Mysid Abundance Research Articles

Does the Ice Age legacy end in Central Europe? The shrinking distributions of glacial relict crustaceans in Lithuania

Abstract Glacial relict crustaceans are characterized by their affinity for cold and well‐oxygenated waters and their limited dispersal ability. They occur in large, deep lakes of Northern and Central Europe and North America, with their distributions shaped by glaciation events. In many countries and especially along the southern distribution edge, glacial relict populations are declining as a result of eutrophication, global warming, and possible adverse interactions with invasive Ponto‐Caspian crustaceans. This study assessed the status of three glacial relict malacostracan species (the amphipods Monoporeia affinis and Pallaseopsis quadrispinosa and the mysid Mysis relicta) in Lithuania and modelled their abundance across environmental variables, including the presence of Ponto‐Caspian malacostracans. The results showed that M. affinis is probably extinct in the country, whereas M. relicta was found in only nine out of 16 locations from which it was previously recorded. The distribution of P. quadrispinosa also seems to be shrinking. The annual water renewal rate (P. quadrispinosa and M. relicta) and lake depth (P. quadrispinosa) were significantly and positively associated with the relative abundance of relict mysids and amphipods, but no association was found with lake size or with the presence of invasive Ponto‐Caspian crustaceans. Both species were less abundant in samples collected in the 21st century compared with the 20th century. Given the biogeographical and ecological importance of glacial relict crustaceans, their widespread declines are of concern and point to the deterioration of habitat quality, essential for other species with similar requirements. Urgent action is needed to improve water conditions and safeguard these communities. In cases where water quality improves, the reintroduction of extirpated relict populations should be considered. One example is Lake Drūkšiai, where these crustaceans became extinct during the operation of the Ignalina nuclear power plant but where conditions improved following the closure of the power plant in 2009.

Community structure and biomass of larval euphausiids and postlarval mysids in relation to anomalous warming events during a weekly time series (2014–2017) at Cabo Pulmo National Park, Mexico

Euphausiids and mysids are important components of pelagic ecosystems due to their high abundance and tendency to form dense aggregations, swarms, and schools, which concentrate biomass that attracts visual predators. Species richness, abundance, and carbon biomass of euphausiids and mysids were investigated from a weekly daytime surface zooplankton time series from 2014 to 2017, in relation to 15 environmental variables at Cabo Pulmo National Park (CPNP, 23°27′ N, 109°25′ W), Mexico. A marine heatwave (MHW; January–August 2014) and El Niño 2015–2016 (May 2015–May 2016) caused anomalously warm temperatures in relation to the 2002–2018 temperature time series. We tested the hypotheses that CPNP is dominated by a tropical euphausiid and mysid species assemblage and that abundance of both taxonomic groups decreased during the MHW 2014 and El Niño 2015–2016. Euphausiids (7.9 ind. m−3) had a higher mean abundance than mysids (3.4 ind. m−3). Eight euphausiid species, mostly of tropical-oceanic affinity, were collected (98 % larvae). Nyctiphanes simplex decreased and Euphausia distinguenda and Stylocheiron carinatum increased in abundance during El Niño 2015–2016. Five mysid species were collected (juveniles and adults). Mysidium rickettsi alone accounted for 99.98 % of total mysid abundance. Although the euphausiids collected were mainly larval stages, due to sampling in shallow coastal waters during the day, they contributed more biomass than juvenile and adult M. rickettsi in the pelagic habitat, though the abundance of this mysid in its epibenthonic habitat was likely underestimated. A canonical correspondence analysis of euphausiid and mysid abundance as a function of 10 environmental variables explained 77.4 % of the total variability, showing greater seasonal (warm vs cold) than interannual changes. The best correlated environmental variables with euphausiid and M. rickettsi abundance were net primary production (−0.419), zooplankton biovolume (0.3177) and the MEI.v2 (−0.278). We concluded that the most evident changes in community structure and abundance were seasonal (e.g. N. simplex dominated during the cold season while E. distinguenda dominated during the warm season) rather than interannual. Moreover, the abundance and biomass of euphausiids decreased during the MHW 2014 and El Niño 2015–2016; paradoxically the highest abundance and biomass of E. distinguenda and the mysid M. rickettsi occurred during El Niño 2015–2016. This implies higher available biomass for benthonic and nektonic predators in this neritic tropical reef ecosystem, contrasting with decreased abundance of euphausiids observed along the west coast of the Baja California Peninsula during this period.

Evaluation of Potential Factors affecting the Distribution of Mysids (Crustacea: Mysidacea) in Puttalam Lagoon, Sri Lanka

Limited information is available on mysids in estuarine systems in Sri Lanka. Present study attempts to find out the distribution of mysids in Puttalam lagoon and also to evaluate the possible factors affecting its distribution. The spatial distributional survey recorded 30 mysid habitats. Mesopodopsis zeylanica, Mesopodopsis orientalis and a new Sirella srilankensis were identified, with M. orientalis being the most abundant and widely distributed species. Survey of temporal variation in mysid abundance was also carried out at two sites for a six months period. This collection of M. orientalis was the first record in Sri Lanka. Distribution of mysid species is primarily dependent on the salinity. Other than the salinity distribution, and abundance of mysids depend on water level and boundary conditions. The occurrence of mysids exhibited a strong positive correlation (P ≤ 0.01) with boundary condition of the lagoon, while presence of garbage, water level, and salinity correlated negatively. Furthermore, species distribution were strongly and positively correlated (P ≤ 0.01) with boundary conditions and negatively correlated with salinity and presence of garbage. The occurrence of M. zeylanica depended on low salinity level while M. orientalis and S. srilankensis depended on high salinity. Lower water levels and boundary vegetation were more favorable for mysids along with sites consisting of mangroves or widespread seagrass beds while soil filled boundary areas and waste disposal sites along the boundary were found to be unsuitable. Reforestation and restoration of mangrove boundary areas can enhance the biodiversity in the lagoon.

San Francisco Estuary mysid abundance in the fall, and the potential for competitive advantage of Hyperacanthomysis longirostris over Neomysis mercedis

San Francisco Estuary mysid abundance in the fall, and the potential for competitive advantage of Hyperacanthomysis longirostris over Neomysis mercedis

Factors influencing the distribution of Mysids (Crustacea: Mysida) in the Negombo lagoon, Sri Lanka

Mysids are minute crustaceans that occur in large numbers in estuarine waters. Their significant role as a main food source for fish populations is becoming apparent. There is currently, limited information on mysids in Sri Lankan estuarine waters. The present study investigated the distribution of mysids in the Negombo Lagoon from June 2012 to May 2013 at 18 randomly selected- sites along the lagoon. Surveys were conducted along the boundary of the lagoon, during day time, using a dip net (opening of 25 × 20 cm2) with 500 μm mesh size. The spatial distribution survey recorded 11 mysid habitats. Mesopodopsis zeylanica, and a potentially new Siriella sp. were identified, M. zeylanica being the most abundant species. Survey of temporal variation in mysid abundance was also carried out at two sites for a six-month period. Mysid species did not show any temporal variation and the distribution and abundance was not affected by any single environmental factor. The spatial distribution of mysids along the lagoon boundary was found to depend mainly on the estuarine boundary conditions. Mangrove vegetation along the estuarine boundary was found to be favorable for mysids while concrete boundaries and accumulation of garbage along boundaries were found to be unsuitable.

Seasonal variation and abundance of Mysids in Auckland region, New Zealand

Seasonal variation and abundance of Mysids in Auckland region, New Zealand

A simulation model for estimating optimum stocking density of cultured juvenile flounder Paralichthys olivaceus in relation to prey productivity

The determination of the optimum stocking density of hatchery-reared juveniles in relation to the carrying capacity of the nursery ground is important to the success of any stock enhancement programs. In order to estimate the surplus productivity of nursery grounds available to support hatchery-reared fish, a population growth model for juvenile Japanese flounder Paralichthys olivaceus was developed by extending an ecophysiology-based model on growth of juvenile fishes. Parameters of the model were adjusted using field-collected data from Ohno Bay in northeastern Japan. This model predicted the optimum stocking density, namely the maximum number of hatchery-reared juvenile Japanese flounder released into the bay without decreasing the growth of conspecific wild juveniles owing to food limitation. The optimum stocking density for juveniles of 89mm total length was estimated to be 2000 individuals per 250,000m2 in Ohno Bay in 1989. Under the model, the most important factors affecting the optimum stocking density of Japanese flounder were the abundance of mysids and wild juveniles at the time of release. The effect of abundance of a competing flatfish species on growth of wild Japanese flounder was less important.

Composition, Abundance, and Life History of Mysids (Crustacea: Mysida) in the Coastal Lagoons of MD, USA

The composition, abundance, biomass, and life history of mysid species were investigated and described for the first time in the Maryland Coastal Bays (38° N, 75° W), Mid-Western Atlantic, using data collected from 2010 to 2013. Three species of mysids were collected, with Neomysis americana being the most abundant species (maximum mean abundance 6.7 ± 6.4 numbers (nos.) m−2 in July 2013 and biomass 2.78 ± 2.76-mg dry weight (DW) m−2 in July 2012). Americamysis bahia was the second most abundant species (maximum mean abundance: 0.7 ± 0.4 nos. m−2 and biomass: 0.23 ± 0.14 mg DW m−2 in March 2012). Metamysidopsis swifti made up 0.02 to 2 % of mysids and were found in samples collected mainly from southern Chincoteague Bay close to that Bay’s inlet in the fall of 2012. The two most abundant mysid species reproduced continuously from March to July (Neomysis) and May to October (Americamysis). N. americana had larger body and brood sizes than A. bahia. Mysids were relatively low in abundance in late summer, a period of relatively high biomass of fish predators, than during other seasons, suggesting that intense predation might be controlling their abundance. The increase in mysid abundance in the fall following their disappearance in late summer without evidence of reproductive activities suggests species migration from coastal waters into the Maryland Coastal Bays. This annual mysid subsidy perhaps helps to sustain their populations within the bays.

Usefulness of deep-ocean water pumping for the seasonal monitoring of mesozooplankton

The Okhotsk Sea is the southernmost seasonally ice-covered ocean in the Northern Hemisphere. Because of the ice coverage during winter, seasonal monitoring of zooplankton is difficult by ordinary ship-board observation. To overcome this issue, zooplankton monitoring of samples collected by deep-ocean water pumping may be useful. In this study, we evaluated seasonal changes in the zooplankton community based on the samples collected by deep-ocean water pumped from a 350-m depth off of Rausu Harbor in the southern Okhotsk Sea at 2.5-day intervals over two and a half years. Zooplankton abundance and biomass ranged from 20–550 inds. m−3 and 6–902 mg WM m−3, respectively. Both parameters showed similar seasonal changes throughout the study period. Copepods were the most dominant taxa throughout the year and accounted for 90% and 76% of the annual mean zooplankton abundance and biomass, respectively. A total of 20 genera and 33 copepod species were observed. Metridia okhotensis and M. pacifica accounted for 61% and 12% of the annual mean copepod abundance, respectively. In general, the abundance and biomass of zooplankton collected by pumped-up deep-ocean water were lower than they were in the epipelagic layer (both characteristics of the former corresponded to approximately 60% of the latter). The combined effects of low zooplankton density at the deep layer (350 m) and the low filtering rate of deep-water pumping are considered to be a possible cause of the low zooplankton abundance and biomass in the pumped-up deep-ocean water. For the zooplankton samples from pumped-up deep-ocean water, there was a high abundance of benthopelagic mysids and copepods (Xanthocalanus spp.). Because the water inlet of the deep-ocean water pumping is located near the sea bottom, the collected samples may be useful for the evaluation of seasonal changes in the population structure of the less-studied benthopelagic species.

Diet overlap between juvenile flatfish and the invasive round goby in the central Baltic Sea

The present study offers a comprehensive analysis of changes in the abundance and diet composition of juvenile flounder (Platichthys flesus) and turbot (Psetta maxima), along with other dominant coastal fish species, before and after the establishment of the alien round goby off an exposed stretch of coast in the eastern Baltic Sea.In the study area, the round goby (Neogobius melanostomus) was recorded for the first time in 2009. After a few years of low abundance, a sharp increase in the population occurred. After the round goby invasion, flatfish juveniles exhibited an increased diet overlap with other species and had a lower feeding success, reflecting an increase in resource competition. For juvenile turbot, the increase was mainly caused by the round goby, while for flounder it was due to both the round goby and the lesser sandeel (Ammodytes tobianus).Juvenile turbot, whose dominant food item before the round goby establishment had been mysids, shifted their diet towards Crangon crangon, reflecting a decrease in mysid abundance by three orders of magnitude and a concurrent doubling in C. crangon abundance in the habitat. At the same time a significant decrease in turbot recruitment was observed.Juvenile flounder had the widest food spectrum of the studied species. When the availability of the primary food item, Bathyporeia pilosa, decreased, flounder juveniles adapted by increasing the share of zooplankton in their diets. No changes in flounder feeding success and recruitment were observed. However, the recruitment estimates of flounder and turbot show an increasing co-variation after the round goby invasion, suggesting that recruitment of the species may currently be regulated by processes in the common nursery habitat.

Abundance and spatial distribution of Mysis diluviana in Lake Ontario in 2008 estimated with 120 kHz hydroacoustic surveys and net tows

Mysids are an important component of Great Lakes foodwebs, both as a prey for fish and as a predator on zooplankton. We monitored mysid abundance in Lake Ontario using lake-wide hydroacoustics data and vertical net hauls collected 1–5 August 2008 during the Ontario Ministry of Natural Resources and New York State Department of Conservation forage fish survey. Acoustic volume backscattering strength was highly correlated with both density and biomass of mysids although the correlation with biomass was stronger. The slopes of these relationships were not significantly different from theory (0.10) indicating a linear relationship between abundance and backscattering strength. Size structure significantly affected the relationship between backscattering strength and density but not between backscattering strength and biomass. Average target strength for areas with small mysids was −91.9 dB per mysid, and in areas with both small and large mysids it was −88.2 dB per animal. Acoustic estimates for Lake Ontario calculated with these regressions provided a lake-wide estimate weighted by lake area within depth intervals, of 228 mysids m−2 (CV 17.6%) and 0.93 g dw m−2 (CV 12.9%). Density and biomass in 14 net hauls averaged 316 mysids m−2 (range 0 to 1113) and 1.35 g dw m−2 (range 0 to 3.89). Mysid biomass was 33% of the biomass of other crustacean zooplankton in the summer of 2008. As in 2005, areas of low mysid abundance were detected in the middle of the transects and along the north shore. Lakewide mysid abundance in the summer of 2008 was higher than in a similar acoustic survey in the summer of 2005 and in whole-lake autumn 2000–2007 net surveys, suggesting that mysids are persisting in Lake Ontario and abundant relative to other Great Lakes.

Flow regime in a restored wetland determines trophic links and species composition in the aquatic macroinvertebrate community

In a restored wetland (South of Spain), where different flow regimes control water exchange with the adjacent Guadalquivir estuary, the native Palaemon varians coexists with an exotic counterpart species Palaemon macrodactylus. This controlled m\\acrocosm offers an excellent opportunity to investigate how the effects of water management, through different flow regimes, and the presence of a non-native species affect the aquatic community and the trophic niche (by gut contents and C-N isotopic composition) of the native shrimp Palaemon varians. We found that increased water exchange rate (5% day−1 in mixed ponds vs. 0.1% day−1 in extensive ponds) modified the aquatic community of this wetland; while extensive ponds are dominated by isopods and amphipods with low presence of P. macrodactylus, mixed ponds presented high biomass of mysids, corixids, copepods and both shrimp species. An estuarine origin of nutrients and primary production might explain seasonal and spatial differences found among ponds of this wetland. A combined analysis of gut contents and isotopic composition of the native and the exotic species showed that: (1) native P. varians is mainly omnivorous (2) while the non-native P. macrodactylus is more zooplanktivorous and (3) a dietary overlap occurred when both species coexist at mixed ponds where a higher water exchange and high abundance of mysids and copepods diversifies the native species' diet. Thus differences in the trophic ecology of both species are clearly explained by water management. This experimental study is a valuable tool for integrated management between river basin and wetlands since it allows quantification of wetland community changes in response to the flow regime.

Variabilidad de agregaciones suprabentónicas costeras de playas arenosas de la costa Caribe de Venezuela

The suprabenthos or hyperbenthos is the macrofaunal assemblage of small-sized organisms that interact for some time in the benthic boundary layer. Information about the taxonomic composition and role of suprabenthic species, especially in littoral zones, is scarce and scattered. This work attempts to contribute alleviate this problem. We analyze the temporal and spatial variations of suprabenthic assemblages in the swash-zone from four beaches of the littoral coast of Venezuela. For each beach, two sites were chosen, and special attention was given to water and sediment characteristics. 12 environmental variables were measured: Dissolved oxygen, oxygen saturation percentage, pH, salinity, surface temperature, total, organic and inorganic suspended solids, total organic carbon, organic matter in sediment, grain size of sediment, and amount of dragged material of sample. All faunal samples were taken on a monthly basis during 2011; these were extracted using a manual suprabenthic sledge towed parallel to the shoreline. Samples were sorted and identified to their lowest possible taxonomic level. A total of 24 141 specimens (mean abundance: 26.16 +/- 55.35 ind./m2) belonging to 21 taxonomic groups were identified. Analysis suggests that seasonality does not explain observed changes either in fauna or environmental variables. It was found that suprabenthic assemblages, total suprabenthos density, richness and environmental variables changed in a dissimilar fashion between months and beaches. The most frequent groups were amphipods and decapods; and at the species/categories level post-larval shrimp (Penaeidae), Grapsidae crab megalopae and Arenaeus cribarius megalopae were common. Dissimilarity between months in each beach was primarily explained by the abundance of amphipods, ctenophores, decapods and mysids. For particular months and selected beaches very high abundances of ctenophores were found. This group dominated the sample even though it is not usually a representative group in suprabenthos. Samples showed low correlations between suprabenthos and environmental variables. A somewhat stronger correlation could be established between water characteristics and dragged material abundance. The studied suprabenthos assemblage was found to have high taxa richness and very dynamic behaviour at spatial and temporal scale. Further analysis suggested that there is no evident pattern of distribution and that causality can not be directly attributed to temporal variation only. Possibly there is an influence of a synergy of environmentals or biological factors, rather than a single variable. The species Americamysis bahia and Americamysis taironana are reported for the first time in Venezuela. This study represents the first ecological research of the suprabenthos in the Caribbean region.

Distribution and abundance of mysid shrimps (Crustacea: Mysidacea) in the Northern Indian Ocean

The distribution and abundance of mysids were studied based on materials collected from the upper 1000 m of the Northern Indian Ocean (Exclusive Economic Zone (EEZ) of India) during the period 2002–2006. A greater abundance of mysids occurred in the Arabian Sea (48.6% of the total mysid population) compared to the Bay of Bengal (24.8%) and the Andaman Sea (26.6%). Both bathymetry and hydrography are important factors influencing mysid distribution and abundance. In the Arabian Sea, maximum mysid abundance occurred during the inter-monsoon (IM) period, but in the Bay of Bengal and the Andaman Sea abundance was highest during the northeast monsoon (NEM). Twenty-seven species belonging to 21 genera were recorded from the study area. Maximum species richness occurred in the Arabian Sea (18) followed by the Andaman Sea (13) and the Bay of Bengal (9). Pseudanchialina pusilla was the most abundant species found (64.8% of the total mysid population) followed by Siriella gracilis. Out of the 27 species found, 13 are restricted to surface waters. Below 500 m depth only two species, Boreomysis plebeja and Giberythrops acanthura, were found. In the Bay of Bengal and the Andaman Sea, mysid abundance did not show much variation between nearshore and offshore regions.

Mysis diluvianapopulation and cohort dynamics in Lake Ontario before and after the establishment ofDreissena spp.,Cercopagis pengoi, andBythotrephes longimanus

We investigated population responses of Mysis to ecosystem changes induced by invasion of dreissenids and predatory cladocerans, Cercopagis and Bythotrephes . Lake productivity declined as dreissenids invaded the offshore region. Whole-lake mysid biomass was compared before (early 1990s) and after (2002–2007) the invasion period; it declined 40%–45%. Abundance of young mysids and presence of a summer cohort increased with summer, epilmnetic, nighttime zooplankton biomass (i.e., food biomass index). Cercopagis + Bythotrephes biomass was negatively correlated with this index, implicating them in the mysid decline. Eggs per gravid female increased with autumn, total-water-column zooplankton biomass, reflecting the greater use of hypolimnetic waters by adults. Reproductive success was below replacement during the period 2002–2005. First-year mysid growth rate was maintained while population abundance declined, suggesting selection for individuals that feed effectively at low food concentrations. Mortality rates in the first and second years were dependent on cohort density, indicating that competition for food limited abundance in the first 2 years. Fish predation indices (smelt and alewife combined) were correlated positively with mortality rates and negatively with abundance in the third year. Thus, mysids cannot support as many fish in invaded compared with non-invaded lakes. They may also not be a stable food resource; unusual cohort losses occurred in some years.

Seasonality and biological characteristics of the shallow-water mysid Mesopodopsis orientalis (Crustacea: Mysida) on a tropical sandy beach, Malaysia

The seasonality in abundance, population structure, and other biological aspects of the tropical shallow-water mysid Mesopodopsis orientalis were studied at an open sandy beach with fine sand on the southern coast of Penang Island, north-western Peninsular Malaysia, from December 2004 to August 2006. The monthly collections ranged from 0.7–534 indiv.m−2, with an overall mean of 49 indiv.m−2. The density, however, could reach as high as 8,656 indiv.m−2. There were no correlations between the abundance of mysids and the temperature, salinity, or tidal cycle, and the wide range in the variations of catches was considered to be mainly due to their heterogeneous distribution. Moreover, no distinct pattern was found in abundance between day and night and again between shady and sunny areas. Males accounted for 33.3–56.5% (mean; 47.3%) of the monthly samples; females predominated over males in the entire shore population. Reproduction of M. orientalis in Penang waters is continuous throughout the year and seasonality in reproductive traits is non-existent or, if present, considerably reduced. The number of eggs/embryos carried by females exhibited a positive correlation with female size, and the maximum brood size was 24. The egg diameter ranged from 0.30–0.42 mm, with a mean of 0.37 mm, and was independent of female size. Thus, the clutch size of the Penang population of M. orientalis was more fecund than that observed in the estuarine counterpart, Mesopodopsis tenuipes, but the former was smaller in size than the latter during all stages in its life cycle.

Seasonal and vertical variations in emergence behaviors of Neomysis americana

Emergence patterns were observed in the macrotidal Damariscotta River estuary in Maine in fall 2005 and summer 2006. High temporal and vertical resolution was achieved with two multifrequency echo sounders sampling from 265 kHz to 3 MHz. Time‐series waterfall plots and spectral analysis revealed that emergence patterns observed at 265‐420 kHz (frequencies that detect animals larger than mesozooplankton) followed combinations of diel and tidal rhythms that differed with both depth in the water column and season. The dominant emergent species at the study site, Neomysis americana, showed nocturnal emergence in summer. Toward the end of its emergence season in fall, however, its dominant rhythm shifted to semidiurnal (12.4‐h period). The timing of major emergence in the fall coincided with low slack tides near the surface but with peak flood speed near the bottom, providing plausible mechanisms of retention and selective tidal stream transport within the estuary. Our results confirm earlier suggestions from net sampling that emergence in this species is best conceived as a broadening of the vertical spread of the population distribution rather than a migration of the bulk of the population far off the bottom: Most individuals spent most of their time near the bottom, even during emergence. Prior acoustic work that did not resolve the lowermost water column reported emergent mysids to overwhelm the holoplankton in biovolume, but it, too, seriously underestimated mysid abundance because it failed to resolve this high concentration of animals in the lowermost water column. In the mesozooplankton size category, emergence reached surface waters only during low slack tides and systematically avoided times of the fastest currents. This study highlights the importance of high temporal and spatial sampling resolution in detecting and understanding components of emergence and points away from simple models of diel vertical migration toward state‐dependent, individual‐based models of habitat utilization.

Hydroacoustic measures of Mysis relicta abundance and distribution in Lake Ontario

Mysis relicta can be observed on echograms as a sound scattering layer when they migrate into the water column at night to feed on zooplankton. However, quantitative measures of mysid abundance with hydroacoustics requires knowledge of mysid target strength (TS), a method of removing fish echoes and contribution from noise, and an understanding of the effect of range on the ability of hydroacoustics to detect mysids (the detection limit). Comparisons of paired net data and acoustics data from July 7, 2005 yielded a mysid TS of −86.3 dB (9 mm animal) and a biomass TS of −58.4 dB (g dry wt)−1. With ambient noise levels (Sv of −125 dB at 1 m depth) and this TS, we can detect a mysid density of 1 m−3 at 60 m depth with a signal to noise ratio of 3 dB. We present a method to remove backscattering from both noise and fish and apply this method and the new TS data to whole lake acoustic data from Lake Ontario collected in July 25–31, 2005 with a 120 kHz echosounder as part of the annual standard fish survey in that lake. Mysis abundance was strongly depth dependent, with highest densities in areas with bottom depth > 100 m, and few mysids in areas with bottom depth < 50 m. With the data stratified in five bottom depth strata (> 100 m, 100-75 m, 75–50 m, 50–30 m, < 30 m), the whole-lake average mysid density was 118 m−2 (CV 21%) and the whole-lake average mysid biomass was 0.19 g dry wt m−2 (CV 22%) in July 2005. The CVs of these densities also account for uncertainty in the TS estimates. This is comparable to whole-lake density estimates using vertical net tows in November, 2005 (93 m−2, CV 16%).

Cross-shore Mysidacea (Crustacea) distribution in two sandy beaches with contrasting morphodynamics

The order Mysidacea comprises small and very common crustaceans that inhabit most coastal ecosystem. The present contribution describes the composition, abundance and distribution of mysids in two sandy beaches representing the extreme conditions of the morphodynamics spectrum. Six species of mysids were found at the studied beaches. Metamysidopsis neritica Bond-Buckup & Tavares, 1992 was the most abundant species with more than 97% of the total individuals collected at the two beaches. At the dissipative beach was dominant in the swash and surf zone, with the greater abundance in the inner surf zone. At the reflective beach, this species was extremely abundant in the surf zone and backward the breaker. Bowmaniella brasiliensis Bacescu, 1968 occurred in low density at the surf zone of the dissipative beach, with the greater abundance in the outer surf zone, and it was practically absent in the nearshore zone. We suggest that mysids zonation in sandy beaches may be first developed as a response to food availability patterns imposed by the nearshore circulation systems, and in the second place as a result of competitive exclusion. The suggestion of partitioning of food and spatial resources shown by the mysid species is surprising in view of the apparent physical harshness and low spatial heterogeneity that seem to characterise the beach environment.

Feeding ecology of juvenile turbot Scophthalmus maximus and flounder Pleuronectes flesus at Gotland, Central Baltic Sea

Food and feeding of juvenile turbot Scophthalmus maximus and flounder Pleuronectes flesus were studied in five nursery areas at Gotland, Central Baltic Sea, ICES SD 27 and SD 28. Ontogeny involved partitioning of available food resources. The food choice of turbot <30 mm standard length (LS) included both planktonic‐hyperbenthic prey (calanoid copepods and mysids) and epibenthic–endobenthic prey (chironomids and amphipods), whereas turbot ≥30 mm LS fed mainly on hyperbenthic species (mysids and fishes). Conversely, for flounder, epibenthic–endobenthic prey were the most abundant prey items throughout development (harpactocoid copepods, oligochaetes and chironomids for fish <40 mm LS and oligochaetes, chironomids and amphipods for flounder ≥40 mm LS). Thus, the highest degree of dietary overlap occurred between turbot <30 mm and flounder ≥40 mm. Food composition for both turbot and flounder varied, however, according to exposure and predominant wind direction in the nursery area. For example, expressed as the ratio between the biomass of mysids and fishes consumed, the relative importance of mysids v. fishes as food source for turbot, varied from <1 in the most sheltered area to 16 and 27 in the more open areas. Considerable differences in feeding incidence were recorded; mean ±s.d. 58 ± 20% for turbot <30 mm LS and 83 ± 8% for turbot ≥30 mm LS, as opposed to ≥85–90% for flounder irrespective of size. The lower feeding success of turbot <30 mm LS was related to mysid abundance, shown to vary spatially and temporally, and to density of flounder, indicating that food availability, and potentially interspecific competition, influence feeding of early juvenile turbot with implications for survival following settlement. Regarding variability in abundance, hyperbenthic prey, as mysids, are considered more variable than epi‐ and endobenthic organisms. Hence, in addition to the ‘nursery size hypothesis’, i.e. the positive relationship between abundance of recruits and extension of nursery areas, variability in food availability may explain the average lower recruitment of turbot as compared to other flatfishes, e.g. flounder.