Influence of turbidity and fluctuating water levels on the abundance and depth distribution of small, benthic fish in New Zealand alpine lakes

Abstract –  Minnow trapping was used to determine the distribution and abundance of two small, benthic species of fish, common bullies ( Gobiomorphus cotidianus ) and koaro ( Galaxias brevipinnis ), in five oligotrophic lakes varying in both turbidity and the extent of water-level variation. Koaro were 5–20 times more abundant in the large turbid lakes than in the clear ones, indicating that turbidity may reduce the extent of salmonid predation on koaro. However, the scarcity of koaro in the clear lakes was related mainly to the prevalence of brown trout ( Salmo trutta ) compared with rainbow trout ( Oncorhynchus mykiss ) as the former were more piscivorous and had a stronger inverse relationship with koaro. Common bullies were more abundant at sites 20 m or more away from the shoreline, and at depths of 9–12 m irrespective of season, year and turbidity level. However, the overall abundance of bullies was reduced by 70–90% in the turbid lakes. As bullies are relatively insensitive to the direct effects of increased turbidity, indirect effects such as reductions in macrophytes from reduced water clarity or smothering of benthic habitats by settled solids are thought to influence their abundance in the lakes affected by glacial turbidity. There was no relationship between the extent of water-level fluctuation and the abundance of koaro or common bullies in the lakes, but the depth distribution of bullies was shallowest in the lake with the least water-level fluctuation and was deepest in the lake with the greatest water-level fluctuation.     

Density and temperature dependence of gill net catch per unit effort for perch, Perca fluviatilis, and roach, Rutilus rutilus

Abstract  The densities of perch, Perca fluviatilis L., and roach, Rutilus rutilus (L.), were estimated in six and three lakes, respectively, using mark–recapture and ranged from 25–1064 perch ha⁻¹ and 865–2749 roach ha⁻¹. Effects of fish density, net type and water temperature on catch-per-unit-effort (CPUE) were analysed by generalised linear models (GLM) and generalised additive models (GAM). GAMs were fitted to estimate the simultaneous linear and nonlinear effects of temperature and density on CPUE. These models showed significant nonlinear effects of density on CPUE – mostly for perch, and also for both species at high densities. The models also revealed that both species had distinct density–temperature criteria of expected maximum CPUE and zero CPUE values. Because of the nonlinear relationship between CPUE and density, it was concluded that CPUE should be used with caution as a proxy of density.     

Brook trout spawning areas in lakes

Abstract— Nine lakes containing self-sustaining populations of brook trout ( Salvelinus fontinalis ) were surveyed to provide quantitative information on the abundance and distribution of redd sites. We measured rates of groundwater flow at redds in four of these lakes, and in one of these lakes we also compared the frequency of site use over four years from spawning observations. The number of redd sites in each lake ranged from 1 to 53 (mean=30) with most found relatively close to shore (∼ 1 m deep). Spawning areas per lake (16.9–829.4 m²) were not related to lake size and were often not continuous areas of littoral zone habitat. Rates of groundwater flow in lakes, averaged across redd sites in each lake, ranged from 20.0 to 107.9 ml. m.⁻² min^−1. The intensive survey of redd site use in one lake demonstrated that only 11% of sites (10 of 92) were used in all four years of observation. The mean rates of groundwater flow at sites used in all four years were significantly greater than at sites used intermittently. Data from this and other studies point to the conservation of subcatchments associated with groundwater discharge areas used by spawning brook trout as a means of maintaining self-sustaining populations.     

Migration pattern of silver eel (Anguilla anguilla, L.) in an obstructed river system

Abstract –  The downstream migration of European silver eels in the River Frémur was examined to determine the potential effects of the numerous migration barriers that exist on waterways in western France. The Frémur has a 14 m high dam which creates a 3 × 10⁶ m³ water supply reservoir, 6 km from tidal limit. Based on 8-year records (1996–2004) of migrant silver eels captured in a Wolf trap located about 1 km below the dam, the influence of this dam on the migration was examined. These records indicate that 91% of silver eels were captured when the dam was overtopped (i.e. when the water reservoir was full). The timings of the overflow periods varied greatly between years mainly because they are function of the duration of the filling period which in turn is dependent on the level of summer water abstraction and annual hydrological conditions. Consequently, migration periods occurred at variable dates (between November and April) which is late in comparison with nonobstructed European river systems (generally between August and December). During overflow periods the migration of silver eels increased markedly during rain events (increasing river flow). This indicates that flow flushes, are essential so that settled silver eels can orientate themselves and pass over the dam. The hypothesis of a 'dam effect' that could stop temporarily or permanently some of silver eels in the reservoir is discussed. Finally, the influence of the migration delays on the condition of silver eels is considered.     

Phosphorus and calcium requirements in the diet of the American cichlid Cichlasoma urophthalmus (Günther)

An experiment was carried out with Cichlasoma urophthalmus (Günther) juveniles to determine the phosphorus requirement and its interaction with dietary calcium. Twelve isoenergetic and isoproteic diets were prepared using a basal artificial diet containing vitamin-free casein, dextrin, starch, corn oil, fish oil, vitamin mixture and a mineral mixture free of calcium and phosphorus. Calcium and phosphorus levels were determined in the casein. To the basal diets were added different concentrations of phosphorus as potassium monophosphate (0.5, 1.0, 1.5 and 2.5 g kg^–1) and calcium as calcium carbonate (0.5, 0.75, 1.0, 1.5, 2.0, 2.5, 3.0 and 4.0 g kg^–1). These concentrations resulted in varying Ca–P ratios (1:1, 1.33:1, 1.5:1, 1.6:1 and 2.0:1). Calcium and phosphorus concentrations in the water were 84 mg kg^–1 and 0.003 mg kg^–1, respectively. The diet with 0.5 g kg^–1 phosphorus resulted in deficiency signs such as reduced growth, high conversion ratio, high fat content and low bone mineralization. Increased levels of dietary calcium and phosphorus both gave improved growth and mineralization. Mineralization continued to increase with dietary phosphorus levels above that required for maximum growth. The optimum level of phosphorus in the diet was 1.5 g kg^–1, the optimum calcium level was 1.8 g kg^–1 and the optimum Ca–P ratio was 1.3. Carcass lipid levels were inversely related to dietary phosphorus.     

Seasonal cycle of body composition and energy of brown trout (Salmo trutta) in a temperate zone lake

Abstract – Seasonal changes in protein and fat were investigated in brown trout ( Salmo trutta L.) of a stunted population from a small, temperate zone lake. The lake was ice-covered during winter (about 200 days), and hypolimnic water temperature during winter was about 4°C. During winter there was an increase in specific fat, in particular among the larger sized fish, while there was a general decrease in specific protein content among both small (<5 winters) and large (age 5 or older) trout. During winter (end of October to mid-May) an average trout gained 32 kJ of fat, but protein energy content decreased by 14 kJ, yielding a net energy increase of 18 kJ or a daily energy gain of 0.09 kJ  ·  day⁻¹. During the ice-free season (mid-May to the end of October) the trout increased mainly in protein content with daily energy gains of about 1.4 kJ  ·  day⁻¹, a value about 14 times higher than the corresponding winter value. Trout living in lakes may store considerable amounts of fat during wintertime in contrast to the depletion of fat reserves found among stream-living trout in the same area, which face lower water temperatures in the winter period. ^Note     

The effects of lakes on growth in yearling brown trout (Salmo trutta L.)

Abstract– Data from the Swedish Electrofishing Register were used to study the effects of lakes on the growth of yearling (0+) brown trout in outlet streams. It was found that growth increased close to lakes. On average, the length of the longest yearlings was 10% greater 0-100 m downstream from lakes as compared with more than 1 km downstream. Growth enhancement was most pronounced downstream from large lakes. This was exemplified with maximum yearling lengths in September in inlet streams (maximum yearlings 50-100 mm) and outlet streams (65-130 mm) of Lake Stora Le (area 200 km²), and inlet streams (50-75 mm) and outlet streams (65-95 mm) of Lake Anten (area 18 km²). It is suggested that this is a combined effect of both increased water temperature and outflow of lake seston. The growth pattern found was not correlated to population abundance or to the abundance of competitors or predators.     

Effects of dietary phosphorus and lipid levels on utilization and excretion of phosphorus and nitrogen by rainbow trout (Oncorhynchus mykiss). 2. Production-scale study

In a 8-week production-scale experiment at a commercial trout farm, the effects of dietary lipid level and phosphorus level on phosphorus (P) and nitrogen (N) utilization of rainbow trout (initial mean weight 99 g) were assessed. A low-phosphorus, high-lipid experimental diet (457 g protein, 315 g lipid, 9.1 g P  kg^–1 dry diet) was compared with a commonly used commercial diet (484 g protein, 173 g lipid, 13.6 g P  kg^–1 dry diet). P and N budgets were constructed using data from the production-scale experiment and digestibility data for the two diets. In addition, orthophosphate and ammonia-N waste were measured in effluent over one 24-h period. Relative to the commercial diet, the experimental diet resulted in significantly increased feed efficiency ratio, N retention and P retention, and substantially reduced dissolved, solid and total P waste (g kg^–1 dry feed). Although N retention resulting from the experimental diet was higher, this was attributable to higher N (protein) digestibility of the experimental diet. Solid N waste (g kg^–1 dry feed) resulting from the experimental diet was substantially lower, but dissolved N waste (g kg^–1 dry feed) was not significantly different relative to the commercial diet. Mean effluent orthophosphate production (mg day^–1 kg^–1 fish) of fish fed the experimental diet was substantially lower than that of fish fed the commercial diet ( P  < 0.05), but effluent ammonia-N production (mg day^–1 kg^–1 fish) was not significantly affected by dietary treatment.     

Creation of artificial upwelling areas for brown trout, Salmo trutta, spawning in still water bodies

Abstract  Brown trout, Salmo trutta L., spawning sites were constructed by creating areas of artificial upwelling water, 252 ± 37 mL m⁻² min⁻¹ (95% CL), through appropriately sized spawning gravel substrate in 3 m² vessels buried in the bottom of a 150-m² pond. Natural spawning occurred in the vessels during autumn 2001–2004, with hatching and alevin swim up the following spring. In areas of upwelling, egg survival was 85–95%, while no live eggs were observed in areas without upwelling. In areas with upwelling, the maximum density of live eggs at the eyed stage was 570–1510 eggs m⁻². In spring 2004 and 2005, the density of alevins was estimated at 322 (±187) m⁻² and 567 (±217) m⁻², respectively, in areas with upwelling water, compared with 35.2 ± 25.4 m⁻² in areas without upwelling water in 2004.     

A preliminary telemetry study of the migration of silver European eel (Anguilla anguilla L.) in the River Mosel, Germany

Abstract  – To study the behaviour of silver eels ( Anguilla anguilla L.) during their downstream migration, particularly near a hydroelectric power dam, we tagged nine eels with ultrasonic transmitters and tracked their paths in the River Mosel, Germany. The onset of migration coincided with the first flood event that followed the full moon but was independent of daytime, because migration and turbine passage occurred during both day and night. During migration eels swam actively downstream with a velocity of 0.3–1.2 m · s⁻¹. When migrating eels arrived at the dam, they either passed through the turbines immediately or stayed upstream of the powerhouse for up to 8 days, showing a characteristic circling behaviour. Circling eels repeatedly approached the trashrack, sprinted upstream, and finally passed through the turbines with the next high water discharge. These observations are discussed with regard to the design of appropriate downstream passage facilities.     

Temporal changes in European eel, Anguilla anguilla, stocks in a small catchment after installation of fish passes

Abstract  Changes in the abundance of European eel, Anguilla anguilla L., in the River Frémur, France, were examined over an 8-year period. Natural connectivity of the river was disturbed by three high dams that inhibited eel upstream migration and reduced recruitment by elvers and yellow eels. After eel passes were installed, fish became more abundant upstream (mean density 0.5 eel m⁻²). Moreover, except in the more upstream areas, no decline in eel numbers and biomass was found, in contrast to the general decline of eel throughout its distribution range. It was concluded that eel passes are important to conserve and/or to recover eel stocks.     

Continuous exposure to infectious pancreatic necrosis virus during early life stages of rainbow trout, Oncorhynchus mykiss (Walbaum)

Rainbow trout, Oncorhynchus mykiss (Walbaum), were exposed continuously to infectious pancreatic necrosis virus (IPNV) at 0, 10¹, 10³ or 10⁵ plaque forming units (pfu) L⁻¹ of water to estimate the effects of chronic IPNV exposure on early life stages. Fish density averaged 35 fish L⁻¹ (low density) or 140 fish L⁻¹ (high density), and the tank flow rate was 250 mL⁻¹ min. Virus exposure began at 6 days before hatch and continued until fish were 44 days old. Cumulative per cent mortality, analysis of survival and hazard functions, and discrete-time event analysis were used to explore the patterns of survival and mortality. In eggs and fish exposed to IPNV, mortality significantly greater than in the 0 pfu L⁻¹ exposure did not occur until IPNV concentration was 10⁵ pfu L⁻¹ at low fish density and 10³ pfu IPNV L⁻¹ at high fish density. These results suggest that in the natural aquatic environment, where rainbow trout densities are likely to be considerably lower than in this study, mortality resulting from infection with IPNV will very likely not occur when ambient concentrations of virus are ≤10³ pfu IPNV L⁻¹. In aquaculture rearing units, trout density is likely to be as high or higher than the densities used in this study. Therefore, continuous inputs of virus at concentrations greater than 10¹ pfu L⁻¹ may result in IPN epidemics in aquaculture facilities.     

Factors influencing the yield of brown trout, Salmo trutta m. lacustris L., in northern Finnish lakes

Abstract Lake-to-lake variation in brown trout, Salmo trutta m. lacustris L., yield from stocking was examined in 34 lakes in northern Finland. The trout were mainly stocked as 2–3-year-old fish. Catch statistics were compiled with information on water quality, water level fluctuations, fishing effort and lake geomorphology. Absolute brown trout yields (kgha^-1) increased with increasing stocking rate, but there was an indication of non-linearity at higher stocking densities. Relative yields (kg per thousand trout released) were highest at low stocking rates. Stepwise multiple regression analysis was used to determine the best predictive model for lake-to-lake variability in brown trout yields. Seventeen measured regressands were used initially, and then replaced with scores obtained in a principal component analysis of highly correlated water quality variables and species-specific fish yields. Three major determinants of brown trout yields in these lakes were found in both analyses: fish community, stocking rate and fishing effort. Brown trout yields from stocking were higher in lakes with proportionally high yields of vendace or vendace and whitefish and proportionally low yields of pike.     

Assessment of catch, effort and species changes in the pair-trawl fishery of southern Lake Malawi, Malawi, Africa

Abstract  Catch per unit effort (CPUE) and species composition in the pair-trawl fishery in commercial Area A of southern Lake Malawi were assessed from 1991 to 2001. CPUE declined from 1.7 t day⁻¹ in 1995 to 0.5 t day⁻¹ in 2001 and the fishery is considered depleted. In 2000/2002, catch composition differed significantly from a 1991 survey, and was dominated by haplochromine cichlids (92% by weight). Of 98 haplochromine cichlid taxa identified in the catch, 18 contributed ≥1% by weight. The artisanal fishery targeted similar species of a similar size. It was recommended that: (1) renewed investment and increased effort in the pair-trawl fishery is not advisable; (2) there was a need for effort limitation in Area A and (3) the pair-trawl fishery should not be managed in isolation of the artisanal fishery.     

Feeding and growth of glass eels, Anguilla anguilla L.: the effect of feeding stimulants on feed intake, energy metabolism and growth

Abstract. Two experiments were conducted in which glass eels, Anguilla anguilla L., were fed with six different diets. Two natural diets, cod roe and minced bovine spleen, and four artificial diets, an unflavoured trout fry crumble and three trout fry crumble diets with added feeding stimulants, were used. The feeding stimulants tested were cod roe extract, spleen extract and a mixture of synthetic L-amino acids. In the first experiment the glass eels were fed at a restricted feeding level and in the second experiment at an ad libitum feeding level. The intake of metabolizable energy (ME) was determined as the sum of retained energy and heat production. Metabolizability of the diets decreased with increasing feeding level and was 75-95% for cod roe, 35-50% for spleen and 40-60% for the artificial diets. Since utilization of ME for maintenance and growth was not affected by feeding level and diet, differences in growth rates could be explained by differences in ME intake. Group ME intake in turn was determined by the fraction of the fish which were actually eating. The uniform feeding response of the glass eels fed the natural diets resulted in the highest ad libitum ME intake (80-90 kJ/kg^0.8/day) and growth rate (4-4-4-9g/kg^0.8/day). Addition of feeding stimulants significantly increased the ad libitum ME intake of the artificial diets from 45 kJ/kg^0.8/day to 50-60 kJ/kg^0.8/day, which resulted in an increase in growth rate from 1·3 g/kg^0.8/day to 24 g/kg^0.8/day.     

Experimental pathogenicity in rainbow trout, Oncorhynchus mykiss (Walbaum), of two distinct morphotypes of long-spined Saprolegnia isolates obtained from wild brown trout, Salmo trutta L., and river water

Juvenile rainbow trout, Oncorhynchus mykiss (Walbaum), were experimentally infected to investigate the pathogenicity of 20 isolates of two morphotypes of long-haired Saprolegnia obtained from wild brown trout, Salmo trutta L., and river water in Spain. The trout were exposed to 2 × 10⁵ and 3 × 10⁵ L^–1 zoospores. Saprolegnia infection could not occur without 'ami-momi' treatment. Pathogenicity varied greatly among isolates as mortality ranged from 0 to 100% of the fish. There was a statistically significant difference ( P  < 0.001) between the mortality caused by morphotype I isolates and that produced by those of morphotype II. The most pathogenic isolates usually belonged to morphotype II, consisting of isolates which had secondary cysts with bundles of hooked hairs which were shorter and less numerous than those of morphotype I; the morphotype I isolates usually had low pathogenicity. Lesions were most frequently found on the fins. Cultures detected the presence of Saprolegnia in internal organs. Histopathology of the intestine suggests that Saprolegnia may reach this and other organs via the blood stream from surface lesions.     

First year growth and survival of common carp in two glacial lakes

Abstract  Cohorts of common carp, Cyprinus carpio Linnaeus, were monitored from hatch through the next spring in two South Dakota, USA lakes to identify factors affecting year-class strength. Hatching occurred over 37- and 47-day periods in the two lakes. Common carp that hatched earlier achieved greater total lengths at the end of the first growing season (Brant Lake: r  = −0.84, d.f. = 77, P  <   0.001; Campbell Lake: r  = −0.87, d.f. = 78, P  <   0.001). Mean daily growth rates were approximately 1.0 mm day⁻¹ and similar between lakes. Total length was positively related to daily growth rate in the two lakes, indicating that members of the cohort that grew faster achieved greater total lengths by time of capture. Hatch date was inversely related to daily growth (fish hatched earlier grew faster) in one lake ( r  = −0.44, d.f. = 77, P  < 0.001) but not the other ( r  = −0.04, d.f. = 78, P  = 0.678), suggesting that length at the end of the growing season can be affected by length of growing season and growth rate. Overwinter survival of age-0 fish appeared to be size dependent because age-1 common carp length–frequency distributions from both lakes comprised only of the larger cohort members present the previous autumn.     

Changes in the fish catches during a trial opening of sluice gates on a run-of-the river reservoir in Thailand

Abstract  Changes in catch-per-unit-effort (CPUE) were evaluated when all the sluice gates on the Pak Mun Dam, Mun River, Thailand, were opened between July 2001 and June 2002. CPUE ranged from 0.38 to 1.70 and 0.61 to 2.71 kg fisherman⁻¹ night⁻¹, downstream and upstream of the dam, respectively. No statistical differences ( P  > 0.05) among months were observed in both areas. The monthly percentage index of relative importance (%IRI) of the fish species caught varied between months.     

Water resources management of the southern rivers of Russia with reference to fisheries requirements

Water consumption from Russia's southern rivers has increased substantially in recent years as a consequence of intensified economic activity. The volume of irreversible river-flow withdrawal from the Azov Sea basin varies between 11 and 13 km³ year^1. The majority of the abstracted water is used for irrigation. By 1990, the decrease in discharge into the Caspian Sea was 41 km³ year^1. Withdrawals of water from the Kura, Terek, Ural and other rivers have so far exceeded the ecologically admissible limits and become critical. The paper discusses methodological approaches for the elaboration of standards for water withdrawal and ecologically safe discharges into the southern seas. New data are presented on the dynamics of fish abundance as a function of the hydrological characteristics of rivers and water salinity. The results of the system analysis and regressive modelling formed the basis of a programme to develop policies for water management for fisheries in the lower Don. The principles and criteria for water withdrawal under variable hydrological regimes for the rivers entering the Azov and the Caspian Seas should be tested for their applicability for other water bodies (e.g. rivers, water reservoirs and lakes).     

Reduced water oxygen levels affect maximal feed intake, but not protein or energy utilization efficiency of rainbow trout (Oncorhynchus mykiss)

This study examined the effect of reduced water oxygen levels on the utilization efficiencies of energy and protein from a diet fed to rainbow trout. An experimental diet was fed at one of the four ration levels with an additional starved treatment also included in each oxygenation regime. Oxygen levels in each oxygenation regime varied with ration level, but averaged 9.3 ± 0.36 mg L⁻¹ for the normal regime and 5.7 ± 1.4 mg L⁻¹ for the hypoxia regime. Significant differences were observed in the apparent satietal feed intake levels in each oxygenation regime, but not at any of the pair-fed restricted levels. No significant effects of oxygenation regime were observed on the utilization of either energy or protein by the fish. Efficiency of protein use varied depending on the protein intake level, but was not significantly affected by oxygenation regime. This study demonstrates that a reduction in the oxygen levels of the water does not affect the utilization efficiency of dietary digestible protein and energy in rainbow trout, but does result in a downregulation of feed intake when the fish is fed to apparent satietal levels.