Annie Duncan

Acoustic size vs. real size relationships for common species of riverine fish

Abstract Target strengths of brown trout, rainbow trout, roach, perch, dace and chub, crucian and common carp, bleak and bream were determined for two ultrasonic frequencies of 200 kHz and 420 kHz using dual-beam sonar directed horizontally at tethered individual stunned fish of known size whilst being rotated through 360° by means of a carousel structure. This provided a data base for each fish of real size in length or weight and target strength for side body aspect, head/tail aspect and mean all-aspect (the average acoustic size of an individual fish rotated through 360°). In all, 182 fish were insonified in this way with more than 800 sonar runs (=replicated rotations). In all individuals, the largest target strength was for side-aspect, the lowest for head/tail-aspect and the mean all-aspect was intermediate. Statistically significant linear regressions of target strength on log length or log weight ( Y = aX + c ) were fitted to the data for individual species with an adequate number of individuals of sufficiently wide size range. Regressions incorporating all three kinds of target strengths, three variants of length (standard, fork and total) and weight have been calculated for each of two frequencies. In addition to individual species regressions, significant pooled all-species regressions ( n =180) were obtained but these were less useful than were family regressions for salmonids ( n =41) and cyprinids plus perch ( n =114). The comparable family regressions differed significantly, either in slope or in elevation (where slopes were in parallel) and the salmonid regression lay lower than the cyprinid plus perch one. In some but not all species, comparable regressions at different frequencies were similar. This study demonstrates the importance of body aspect for describing relations between acoustic size and real size in freshwater fish species.

Brown trout populations of three Scottish lochs estimated by horizontal sonar and multimesh gill nets

Abstract Brown trout (Salmo trutta) populations of the three Orkney freshwater lochs—Boardhouse, Harray and Swannay—were studied in late September 1991 using horizontalll oriented sonar. Dual-beam echo counting during mobile surveys gave the following densities of fish per 100 m3: 2.94 in Boardhouse, 1.94 in Harray and 1.08 in Swannay. Multiple fish targets and reflections from submerged macrophytes were filtered out. The ranking of the estimated densities corresponds with the anglers yield which was the highest in Boardhouse and the lowest in Swannay. The acoustic signal size distribution obtained by mobile survey was interpreted by a deconvolution procedure analogous to analysis of target strengths by single-beam sonar. This revealed different fish length frequency distributions between the lochs. The sonar technique was sampling the same length range of fish as the multimesh gill nets earlier in the year but it estimated a higher proportion of the smallest fish. Results from the application of the deconvolution procedure to mobile survey data were compared with measurement of the target strengths of side-aspect tracked fish from fixed location data.

Diurnal changes of fish behaviour in a lowland river monitored by a dual-beam echosounder

The behaviour of fish was monitored in one of the lower reaches of the River Thames (England), using a BioSonics 102 dual-beam echosounder (420 kHz) with two transducers. One transducer was located in littoral (L) depths of the river whilst the other was sited in 3-m depth of water in mid-river and placed on a especially designed lifting device which allowed different 1-m depth strata to be sampled: surface (S), middle (M) and deep (D). These depth strata were sampled hourly over 24 h. The highest fish densities and biomass were recorded in the S and L strata during the night and early morning hours from 2230 to 0700 when the fish density in the M and D strata was much lower. During the night hours, the largest fish tended to move to the surface (S) and towards the littoral (L), resulting in a marked increase in fish biomass. Most of these fish seemed to move to the deeper strata during the daytime so that many of them became undetectable by horizontal sonar as they were too close to the bottom. Swimming upstream and downstream was more marked in the open water (S,M and D strata) than in the littoral (L), where movements were more random.

Assessment of factors influencing the composition, body size and turnover rate of zooplankton in Parakrama Samudra, an irrigation reservoir in Sri Lanka

This paper considers which of the following factors influenced the taxonomic composition, body size, fecundity and birth rates of the zooplankton inhabiting a tropical irrigation reservoir: (a) wash-out and/or dilution of the population which arose from the water regime to which the reservoir was subjected by management; (b) the size-selective or species-selective predation; (c) the effect of food limitation upon the body size, egg size, post-embryonic duration and fecundities of the planktonic species and (d) the metabolic consequences of high tropical temperatures. Many of these impinge upon the characteristics of tropical zooplankton and it is appropriate that this synthesis of published results on the zooplankton of Parakrama Samudra, Sri Lanka, was prepared for the Symposium on Tropical Zooplankton held at Sao Carlos, Brazil, in December 1982.Daily losses of planktonic rotifers due to wash-out or by dilution were low and not significant compared with their capacity for recruitment but this may be a serious source of loss for the planktonic crustaceans which were virtually absent from this reservoir. Daily death rates (estimated by subtraction) were much higher and were mainly due to predation by a planktonic fish, Ehirava fluviatilis, and by Asplanchnella brightwelli. The main loss occurred during the day and on larger individuals of the brachionid species, Trichocerca spp. and Filinia longiseta, thus resulting in a small-sized rotifer community. This appeared to be due mainly to predation by the fish which was responsible also for the further reduction of rotifer body size in 1980 compared with 1979. Small rotifer body size was not caused by conditions of food deficiency because fecundity and birth rates were relatively high though not optimal. High tropical temperatures aggravate the cost of cumulative respiration resulting from food-limited prolongation of the juvenile phase and will raise the food threshold levels for growth and for reproduction above those adequate for temperate situations.

Land/water ecotone effects in reservoirs on the fish fauna

The morphology and function of one tropical and 25 temperate reservoirs are examined in relation to their effect upon the nature of the land/water interface and, further, to what extent the features of these ecotones satisfy the ecological requirements of the reservoir fish species throughout their life cycle during spawning, larval, juvenile and adult stages. The two main conclusions are that (1) reservoir fish species are especially dependent upon land/water ecotones during their early life history and (2) there exists a strong relationship between the extent of the littoral area and the nature of the fish stocks. Several examples are given to show that manipulation of the land/water ecotone is a major tool for the management of reservoirs advantageously for their major functions.

A review: limnological management and biomanipulation in the London reservoirs

Low algal biomasses and high water transparencies are a feature of the storage reservoirs that supply most of London’s treated water. This is a result of knowledgeable limnological management and biomanipulation and despite the eutrophic nature of the River Thames with its high nutrients (7 gN m− 3; 1 gP m−3) and particulate organic carbon (2 gC m−3). Built-in possibilities of jetting input water are managed to prevent stratification, to ensure isothermy, to mix chemicals and plankton vertically and horizontally and to manipulate the mixed-depth of the algal populations such that their potential for biomass growth is reduced by light-energy limitation. Spring algal growth is delayed and the spring peak is reduced and curtailed by the grazing impact of considerable biomasses of large-bodied daphnid populations (Daphnia magna, pulicaria & hyalina) whose development is also supported by the continuous input of high riverine algal crops. The existence of a large-bodied daphnid zooplankton in the reservoirs is associated with low levels of fish predation since the late 1960s. Variations in the intensity and nature of this vertebrate predation during the subsequent twenty years (1968–88) are illustrated by the changes that have occurred in the relationship between the phytoplankton and zooplankton biomasses of the April-May-June quarter of the year. This example of the London reservoirs serves to illustrate biomanipulation in deep water bodies by bottom-up as well as top-down effects.

Modelling the ecological aspects of bankside reservoirs and implications for management

Bankside storage reservoirs are used as a major water supply resource in the lower Thames Valley, England. They form the link between the River Thames and the water treatment works of the Greater London area. The reservoirs act as both a water reserve in times of low river flows, and a quality ‘buffer’ between the river and the treatment works. The load on the water treatment works (particulate material, physico-chemical characteristics) primarily reflects the water qualities of the reservoirs. Management of such reservoirs thus seeks to reduce the adverse impacts which would otherwise arise from direct river use, and to ensure as far as possible that the ecological processes within the reservoirs do not introduce new challenges to the water treatment. Reservoir management clearly needs a good understanding of those ecological processes and their interactions, and, hopefully, a means to exploit that understanding in hindcasting to explain past events, in forecasting near- or far-future events, and to help in exploring operational options to ameliorate any foreseable difficulties. The reservoirs consist of a variety of configurations, physical dimensions and operational circumstances. They have, importantly, basically simple morphologies, known hydraulic regimes and physico-chemical qualities. Nonetheless, they appear to behave essentially as small (1–50 Mm3), eutrophic lakes; and various aspects of their ecology has been studied for the past 65 years. Their attributes and operational involvement make them ideal candidates for ecological modelling, which has been applied to them in varying extents for the past 30 years. The major conclusion which may be drawn from these studies is that even in such relatively simple water bodies, current (and probably future) models can only encompass their broad ecological characteristics. Detailed operational needs have to be met by a variety of modelling approaches, mainly predicated on the basis of only being able to know a lot about a little or a little about a lot. The operational needs for modelling fall into the following broad types: (a) understanding: why did those events occur, or where is our ignorance greatest? (b) short-term forecasts: how will the current situation develop in the short-term (weeks)? (c) what-if considerations: what would happen if some management facility were employed or used differently? (d) optimisation: what are the optimal volume– quality supply arrangements? (e) long-term prediction: what is the longer-term (years) outlook under foreseeable scenarios? (f) projective evaluation: how would potential, as yet non-existant reservoirs behave under prescribed circumstances? Examples of how these needs have been met are outlined, with examples ranging from simple models of the diatom ecology of the reservoirs to much broader trophic–dynamic descriptions which can allow expression of fish–zooplankton–phytoplankton interactions. This is crucial for present and future management of cyanobacterial phases. It is clear that considerable management insight and control can result from modelling assistance, but only if the appropriate questions are asked. Whilst simple short-term modelling is less demanding, any attempt to model the full complexity of the ecology of even these relatively simple water-bodies is probably doomed to founder on complexity–understanding difficulties, unless these are resolved to much more constrained system aspects. This is particularly so for the qualitative biology. The best that may presently be foreseen is for development of the newer multi-biological type models, with reasonably realistic and dynamic physical and chemical environment sub-models, being able to manifest the general characteristics of the ecosystem in question. Despite such difficulties, new reservoir management insights and approaches will inevitably be founded on critical modelling of those ecosystems.

Low fish predation pressure in London reservoirs: II. Consequences to zooplankton community structure

The 1992 survey of zooplankton structure in fourteen London supply reservoirs showed the overall dominance of large-bodied zooplankton, mainly species of Daphnia. These reservoirs can be considered as ‘anti-fish’ by virtue of their steeply sloping concrete or brick sides. The average biomass of large Daphnia spp (retained on a 710 µm sieve) in the total zooplankton biomass was higher than 20% for twelve out of fourteen reservoirs. The cladoceran-copepod ratio was inversely correlated with both dominance of large-bodied Daphnia magna and cladoceran body-size structure. Parallelly, there were tendency of more efficient utilization of lowered algal crops in reservoirs dominated by large-bodied Daphnia spp.A graphical model is presented which relates daphnid species composition and zooplankton size structure to a presumed gradient of fish biomass in these reservoirs.

Promotion of sustainable capture fisheries and aquaculture in Asian reservoirs and lakes

A collaborative international project funded by the European Unions INCO-DC programme is undertaking limnological, fish biological, environmental and socio-economic research in five tropical lakes and reservoirs in Sri Lanka, Thailand and the Philippines over the period 1998–2001. The aim is to determine their trophic structure and their capacity to sustain both their existing fisheries and present and future aquaculture. In some cases, these activities could potentially be expanded for the benefit of rural communities and of the local market within the bounds of social and environmental sustainability. This paper describes the concepts and methods involved in this innovative multidisciplinary project which aims to integrate limnological, fisheries and socio-economic issues in a comparative approach involving Asian and European research teams.