahi migrated inshore to spawn and disappeared from the toothfish diet, being substituted by M. australis australis which dispersed on the shelf after spawning. After its ontogenetic descent to the lower part of the continental slope (500–1000 m depths), toothfish took less active (than on the shelf) fishes such as Antimora rostrata whilst also feeding on active near‐bottom macrourids and skates. In their deepest habitat (>1000 m depths), toothfish became a typical opportunistic predator, feeding mainly on relatively small and inactive fishes, squids and prawn‐like crustaceans Acanthephyra pelagica and Thymops birsteini. Decrease in hunting activity with depth could be related to a specific adaptation to keep neutral buoyancy by increase of lipid content in white muscles of D. eleginoides with size.     

Depth telemetry from the sixgill shark,Hexanchus griseus,at Bermuda

Synopsis Transmitters were attached to two female sixgill sharks, Hexanchus griseus, which were followed in course and depth for periods of 4 and 2 days. The sharks swam slowly (10 to 20 cm s^-1) and continuously, generally moving parallel to the bottom contours within a limited (less than 10 km long) area. The greatest depth was an excursion to 1500 m, but otherwise the sharks remained near the bottom at depths ranging from 600 to 1100 m.     

Population dynamics of an infaunal polychaete: The effect of predators and an adult-recruit interaction

The effect of predation by fish and wading shore birds on the population dynamics of a nereid polychaete (Ceratonereis pseudoerythraeensis Hutchings & Turvey) was studied. The species occurs in high densities on the site studied probably owing to high nutrient input from a nearby sewage treatment farm. Predators were excluded from experimental areas with wire mesh cages. The effects of cage structures on predator behaviour, algal deposition and sediment depth and composition were studied, and the possibility that C. pseudoerythraeensis might migrate in or out of caged areas was experimentally tested. These effects appeared to be of minimal importance. Analyses of size classes of C. pseudoerythraeensis showed that the density of adult worms in control areas decreased relative to caged areas. This was attributed to predation, mainly by flounder and curlew sandpipers. Recruitment of juvenile C. pseudoerythraeensis was greater in control than caged treatments. The hypothesis that increased adult density decreases juvenile recruitment was confirmed by manipulating the densities of adults. The adult-recruit interaction effect apparently counter-balanced losses due to predators, so that if only the total densities of the polychaetes had been studied no effect of predation would have been apparent.     

Spatial and length distribution,sex composition,and feeding of the notched-fin eelpout <Emphasis Type="Italic">Zoarces elongatus</Emphasis> (Perciformes: Zoarcidae) in the Sea of Okhotsk

Based on the data from six bottom trawl surveys carried out on the shelf of the Sea of Okhotsk in 1997–2007, the features of the distribution, size and sex structure, and food composition of the notched-fin (east) eelpout Zoarces elongatus were analyzed. It was established that this species regularly occurs in trawl catches only in the eastern part of the sea and that the density of its schools decreases with depth. It was recorded that females of the notched-fin eelpout are larger than males. The sex ratio of the fishes varies from year to year. Females prevail at depths less than 25 m and males prevail at depths greater than 25 m.     

Colonization of artificial substrata by macro-invertebrates in a stream and variations according to stone size

SUMMARY. Plastic cages containing artificial substrata were placed on the stony bottom of a stream in a section where the environmental conditions appeared to be homogeneous. The section was divided into eight equal sub-sections (A-H) and four cages were placed in each sub-section. The stone-size of the artificial substrata was similar within each cage but differed between the four cages with average diameters of 14, 24, 48 and 96 mm, respectively. The baskets were removed one month later and the catches of macro-invertebrates were recorded.
Analysis of the catches (density and number of taxa in each cage) revealed no significant differences in connection with the position of the cages in the section of stream. Cages with 48-mm stones contained the least abundant fauna. The taxa which colonized cages with 14- or 24-mrn stones were more numerous than those collected from cages with 48- or 96-mm stones.
Statistical analysis by laxon confirmed the preceding results. Variability in connection with the position of the cages along the section of stream was never significant. A little more than half the taxa were distributed independently of stone size but always showed a contagious type distribution. The density of the other taxa was higher, either in cages with 14- or 24-mm stones (particularly Gammarus ) or only in cages with 96-mm stones (especially Hydropsyche ). Only one taxon was more abundant in cages with 48-mm stones, the type of substratum in which the overall density was lowest.
Catches in the cages were not the same as those taken with a Surber sampler because the two samplers did not take samples from the same habitats and also because the baskets offered a more specialized habitat than the surrounding bottom. The basket with 24-mm stones gave the least distorted picture of the natural population.     

Net mesh size affects production of giant freshwater prawn Macrobrachium rosenbergii cultured in lake‐based cages

Cage culture of freshwater prawns in open waters is prone to the entry of predators and competitors that particularly hamper production. This study was conducted to determine how smaller net mesh sizes to reduce entry of unwanted species inside the cages affects the production of Macrobrachium rosenbergii in lake‐based cages. Juvenile prawns were stocked in cages (7 × 7 × 1.5 m) of two net mesh sizes at 10 individuals m^?2 and cultured for 10 months in a shallow eutrophic lake in the Philippines. The two net mesh sizes were either 5 mm‐mesh B‐nets or and 1 mm‐mesh Hapa nets. Each treatment had four replicates each and was fed based on biomass with commercially formulated feed. Monitoring of various production parameters was done during the two phases of culture: batch phase on days 63 and 127 and the selective harvest phase on days 187, 219, 253, 281 and 313, when the experiment was terminated. For the first 127 days of culture, the weight, percent weight increase, daily growth rate (DGR), specific growth rate (SGR), yield and feed conversion ratio (FCR) were significantly better in prawns reared in the Hapa compared to the B‐nets. During the selective harvest phase the blue claw, orange claw and berried females were selectively harvested and the remaining prawns returned to the cages. After changes in stocking density through culling, ancova was used to compare the effect of mesh size with the total number of prawns returned to the cages as a covariate. Yield was significantly higher in the Hapa nets. Weight, DGR, SGR and FCR were also consistently higher in the Hapa nets, although not always significantly different. The overall better performance of prawns reared in the Hapa net cages was due to: (i) the reduction in the entry of predator and competitor species in the finer‐meshed Hapa compared to the larger mesh B‐net, (ii) more natural food trapped inside the Hapa cages, and (iii) a higher number of selectively harvested prawns, which decreased stocking density in the cages and improved growth. Use of small mesh size nets is recommended in the cage culture of M. rosenbergii in inland natural water bodies.     

Diel vertical migrations of zooplankton in a shallow, fishless pond: a possible avoidance-response cascade induced by notonectids

1. Day (noon) and night (midnight) vertical distributions of zooplankton and phytoplankton in the water column (1.5 m) of a Vermont pond were determined on two consecutive days from 470 mL water samples taken at three depths (0.1, 0.5 and 1.0 m) at three sites. There was little variation across depths in temperature, dissolved oxygen concentration and phytoplankton. All individuals of each zooplankton species (a small copepod, Tropocyclops extensus and six rotifers) were counted. 2. A three-way ANOVA on the zooplankton data showed no effect of date or time of day on the abundance of any species. Significant diel shifts in vertical distribution (depth × time-of-day interactions) were found for T. extensus (nauplii, as well as copepodites and adults) and Polyarthra remata, but not for Hexarthra mira, Keratella cochlearis, Anuraeopsis fissa, Ascomorpha ovalis and Plationus patulus. Tropocyclops extensus showed a pronounced, typical diel vertical migration, avoiding the surface and occurring most abundantly near the bottom during the day. Polyarthra remata showed an equally pronounced, reverse diel vertical migration, avoiding the bottom and being most abundant near the surface during the day. 3. The diurnal descent of Tropocyclops is interpreted as an avoidance response to Buenoa macrotibialis, a notonectid which feeds on this copepod at the surface during the day but not at night. The diurnal ascent of Polyarthra is thought to be an avoidance response to Tropocyclops, which strongly suppresses this rotifer in field enclosures and laboratory vessels. Thus, these out-of-phase migrations may be coupled and represent a behavioural cascade initiated by Buenoa. 4. At night, Tropocyclops and Polyarthra both were uniformly distributed across depths. This is believed to reflect the absence of appreciable depth-related variation in temperature, algal food resources (biovolume of cryptomonads and chrysophyte flagellates) and predation risk at this time. 5. The five rotifer species that did not exhibit diel vertical migrations may be less susceptible to Tropocyclops predation than Polyarthra.     

Exclusion cage studies on the impact of predation on Helicoverpa armigera in cotton

The impact of predation on Helicoverpa armigera was studied in four field cage exclusion trials on cotton in Kenya. H. armigera egg cohorts were introduced inside predator‐free and open control cages, and the impact of local predator populations on the cohort was examined. Fourteen days after inoculation, exclusion cages had four times more larvae than controls, indicating a strong impact of predation. Ants and Anthocoridae were the predominant predator groups. Exclusion cages had more damaged fruiting plant parts (squares, flowers and bolls) than the control. In the absence of predators, natural mortality of H. armigera was greater as cotton matured, and is likely to be linked to the host plant condition.     

Habitat structure and fish predation: effects on invertebrate colonisation and breakdown of stream leaf packs

1. We investigated the effects of two features of leaf‐pack habitat structure (i.e. mass of a leaf pack and surface area of leaves comprising a leaf pack) and fish predation on colonisation of shredders and leaf breakdown rates in a coldwater stream. Packs were constructed of red maple (Acer rubrum) leaves. 2. A 2 × 3 × 3 factorial experiment was used to manipulate fish predation (exclusion and control cage), leaf‐pack mass (1, 3 and 5 g dry mass) and leaf surface area (small: approx. 17.9 cm², medium: approx. 34.6 cm², large: approx. 65.6 cm²). Exclusion cages had mesh on all sides, whereas control cages lacked mesh on two sides to provide access to fish. 3. Common shredders were Gammarus pseudolimnaeus, Pycnopsyche and Lepidostoma. Shredder biomass per leaf pack increased with the mass of a leaf pack (P < 0.001), but biomass per unit mass of leaf pack did not differ with leaf‐pack mass (P = 0.506). Shredder densities did not respond to the exclusion of fish (P > 0.7) or leaf surface area (P > 0.7), and interactions among treatment factors were not significant (P > 0.2). 4. Breakdown rates were lower for leaf packs comprised of small leaves (P < 0.001) and leaf packs with high mass (P = 0.001). Excluding fish did not significantly affect leaf breakdown rates (P = 0.293), and interactions among treatment factors were not significant (P > 0.3). Breakdown rates were highest when packs consisted of few leaves (i.e. leaf packs with large leaves and low mass) and were colonised by many shredders. 5. Fish predation was not an important factor controlling shredder densities in leaf packs over the spatiotemporal scale of our experiment. Nevertheless, we found shredder colonisation was proportional to leaf‐pack mass and breakdown rates were affected by leaf‐pack size (i.e. number of leaves in a pack). We suspect that fragmentation is the primary mechanism causing the breakdown rates to be dependent on leaf‐pack size.     

Mortality of non‐target flatfishes escaping from demersal trawl codends

Mortality of non‐target flatfishes that had escaped from various sizes (40, 44 and 50 mm) of codend meshes was determined by capturing and holding the escapees in cages for 7 days. Although three flatfish species were studied, sufficient data for detailed analyses were obtained only for Buglossidium luteum (Risso, 1810) (solenette). Trawl hauls (48 in total) with 15 min towing durations were conducted in the summer and winter seasons of 2011 and 2012. A tailored bottom trawl with 800 meshes around the mouth was used with covers to capture fish. Covers were used as an observation cage, and were fixed to the sea bottom at depths of 3.0–8.2 m. The monitoring period was 7 days. It was estimated that average escape mortality of B. luteum for all test cages (96.5%) in summer was significantly higher (p < .01) than that in winter (56.5%). Mesh size and shape was not a significant factor (p > .01), with the exception of the 2011 winter; however, the mortality rate was inversely related to fish length in most cases (p < .01). High mortalities (~100%) were observed for Arnoglossus laterna (Walbaum, 1792) (Mediterranean scaldfish) and Citharus linguatula (Linnaeus, 1758) (spotted flounder) in all hauls in both summer and winter.