Surface Tows Research Articles

Distribution of myctophid fishes in the Eastern Pacific Ocean off northern Chile (18°30´ TO 30°30'S)

Thejuvenile and adult myctophid fishes from two cruises off northern Chile (from 18°30' to 30°30’S) captured with a HPN midwater trawl in oblique tows and a Bongo net in double oblique tows, 0 to 200 m and a CalCOFl net in surface tows, were identified and their distributions plotted. Eleven species were caught during the winter cruise and fourteen during the summer cruise. Triphoturus mexicanus was the most numerous and widespread species in both cruises. Diogenichthys atlanticus, D. latematus, Hygophum bruuni and Myctophum nitidulum were alsocommon. Almost ail specimens of all species were caught during night-time sampling, suggesting different day and night vertical distributions. The length-frecuency distribution of most of the species captured is analyzed.The apparent relationship between northward or southward extensions of the seasonal distributions of several speciesand the physical parameters recorded during the cruises is discussed.

The structure of zooplankton and its relationship to marine conditions in the northern Colombian Caribbean

During the dry and rainy seasons of 2019, two research cruises were conducted to determine the composition, abundance, biomass, and diversity of the zooplankton assemblage, and its relationship with the water conditions, along the northern Colombian Caribbean coast. The 21 sampling locations were situated in two of the Colombian Caribbean’s physicochemical provinces: Magdalena-Ciénaga Grande de Santa Marta (Mag-CGSM) and Parque Nacional Natural Tayrona-Guajira (PNNT-Gua). At each location, samples were collected using a bongo net through circular surface tows. Eighty-seven taxa (72 families and 15 morphotypes) were recorded, with the highest number of families found in Mag-CGSM during the dry season. The highest abundance and biomass were observed in PNNT-Gua during the rainy season while the highest species diversity was recorded in PNNT-Gua during the dry season. Conversely, the lowest species diversity was detected in the same province during the rainy season, coinciding with an increase in dominant families. This increase in dominance resulted in a decrease in uniformity. The spatiotemporal dynamics of the assemblage were most closely associated with the dissolved oxygen and salinity, highlighting their crucial role in shaping the structure of the zooplankton community.

Daily and monthly ichthyoplankton assemblages of La Azufrada coral reef, Gorgona Island, Eastern Tropical Pacific

The description of the daily and seasonal variation of the species composition and abundance on the early stages of fishes in coral reefs can lead to a better understanding of fish population dynamics in these ecosystems, because settlement and recruitment processes depend on larval fish supply. Daytime and nighttime samples of ichthyoplankton were collected through circular surface tows using a bongo net between October 2010 and November 2011 at La Azufrada, the largest coral reef on the Colombian Pacific coast, located in Gorgona Island. A total of 2,171 fish larvae representing 87 species in 42 families were identified. The most important species by their frequency of occurrence and density were Anisotremus sp., Cetengraulis mysticetus, Bregmaceros bathymaster, and Gobiidae sp.4. Daytime and nighttime samples showed variations in the composition of the assemblage, with a greater number of species at night, evidencing short-scale variations over a diel cycle. On the other hand, environmental parameters such as oxygen, salinity, pH, temperature, and zooplankton biomass were also measured; however, none of them had a strong relationship with the larvae assemblage structure throughout the year. In conclusion, this study suggests that night conditions had a higher effect over the ichthyoplanktonic species found in La Azufrada coral reef than the observed changes in the environmental parameters.

Indexing Age‐0 Walleye Abundance in Northern Wisconsin Lakes before Fall

AbstractRecruitment of age‐0 Walleye Sander vitreus is often indexed using fall electrofishing surveys. However, collecting fish before fall may provide timely information regarding stocking decisions and factors influencing recruitment. We evaluated sampling methods for age‐0 Walleye in northern Wisconsin lakes that could be used to assess recruitment in spring and summer. Initial assessments on two lakes indicated surface tows of ichthyoplankton nets at night during May to early June and 0.64‐cm‐micromesh gill nets set in July provided the highest catches of age‐0 Walleye among the methods we evaluated. Additional sampling on 13 lakes over 2 years indicated catch per effort (CPE) of age‐0 Walleye using these two methods did not correlate with age‐0 CPE in fall electrofishing. However, presence or absence of age‐0 Walleye in micromesh gill nets was 92% accurate in predicting whether age‐0 CPE in fall electrofishing was ≥15 fish/h, the threshold above which eventual recruitment to the fishery is expected to occur. Micromesh gill netting may provide a useful tool for allocating fingerling Walleye that are stocked in fall, a resource that is often limited due to space and forage constraints associated with propagation. Additionally, our sampling protocol may help to identify timing of potential recruitment bottlenecks occurring in some lakes.

Litter Windrows in the South-East Coast of the Bay of Biscay: An Ocean Process Enabling Effective Active Fishing for Litter

Large scale convergence regions of floating marine litter are commonly observed in semi-enclosed seas as the Bay of Biscay. However, clean-up activities on such accumulation regions are limited by the spread of the large-size floating litter on the sea surface. Data gathered by a small-scale fishing vessel devoted to active fishing for floating litter activities during the spring and summer of 2018 reveals that the linear streaks of high concentration of floating litter (so-called litter “windrows”) are common accumulation structures in the south-east coast of the Bay of Biscay. The random search of litter windrows for their collection through surface tows of macro-nets was proved to be an effective action for floating litter mitigation. A total of 196 tows collected 16.2 tons of floating marine litter in 68 working days. Most of the litter windrows were around 1 km length and, on average, accumulated 77.75 kg of floating marine litter. Fishing, shipping and aquaculture sectors were the source of 35% of the 4,130 litter items analyzed (55% in weight of the sourced items), and plastic was the most common type of material (96% in terms of items). A better understanding of the phenomenon of the litter windrows, capable to guide clean-up efforts in space and time, would provide a considerable improvement in the efficiency of mitigation actions to reduce the marine litter pollution. The observations of litter windrows in the coastal area of the south-east of the Bay of Biscay demonstrate the key role of submesoscale processes in the distribution of FML. The present work provides a thorough description of floating litter windrows in nature, which it was nonexistent to date. The results are the kind of proof necessary to boost the research addressed on the submesoscale aggregations of FML. Coupling litter windrows observations with remote-sensing technology and high-resolution modelling techniques offer great opportunities for the mitigation actions against marine litter.

Marine Debris Polymers on Main Hawaiian Island Beaches, Sea Surface, and Seafloor.

Polymeric differences of plastic debris were assessed across four compartments of the Main Hawaiian Islands (sea surface, windward beaches, leeward beaches, and seafloor) to better describe sources and fate. Plastic debris pieces (n = 4671) were collected from 11 beaches, three sea surface tows, and three seafloor dives. Fourier transform infrared spectroscopy identified the polymers of 3551 pieces. Significant differences (p < 0.05) in concentration, types, polymer composition, and weathering were found among four compartments. Windward beaches had 1-2 orders of magnitude more plastic pollution (g/m2) than leeward beaches, despite smaller human populations on windward sides. Sea surface and windward beaches were dominated by severely weathered, less dense floating polymers (polyethylene and polypropylene comprised 92.7 and 93.5% on average, respectively, of the total debris mass), while leeward beaches and the seafloor debris consisted of less weathered and more dense sinking polymers (e.g., 41.0 and 44.7% of total mass consisted of the sum of polystyrene, nylon, cellulose acetate, polyethylene terephthalate, and additive-masked debris). These results are some of the first to provide evidence of polymeric stratification in the marine environment and emphasize that the majority of marine debris in Hawaii is floating in from distant sources rather than from Hawaii's residents or tourists.

Seasonal changes in paralarval cephalopod communities on the southwest coast of Baja California Sur (spring and autumn 2003)

We recorded seasonal changes in the paralarval cephalopod communities collected off the southwestern coast of Baja California Sur, Mexico, during 2003. Simultaneous surface and oblique trawls were carried out with CalCOFI-type nets and CTD casts during spring and autumn 2003 on board the R/V Río Suchiate (Secretaría de Marina), at 85 sampling stations (Magdalena Bay to Cape San Lucas). The paralarval cephalopod community predominantly showed tropical affinity in both seasons. During autumn, 11 tropical taxa, 2 cosmopolitan taxa, and one temperate taxon were incorporated to the paralarval community recorded previously in spring and were associated with the decrease in Subarctic Water and the entrance of warm water masses to the study region, modifying the community indexes between seasons. The canonical correspondence analysis showed 2 groups significantly correlated with chlorophyll a concentrations and zooplankton volumes. Oblique tows provide better information on the paralarval cephalopod community, while surface tows can be an efficient method for monitoring and evaluating the hatching areas of the jumbo Pacific squid Dosidicus gigas through their paralarvae, which are included in the Sthenoteuthis oualaniensis–Dosidicus gigas complex (SD complex).&#x0D; Key words: paralarvae, community structure, SD complex, Baja California Sur.

A Covered Cod-End and Tow-Path Evaluation of Midwater Trawl Gear Efficiency for Catching Delta Smelt (Hypomesus transpacificus)

Author(s): Mitchell, Lara; Newman, Ken; Baxter, Randall | Abstract: For nearly 50 years, the California Department of Fish and Wildlife has used a midwater trawl to intensively monitor fish populations in the San Francisco Estuary during the fall, sampling over 100 locations each month. The data collected have been useful for calculating indices of fish abundance and for detecting and documenting the decline of the endangered fish species delta smelt (Hypomesus transpacificus). However, efforts to calculate estimates of absolute abundance have been hampered by the lack of information on gear efficiency, in particular questions about contact selectivity and the effect of tow method on catches. To answer these questions we conducted a study that used a covered cod end on a net towed either near the surface, referred to as a surface tow, or throughout the water column, referred to as an oblique tow. A contact selectivity model was fit to estimate the probability that a delta smelt that has come into contact with the net is retained in the cod end of the net conditional on its body length. Full retention of delta smelt was found to occur around 60 mm fork length. Delta smelt catch densities for the surface tows were an order of magnitude greater than densities in the oblique tows, suggesting a surface orientation at the sub-adult life stage. These results represent an important step in being able to calculate absolute abundance estimates of the delta smelt population size using decades’ worth of monitoring data.

Molecular identification and morphological description of Micropogonias megalops, Cynoscion othonopterus, C. reticulatus and Menticirrhus nasus larvae, collected in the upper Gulf of California during Summer 2012

Sciaenidae fish larvae were collected from the upper Gulf of California during September 2012 using a conical net (505 μm) through surface tows. These were pre-classified into four larval morphotypes, based on external characteristics (mainly meristic and pigmentation). Partial sequences of cytochrome c oxidase, subunit 1 and 16S rRNA (16S) genes of mitochondrial DNA, were used in molecular genetic identification from each larval morphotype. Genetic results indicated the identification of four larval morphotypes as Micropogonias megalops, Cynoscion othonopterus, C. reticulatus and Menticirrhus nasus. Pigmentation patterns of larvae described after molecular genetic identification made it possible to distinguish between M. megalops, M. nasus and C. othonopterus (postflexion). However, pigmentation was not reliable for differentiating between preflexion larvae of C. othonopterus and C. reticulatus. From these results, both morphological and genetic approaches were proposed as complementary tools in taxonomic studies of ichthyoplankton, particularly in early fish larvae identification of congeneric species with similar morphological characteristics.

The influence of environmental parameters on fish larval distribution and abundance in the mangrove estuarine area of Marudu bay, Sabah, Malaysia

The aim of this study was to estimate the influence of selected environmental parameters on the fish larval distribution and abundance between October 2012 and September 2013 in the Mangrove estuarine area of Marudu bay, Sabah, Malaysia. Fish larvae samples were collected monthly using plankton net of 350 m of mesh size with a mouth opening of 40.5 cm in diameter through 20 min surface tows. Five different stations were chosen in this study. In total 3,879 larvae, belonging to 20 families were recorded, with a mean abundance of 118 larvae per 100 m 3 . Among 20 families, top four families such as Sillaginidae (44%), Engraulidae (14%), Mugilidae (12%) and Sparidae (10%) occurred consistently around the year in the study areas. Sillaginidae was the most abundant larval fish families. In situ environmental parameters such as water temperature, dissolved oxygen (DO), salinity, pH and conductivity were recorded during the sampling times. Significant variations were found in the environmental parameters among the five stations except DO and pH (p < 0.05). It is revealed that mainly four families (centriscidae, Engraulidae, Mugilidae, and Sillaginidae) were significantly influenced by the abiotic factors. Of the environmental parameters recorded, salinity appeared to be the main factor affecting the distribution and abundance of Mugilidae families. Overall, regression analysis test indicated a weak overall correlation between larval assemblage and environmental parameters in the estuary of Marudu Bay.

CERATIUM SCHRANK (DINOFLAGELLATAE, PERIDINIALES) DE LAS ISLAS DEL ROSARIO, CARIBE COLOMBIANO

The species of Ceratium Schrank (Dinoflagellatae, Peridiniales) of Rosario Islands, Colombian Caribbean Sea, are pointed out in relation to physical parameters of sea ÿwater for three climatic periods. It is found that the species also correspond to three different populations, some of them being common for the three periods.29 species are reported, 12 are for the first time for the Colombian Caribbean Sea: Ceratium praelongum, C. kofoidii, C. incisum, C. longiroztrum, C. contrarium, C. inclinaturn, C. vultur, C. ranipes, C. reflexum, C. gibberum, C. limulus and C. schranki).Species like C. praelongum, C. ranipes, C. tenue, C. inclinatum, C. vultur and probably C. reflexum and C. lunula usually regarded as shade species, captured in surface tows may show a renewed water caused by strong winds or by small upwellings.Finally, the conservation of C. tenue and C. inclinatum as valid species and not as synonyms of C. horridum is proposed.

Spatial and temporal occurrence of fin whales (Balaenoptera physalus) and euphausiids (Nyctiphanes simplex) in the Ballenas Channel, Gulf of California, Mexico

Fin whales (Balaenoptera physalus) are observed year-round in the Ballenas Channel and Bahía de los Ángeles region, Gulf of California, where their main food item is the euphausiid Nyctiphanes simplex. This investigation was designed under the hypothesis that the spatial and temporal occurrence of whales and euphausiids coincides. The aim was to establish the geographic, seasonal, and interannual changes in the abundance of the fin whale and its main prey in the region during 2003 and 2004. Four field trips were carried out per year. Surveys to search for whales and carry out horizontal surface tows were conducted from small boats. Fin whales and euphausiids were mainly observed off the coast of Baja California and in the southern Ballenas Channel. Their abundance varied significantly from one season to another in both years. The highest relative abundance of whales occurred in July (warm season), followed by June (cold–warm transition period) and March–April (cold season), and the lowest in October (late warm season). Euphausiid abundance was significantly higher in June (cold–warm transition period), followed by July (warm season). There was no direct correlation between weekly abundances of both species, probably because whale peak abundance occurred four weeks later than euphausiid peak abundance. Fin whale relative abundance was significantly higher in 2004 than in 2003 (2.22 and 0.46 whales h–1, respectively), but euphausiid abundance remained almost the same (geometric means 255 and 174 ind/1000 m3). Even though there was no weekly correlation between both species, their geographic distribution did overlap.

Paralarva of the sharpear enope squid Ancistrocheirus lesueurii (Oegopsida: Ancistrocheiridae) in the southeastern Arabian Sea

First morphological description of an early stage paralarva of the sharpear enope squid Ancistrocheirus lesueurii from southeastern Arabian Sea is presented. The paralarva was collected at dusk by using surface tows of zooplankton nets in February 2012. The presence of paralarva A. lesueurii indicates that this species completes its life cycle in this sea. A comparison of the arm formula of paralarva and adult stages shows a major change in the formula during metamorphosis to adult.

Hidromedusas del Pacífico Colombiano: Aspectos Básicos de su Dinámica Ecológica

&lt;p&gt;Este estudio examina la dinámica espacio-temporal de la comunidad de hidromedusas en el Océano Pacífico Colombiano (OPC) en relación con la temperatura, salinidad, biomasa mesozooplanctónica y variación día/noche en la superficie, usando algunos análisis estadísticos e índices ecológicos. Un total de 128 muestras de mesozooplancton fueron obtenidas mediante arrastres superficiales con una red cónica (50 cm de diámetro, malla de 363 μm provista de un flujómetro para estimar el volumen de agua filtrada) durante cinco cruceros (23 junio-12 julio 2001, 27 agosto-15 septiembre 2001, 03-22 septiembre 2002, 01-21 septiembre 2003 y 18 septiembre-08 octubre de 2004). Las densidades más altas se presentaron durante septiembre de 2002 y septiembre-octubre de 2004, quizás principalmente por la mayor oferta de alimento. Se identificaron nueve especies meroplanctónicas y nueve holoplanctónicas, cuyos ciclos de vida aún son desconocidos o poco comprendidos; las especies holoplanctónicas dominantes &lt;em&gt;Aglaura hemistoma, Liriope tetraphylla &lt;/em&gt;y &lt;em&gt;Cytaeis etrastyla &lt;/em&gt;fueron definidas como euritípicas en el OPC. La diversidad más alta (H´= 1,65) fue menor que en otras regiones de Pacífico americano. No se observó una secuencia clara en la aparición de especies exclusivas, características o generalistas, ni un patrón de distribución espacio-temporal, aunque si indicios de migración nocturna a aguas superficiales. La similaridad entre las estaciones se explicó por las variables abióticas o bióticas, más que por su ubicación costera, intermedia u oceánica, o por la hora de los registros. Esta aproximación ecológica preliminar refleja la complejidad ambiental del OPC y el escaso conocimiento del comportamiento de la comunidad de las hidromedusas en el OPC.&lt;/p&gt;

Diversity and abnormalities of cyclopoid copepods around hydrothermal vent fluids, Kueishantao Island, north-eastern Taiwan

The diversity and abundance of cyclopoid copepods were investigated at four stations along the hydrothermal vent at Kueishantao Island, north-eastern Taiwan. Zooplankton samples were collected by surface tows. Qualitatively, 21 species of cyclopoids belonging to five genera and three families were observed. Our analysis revealed a number of abnormal protrusions (TLAs) on cyclopoid copepods, which were more frequent on the dorsal body surface and in members of the family Corycaeidae. The number of TLAs was highest at the hydrothermal vent plume (HVP) zone and among adult copepods. The potentially high levels of toxic minerals erupting from the hydrothermal vents as well as low pH and high temperatures may weaken the copepods' exoskeletons, thereby increasing susceptibility to infections, indicated by the HVP-related increase of tumoural processes. It is suggested that some of these abnormalities are related to the occurrence of symbiotic tantulocarids, a topic yet to be studied in this interesting environment.

Mesh size affects abundance estimates of Oithona spp. (Copepoda, Cyclopoida)

Retrieval by different mesh-sized plankton nets is compared, using the abundance and distribution patterns of cyclopoid copepods of the genus Oithona W. Baird, 1843 as a case study. Mesozooplankton was sampled during spring, summer, and autumn of 2000-2002 in the South China Sea. Plankton samples were collected by surface tows (5-0 m) and by oblique tows in three different depth layers (0-50 m, 0-100 m, and 0-150 m), with Norpac zooplankton nets of 333 and 100 mu m mesh size. The oithonid density and diversity in the plankton samples collected through a 100 mu m mesh was about one order of magnitude higher than those collected through a 333 mu m mesh. The mean occurrence frequency of Oithona spp. in the plankton samples collected through a 100 mu m mesh net was > 85% and > 60% through a 333 mu m mesh net, with a mean total density of 45.86 +/- 206.97 m(-3) for the 100 mu m mesh net compared to 2.28 +/- 21.94 m(-3) for the 333 mu m mesh net [which indicates the enormous patchiness in occurrence of these copepods]. The density of smaller mesozooplankton, as evidenced by Oithona spp., has thus been underestimated by previous researchers using larger meshed nets. Samples collected through a 100 mu m mesh net demonstrated the numerical importance of the genus Oithona and its distinct seasonal and interannual patterns of abundance and distribution in the South China Sea.

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.

Shrimp pond zooplankton dynamics and the efficiency of sampling effort

The abundance and species composition of zooplankton assemblages were examined in a commercial Penaeus monodon (Fabricius) aquaculture pond in subtropical Australia. Numbers and biomass of zooplankton in the surface tows (142 µm mesh) varied from 2080 L−1 (2466 µg L−1) to < 1 L−1 (15 µg L−1). Peak zooplankton abundance and biomass occurred at the beginning of the shrimp farm season. The small copepod Oithona australis Nishida was dominant during these peaks in abundance although other species were common, including small, low-salinity copepods in the first year and larger marine copepods in the second year. Irrespective of taxa or size, zooplankton abundance declined rapidly after the stocking of P. monodon postlarvae, indicating high levels of non-selective predation. For the remainder of the grow-out season, mean zooplankton abundance was below 1 L−1 in the first season and below 3 L−1 in the second season. The factors that maintain the abundance of these zooplankton assemblages at these relatively low levels are poorly understood but may include a deterioration pond water quality. At these lower levels of abundance, the copepods Acartia pacifica Steuer and Acartia sinjiensis Mori were relatively common during the middle phase of the production season, with barnacle nauplii increasing in abundance during the later stages of the grow-out season. Analysis of fixed effects showed that there were significant differences in total zooplankton abundance between farm production seasons and occasion within season, and time (day or night). Analysis of random effects demonstrated that the dominant source of variation, in total zooplankton abundance, was day-to-day changes within sampling occasions. The complexity of temporal and spatial patterns in the abundance, distribution and composition of zooplankton assemblages in shrimp ponds presents significant challenges in designing sampling programmes that accurately quantify temporal or spatial trends. Our results have shown that sampling for more than four consecutive days, at more than one site, is necessary to accurately assess such trends.

Early Life History of the Northern Pikeminnow in the Lower Columbia River Basin

The northern pikeminnow Ptychocheilus oregonensis is a large, native cyprinid in the Columbia River basin that has persisted in spite of substantial habitat alterations. During the months of June to September 1993–1996, we investigated the temporal and spatial patterns of northern pikeminnow spawning, along with describing larval drift and characterizing larval and early juvenile rearing habitats in the lower Columbia River (the John Day and Dalles reservoirs and the free-flowing section downstream of Bonneville Dam) as well as in the lower sections of two major tributaries (the John Day and Deschutes rivers). The density of newly emerged drifting larvae was higher in dam tailraces (a mean of 7.7 larvae/100 m3 in surface tows) than in the lower reservoirs (0.3 larvae/100 m3), indicating that tailraces were areas of more intense spawning. Density was particularly high in the Bonneville Dam tailrace (15.1 larvae/100 m3), perhaps because adult northern pikeminnow are abundant below Bonneville Dam and this is the first tailrace and suitable main-stem spawning habitat encountered during upriver spawning migrations. Spawning also occurred in both of the tributaries sampled but not in a backwater. Spawning in the Columbia River primarily took place during the month of June in 1993 and 1994, when the water temperature rose from 14°C to 18°C, but occurred about 2 weeks later in 1995 and 1996, possibly because of cooler June water temperature (14–15°C) in these years. The period of drift was brief (about 1–3 d), with larvae recruiting to shallow, low-velocity shorelines of main-channel and backwater areas to rear. Larvae reared in greatest densities at sites with fine sediment or sand substrates and moderate- to high-density vegetation (a mean density of 92.1 larvae/10 m3). The success of northern pikeminnow in the Columbia River basin may be partly attributable to their ability to locate adequate spawning and rearing conditions in a variety of main-stem and tributary locations.

Planktonic Dispersion of Larval Salmon-Lice,Lepeophtheirus Salmonis, Associated with Cultured Salmon,Salmo Salar, in Western Ireland

Dispersion ofLepeophtheirus salmonisKreyer 1838 (Crustacea: Caligidae) larvae from a single cage and also from a farm system was investigated in Ardmore Bay, west coast of Ireland. Surface plankton tows were taken from within a cage and also at various distances from the farm. Highest densities of larvae (maximum, 66·1 m−3) were consistently recorded within the cage. Less than 10% of the larval density recorded inside the cage was found outside at any one time, indicating a high retention of larvae within the cage throughout the sampling period. This was due to a reduction of water movement inside the cage caused by the physical barrier of the net which was fouled during the sampling period. Highest densities of larvae were consistently found in samples taken at 10 m (maximum, 4·8 m−3) from the last cage on the farm in comparison to those taken further away from it (maximum, at 1 km; 0·4 m−3). Regression analysis of the data from each of the sampling dates showed a significant inverse relationship (P=0·001) between distance and larval numbers. The models produced suggest that few louse larvae would have been recovered in samples (i.e. 100 m plankton surface tows) taken more than 2 km from the last cage on the farm. Although the ratio of copepodids to nauplii increased with distance from the farm, the densities of the copepodids did not increase.