Trawl Samples Research Articles

Growth and production of Ophiocten gracilis (Ophiuroidea: Echinodermata) on the Scottish continental slope

Growth and production of the bathyal ophiurid brittle star Ophiocten gracilis was studied from skeletal growth bands and disc size frequencies of specimens collected in sled and trawl samples taken on the continental slope off Scotland. Growth bands showed up in SEM examination as ring-like zones in surface relief and texture of the stereom microstructure of the intervertebral muscle insertions on the arm ossicles. Seasonal variability in somatic growth, presumed to underlie this growth pattern, may reflect reproduction and/or a possible non-feeding period during gonad maturation. Disc size-at-age was back-calculated from size-at-age interpreted from growth-band series on the vertebral ossicles from arms of O. gracilis. Pooled growth-band frequency data and normal-distribution mixtures based on size-at-age data were used to test for overgrowth of early growth bands on the ossicles from larger individuals. Von Bertalanffy and Gompertz growth models were fitted to the finalised back-calculated disc size-at-age data. These were used along with the modal structure of the observed disc size frequencies to develop a demographic model based on normal-distribution mixtures constrained by the growth model. These and other defining parameters were fitted by non-linear regression to size structure observed in a sample of the breeding population from 997 m depth on the Hebrides Terrace. Recruitment was estimated according to available data from sediment-trap time series. A ratio of somatic production/biomass, P S /B, in the range of 0.43–0.54 was estimated using a fitted size/mass relationship and the increment summation method (ISM) applied to the fitted growth models. A narrower, but otherwise similar, range in estimated P S /B ratios (0.48–0.49) was obtained in a parallel approach using the mass-specific growth rate method (MSGRM), whereby the same size/mass relationship was applied to the observed frequencies and growth parameters fitted to growth banding. Using previously obtained data on population density, a standing crop of 4.8 g wet weight (~0.58 mg AFDW) m−2 would provide annual wet weight production in the range of 1.9–2.4 g (~0.23–0.29 mg AFDW) m−2 in the population between ca. 700–1000 m depth. Somewhat greater production estimates (P S /B=0.73–0.98) were obtained from MSGRM by pooling the sample with size frequencies from other large samples in which postlarval sizes were more numerous, but larger sizes less numerous. Similarly high production was estimated by MSGRM from a box corer sample from the Wyville-Thomson Ridge. Explanations for variability in size structure are discussed, but even the lower estimates are comparable to boreal shallow-water brittle stars. The high rate of growth and production by accepted deep-sea standards may be related to a capability for interface feeding.

Zooplankton and epibenthic fauna in shrimp ponds: factors influencing assemblage dynamics

The assemblage composition, biomass and dynamics of zooplankton and epibenthos were examined in a commercial shrimp (penaeid prawn) pond in subtropical Australia. Physicochemical characteristics of the pond water were measured concurrently. Numbers and biomass of zooplankton in the surface tows (140 μm mesh) varied from 111.7 ind. L−1 (324 μg L−1) to 8.3 ind. L−1 (44.2 μg L−1). Immediately after the ponds were stocked with shrimp postlarvae there was a rapid decline in zooplankton numbers, particularly the dominant larger copepods. We attributed this to predation by the shrimp postlarvae. Subsequent peaks in zooplankton numbers were principally due to barnacle nauplii. Changes in abundance and biomass of the zooplankton assemblage were not correlated with physicochemical characteristics. Epibenthic faunal abundance in the beam trawls (1 mm mesh) peaked at 14 ind. m−2 and the biomass at 0.8 g m−2. Unlike zooplankton, the peaks in abundance of epibenthos did not correspond to the peaks in biomass. This was due to the large differences in the size of the dominant taxa across the season. Sergestids (Acetes sibogae) and amphipods were the most abundant taxa in beam trawl samples, with amphipods abundance increasing towards the end of the growout. Negative correlations were found between epibenthos abundance and pH and temperature. These relationships were strongly influenced by the high abundances of amphipods and may reflect an effect on the growth of macroalgae in the pond rather than a direct effect on the epibenthos. No correlations were found between epibenthic fauna biomass and physicochemical parameters. Abundances of epibenthic fauna were not related to zooplankton densities, indicating that this source of food was not likely to be a limiting factor. Neither the pond water exchange regime nor moon phase could explain changes in abundances of zooplankton or epibenthos assemblages. Zooplankton clearly contribute to the nutrition of shrimp postlarvae immediately after stocking. The establishment of an abundant assemblage of zooplankton before stocking shrimp postlarvae would appear to be beneficial, if not essential. Later in the season, zooplankton and epibenthos apparently contribute little to shrimp biomass. Owing to their relatively low biomass, the consumption of shrimp feeds by epibenthos is likely to be insignificant compared with that of the shrimp.

An assessment of biases associated with caging, tethering, and trawl sampling of summer flounder (Paralichthys dentatus)

We provide an example of the type of bias assessment that should, but often is not, used in ecological studies using techniques such as caging, tethering, and trawl sampling. Growth rates of summer flounder (Paralichthys dentatus) from cage enclosures were compared to those from mark-recapture trials, and prey types (identified through stomach content analysis) compared between caged fish and wild fish collected from nursery sites over a 2-yr period. Site-specific growth rates estimated from the caging method were similar (± 15%) to those estimated from the mark-recapture trials. Prey types were nearly identical between caged and wild fish, although selectivity may have varied quantitatively. Caging summer flounder will generally be an appropriate tool with which to measure growth rates in the wild, but comparisons with an independent measurement method are necessary for validation. In tethering trials, predation was significantly greater on tethered than on untethered fish, indicating that tethering is not an appropriate tool with which to measure absolute rates of predation on juvenile summer flounder. The lack of effects of substrate (sand versus mud) and fish origin (hatchery-reared versus wild) on predation of tethered versus untethered fish indicates that tethering trials will not indicate treatment-specific differences when none exist. Tethering may be an acceptable method for comparing relative rates of predation on different substrates and between hatchery-reared and wild juvenile summer flounder in the field, although true differences in treatment levels could be masked by tethering. Beam trawl efficiency estimates for juvenile summer flounder were similar between beach and marsh habitats, but differed significantly between marsh sites, indicating that site-specific trawl efficiency estimates may be critical to accurately assess juvenile flounder the appropriateness of comparisons of size-frequency information between the sites and habitats used in this study. Caging, tethering, and beam trawl sampling are appropriate tools for measuring ecological parameters of juvenile summer flounder, but only if possible biases of each method are identified and compensated for when interpreting data collected using these methods.

High-Antarctic regular sea urchins – the role of depth and feeding in niche separation

Regular sea urchins of the families Cidaridae and Echinidae are widespread and sympatrically occurring epibenthic species in Antarctic waters. Food preference and water depth distribution of the five most abundant species (Ctenocidaris gigantea, C. spinosa, Notocidaris mortenseni, Sterechinus antarcticus, S. neumayeri) were analysed based on trawl and photograph samples. Both diet and water depth contribute to niche separation among these species. All sea urchins consume bryozoans and sediment, but echinids feed predominantly on diatoms in the fluff, when available. Cidarids do not consume diatoms, most likely owing to morphological constraints; their typical food consists of sponges and hydroids. C. spinosa and S. neumayeri prefer shallow water depths, whereas N. mortenseni and S. antarcticus prefer deeper regions. C. gigantea is the most variable species regarding food composition and living depth.

The biology of the giant red shrimp ( Aristaeomorpha foliacea) at an unexploited fishing ground in the Greek Ionian Sea

Experimental trawl sampling was carried out along the Greek coast of the Ionian Sea between January and December 1997 at three depth zones: >500, 500–600 and <600 m, to study the distribution and population structure of an unexploited stock of the giant red shrimp ( Aristaeomorpha foliacea). The overall length–frequency distribution ranged between 16 and 62 mm (carapace length, CL), while the most abundant classes were between 40 and 48 mm CL. Length–frequency distribution was significantly dependent on depth. Medium-sized adults appeared in the shallower zone (>500 m), the young-of-the-year in the middle (500–600 m) and larger adults in the deepest (<600 m). The highest abundance was observed in the 500–600 m depth zone. Growth analysis indicated that the population was composed of two to three annual cohorts. The VBGF parameters were CL ∞=47.2 mm, K=0.564 per year, t 0=−0.13 years and CL ∞=64.086 mm, K=0.460 per year, t 0=−0.192 years for males and females, respectively. The asymptotic weight estimated from length–weight relationships was 32.6 and 67.3 g for males and females, respectively. The reproductive period, estimated from the maturity stages of females, is from summer to early autumn with a peak from June to August. The smallest mature female was 37.5 mm CL. Sex ratio was significantly higher than 1:1 in favor of males and no correlation with depth was found. Natural mortality was estimated at 0.57–0.66 and 0.47–0.55 per year for males and females, respectively. Total mortality was 0.58–0.62 and 0.53–0.62 per year for males and females, respectively. The abundance and the population dynamics of this species suggest that the population of the giant red shrimp in the Ionian Sea could provide a new target species to the deep-trawl fishery of the area.

Seasonal and Spatial Patterns of Experimental Trawl Catches in the Southwest Arm of Lake Malawi

Experimental demersal trawl samples were collected in the same manner at the same sites on a monthly basis over an annual cycle in the southwest arm (SWA) of Lake Malawi. Catch composition in terms of species representation and mass was compared over time and depth (10, 30, 50, 75, 100 and 125 m). The average catch per unit effort was calculated per species and depth. Haplochromine cichlids dominated the catches at every depth, making up 75 to 92% of the biomass. The remainder was made almost exclusively of catfishes. Despite catching more than 140 species in the trawls, 60 to 80% of the catches consisted of ten or fewer species, including three catfishes. About twenty species accounted for 90 to 95% of the catches at any depth, suggesting that many species are uncommon or rare. Previous authors reported dramatic changes in species composition at 50 m in the southwest arm. Data presented here suggest that this might be due to a change in the nature of the substratum at 50 m. The greatest diversity of species is in shallow waters, but the highest catches in terms of biomass were recorded between 50 and 125 m and peaked at 75 and 100 m. As catches in the deep waters were dominated by fishes with favorable life history characteristics and which are large relative to species in the shallow waters (though the largest species of cichlid occur in the shallow water, the catches are dominated by small species), it is recommended that the possibility for increased exploitation of the SWA deep demersal stocks should be explored by encouraging controlled development of a demersal commercial fishery in the SWA.

Multibeam volume acoustic backscatter imagery and reverberation measurements in the northeastern Gulf of Mexico.

Multibeam volume acoustic backscatter imagery and reverberation measurements are derived from data collected in 200-m-deep waters in the northeastern Gulf of Mexico, with the Toroidal Volume Search Sonar (TVSS), a 68-kHz cylindrical sonar operated by the U.S. Navy's Coastal System Station. The TVSS's 360-degree vertical imaging plane allows simultaneous identification of multiple volume scattering sources and their discrimination from backscatter at the sea surface or the seafloor. This imaging capability is used to construct a three-dimensional representation of a pelagic fish school near the bottom. Scattering layers imaged in the mixed layer and upper thermocline are attributed to assemblages of epipelagic zooplankton. The fine scale patchiness of these scatterers is assessed with the two-dimensional variance spectra of vertical volume scattering strength images in the upper and middle water column. Mean volume reverberation levels exhibit a vertical directionality which is attributed to the volume scattering layers. Boundary echo sidelobe interference and reverberation is shown to be the major limitation in obtaining bioacoustic data with the TVSS. Because net tow and trawl samples were not collected with the acoustic data, the analysis presented is based upon comparison to previous biologic surveys in the northeastern Gulf of Mexico and reference to the bioacoustic literature.

Composição das populações de linguados (Osteichthyes, Pleuronectiformes) da Baía de Sepetiba, Rio de Janeiro, Brasil

Bi-monthly otter trawl sampling programme using a stratified random design was carried out at Sepetiba Bay (22o54'-23o04'S; 43o34'-44o10'W), from August 1994 to July 1996, aiming to assess the flatfish species composition and their relative abundance. Fourteen species in eight genera, four families and one suborder were identified. Paralichthyidae is the most diverse and abundant family, contributing to 70% of total number of individuals and to 58,8% of total biomass, being comprised of nine species: Etropus crossotus (Jordan & Gilbert, 1882); E. longimanus (Norman, 1933); Citharichthys spilopterus (Gunther, 1862); C. arenaceus (Evermann & Marsh, 1902); C. macrops (Dresel, 1889); Paralichthys orbignyanus (Valenciennes, 1847); P. brasiliensis (Ranzani, 1840); P. patagonicus (Jordan, 1889) e Syacium papillosum (Linnaeus, 1758). Etropus crossotus was the most abundant species amounting 52,5% of the total number and 31,3% of the biomass of Pleuronectiformes, being widespread all over the study area. Additional comments on the species occurrence and their distribution over the Western Atlantic are given and compared to their relative abundance in the Sepetiba Bay.

Comparison of Deep-River and Adjacent Sandy-Beach Fish Assemblages in the Napo River Basin, Eastern Ecuador

We sampled fishes with a small trawl in the deep midchannel and with beach seines on nearby sandy beaches at five sites along two parallel headwater tributaries of the upper Amazon in Eastern Ecuador. We quantified ecological distance between fish assemblages of deep-river and beach habitats and compared assemblage composition, species richness, and diversity. Results of detrended correspondence analyses (DCA) and two-way indicator species analyses (TWINSPAN) clearly revealed the presence of a striking faunal change (i.e., about 92% dissimilarity) between beach-zone and off-shore fishes. The boundary between these assemblages occurred at about 2 m depth. Beach samples collected as far apart as 325 km were more similar to each other than to trawl samples taken only 10s of meters away. The beach-zone fish assemblage was strongly dominated by small characins, whereas the deep-river habitat was dominated by catfishes and weakly electric gymnotiforms. Apparent adaptations to deep-water habitats included small size, flattened or elongated body, ventral mouths, reduced eyes, and sensory specializations for life in what may be a nearly light-less environment (e.g., chemo- and electroreceptors). Visually oriented fishes and those dependent on algal or detrital foods were mostly absent from the deep-river samples. We caught many more species along the beaches, but after correcting for higher numbers of individuals collected in beach habitats, species richness was not noticeably different between the two habitats. Species diversity also averaged slightly higher for beach samples, but the difference was not significant. We infer that the deep-river habitat was undersampled and that further effort in the deep river would reveal the presence of many more species.

Extreme mitochondrial DNA divergence within populations of the deep-sea gastropod Frigidoalvania brychia

The deep sea supports a diverse and highly endemic invertebrate fauna, the origin of which remains obscure. Little is known about geographic variation in deep-sea organisms or the evolutionary processes that promote population-level differentiation and eventual speciation. Sequence variation at the 16 S rDNA locus was examined in formalin-preserved specimens of the common upper bathyal rissoid Frigidoalvania brychia (Verrill, 1884) to examine its population genetic structure. The specimens came from trawl samples taken over 30 years ago at depths of 457–1,102 m at stations in the Northwest Atlantic south of Woods Hole, Massachusetts, USA. Near the upper boundary of its bathymetric range (500 m), extremely divergent haplotypes comprising three phylogenetically distinct clades (average uncorrected sequence divergence among clades ~23%, ~3% within clades) were found at stations separated by a maximum distance of ~80 km, suggesting the presence of high levels of intraspecific divergence or the possibility of morphologically cryptic species. Only one of these clades was found at two stations in the mid- to lower part of F. brychia's depth distribution (800–1,100 m), suggesting lower clade diversity with increasing depth, although among-sample divergence, with a single exception, was minimal. One station was genetically divergent from all others sampled, containing a unique suite of haplotypes including two found only at this site. Steep vertical selective gradients, major oceanographic changes during the late Cenozoic, and habitat fragmentation by submarine canyons might have contributed to an upper bathyal region that is highly conducive to evolutionary change.

Species identification of pelagic fish schools on the South African continental shelf using acoustic descriptors and ancillary information

Stock assessment of exploited pelagic species often relies on hydroacoustic surveys, but a major limitation of this approach stems from an inability to distinguish between the echoes of co-occurring species. We make acoustic measurements of morphometric, energetic, and bathymetric features of anchovy (Engraulis capensis), sardine (Sardinops sagax), and round herring (Etrumeus whiteheadi) schools, to test the hypothesis that schools of each species can be distinguished on the basis of such acoustic descriptors. School measurements were extracted using commercially available software from acoustic data collected by a vertical echosounder during trawling operations of pelagic stock surveys of the South African continental shelf, November 1997, 1998 and 1999. Principal components analysis of 18 school descriptors of the 214 schools for which the species composition was satisfactorily determined by trawl samples suggested that schools of the three species differed in energetic features and position in the water column, but not in morphometrics. Discriminant function analysis indicated that schools could be correctly identified to species in 88.3% of all cases (94.9% for anchovy, 82.6% sardine, 82.6% round herring). Sardine were distinguished from anchovy and round herring primarily on the basis of school depth and energy, and anchovy from the two other species based on an index of school altitude. Correct classification was not improved by including the ratios of measurements of school features made at a 65 dB processing threshold relative to measurements made at 60 dB. Inclusion of ancillary information (latitude, longitude, sea surface temperature, bottom depth, and time of day) improved the accuracy of school identification to 94.9%, since such variables allowed discrimination on the basis of inter-specific differences in habitat use, as well as in schooling behaviour. 2001 International Council for the Exploration of the Sea

Monitoring Round Goby ( Neogobius melanostomus) Population Expansion in Eastern and Northern Lake Michigan

In the Great Lakes basin, round gobies ( Neogobius melanostomus) were first reported from the St. Clair River in 1990. Reported here are the details of range extension and establishment of round gobies in eastern Lake Michigan at Grand Haven, Michigan and in northern Green Bay at Escanaba, Michigan. Round gobies were first collected in survey samples at Grand Haven in 1997 (0.0 to 0.5 fish per trawl hour). Catch rates increased in 1998 (1.5 to 3.0 fish/h) and 1999 (38.0 to 69.0 fish/h). Individuals collected by trawling (N = 207) ranged in total length from 18 to 94 mm. Round gobies were first collected in survey (trawl) samples at Escanaba in 1998 (7.5 fish/h; total length range 43 to 58 mm). Round gobies were also collected during 1999 in trawls at Muskegon and in gill nets at Grand Haven and Saugatuck. No round gobies were collected at five other eastern Lake Michigan ports sampled during this same time period (1995 to 1999). Numerous adult round gobies were captured by anglers at Grand Haven and Escanaba, indicating that these populations had been established for some time prior to being detected in survey sampling. Additional populations were verified from angler reports at Charlevoix (1998) and Kipling (1999), Michigan. Round gobies collected by anglers were generally larger, and taken in areas that are difficult to sample using conventional inshore sampling gear. These results indicate that survey and angler samples are complementary, but that up-to-date angler reports of exotic species that bite readily (round gobies) or are otherwise collected by anglers ( Bythotrephes cederstroemi, Cercopagis pengoi) are critical to tracking the spread of these organisms.

Acoustic observations of jellyfish in the Namibian Benguela

Multi-frequency acoustic data (18, 38 and 120 kHz) were collected in conjunction with pelagic trawl sampling for gelatinous macrozooplankton during a cruise to the Namibian Benguela in September 1999. Sampling focused specifically on the scyphozoan Chrysaora hysoscella and the hydrozoan Aequorea aequorea, both of which occur in large numbers, are probably of major ecolog- ical importance, and physically hamper pelagic fishing and diamond extraction activities. C. hyso- scella was detected predominantly at an inshore station and A. aequorea was found in greatest abun- dance further offshore in deeper water. Echo-sounder observations were linked directly to net catches, and relationships between catch density (number of individuals m -3 ) and nautical area scat- tering coefficients (sA) at each frequency were determined for both species in order to estimate target strength (TS) using the comparison method. TS for C. hysoscella (mean umbrella diameter 26.8 cm) was -51.5 dB at 18 kHz, -46.6 dB at 38 kHz and -50.1 dB at 120 kHz; for A. aequorea (mean central umbrella diameter 7.4 cm) TS was -68.1 dB at 18 kHz, -66.3 dB at 38 kHz and -68.5 dB at 120 kHz. These TS values compared favourably with previously published estimates for related species. Jelly- fish were caught at high numerical densities (maxima 3 C. hysoscella per 100 m 3 , 168 A. aequorea per 100 m 3 ). These high densities, combined with the not unsubstantial TS at frequencies used for fish- eries surveys, imply that jellyfish could potentially bias acoustic estimates of fish abundance. We sug- gest a simple multifrequency approach that could be used to discriminate between echoes from jelly- fish and some commercially important pelagic fish in the northern Benguela ecosystem.

Species identification of pelagic fish schools using acoustic descriptors and ancillary information

It is shown that schools of similarly sized and behaving pelagic fish species can be accurately identified to species over a large spatial area, on the order of the broad spatial-scales typical of standard assessment surveys, and over a time-span of three years. Acoustic measurements of morphometric, energetic, and bathymetric features of anchovy, sardine, and round herring schools were extracted using commercially available software from acoustic data collected by a conventional single-frequency narrow-band echosounder during trawling operations of pelagic stock surveys of the South African continental shelf (November 1997, 1998 and 1999). Discriminant function analysis of 18 descriptors of the 214 schools for which the species composition was satisfactorily determined by trawl samples indicated that schools could be correctly identified to species in 88.3% of all cases (94.9% for anchovy, 82.6% sardine, 82.6% round herring). The three species were differentiated primarily on the basis of school bathymetric position and backscattered energy. Including ancillary information (latitude, longitude, sea surface temperature, bottom depth, and time of day) in analysis improved the accuracy of school identification to 94.9%, since such variables allowed discrimination on the basis of interspecific differences in habitat use, as well as in schooling behavior. a)Currently at MIT/Woods Hole Joint Program.

Comparison of Two Otter Trawls of Different Sizes for Sampling Black Crappies

Abstract Young black crappies Pomoxis nigromaculatus are often difficult to collect, which hampers fishery managers' ability to detect weak year-classes and study early life history. Trawls have proven effective for sampling adult crappies in Florida lakes; however, this gear has received little attention as a tool to collect juvenile fish in freshwater systems. I compared two otter trawls of different sizes in two Florida lakes and found that a large (31.8-mm knotted stretch mesh) trawl was less effective than a smaller trawl of similar design (6.8-mm knotless stretch mesh) for collecting juvenile black crappies. Therefore, where it is possible to use trawl samples, the smaller mesh trawl is recommended for collection of juvenile black crappies and as a tool to assess year-class strength in systems.

Do some like it hot? Temperature as a possible determinant of variability in the growth of the Patagonian squid, Loligo gahi (Cephalopoda: Loliginidae)

Loligo gahi were collected from bottom trawl samples taken around the Falkland Islands on both commercial and research vessels from January 1994 to November 1995. Commercial samples were taken from both seasons of the fishery (February–May, August–October), research samples were taken in January, June and November. A sub-sample of squid was taken from each sample to provide statoliths for age determination by statolith increment analysis. Sea surface temperature data were derived for an area encompassing the southerly portion of the fishing grounds for L. gahi to provide an indicator of seasonal temperature variations for that region. Age data were derived for 486 females and 445 males. Exponential and power growth models explained the fit to the size-at-age data to a similar extent. Age data were grouped by month and by season of hatch to test the hypothesis that seasonal temperature variability influences the size-at-age relationship in L. gahi. Analyses of covariance showed that the relationships of both predicted mean mantle length and predicted mean body mass per month of hatch, and per season of hatch resembled closely the trends in mean monthly sea surface temperature, with the squid hatched in the summer, i.e., at higher temperatures, being significantly larger than squid of the same age but hatched in the winter months. The growth rates for adult squid in this study were similar over the age range analysed. This study provides evidence that increased temperature during a squid’s early growth period may accelerate growth markedly giving rise to significant differences in size-at-age for adult squid hatched at different temperatures.

Patterns in deep-sea macrobenthos at the continental margin: standing crop, diversity and faunal change on the continental slope off Scotland

Depth-related patterns of macrobenthic community structure and composition have been studied from box-core samples from the Scottish continental slope where deep-sea trawling and oil exploration are becoming increasingly important. There is a strong pattern of declining biomass and faunal abundance with increasing depth, but results also indicate reduced biomass and numbers of macrobenthos in the shallowest samples from just below the shelf edge where there are coarse sediments and a regime of strong bottom currents. There is also reduced species diversity at the shallowest stations, probably caused by hydrodynamic disturbance, but no clear mid-slope peak in species diversity as described from the northwest Atlantic. Taxonomic composition of the macrobenthic community shows most change between about 1000 and 1200 m, expressed as a major dichotomy in multivariate analysis by cluster analysis and ordination. It also shows up as a step-like increase in the rate of accumulation of new macrofaunal species. This corresponds to a change in hydrodynamic regime, from a seabed rich in suspension- and interface-feeding epifauna, to one where biogenic traces from large, burrowing deposit feeders are well developed, and visible epifauna rare in seabed photographs. It also corresponds to the depth zone where earlier study of megafaunal echinoderms in trawl and epibenthic sled samples also shows a clear peak in across-slope rate of change in faunal composition.

Size structure and sex composition of &lt;i&gt;Eurypodius latreillii&lt;/i&gt; Guérin, 1828 (Brachyura) and &lt;i&gt;Pagurus comptus&lt;/i&gt; White, 1847 (Anomura) in southern Chile

SUMMARY: The decapod crustaceans Eurypodius latreillii (Brachyura) and Pagurus comptus (Paguridae) were the most frequently occurring species in Agassiz trawl (AGT) samples obtained during the Chilean - German - Italian “Victor Hensen” Expedition in the Straits of Magellan and south of the Beagle Channel. Sex composition and length structure aspects were studied using the material from 15 stations in the two areas, which comprised 773 specimens of E. latreillii and 647 P. comptus. The size frequency of both species was characterized by the presence of small individuals in shallow waters. The sex composition varied with depth, indicating a dominance of E. latreillii females in deep waters, whereas no such pattern was observed in P. comptus. These differences may be caused by epizoites of E. latreillii and the availability of vacant gastropod shells for P. comptus, in addition to geographical aspects and life-cycle patterns of these species.

Comparison of Trap Nets and Otter Trawls for Sampling Black Crappie in Two Florida Lakes

We compared the effectiveness of a recreational shrimp trawl (used for Penaeus spp.) with that of trap nets for assessing populations of black crappie Pomoxis nigromaculatus. Lakes Griffin and Monroe were sampled with both gears simultaneously during Oct–December 1997. Coefficients of variation (CV = 100 × SD/mean) on mean catch per effort (CPE) values ranged from 105 to 161 for trap nets and from 62 to 96 for trawls. Both trawls and trap nets collected fish less than 150 mm total length (TL), but trawls sampled significantly more adult fish (>250 mm TL). Variable catches in trap nets would require more sampling effort than trawl sampling (up to 4 times as much) to obtain precise estimates of mean CPE. Trawl sampling was preferable to trap nets based on size of fish captured, precision of abundance estimates, cost of the gear, and required sampling effort to estimate mean CPE. However, trawl sampling may be impractical in water bodies with excessive submerged structures, debris, and submersed macrophytes. We are encouraged by the effectiveness of the trawl for assessing black crappie populations in lakes where it is possible to sample with a bottom trawl.

The mysid‐feeding guild of demersal fishes in the brackish zone of the Westerschelde estuary

The demersal fish fauna of the mesohaline zone of the Westerschelde estuary (south‐west Netherlands) was sampled intensively in the period 1990–1992. Almost 500 beam trawl samples were taken in both subtidal (330 samples) and intertidal (144 samples) habitats. These yielded 44 fish species, mostly as juveniles. The area functioned as a nursery for several demersal fish species, and harboured large populations of hyperbenthic mysids. Three gobies, three flatfish, one clupeoid and one gadoid dominated the fish fauna, while three mysid species were important components of the holohyperbenthos. From c. 1500 stomach contents of 25 fish species, 44 prey species were identified, the most abundant of which were also common in the hyperbenthal. The demersal fish community consisted of a group that foraged subtidally on fast‐moving epi‐and hyperbenthic prey (for example gadoids, gobies and clupeoids) and a group that foraged on slow‐moving or sessile endobenthic organisms, mainly in intertidal areas (for example most flatfish species). Mysidacea occurred in &gt;50% stomachs analysed and were taken as prey by 19 of the 25 fish species. Mysids were most important in the diets of Pomatoschistus minutus, P. lozanoi, Trisopterus luscus and Merlangius merlangus, and were present in appreciable numbers in Pleuronectes flesus, Trigla lucerna, Clupea harengus and Pleuronectes platessa. These species fed mainly on the brackish water endemic Neomysis integer. Mesopodopsis slabberi (present in 35% of the gobiid stomachs) and Gastrosaccus spinifer(present in 25% of the gadoid stomachs) were of secondary importance. P. minutus and T. luscus showed a diet shift from calanoids (Eurytemora aYnis and Temora longicornis) to mysids at LS of 30 and 50 mm, respectively. Only 1% of the standing stocks of the N. integer and M. slabberi populations was removed by the local demersal fish community, so top–down control of mysid populations in estuaries seems unlikely.