Low Fish Densities Research Articles

Fish-induced changes in zooplankton grazing on phytoplankton and bacterioplankton: a long-term study in shallow hypertrophic Lake Søbygaard

The impact of fish-mediated changes on the structure and grazing of zooplankton on phytoplankton and bacterioplankton was studied in Lake S0bygaard during the period 1984-92 by means of in vitro grazing experiments (C-labelled phytoplankton, H-labelled bacterioplankton) and model predictions. Measured zooplankton clearance rates ranged from 0-25 ml I' h on phytoplankton to 0-33 ml H h on bacterioplankton. The highest rates were found during the summer when Daphnia jpp. were dominant. As the phytoplankton biomass was substantially greater than that of bacterioplankton throughout the study period, ingestion of phytoplankton was 26-fold greater than that of bacterioplankton. Multiple regression analysis of the experimental data revealed that Daphnia spp., Bosmina longirostru and Cyclops vidnus, which were the dominant zooplankton, all contributed significantly to the variation in ingestion of phytoplankton, while only Daphnia spp. contributed significantly to that of bacterioplankton. Using estimated mean values for clearance and ingestion rates for different zooplankters, we calculated zooplankton grazing on phytoplankton and bacterioplankton on the basis of monitoring data of lake plankton obtained during a 9 year study period. Summer mean grazing ranged from 2 to 4% of phytoplankton production and 2% of bacterioplankton production to maxima of 53 and 88%, respectively. The grazing percentage decreased with increasing density of planktrvorous fish caught in August each year using gill nets and shore-line electrofishing.The changes along a gradient of planktivorous fish abundance seemed highest for bacterioplankton. Accordingly, the percentage contribution of bacterioplankton to the total ingestion of the two carbon sources decreased from a summer mean value of 8% in ZJap/tnia-dominated communities at lower fish density to 0.7-1.1% at high fish density, when cyclopoid copepods or Bosmina and rotifers dominated. Likewise, the percentage of phytoplankton production channelled through the bacteria varied, it being highest (5-8%) at high fish densities. It is argued that the negative impact of zooplankton grazing on bacterioplankton in shallow lakes is highest at intermediate phosphorus levels, under which conditions Daphnia dominate the zooplankton community.

Spatial and Temporal Distribution of Threadfin Shad in a Southeastern Reservoir

The spatial and temporal patchiness of limnetic fishes is not well understood but has important implications for biological interactions as well as for sampling strategies. We used hydroacoustics to assess the distribution of threadfin shad Dorosoma petenense in Lake Norman, North Carolina, a large, multipurpose reservoir. We sampled three transects on four consecutive days and nights in April, July, and October to enable us to characterize threadfin shad on both daily and seasonal time scales. We used a patch recognition algorithm to analyze echo-integrated hydroacoustic data in order to define patches and related patch characteristics (numbers, density, area, and mean depth). Threadfin shad in Lake Norman were patchily distributed and exhibited strong, generally consistent patterns in patch number, fish density, size, and depth. During most surveys we observed 12–16 patches/km with fish densities exceeding twice the average background density, and 1–2 patches/km with fish densities 50 times the average background density. Most patches were small (<50–100 m2 in the vertical plane); less than 10% of the area contained 50% of the fish. In October night surveys, patches tended to be larger and less numerous and to have lower fish densities than at other times. Depth distribution of patches varied among seasons and was influenced by temperature, by oxygen concentration, and possibly by vertical distribution of predators and prey. Temporal changes in patch distribution suggested that fish composing the patches were highly mobile, spatially integrating threadfin shad impacts on their zooplankton prey. Large differences in day and night average density estimates in October suggested strong diel habitat shifts by threadfin shad during fall.

Effects of dams, regulation and pollution on fish stocks in the Vltava river in Prague

AbstractFish abundance and size composition in a 28 km stretch of the Vltava River in the Czech capital of Prague were studied using horizontal dual‐beam scientific sonar. Very low fish densities (0.6–0.7 fish/100m3) were found above Prague where the river is influenced by a cascade of reservoirs. These have a retention time of more than 100 days and release hypolimnetic water. A significant increase in fish density was found in the Berounka River confluence (a tributary with a normal, seasonal temperature pattern) just above the city (about 10 fish/100 m3). The fish stock is fairly low in the city itself because of channelization of the river bed (1.3–3.5 fish/100 m3), but much greater densities (14.9–16.9 fish/100 m3) were found below the city where the river receives a high organic load of partially treated sewage and has a semi‐natural shoreline. The largest average fish size and biomass were also reported from this section.

Artificial deposition of eggs of Atlantic salmon (Salmo salar L.) in a regulated Norwegian river: Hatching, dispersal and growth of the fry

AbstractFertilized eggs of Atlantic salmon (Salmo salar) were artificially deposited in the regulated River Ekso, western Norway. This was done by placing 320–2400 eggs in perforated platic baskets, together with small stones and gravel, buried in the river. To evaluate development of the eggs, some baskets were removed and examined with respect to fertilization and mortality at the eye‐roe stage. After swim‐up of alevins, all baskets were examined for the number of dead eggs. The hatching of eggs was between 55 and 98% in the baskets. There was no correlation between the egg number in the basket and survival. The alevins were restricted to their hatching area for the first month; after the first season they migrated 50 m upstream and 175 m downstream of their birthplace. One year later they occupied the whole area (375 m) between the two weir basins. Mortality was 80% in the first season and 87% in the second (regarded as normal and high, respectively). High mortality was probably due to acid episodes, independent of regulation. Growth was fast and salmon became smolt at age 2+, explained by favourable temperature and low fish density. Competition between salmon and native brown trout (Salmo trutta) seemed to be low as they occupied different habitats.

Effect of young-of-the-year walleye (Percidae: Stizostedion vitreum) on plankton dynamics and water quality in ponds

To understand the impact of young-of-the-year (YOY) fish on food web dynamics and water quality, we stocked larval walleye (9 mm TL) (Stizostedion vitreum) in six experimental ponds using two fish densities (10 and 50 fish m−3) with three replicates. At high fish density, the average abundances of cladocerans and copepods and the Secchi depth were lower whereas abundances of rotifers and algae, gross primary productivity (GPP), pH and total phosphorus concentration were higher than at low fish density. Fish impact on bacterial abundance, dissolved oxygen, nitrogen and phosphorus concentrations, however, was not significant. The within treatment measurements of all variables except GPP were significantly different over time. Our results indicate that YOY walleye predation at high density can affect plankton community by reducing large zooplankton biomass and water clarity, and increasing phytoplankton abundance. The impact of YOY piscivorous fish on plankton should be considered when biomanipulation is applied for improvement of water quality.

Effect of larval walleye ( Stizostedion vitreum) and fertilization on the plankton community: implications for larval fish culture

Plankton abundance which greatly affects larval fish growth and survival can be regulated by fish predation and nutrient input. In this study, we examined community effects of larval walleye and fertilizer type on plankton in ponds involving three densities (0, 250 000, and 500 000 fish/ha) of larval walleye ( Stizostedion vitreum) and two fertilizer types (inorganic or organic). Walleye survival was not affected by fish density or fertilizer type, and more fish yield was gained in high fish density ponds. The best fish growth was obtained in ponds with low fish density and inorganic fertilizer. As fish density increased, the abundances of Daphnia, Diaptomus and nauplii, Secchi depth, NH 3-N, NO 3-N and chironomids were reduced, whereas primary productivity, Scenedesmus, flagellates, colonial algae, Anabaena, rotifers and pH were enhanced. Compared to organic fertilizer, inorganic fertilizer increased the levels of primary productivity, algal abundance, dissolved oxygen, pH and total phosphorus. Inorganic liquid fertilizer with N:P ratio at 20:1 and a stocking density of 250 0005̄00 000 fry/ha is recommended for larval walleye culture. The stocking and fertilization protocol for larval walleye culture could be applied to the larval culture of other planktivorous fish species.

Occurrence of white sturgeon iridovirus infections among cultured white sturgeon in the Pacific Northwest

The white sturgeon iridovirus (WSIV) was detected in cultured white sturgeon Acipenser transmontanus from the lower Columbia River in Oregon, the Snake River in southern Idaho and the Kootenai River in northern Idaho, USA. In Oregon, WSIV was consistently detected in young sturgeon that were progeny from Columbia River adults and cultured in river water but not detected in sturgeon cultured in well water. In Idaho, WSIV was detected in sturgeon that were progeny from wild Snake River and Kootenai River adults after being subjected to stressful conditions of low spring water flows and high fish densities. When densities were reduced and water flows increased, mortality subsided. These observations suggest that WSIV may occur in wild sturgeon and that the virus may be present in many Northwest populations due to the long life span of the species, migratory patterns, and continuity of the river systems. Additionally, since the disease appears size(age)-specific and stress-mediated, fish culture management strategies could be used to avoid or minimize epizootics. These include iodophor disinfection of eggs, sustaining low fish densities and loadings, maintaining virus-free water supplies, minimizing adverse environmental conditions, and reducing the handling of sturgeon younger than 1 year.

Historical Status of Fish Populations in Canadian Rocky Mountain Lakes Inferred from Subfossil Chaoborus (Diptera: Chaoboridae) Mandibles

We used subfossil Chaoborus mandibles preserved in sediments of Cabin, Caledonia, and Celestine lakes, Alberta, to determine whether fish were present in the lakes (which now contain rainbow trout (Oncorhynchus mykiss) and lake chub (Couesius plumbeus)) prior to the first fish stockings early this century. We first tested whether Chaoborus were good indicators of the presence of fish in montane lakes in a survey of 43 lakes in jasper and Banff national parks. Chaoborus americanus was the only species inhabiting fishless lakes and was also found in three lakes with low fish density (co-occurring with C. flavicans in two of the three lakes). Other lakes containing fish had either C. flavicans or no Chaoborus species. Subfossils revealed that C. americanus was the predominant species prior to this century in Cabin and Celestine lakes, suggesting that these lakes were originally fishless; C. americanus (and C. trivittatus in Cabin Lake) were eliminated early this century, contemporarily with the first recorded fish stockings. Chaoborus flavicans was the only species recovered from Caledonia Lake, suggesting that the lake had fish prior to this century; while it is not possible to identify the species originally present, historical evidence suggests that rainbow trout is native to Caledonia Lake.

Demographic and life history response of the cladoceranBosmina longirostris to variation in predator abundance.

Population dynamics, demography and body size of the cladoceranBosmina longirostris were examined in an experimental study in which the abundance of its predator (the cyprind fishPhoxinus eos) was varied in an unproductive lake. Four densities of fish were used, encompassing the biomass of fish in the lake.Bosmina was most abundant at low and medium fish densities (1.06 and 2.12 g fish biomass · m-3) and less abundant when fish were either absent or present at high density (3.71 g fish biomass · m-3). The unimodal response to predator abundance resulted from effects on both birth and death rates.Bosmina birth rates increased as fish biomass increased, in response to increasing food (phytoplankton) biomass. Death rates were highest at high fish biomass (because of fish predation) and in the absence of fish (because of predation by the dipteranChaoborus, which was most abundant in the absence of fish). Size-frequency distributions revealed that fish eliminated the larger size classes ofBosmina, and mean carapace length ofBosmina populations was inversely proportional to fish biomass.Bosmina initiated reproduction at smaller size in the presence of fish than in their absence, and size at maturity was inversely proportional to fish biomass. Size at birth also tended to decrease with increasing fish biomass, but this trend was not as strong as that of size at maturity. Decreased size at maturity apparently allowedBosmina individuals to reproduce before becoming vulnerable to fish predation. Flexibility in size at maturity, together with low abundance of invertebrate predators and large herbivores (which were preyed upon by fish), allowedBosmina to become abundant in low and medium fish treatments. In the high fish treatment, mortality due to fish predation was too severe to be offset by decreased size at maturity, andBosmina population density was low. The net response ofBosmina populations to fish predation results from interactive effects of predation on mortality, natality, and life history traits.

Nutrient Regeneration by Sockeye Salmon (Oncorhynchus nerka) Fry and Subsequent Effects on Zooplankton and Phytoplankton

Planktivorous fish can affect phytoplankton indirectly by control of herbivore communities, and directly by excretion of soluble nutrients. A field mesocosm study showed that nutrient regeneration by sockeye salmon (Oncorhynchus nerka) fry substantially increased biomass of zooplankton and phytoplankton relative to fishless controls. Plankton communities in enclosures treated with only fish-regenerated nutrients but not fish predation responded differently to two densities of fish. Enclosures with low fish densities exhibited significant increases in zooplankton biomass and small increases in phytoplankton biomass. Nutrients resulting from high fish densities stimulated blooms of ungrazable Dinobryon, and extremely low zooplankton biomass when compared with fishless controls. Phosphorus excretion rates by zooplankton and fish explained 55% of the variance in total plankton biomass in predation-free enclosures and 95% of the variance in chlorophyll a concentrations for enclosures with predators and fish-regenerated nutrients. The results of this experiment suggest that algal production is greatly enhanced by excretion of nutrients by fish, while grazer control is less important.

Composition and seasonal fluctuations of the ichthyoplankton community in a shallow tidal channel of Cadiz Bay (S.W. Spain)

The composition and seasonal variation of the ichthyoplankton community in a shallow tidal channel in Cadiz Bay was studied for 5 years (1984 1988), taking fortnightly samples with a conical tide‐strained net (0.5 mm mesh). Thirty‐nine species belonging to 21 families of teleosts were caught. Four species (Pomutoschistus microps Kroyer, Atherina hoyeri Risso, Liza rumuda Risso and Lizu a lira Ia Risso) totalled more than 90% of the annual ichthyoplankton abundance. Three species, assumed to be permanent residents, dominated the number offish (837%), and were represented by all their developing stages; 26 species (66.7%) were temporary residents, contributing 17.2% to the total abundance and were represented only by early developing stages; the remaining 10 species were classified as rare. Monthly numbers of species and individuals showed seasonal variation with a main annual peak in spring and in late spring‐early summer, respectively. The diversity (H’) and evenness (S) indices of this community were low (0.13 to 1.83 and 0.03 to 0.71. respectively), with higher values in winter and late summer, and both were significantly correlated with the log‐transformed density of temporary residents (r = 0.43 and 0.41, respectively; P&lt;0.0l) and permanent residents (r=−0.75 and −0.79, respectively; P&lt;0.01). Multivariate classification analysis of samples revealed that similar cyclical changes were taking place in the structure of this ichthyoplankton community every year; three periods could be differentiated: (1) a cool period of low fish density but relatively high diversity; (2) moderate temperature and salinity with high numbers of species and density but low diversity and (3) a warm period of moderate diversity and fish density.

Determination of fish number density by a statistical analysis of backscattered sound

A theory is presented for estimating the density of fish aggregations from the second normalized moment of intensity of overlapping backscattered acoustic pulses. The chief advantage over the normal echo-integration method is that no knowledge of fish target strength is required. The model assumes that the probability distribution function (PDF) of backscattered amplitudes from the individual fish is Rician, and that within regions of constant density the fish have a Poisson volume distribution. Problems arising from variations in density can largely be overcome by grouping together intensity values from regions of like density and estimating the density for each region separately. It is shown theoretically that the error in estimated density increases with the number of scatterers per resolution cell, which implies that the method is most useful at low fish densities. The theory has been tested at sea on 38-kHz backscattered pulses from two nonhomogeneous layers of small pelagic fish. The results are encouraging judged by their self-consistency and by comparison with estimates obtained by echo integration of the same signals. The method is considered to be a potential alternative or supplement to echo integration in low-density regions.

Dynamic relationships between the dominance of male dark chub, Zacco temmincki, and their acquisition of females

The mating system of the dark chub is a form of male-dominance polygyny. The principal spawning ground in this study was at the shallow end of pools where males fought to obtain females. Mating was promiscuous with little female choice, and dominant males commonly took females from other males. Spawning was performed by a female and a single large dominant male, while other individuals dashed to the spawning pair to release sperm or eat eggs. At sites of low fish density, spawning was infrequent but was generally accomplished successfully because of the scarcity of satellite individuals, and the males' dominance hierarchy was stable throughout the reproductive season. Conversely, at sites of high density, although many females spawned, spawning was frequently interfered with. Furthermore, the dominance hierarchy was unstable and the most dominant male tended to become exhausted after frequent reproductive activities. In addition to his body size and dominance, how long he spent around the spawning grounds, his skill at spawning and the selection of a spawning site affected a male's reproductive success. While the larger dominant males mainly used the most frequently used spawning grounds where many females spawned, the smaller sub-dominants tended to use the less frequently used spawning grounds.

Fish community structure, spatial distribution and feeding ecology in a beaver pond

The fish community of a small (2.7 ha) Ontario beaver pond was analyzed relative to predictions based on its small size, shallow depth, brief existence and isolation from more permanent water bodies. The predictions were: (1) species richness will be lower than that of more permanent water bodies in the area, (2) fish will be mainly of small body size, (3) species will be randomly distributed across habitats, and (4) there will be a high degree of diet overlap between species and age classes. The first and second predictions were supported. The pond consisted of 10 resident species in 1985, and at least seven in 1988. Species richness was below the average of 13.2 found in four lakes in the vicinity, but greater than the 3.1 predicted by a species-area curve for non-acidified lakes in Ontario. All species except pumpkinseed, yellow perch and brown bullheads were small-bodied with short life spans and high population turnover rates, and few fish above 100 mm were present. Predictions 3 and 4 were not supported. Habitat occupation was nonrandom, and high diet separation occurred, particularly in August when food limitation was evident. While beaver ponds lack the range of habitats and the diversity of species of lake environments, low prey density and high fish density nevertheless appear to foster resource partitioning.

Expanded Bed Treatment of Complete Recycle Aquaculture Systems

The popular dietary shift to fish and the limits on natural fish populations have caused rapid growth in fish farming and aquaculture. Optimized aquaculture systems are attempting to use as complete recycle as possible to obtain maximum control over the fish populations, and limit water and energy requirements. Closed systems aquaculture is severely limited by water quality since pollutants accumulate in the system. This study examined the possibility of simultaneous BOD5 reduction, SS removal, nitrification and denitrification with an expanded bed in complete recycle growth systems with Atlantic salmon and brook trout. The fish rearing tank volume was 10 m3 and the expanded bed volume was 2.6 m3. The total duration of the study was 180 days. The expanded bed developed a film capable of nitrification and denitrification within 40 days. After this time, the SS and BOD were maintained at less than 6 mg/l. Ammonia, nitrates, and nitrites were below detectable levels, thus confirming the simultaneous one-stage removal potentials. Maximum nitrogen removal rates of 40 mg per liter of AFEB per day are similar to the higher removal rates predicted for maximum removal efficiencies at bulk solution ammonia-nitrogen concentrations of 0.5 mg/l. Dissolved oxygen concentrations in the expanded bed appear to be the most sensitive control parameter for these simultaneous reactions. This one-stage pollution control process for complete recycle aquaculture would be useful in both freshwater and seawater applications. The system is easily modularized for fish rearing tank capacities of 1,000 to more than 10,000 kg. The treatment system volume would be less than half the rearing volumes at low fish densities and low ammonia concentrations (less than 0.5 mg/l of ammonia nitrogen).

Improved growth in stunted brown trout (Salmo trutta L.) after reliming of Lake Hovvatn, Southern Norway

The chronically acidic Lake Store Hovvatn and the adjoining pond Pollen in southernmost Norway were limed in March 1981. The two locations were stocked with brown trout (Salmo trutta L.) at low and high densities in Hovvatn and Pollen, respectively. After 6 yr of reacidification, the locations were relimed in July 1987. Growth depression during the reacidification process in spite of low fish densities and superabundance of food was observed in Lake Store Hovvatn. Three months after reliming, a substantial growth response was found in trout from Lake Store Hovvatn; Mean annual length increment was 68% higher than that of the preceding year. In Pollen, reliming had no apparent effect on growth. In both populations reliming caused increased swimming activity measured as an increase in CPUE-values. These results show that the growth response to liming depends on population density and food availability. Moreover, the results indicate that the food conversion rate of the trout is negatively affected in acid waters.

Effect of Rearing Density on Poststocking Survival of Lake Trout in Lake Ontario

Abstract Six paired lots of yearling lake trout (Salvelinus namaycush) reared at densities of 41,000 and 51,000 fish per raceway during their last 9 months in the hatchery were stocked in Lake Ontario. Poststocking survival of the high-density (HD) and low-density (LD) fish was not different for the 1982 year-class. However, for the 1983 year-class, mean survival was significantly different between HD and LD fish (P < 0.01). Mean survival of HD fish was only 76% that of LD fish (P < 0.01), and most of the mortality attributable to rearing conditions had apparently occurred within 2 months after stocking. Mean size at stocking was not different for HD and LD fish of the 1982 year-class, but for the 1983 year-class, the LD fish were 6% longer and 22% heavier than the HD fish. Mean lengths and weights of LD and HD fish were not different in samples collected in Lake Ontario at age 2 and older. Size at stocking was not likely the factor that caused the difference in survival. Rather, the rearing conditions (p...

Daytime Exposure to Fish Predation for Littoral Benthic Organisms in Unproductive Lakes

Fish activity near the bottom was monitored in two central Ontario lakes using time-lapse video equipment. Predation risk for benthic invertebrates was measured as the frequency of fish entries into a hypothetical cylinder with dimensions such that an imaginary 2–3 mm invertebrate at the bottom-center of the cylinder would be within the detection radius of any fish entering the space. By this criterion, risk of predation is high, even in lakes with low fish densities. For 7 d of data spread over different lakes, sites, and months, the median rate of exposure of the hypothetical invertebrate to passing fish was 14.2 times/h; the median rate of exposure to fish that stopped or turned as if foraging was 3 times/h. Exposure to benthivorous fish varied markedly between two lakes in June and within a lake between June and August. Changes in the fish species, sizes, and level of activity all contributed to the variance. Exposure to fish also varied markedly among days, within a day, and among sites within a lake. Differences in predation risk between two neighboring sites were consistent for several days, suggesting that some of the patchiness in benthic invertebrate composition might be generated by patchy fish exploitation. The high median levels of exposure to fish suggest that benthic organisms should spend a high proportion of their time in risk-reducing positions or behaviors, so that competition among fish caused by resource depression will be important.

Seasonal changes in territoriality and frequency of agonistic behavior in two densities of juvenile brown bullhead, Ictalurus nebulosus

Incidence of territoriality and frequency of six agonistic action patterns of juvenile brown bullhead, Ictalurus nebulosus, were observed in the laboratory over a two year period at two fish densities under conditions of constant light and temperature. High density fish showed little territoriality or agonistic behavior while low density fish displayed an annual cycle of agonistic behavior and territoriality. Agonistic behavior frequency and incidence of territoriality peaked in February and March and were higher the second year when compared with the first.

Effects of Nutrients and Zooplankton Size on the Structure of a Phytoplankton Community

In situ enclosure experiments were conducted over two summers (1980—1981) to assess the effects of nutrient (N and P) enrichment and zooplankton size structure on the phytoplankton community of an oligo—mesotrophic lake containing planktivorous fish and small zooplankton species. Exclusion of fish from the community resulted in an increase in mean individual zooplankton size, primarily because the cladocerans Ceriodaphnia and Diaphanosoma attained larger sizes. In 1980, total cladoceran biomass was also greater in the absence of fish. In 1980, total phytoplankton density was significantly lower in the fishless enclosures (where zooplankton were larger and grazing rates presumably higher), at a given nutrient level. The proportion of phytoplankton density comprising species with gelatinous sheaths or other protective coverings was greater in the fishless enclosures, under both enriched and unenriched conditions, presumably because these species are relatively resistant to zooplankton grazing. In 1981, when lower fish densities were used (than in 1980), total phytoplankton density was lower in the fishless enclosures only toward the end of the experiment and only in unenriched enclosures. In enriched enclosures the presence of fish had no effect on total phytoplankton density. However, the proportion of phytoplankton density made up of small, edible (to zooplankton) phytoplankton species was lower in the absence of fish throughout the experiment in both unenriched and enriched treatments. In both years, nutrient enrichment caused phytoplankton density to increase greatly, in the presence and abscence of fish. Nutrient enrichment increased phytoplankton density a much greater amount than did reduction in zooplankton size. The results of these experiments demonstrate that even relatively small changes in zooplankton size (i.e., shifts in size within only small species) can result in significant alterations of the phytoplankton community. However, the increase in zooplankton body size resulting from fish exclusion could not buffer all effects of nutrient enrichment on total phytoplankton density.