Annual Average Abundance Research Articles

Dynamics and Potential Production of White Sturgeon in the Unimpounded Lower Columbia River

The unimpounded lower Columbia River downstream from Bonneville Dam (LCR) supports the greatest abundance and density of white sturgeon Acipenser transmontanus reported in the speciesˈ range. The high productivity of the population resulted from growth that was as good as or better than reported for other populations, the highest mean relative weight reported for any white sturgeon population, and a relatively low median age (24 years) of first maturity for females (95% of females matured between 16 and 35 years of age). Estimated instantaneous natural mortality was 10%. During 1986–1990, the estimated instantaneous fishing mortality for age-12–17 fish averaged 36% in LCR fisheries. Over the same period, the average annual abundance estimate of LCR white sturgeon 54 cm fork length (FL) or longer was 895,500 fish, and the average density was 14.6 fish per ha. Population simulations, under the assumption of constant recruitment, predicted a maximum sustainable yield (MSY) of 3.0 kg/recruit at an 18% exploitation rate (of the 82–166-cm FL population). Reproductive potential was 93,400 eggs/recruit for an unexploited population and 4,800 eggs/recruit at the predicted MSY exploitation rate. The factors most responsible for the favorable production potential of the population were access to marine areas, abundant food resources, and consistently favorable hydrologic conditions during the spawning period.

Macrozoobenthos abundance and community composition in three lower Rhine floodplain lakes with varying inundation regimes

AbstractThe relative importance of lake inundation regime, littoral substrate and season for macrozoobenthos abundance and community composition was studied in three Lower Rhine floodplain lakes situated along a flood duration gradient. In each lake the macrozoobenthos density, biomass and community composition were determined monthly over one year in two contrasting littoral substrates (sandy‐mineral sediments and clayey‐organic sediments). The influence of the lake inundation regime on the macrozoobenthos abundance was tested against those of littoral substrate and season with a three‐way analysis of variance (ANOVA). Between‐lake variation in macrozoobenthos abundance was clear for Bivalvia, Ephemeroptera, Heteroptera and Trichoptera. Bivalvia (Pisidium sp.) were most abundant in the frequently flooded lake, Heteroptera [Micronecta sp., Sigarafalleni (Fieber), S. striata (L.)] in the rarely flooded lake and Trichoptera [Mystacides longicornis (L.), M. nigra (L.)] and Ephemeroptera [Caenis horaria (L.), C. luctuosa Burmeister] in the isolated, non‐flooded lake, suggesting an impact of inundation regime on the abundance of these dominant taxa in Lower Rhine floodplain lakes. In each lake the annual average abundance of macrozoobenthos was dominated by Gastropoda [Bithynia tentaculata (L.), Potamopyrgus antipodarum (Gray), Valvata piscinalis (Müller)].Between‐lake variations in the relative densities of macrozoobenthos functional feeding groups suggests an impact of the inundation regime on the macrozoobenthos communities via the availability of nutritional resources. Relative densities of filter‐feeders (e.g. Pisidium sp.) were highest in the frequently flooded lake with its high phytoplankton biomass and poorly developed vegetation. Shredders (e.g. Mystacides sp.) were most numerous in the non‐flooded lake with its well developed aquatic vegetation and low phytoplankton biomass.Within‐lake variations in macrozoobenthos abundance could be attributed to littorai substrate and season. The importance of substrate on macrozoobenthos abundance was most clear for Oligochaeta and Trichoptera. The importance of season on macrozoobenthos abundance was clear for the total macrozoobenthos and for all taxonomic groups, except Oligochaeta. Correlations between the total macrozoobenthos abundance and seasonally fluctuating parameters, i.e. flood pulse duration, water level, water temperature and vegetational coverage in the lakes, suggest that hydrological parameters were most important in the frequently flooded lake, whereas vegetational coverage and water temperature were most important in the isolated, non‐flooded lake.

Nephrops norvegicus in the Galician continental shelf (NW Spain): abundance and distribution

This paper examines the abundance and distribution of Nephrops norvegicus during 13 fishery survey cruises carried out between 1980 and 1991 on the Galician continental shelf (NW Spain). Average annual abundance values fluctuate between 3 and more than 90 specimens per 30 min tow, with the highest densities found between 100 and 200 m in the Miño to Fisterra area, and at over 200 m in the Fisterra to Estaca de Bares area. Catches of this species were very scarce in the area between Estaca and Ribadeo. Mapping of N. norvegicus distribution, carried out using geostatistical analysis techniques, points to the existence of patches ranging in size between 20 and 117 km in length in the different cruises, which are located in two major areas, off the Rías Baixas and north of Fisterra. Depth is not a determining factor of N. norvegicus abundance and distribution within the area under study. There is a coincidence, on a medium scale, between the presence of this species and areas having fine sediment (between 20 and 60% of fine fraction less than 62.5 μm) on the shelf, and the distribution was restricted to the area between Miño and Estaca.

The seasonal and global behavior of water vapor in the Mars atmosphere: Complete global results of the Viking Atmospheric Water Detector Experiment

The water vapor content of the Mars atmosphere was measured from the Viking Orbiter Mars Atmospheric Water Detectors (MAWD) for a period of more than 1 Martian year, from June 1976 through April 1979. Results are presented in the form of global maps of column abundance for 24 periods throughout each Mars year. The data reduction incorporates spatial and seasonal variations in surface pressure and supplements earlier published versions of less complete data. Column abundances vary between 0 and about 100 precipitable microns (pr μm), depending on location and season, while the entire global abundance varies seasonally between an equivalent of about 1 and 2 km3 of ice. The first appearance of vapor at nonpolar latitudes as northern summer approaches and the drop in abundance at mid‐latitudes as summer ends both strongly imply the existence of a seasonal reservoir for water within the regolith. There appear to be no net annual sources away from the poles that contribute significant amounts of water. However, the strong annual gradient of vapor from north to south implies a net annual flow of vapor toward the south. This southward flow may be balanced by a northward flow during the global dust storms, by transport in the form of clouds or adsorbed onto dust grains, or during other years. The perenially cold nature of the south polar residual cap, along with the relatively large summertime vapor abundances over the cap, implies a net annual condensation of vapor onto the cap. Comparison with earlier (Earth‐based) observations of vapor in the south during the local summer indicates that all of the seasonal CO2 cap may sublime away in some years to reveal the water ice cap which must lie underneath, with corresponding large southern summer vapor abundances. The global distribution of the annual average abundance of vapor correlates well with Martian topography, as might be expected for a uniform constant atmospheric mixing ratio. If this topographic effect is divided out, the resulting residual map correlates well with maps of surface albedo and thermal inertia; this correlation may be related to the control exerted by the surface and subsurface temperatures on the adsorption/desorption process and on the atmospheric temperature profile and, hence, the vapor holding capacity of the atmosphere.