The fish community of the Loosdrecht Lakes and the perspective for biomanipulation

In a two-year-study, the fish community of the Loosdrecht Lakes area was characterized with regard to the professional fishery and the forage base. The lakes are shallow (mean depth 2 m) and eutrophic to hypertrophic. The bream,Abramis brama, dominates the fish biomass. The pikeperch,Stizostedion lucioperca, is the main predator.Bream up to 30 cm have a slow growth rate and are in a bad condition. A faster growth rate and a better condition are shown by bream of 30 cm and more. The small bream feeds on chironomid larvae, benthic cladocerans and zooplankton. The better condition of bream over 30 cm is explained by the more efficient feeding of larger bream onChironomus plumosus larvae. Pikeperch show a fast growth rate and a good condition. Recruitment is limited by the low densities of smelt,Osmerus eperlanus, leaving cannibalism as the most important way for the 0+ pikeperch to become piscivorous and to manifest a fast growth.The impact of the professional gillnet fishery on bream and pikeperch is small because the mesh sizes in use are as large as 75–100 mm bar mesh.The planktivorous 0+ pikeperch consumes mainly the carnivorous zooplanktersLeptodora kindtii and cyclopoids. The zooplankton community lacks large herbivorous species likeDaphnia hyalina, capable of consuming bluegreens. A possible experiment in biomanipulation with a view to find out whether the development ofD.hyalina is depressed by the small planktivorous cyprinids, is predator enhancement with the aid of a stocking programme for indoor-raised 0+ pikeperch in early summer.     

Diet patterns and ontogenetic diet shift of pikeperch, <Emphasis Type="Italic">Sander lucioperca</Emphasis> (L.) fry in lakes Peipsi and Võrtsjärv (Estonia)

Pikeperch is a major predator in the pelagic zone of eutrophic waters, such as the large north-temperate lowland lakes Võrtsjärv (Estonia) and Peipsi (Estonia/Russia). The size and structure of the pikeperch population is strongly influenced by their success at the juvenile stage. Therefore, we investigated the diet and prey selection of pikeperch fry caught in the ice-free period in lakes Peipsi and Võrtsjärv in 2007 and 2008. We analysed the stomach contents of 635 pikeperch from Lake Peipsi and 202 pikeperch from Lake Võrtsjärv, and compared our findings with similar data from the 1950s (Erm, About Biological and Morphological Differences of Pikeperch. Hydrobiological Researches II (in Estonian), 1961). Analysing 4–20 cm long fry, we studied differences in prey size, seasonal diet patterns and the ontogenetic diet shift. In both lakes, 0+ pikeperch feed mostly on large predatory zooplankters. However, in Lake Peipsi the stomach content weight and the average number of food items in stomach were higher, and the food spectrum was wider than in Lake Võrtsjärv. There was also a difference in the type of food that dominated fry’s stomach content (calculated by weight) in the two lakes. In Lake Peipsi, chironomids larvae, as well as zooplankters Daphnia galeata and Bythotrephes longimanus dominated, while in Lake Võrtsjärv zooplankters Mesocyclops leuckarti and Leptodora kindti. Seasonal analysis showed that cladocerans dominated in pikeperch fry stomach content in summer and at the beginning of September, but copepods were dominant in autumn and spring. In contrast to the studies carried out from 1952 to 1958 (Erm, About Biological and Morphological Differences of Pikeperch. Hydrobiological Researches II (in Estonian), 1961), the shift from planktivory to piscivory at the end of the first growing season was hardly ever observed during our investigation. We believe this is due to the lack of suitable prey fish as there was a collapse of the smelt, Osmerus eperlanus (L.), population in both lakes. The transition of pikeperch from planktivory to piscivory was delayed till the next summer.     

Comparison of losses of planktivorous fish by predation and seine-fishing in a lake undergoing long-term biomanipulation

SUMMARY 1. Piscivore stocking at artificially high densities and fishing are the two common approaches to reduce the amount of planktivorous and benthivorous fish in lake biomanipulation programmes. Both measures have advantages and disadvantages, but their relative efficacy has not previously been directly compared.
2. We calculated the average annual catch of roach and bream in a lake undergoing long-term biomanipulation (Feldberger Haussee, Germany) by seining each year between 1992 and 1998. We compared this value with a bioenergetics estimate of annual consumption rates of the dominant cohorts of piscivores, pikeperch and pike, in 1997 and 1998. We also determined species composition and length distribution of prey fish in stomachs of the piscivores.
3. Roach was the dominant prey species of both pikeperch and pike, whereas bream was rarely taken by either piscivorous species. Seining removed on average larger specimens of roach than were found in the stomachs of the piscivores.
4. Based on stocking densities of the piscivores, published mortality rates, and individual consumption rates, feeding of pikeperch and pike on roach exceeded the manual removal of roach by seining by a factor of 4–15 (biomass) in 1997 and 1998.
5. Based on these results, a combination of fishing and piscivore enhancement is recommended. Whereas the stocks of adult roach and bream have to be reduced mainly by fishing, the predation of piscivores should be directed predominantly towards the juvenile zooplanktivorous fish. Therefore, small size-classes of piscivorous fish should be promoted by fisheries management, including stocking and harvest regulations.     

The effect of biomanipulation of the zooplankton on the growth, feeding and survival of pikeperch (Stizostedion lucioperca) in nursing ponds

The following treatments were evaluated for use in rearing pikeperch larvae (Stizostedion lucioperca L.) in nursing ponds: treatment with Dipterex (Bayer); concurrent stocking with common carp (Cyprinus carpio L.); short flooding periods and prefeeding fish larvae. Treatment with Dipterex (active ingredient trichloorfon, i.e. 00-dimethyl-2,2,2, trichlorohydroxyethyl phosphonate) gave zooplankton populations dominated by rotatoria and nauplii at the onset of exogenous feeding and dominated by copepods later on. Cladocerans dominated the plankton in untreated ponds but were never prominent in treated ones. Stomach analysis revealed a fast changing feeding preference in pikeperch from nauplii and copepodites towards cladocerans and chironomids. The excellent food conditions for first feeding pikeperch larvae in Dipterex treated ponds resulted in a significant higher number of harvested fingerlings. In spite of the more favourable food conditions in untreated ponds during the second half of the culture period, a significant growth difference was not found. Concurrent carp stocking did not affect significantly the survival and growth of the pikeperch, although it decreased the accessability of chironomids for pikeperch. Short flooding periods and prefeeding pikeperch larvae failed as useful management strategies for pikeperch nursing. Pikeperch fingerlings cultured in ponds are larger than those found in natural populations at the same time, giving them a high chance to become piscivorous and to increase the year class strength when stocked in natural waters.     

Restoration by biomanipulation of lake Bleiswijkse Zoom (The Netherlands): First results

In 1987, the Bleiswijkse Zoom, a small, shallow lake in The Netherlands, was divided into two compartments to investigate the possible use of biomanipulation as a tool for restoring the water quality of hypertrophic lakes. The density of the fish stock before restoration was about 650 kg.ha^–1, composed mainly of bream, white bream and carp. Pikeperch was the main fish predator in the lake. In April, 1987, in one compartment (Galgje) all planktivorous bream and white bream and about 85% of the benthivorous bream and carp were removed. Advanced pikeperch fry were introduced as predator during the transient period. The other compartment (Zeeltje) was used as a reference. Removal of the fish in Galgje resulted in low concentrations of chlorophyll-a, total phosphorus, nitrogen and suspended solids. The absence of bottom-stirring activity by benthivorous fish and the low chlorophyll-a concentrations led to an increase in the Secchi disk transparency from 20 to 110 cm. Within two months after removal of the fish, macrophytes, mainly Characeae, became abundant. Until July the high density of large zooplankton species caused low algal biomass. From June onwards, the zooplankton densities decreased, but the algal concentrations remained low. This is probably because of nutrient limitation or depression of algal growth by macrophytes or both. Compared with the non-treated compartment the number of fish species in the treated compartment was higher. Perch, rudd and roach, i.e. the species associated with aquatic vegetation, were found in the samples. The survival of the O⁺ pikeperch was poor. The pikeperch could not prevent the growth of young cyprinids. Within two months after the removal of the fish a habitat for northern pike was created.     

Variation in abundance and survival of fish larvae in shallow eutrophic lake Tjeukemeer

Synopsis Variations in abundance and survival of larvae of the major fish species, bream, roach, pikeperch, perch and smelt were studied in shallow eutrophic lake Tjeukemeer (1986–1988). Except for smelt, the size of the spawning stock and the abundance of larvae were not related. Both the timing of appearance of larvae and their growth rate were determined by water temperature as was shown by a growth model developed earlier for Tjeukemeer fishes. In cyprinids and percids the survival of the larvae was hypothesized to be negatively related to the duration of the earliest stages. Except in cyprinids, the abundance of 0+ juvenile fish could be explained by the number of larvae. In cyprinids 0+ juvenile abundance at the end of the year was not related to the number of larvae in May and June. The cyprinid juvenile survival rates are hypothesized to be related to the size of the main cyprinid predator, 0+ pikeperch.     

Diel activity of adult pikeperch <Emphasis Type="Italic">Sander lucioperca</Emphasis> (L.) in a drainage canal in the Mediterranean basin during spring

Pikeperch (Sander lucioperca L.) is a broadly distributed fish species in Europe but little is known about its ecology in the southern part of its distribution area in warm climatic conditions. The aim of this study was to analyse pikeperch rate of movement and to assess whether it displayed a diel pattern related to temperature. Thus acoustic telemetry was used to track adult pikeperch in a drainage canal located in south of France. The survey was carried out in spring, during the spawning period. The results showed that females were more active than males. This is in accordance with previous data on the nest guardian behaviour of the males. For both genders, the activity rates increased during the study period as water temperature rose. Males and females displayed the same diel activity with a maximum at dusk, thereby confirming many indirect observations. Nevertheless, inter-individuals variations were observed. Thus, these results on diel activity are rather a general trend than a strict rule and suggest the involvement of other factors than light intensity in the control of diel activity. This diel rhythm is positively correlated to water temperature for females. Pikeperch activity may be the result of a trade-off between physiological requirements of temperature and light, satisfaction of energy needs and avoidance of predators.     

Effects of water temperature on year-class strengths and growth patterns of pikeperch (<Emphasis Type="Italic">Sander lucioperca</Emphasis> (L.)) in the brackish Baltic Sea

Pikeperch (Sander lucioperca), the studied species, is the most important amongst freshwater fish species, commercially, in the brackish Baltic Sea. We studied the effects of water temperature on three aspects of pikeperch growth in Haminanlahti bay, Finland. First, annual length increments were related to age and temperature using a non-linear growth model. Since length increments were based on back-calculated lengths, i.e. repeated measurements, the first order autoregressive covariance structure was used. The model showed that when temperature increased annual length increments also increased, whereas increased age reduced annual length increments. In the modelling, the best fit was found with water degree-days over 10°C. Second, the onset of growth of pikeperch in spring had a positive relationship with water temperature and a negative one with the length of pikeperch. The latter finding suggests that smaller, non-mature pikeperch starts to grow earlier than larger, mature pikeperch. Third, the year-class strength was positively correlated with water degree-days, and the year-class strength negatively affected annual length increments. This indicates that within a strong year-class subsequent growth is reduced more that within a weak year-class, suggesting density-dependent growth.     

Long term study on the influence of eutrophication, restoration and biomanipulation on the structure and development of phytoplankton communities in Feldberger Haussee (Baltic Lake District, Germany)

Feldberger Haussee provides a classic example of eutrophication history of hardwater lakes in the Baltic Lake District (Germany) and of changes in their algal flora during the 20th century. The lake originally was regarded as slightly eutrophic. A process of drastic eutrophication from the 1950s until the end of the 1970s caused mass developments of blue-green and green algae. A restoration program was started in the 1980s to improve the water quality of the lake using both diversion of sewage outside the catchment area, and biomanipulation by altering the fish community. This restoration program led to positive changes in the lake ecosystem. Direct effects of biomanipulation resulted in an increase of herbivorous zooplankton, a decrease of phytoplankton biomass, and an increase of water transparency. The recovery of Feldberger Haussee also may have been indirectly enhanced by an increase in nutrient sedimentation as a consequence of intensified calcite precipitation, decrease in phosphorus remobilization due to a pH-decrease, increased NIP-ratio, and recolonization of the littoral zone by macrophytes. This paper concentrates on the long term development of the phytoplankton community as a response to changes in the food web structure as well as to alterations in the chemical environment of the algae. Both are reflected in four major stages passed by the algal assemblage between 1980 and 1994: (1) From 1980-summer 1985 dense green algal populations were found indicating similar conditions as in the 1970s during the period of maximum eutrophication. (2) A diverse phytoplankton community during summer 1985–1989 showed the first effects of a recovery. (3) From 1990–1992 the phytoplankton was characterized by ungrazeable filamentous blue-green algae first of all as a response to increased herbivory of zooplankton on edible species and to increasing N/P-ratios. (4) Finally, the algal species diversity increased in 1993 and 1994 whereas the phytoplankton biomass decreased showing the success of the combined restoration measures.     

Pikeperch habitat use within a canal network in spring

Thirteen pikeperch Sander lucioperca were implanted with acoustic transmitters and 10 were successfully tracked twice a week from March to early July in a drainage canal connected to a brackish lagoon located in the Rhône delta. Pikeperch was found in the main canal and in narrow tributaries, but never in the lagoon. Pikeperch avoided shallow parts of the canal and selected areas with high riparian vegetation (mainly trees). No between sexes differences occurred in habitat selection. At the basin scale, while both sexes remained in the upstream part of the canal during the spawning period, after spawning, females migrated downstream and also into the tributaries. This migration may have been induced by the movement of prey while males remained guarding the nests.     

Spawning strategies in cypriniform fishes in a lowland river invaded by non-indigenous European barbel Barbus barbus

Spawning strategies of lowland river fishes include single spawning, where reproduction generally occurs in early spring to provide 0+ fish with an extended growth season through the summer, but with a high risk of stochastic mortality events occurring, such as early summer floods. This risk can be reduced by multiple or protracted spawning strategies, where 0+ fish are produced over an extended period, often into mid-summer, but with the trade-off being a shorter growth season. The spawning strategies of cypriniform fish were explored in the River Teme, a spate river in Western England, which has non-indigenous European barbel Barbus barbus present. Sampling 0+ fish in spring and summer and across three spawning periods, B. barbus, chub Squalius cephalus and minnow Phoxinus phoxinus always revealed multiple spawning events, with 0+ fish of < 20 mm present in samples collected from June to August. Fish below 20 mm in August remained relatively small by the end of their growth season (October). For dace Leuciscus leuciscus, only single spawning events were evident, but with 0+ dace always being relatively large. Therefore, multiple spawning appears to be a common strategy that provides resilience in 0+ fish against stochastic mortality events in lowland rivers.

     

Changes in the fish community and water quality during seven years of stocking piscivorous fish in a shallow lake

SUMMARY 1. Piscivores (annual stocking of 1000 individuals ha^?1 of 0+ pike and a single stocking of 30 kg ha^?1 of large 20–30 cm perch) were stocked in seven consecutive years in a shallow eutrophic lake in Denmark. The stocking programme aimed at changing food‐web structure by reducing zooplanktivorous and benthivorous fish, with resultant effects on lower trophic levels and ultimately water quality. 2. The fish community and water quality parameters (Secchi depth, concentrations of total phosphorus, chlorophyll a and suspended solids) were monitored between 1996 and 2000 and relationships were evaluated between predatory fish and potential prey and between zooplanktivorous or benthivorous fish and water quality parameters. In addition, potential consumption of piscivorous fishes was calculated. 3. The density of fish feeding on larger zooplankton or benthos (roach >15 cm, crucian carp >15 cm) declined distinctly during the study period. This effect was attributed to predation by large (>50 cm) pike. Based on scale readings, we cautiously suggest that the stocking of 0+ pike boosted the adult pike population to produce an unexpected impact in later years. Conversely, no direct impact of stocked 0+ pike was detected on 0+ roach. 4. A major decline in the recruitment strength of 0+ roach was observed in 2000. A combination of (i) the indirect effect of large pike preying on adult roach, with negative effect on roach reproduction and (ii) the direct predation effect of 0+ pike and/or 1+ and 2+ perch recruited in the lake, provides the most likely explanation of this phenomenon. 5. A marked increase in Secchi depth in 2000 and declining trends in suspended solids, chlorophyll‐a and total phosphorus concentrations were observed. These changes may also be attributable to changes in the fish community, although the relationships were not straightforward. 6. This 7‐year study indicates that piscivorous fish may be a significant structuring force in shallow eutrophic lakes, suggesting that stocking piscivores can increase predation pressure on cyprinids. However, the general lack of impact of 0+ pike points to the need of refining current stocking practices in several countries across Europe.     

P-load,phytoplankton, zooplankton and fish stock in Loosdrecht Lake and Tjeukemeer: confounding effects of predation and food availability

The fish community in the Loosdrecht lakes is dominated by bream, pikeperch and smelt and is characteristic of shallow eutrophic lakes in The Netherlands. The biomasses of the respective fish species amount to ca. 250, 25 and 10 kg ha^–1 and correspond to those in Tjeukemeer, another lake in The Netherlands. The average size of bream, however, is much smaller in the Loosdrecht lakes as a consequence of poorer feeding conditions. The zooplankton community in the Loosdrecht lakes is predominantly composed of relatively small species such as Daphnia cucullata, Bosmina coregoni and cyclopoid copepods, whereas in Tjeukemeer, Daphnia hyalina is permanently present in relatively high densities and the other species show a larger mean length. In the Loosdrecht lakes, the absence of D. hyalina and the smaller sizes of the other zooplankton species could be the consequence of a higher predation pressure, in combination with unfavourable feeding conditions for the zooplankton including the low density of green algae and the high density of filamentous cyanobacteria. A biomanipulation experiment in Lake Breukeleveen, one of the Loosdrecht lakes, indicated that feeding conditions were too unfavourable for large zooplankton to develop in spring, when the reduced fish biomass was not yet supplemented by natural recruitment and immigration.     

Genetic effects of supportive stockings on native pikeperch populations in boreal lakes--cases, three different outcomes

The genetic consequences and gene flow of pikeperch (Sander lucioperca) stocking were assessed in three boreal lakes based on admixture model analysis and comparison of the pre- and post-release patterns of genetic variability at 9 DNA microsatellite loci in the recipient populations. In two out of the three cases, the releases of fish from foreign populations caused significant changes in the genetic structure of the recipient population. The largest changes were observed in Lake Ouluj?rvi, where the post-release sample was almost identical to the released Lake Vanajanselk? population, and about 90% of the catch was composed of the released population. The genetic composition of Lake Lohjanj?rvi pikeperch also shifted markedly towards that of the released Lake Vanajanselk? population, and about half of the later catch was of released Vanajanselk? origin. In Lake Vanajanselk?, in contrast, releases of pikeperch from lakes Painio and Averia had only a small impact on the genetic structure of the pikeperch population. These results indicate that the current stocking practices create an effective artificial gene flow that may strongly shape and reduce the genetic differentiation among the remaining native pikeperch populations. A common feature of all three cases was the lack of prior appraisal of the potential genetic and ecological risks in relation to the expected benefits of the release programmes.     

Changes in aquatic insect emergence in response to whole-lake experimental manipulations of introduced trout

1. Insects emerging from mountain lakes provide an important food source for many terrestrial predators. The amount of insect subsidy that emerges from lakes is influenced by predator composition, but predator effects could be ameliorated by greater habitat complexity. We conducted a replicated whole‐lake experiment to test the effects of introduced fish predators on the abundance and composition of aquatic insects within and emerging from the littoral zone of 16 mountain lakes in the Trinity Alps Wilderness in northwestern California. 2. Study treatments matched the fisheries management options being implemented in California’s wilderness areas: (i) continued stocking with non‐native trout, (ii) suspension of stocking, and (iii) suspension of stocking and removal of fish. We also included four naturally fishless ‘reference’ lakes. We compared abundances and biomass of emerging aquatic insects before treatments were initiated and for 3 years following their establishment. Abundances of benthic insects were also compared in the third year post‐treatment. 3. Trout removal rapidly increased abundances of mayflies, caddisflies, and insect predators, and overall insect biomass emerging from lakes. No significant differences were found between the suspension of stocking lakes and continued stocking lakes. Fish density was a more important predictor of aquatic insect emergence than habitat complexity. 4. Mayfly larvae responded positively to fish removal and caddisfly larvae tended to be more abundant in lakes without fish, but we did not detect effects on abundance of predatory insects. However, we found large insect predators in shallower water in lakes with fish compared to fish removal or fish‐free reference lakes. 5. These results provide insights into the continuing effects of past and current fish stocking practices on the flow of insect prey from mountain lakes into the neighbouring terrestrial environment. We also show that these consequences can rapidly be reversed by removing non‐native fishes.     

A bioenergetic assessment of the influence of stocking practices on rainbow trout (Oncorhynchus mykiss) growth and consumption in a New Zealand lake

Summary

• 1 To investigate the carrying capacity and factors affecting growth of rainbow trout in Lake Rotoiti, we employed a bioenergetics model to assess the influence of stocking rates, timing of releases and prey abundance on growth and prey consumption. We hypothesised that stocking rates and prey abundance would affect growth and prey consumption by influencing per‐capita prey availability, and that the environmental conditions encountered by fish at the time of stocking would affect growth and consumption.
• 2 Prey consumption of stocked rainbow trout was calculated with the Wisconsin bioenergetics model. We calculated growth trajectories of released trout based on data from stocked trout that were released in spring and autumn from 1993 to 2009 and then re‐captured by anglers. Diet, prey energy density, body mass lost during spawning and lake temperature were measured locally.
• 3 Stocking timing had no effect on return rates to anglers or length or weight of caught fish. Although trout released in autumn were smaller than those released in spring, autumn‐released trout grew at a faster rate and had similar lengths and weights to spring cohorts after 2 years of growth in the lake. Modelled consumption parameters were negatively correlated with trout population size, suggesting that stocking rates (347–809 fish ha^?1 year^?1) caused density‐dependent effects on growth. Although common smelt (Retropinna retropinna) accounted for 85% of total prey consumption, no significant relationship was found between prey consumption by individual trout and adult smelt abundance, possibly because trout are targeting smaller smelt that our abundance estimate did not account for.
• 4 Releasing trout in autumn appears to be advantageous for growth, possibly because (i) temperature is more suitable for growth in autumn–winter than in spring–summer and (ii) prey for small trout is abundant in autumn. Mild winter conditions appear to enhance overwinter survival and growth of rainbow trout in warm‐temperate lakes compared to higher latitudes. This implies that moderately productive warm‐temperate lake ecosystems are highly suitable for trout growth in winter, but less so in summer, when lake stratification and high nutrient levels may create conditions suitable for algal blooms and hypolimnetic deoxygenation. High growth rates of trout in warm‐temperate lakes can therefore be supported by timing releases to coincide with favourable winter conditions.

     

Migratory behaviour of adult pikeperch (Stizostedion lucioperca) in a lowland river

The behaviour of radio-tagged adult pikeperch (Stizostedion lucioperca (L.)) from two areas in the Danish River Gudenaa were recorded prior to, and during the spawning period. Eight of 13 tagged fish in the lower reaches of the river were located throughout the whole study. Five of these fish moved upstream to various sites in the river prior to spawning, which occurred from late April to June. The three remaining fish moved to the fjord. These movements were interpreted as a spawning migration, and it is suggested that the lower reaches of the River Gudenaa constitute an over-wintering area for pikeperch, which use different spawning areas. Ten pikeperch caught just downstream of an impassable hydropower plant were radio-tagged and translocated upstream of the dam to a reservoir. Within a week, half of the fish moved to a lake situated more than 30 km upstream the reservoir. This behaviour is hypothesised to be a homing response. The study reveals that the pikeperch is a highly mobile species with a complicated migration pattern, even in relatively small river systems.     

The effects of diet and stocking density on the growth and behaviour of pond pre-reared pikeperch under intensive conditions

Experiments were carried out between 1999 and 2001 on pikeperch fingerlings using two feeds at four stocking densities to evaluate the success of training 35‐day‐old pond‐reared pikeperch, Stizostedion lucioperca L., to lifeless feed in intensive culture. Training periods of 28 days for each of three cohorts (1999–2001) were applied to introduce pikeperch from zooplankton to minced feed. In a preliminary study (1999), two feeds (live prey and minced fish meat) were compared at two stocking densities. Two higher densities were investigated in 2000 and 2001 and matched with the minced fish diet. Different feeds had significant effects on feed consumption and, as a consequence, on body weight gain; fish fed live prey showed highest growth rates. Unexpectedly, the stocking density had no significant effect on growth, and at the highest density there was a significantly betterfeed conversion rate. Stocking density and feed type significantly influenced the feeding behaviour. Three characteristic foraging techniques are described.     

The relations between 0+ fish density,zooplankton size and the vulnerability of pikeperch,Stizostedion lucioperca,to angling in the Frisian lakes

During a five year study, the size and abundance of zooplankton and 0+ fish in September were registered in a number of Frisian lakes. The mean size of Daphnia was related to the abundance of planktivorous 0+ fish in September. Immigration of fish larvae into the Frisian Lake District from the IJsselmeer in June was observed in 1979 and 1980. The abundance of 0+ fish was negatively related and the size of Daphnia was positively related to the vulnerability to angling of predatory pikeperch.     

Connectivity of a large embayment and coastal fishery: spawning aggregations in one bay source local and broad-scale fishery replenishment

Ichthyoplankton sampling and otolith chemistry were used to determine the importance of transient spawning aggregations of snapper Chrysophrys auratus (Sparidae) in a large embayment, Port Phillip Bay (PPB), Australia, as a source of local and broad-scale fishery replenishment. Ichthyoplankton sampling across five spawning seasons within PPB, across the narrow entrance to the bay and in adjacent coastal waters, indicated that although spawning may occur in coastal waters, the spawning aggregations within the bay were the primary source of larval recruitment to the bay. Otolith chemical signatures previously characterized for 0+ year C. auratus of two cohorts (2000 and 2001) were used as the baseline signatures to quantify the contribution that fish derived from reproduction in PPB make to fishery replenishment. Sampling of these cohorts over a 5 year period at various widely dispersed fishery regions, combined with maximum likelihood analyses of the chemistry of the 0+ year otolith portions of these older fish, indicated that C. auratus of 1 to 3+ years of age displayed both local residency and broad-scale emigration from PPB to populate coastal waters and an adjacent bay (Western Port). While the PPB fishery was consistently dominated (>70%) by locally derived fish irrespective of cohort or age, the contribution of fish that had originated from PPB to distant populations increased with age. At 4 to 5+ years of age, when C. auratus mature and fully recruit to the fishery, populations of both cohorts across the entire central and western Victorian fishery, including two major embayments and c. 800 km of coastal waters, were dominated (>70%) by fish that had originated from the spawning aggregations and nursery habitat within PPB. Dependence of this broadly dispersed fishery on replenishment from heavily targeted spawning aggregations within one embayment has significant implications for management and monitoring programmes.