UTILISATION OF TREATED SEWAGE GROWN ZOOPLANKTON IN WHITEFISH COREGONUS LAVARETUS (L.) FRY REARING

Wojciech Pelczarski
Sea Fisheries Institute, Gdynia , Poland

ABSTRACT

The reintroduction of whitefish to Puck Bay generates a great demand for its autumn fry. This results in attempts to find new solutions that would enhance local stocking potential. One of such solutions is to use the zooplankton grown in treated wastewater as the food for whitefish fry. This report presents the variability of zooplankton species composition and biomass growing in the effluent of the sewage treatment plant located in Swarzewo, Poland. Filtered zooplankton was used to feed whitefish fry reared in a seawater-filled pond during May to September, 2003. Food analysis of the fry has demonstrated that the zooplankton was highly preferred as food, both in terms of quality and quantity. Daphnia longispina was the dominant food item over the entire period; however, in September Neomysis integer played a similar role. The weight of the supplied zooplankton outweighed the daily requirement of the fry many fold during May-June, providing enough food for the current stocking density over the remaining months. At the end of September, the juveniles with a mean length of 128.4 mm and a body weight of 20.6 g were released to Puck Bay.

Key words: feeding , treated sewage, whitefish, zooplankton.

INTRODUCTION

The use of zooplankton for fish feeding is a recognised and widely applied technique both in the freshwater (rivers, ponds, lakes, pools) [6, 10, 27, 28, 29, 32] and marine [18, 19, 23, 30, 33] environments. In Finland, millions of whitefish fry are reared in freshwater ponds entirely on natural food [12, 15]. Research on zooplankton in terms of fish food usually involve larval stages of less than 3 g in weight [8, 13, 31]. Technical limitations underlie the fact that there is scant research on whitefish juveniles of 1-30 g feeding on zooplankton in the seawater environment.

The whitefish (Coregonus lavaretus) belongs to one of the most seriously endangered species of the southern Baltic waters [24], hence the necessity to support its population with stocking. There is a great demand for whitefish stocking material for Puck Bay in the form of autumn fry, which are much more efficient in stocking than the summer fingerlings that have been used so far. It has been decided to use the trophic potential of the sewage treatment plant in Swarzewo (Fig. 1) to enhance the production of whitefish autumn fry for the purpose of Puck Bay stocking.

High concentrations of phosphorus and nitrogen in reclaimed sewage water during May – -September together with high temperature contribute to intensive zooplankton bloom, its biomass reaching 150 g·m^-3 [20]. Within the normal operation of the treatment plant, up to 50% of the zooplankton biomass can be harvested from the pond every week [6].

Obtaining whitefish autumn fry using cost-free food in the form of treated sewage-grown zooplankton creates a great rearing potential, especially as live zooplankton has so far been the best feed in mass rearing of whitefish fry [5]. On the premises of the sewage treatment plant in Swarzewo, a pond was built, supplied with the brackish water (salinity 5-8‰) from nearby Puck Bay (300 m), without any addition of sewage or reclaimed sewage water. This is an innovation in relation to cultivating fish directly in reclaimed sewage water, which also took place in Swarzewo until the early 1990s. However, with a growing amount of sewage obtained from surrounding towns and villages it became clear that the parameters of the effluent of the sewage treatment plant did not allow rearing fish in it; therefore, with a limited supply of fresh water, it was decided to use seawater for this purpose. It was also taken into account that rearing whitefish fry from early stages in brackish water and feeding it on living zooplankton created conditions more similar to what the fish find in their natural environment, which should have led to better stocking efficiency.

The flow chart of treated sewage flow in the sewage treatment plant and the fry pond functioning is presented in Fig. 2. The fry was fed on living zooplankton filtered out from the effluent of the plant. Previously, the zooplankton had been lost with the effluent being discharged to the bay. If utilised, it represents a virtually cost-free feed, which is crucial since the cost of feeding in a fish culture amounts up to 50% of the total cost. Although this method of food provision is not frequently used in aquaculture, previous rearing experience with whitefish and rainbow trout fry in Swarzewo [20, 21] has created prerequisites for this type of culture. Availability, kind, and biomass of the zooplankton fed, besides the environment, represent the main factors affecting the growth and mortality of the juveniles.

The aim of this study was (1) to determine the variability of species composition and biomass of zooplankton grown in treated sewage as a chief food source for whitefish fry, (2) to perform food analyses in terms of utilisation of the zooplankton by the fry, and (3) to determine its growth rate with consideration to environmental factors. The knowledge in this areas, besides bringing cognitive values, can be used to adjust the production output of whitefish, or other species, juveniles to the trophic potential of the treated sewage effluent produced by the sewage treatment plant.

MATERIAL AND METHODS

Zooplankton in the stabilisation pond

The stabilisation pond of the sewage treatment plant in Swarzewo, from which zooplankton was filtered out, is 11 220 m³ in volume. It is the last in the chain of three ponds through which the effluent flows before being discharged to the bay (Fig. 3). The zooplankton was separated from the effluent 24 hours a day using a microsieve HYDROTECH Drumfilter 1203 with 200ľm mesh size installed between the outlet of the last pond and the discharge channel.

Virtually all the treated sewage was filtered with the microsieve. From May to September, an integrated zooplankton sample was collected once a week in a certain time interval at the outlet of the pipe transporting the zooplankton from the sieve to the fry pond. The samples were preserved in 4% formalin and transported to the Sea Fisheries Institute. In the laboratory, species composition of the zooplankton was analysed and, basing on the unit weights [25], the zooplankton biomass per 1 dm³ of water was estimated. The volume of water filtered during the samplings was measured with an automatic Endress Hauser flow meter installed in the measuring flume at the outlet where the effluent was discharged to the bay. Knowing the zooplankton biomass in a sample allowed estimating the entire quantity of zooplankton supplied from the microsieve to the fry pond. The fish were fed on the zooplankton ad libitum. On some days, the zooplankton was caught manually from the stabilisation pond using a plankton skimming net with 200ľm mesh size. The wet mass of this zooplankton was weighed after draining out the excess of water.

Fry pond

The fry pond is located on the premises of the “Swarzewo” Water and Sewage Company (Spółka Wodno-Sciekowa “Swarzewo”); however, it organisationally belongs to the Municipal Association of Communes (Komunalny Zwiazek Gmin, KZG) in Władysławowo. It is a man-made pond, partly excavated, with earth embankments around (Fig. 4). The pond was put into service in 2002 and has been used ever since for whitefish fry rearing during the summer, while since 2005 – also roach fry. The area of the pond is 2 600 m², the average depth 1.5 m ranging from 1.25 m, at the inlet of the seawater water, to 1.75 m at the outlet box. The seawater was pumped from Puck Bay and filtered through a gravel-sand filter before entering the pond. The pond was aerated 24 hours a day with an air blower and a system of 40 membrane diffusers placed on the bottom.

Water temperature and dissolved oxygen concentration were recorded daily, at 7.00 a.m. and 3.00 p.m., as measured with a WTW TriOxmatic 700 automatic sensor located 50 cm below the surface. In the fry pond, pH and salinity were measured weekly.

Fry

On 29 April 2003, 300 000 whitefish fry were stocked to the fry pond; the fry had been obtained from the Department of Salmonid Research of the Inland Fisheries Institute in Rutki. Due to an event that killed nearly all the fry at the end of June, on 11 July the Municipal Association of Communes decided to additionally stock 10 000 of 1.2-g fry originating from the same lot as the original stocking material. From 1 May till 30 September 2003, about 30 fish were sampled fortnightly to be measured for length and weight. Due to technical and organisational reasons, one sample only was collected in July and August. The mean daily gain in length (mm·day^-1) was calculated in each measuring period and at the end of the rearing cycle, according to the formula of Støttrup et al. [26]:

GL = (L_t – L₀)/(T_t – T₀)

Specific growth rate (G, % day^-1) for the same periods was calculated according to the formula of Houde and Schekter [9]:

G = 100(e^g – 1)

where:

g = [(lnW_F – lnW_I)t ^-1], W_I and W_F are, respectively, the initial and final mean wet weights, and t is the duration of the growth period in days [1].

Diet

The stomachs of the examined fish were collected for diet analysis, which included identification of zooplankton taxa and estimating the numbers and mass of the organisms. Diet composition was described as numerical share of given components, their frequency of occurrence, and their weight share using standard masses. The relative importance index was calculated for each food component. In order to calculate the possible fry production on the available zooplankton biomass during May – September, the method described by Szlauer [29] was applied, where the daily gain between the analyses was multiplied by the food conversion ratio, which averages 6 for the fry of salmonid fishes and whitefish. The value arrived at was assumed as a daily food ration of an individual fish during the given time. The daily biomass of the supplied zooplankton was divided by the daily food ration, arriving at the number of fry of the given average weight that can be produced on the available amount of food. Since the biomass of the zooplankton varied in time, these calculations should be treated as estimates.

Taxonomic similarity between the zooplankton components in the stabilisation pond (A) and the diet components of the whitefish in the fry pond (B), during May – September 2003, was calculated according to the formula of Marczewski and Steinhaus [3]:

S = W/a + b – W

where:

S – statistical probability of two component sets compared one to another, a – number of elements for set A, b – number of elements for set B, W – number of the elements common for A and B.

RESULTS

Zooplankton in the stabilisation pond

Over the entire period, the zooplankton was dominated, both in relation to numbers and biomass, by adult and juvenile forms of cladocerans, mainly Daphnia longispina and, to a lesser extent, D. magna, which both comprised more than 90% of the total zooplankton biomass. The rest was composed of small quantities of rotifers (Brachionus spp. and B. calyciflorus) and cyclopoid copepods (Mesocyclops and Cyclops strenuus). Other components, such as ostracodes, Hirudinea leeches, and larvae of Culicidae mosquitoes and dipteran flies where less frequent. Turbellaria were found only in September, while Moina spp. only in August.

The changes in the zooplankton total biomass (g·m^-3) and the percentage of its two main components, Daphnia longispina and D. magna, are presented in Fig. 5, whereas the percentages of other major components and their peak abundances during the studied period are presented in Fig. 6.

The highest levels of the zooplankton total biomass, 18.5 g·m^-3, and abundance, 49 122 organisms·m^-3, were recorded on 20 May, with slightly lower values recorded on 3 June, respectively 16.6 g·m^-3 and 37 000 organisms·m^-3. During the period July – September, the biomass of the zooplankton reached much lower levels, from 2.5 to 7.2 g·m^-3. At the beginning of July, at the end of August, as well as at the beginning of September, temporary yet clear drops in the zooplankton production were observed in the stabilisation pond. The total biomass was dominated by D. longispina representing 98% of the biomass in May and June; 19 August was the only day when this cladoceran was outnumbered by D. magna.

The total density of the zooplankton (organisms·m^-3) and separately of Daphnia longispina and D. magna during the studied period are presented in Fig. 7. The highest density, 35 561 organisms·m^-3, was achieved by D. longispina on 20 May, whereas the highest density of D. magna, 970 organisms·m^-3, was recorded on 19 August. Among the other components, Chydorus sphaericus was the most abundant species, 11 572 organisms·m^-3 counted in July, Mesocyclops, 11 310 organisms·m^-3 in May, and juvenile forms of Daphnia, 8546 organisms·m^-3 – also in May. Moreover, Moina spp. comprised a considerable proportion in the biomass and abundance of the zooplankton, reaching 6282 organisms·m^-3 in the second half of August.

Towards the end of May, a considerable percentage of D. longispina with the largest unit weights (2.832 and 2.941 µg) (1 µg = 10-6 g = 0.000 001 g) was noted; these were rarer during the rest of the period. During the period May – September, the most common specimens were from 177 to 1.416 µg in weight. The largest D. magna (11.328 µg) were found from 27 May to 29 July. In June, specimens of 5.664 µg were very abundant.

Total biomass filtered during 24 hours and supplied to the fry depended on both the unit biomass (g·m^-3) and the volume of water (m³) flowing out of the pond. The largest flow rates, from 10 000 to 12 500 m³ per day, occurred between 1 July and 15 September (Fig. 8). This much larger water volume flowing through the microsieve during this time compensated for a lower unit biomass of the zooplankton, hence the total biomass of the microsieve-filtered zooplankton in July and August was only slightly lower compared with the period May–June (Fig. 9). During the remaining periods, the flow rates ranged between 4000 and 6000 m³ per day.

Fry pond – hydrological conditions

The course of changes in air temperature, water temperature, and water oxygen concentration is presented in Fig. 10. When the fry rearing was started, water temperature was 14°C. With growing air temperature and increasing insolation, water temperature in the pond increased too. The highest water temperatures, up to 25.5°C, were recorded in mid-July and at the beginning of August, which was also due to windless weather. From this point on, water temperature gradually decreased down to 12-13°C on the completion of the rearing cycle.

Continuous bubble aeration resulted in well oxygenated and well mixed water column in the pond. Oxygen concentration in May remained at a preferred level of 8-13 mg O₂·dm³, while dropping to 5.7 mg O₂·dm³ on some days during the morning hours in the summer (beginning and end of July). Those were also the days when the highest diel variations of oxygen concentration were recorded, with amplitudes up to 6.7 mg O₂·dm³.

Salinity ranged between 8.0 to 9.5‰ in May, 8.0-8.5‰ in June, remained between 7.0-7.5‰ in July and August to increase to 9‰ at the end of September after the last refill of the water.

Liming the bed of the pond in April led to a slightly elevated pH level at the beginning of May, remaining between pH 7.8 and pH 8.5. Later on, it ranged between pH 7.2-7.6.

The pond was filled with seawater at the end of April. Total water exchange in the pond during May – September was 12 600 m³, or nearly fourfold in relation to the capacity of the pond. The largest water exchange occurred at the end of June and in July, when the entire water content was replaced due to the event of mass mortality of the fish. In August and September, only natural loss of water was compensated for and partial water exchange took place.

Diet

Food supplies to the pond started on the second day of the rearing cycle. The zooplankton was supplied 24 hours a day in the cycle of the microsieve operation: 10-minute break followed by 1-minute rotating and flushing. This was due to the need to save freshwater used for flushing down the zooplankton. The fish fed intensively, except during breaks in the operation of the microsieve resulting from reduced food demand caused by high temperatures, when the pond was emptied of water, or when the quality of the zooplankton was questionable. Additional portions of zooplankton, caught by hand and dispersed near the bank of the pond, were also immediately eaten up. The poorest feeding occurred during the second half of June, which was reflected by a high rate of empty stomachs observed among the sampled fry (up to 50%), as well as by lower numbers of organisms present in the diet compared with the other periods (Table 1).

The analysis of taxonomic similarity has demonstrated that the diet contained more components from the stabilisation pond during May – July, while during August – September the diet was dominated by the components originating from the brackish biocenosis of the fry pond. The highest similarity was found in July and August (Table 2).

The frequency of occurrence (FO) of particular diet components in each month is presented in Table 3, their numerical share (NS) in Table 4, while the weight share of the food components are displayed in Table 5.

Frequency of occurrence. Cladocerans predominated over the entire period, including Daphnia longispina, which from May to August were the most frequent in 92-100% of fry. In September, D. longispina was found only in 60% of fish. Other cladocerans, such as D. magna, Chydorus sphaericus, or Ceriodaphnia quadrangula, were encountered in the diet of 20-40% of fish during May – August. During the period August – September, cyclopoids (mainly Mesocyclops and Cyclops strenuus), coleopterans, dipteran larvae, and Neomysis integer become more frequent in the diet.

Numerical share. In terms of abundance, the highest numerical percentage of the diet was represented by Daphnia longispina (80-90%) during June – August, Brachionus sp. (approx. 82%) in May, and Chydorus sphaericus and D. longispina, 23 and 32% respectively, in September. Also in September, Acartia tonsa (11%), Mesocyclops (15%), Eurytemora affinis (6%), and Neomysis integer (5%) comprised a higher numerical percentage.

Weight share. During May – August, D. longispina was the most important in terms of weight, comprising 69-97% of the diet, which dropped to 42% only in September due to a nearly equal importance of Neomysis integer (42%). The share of gastropods, gammarids, and coleopterans was higher in September as compared with the other months.

Relative importance index. The values of the relative importance index show (Table 6) that D. longispina was the most important and definitely predominant food component of the whitefish fry throughout the entire period of rearing. In relation to this species, the index was ten- to twentyfold higher than that for the other components in turn, i.e. D. magna, Chydorus sphaericus, Mesocyclops C IV-V, except for September, when the index for D. longispina was threefold higher than that of the subsequent component, Neomysis integer.

Growth and survival of fry

The mean length of the whitefish juveniles at the end of September was 128.4 mm, which means that an average fish increased its length nearly tenfold during five months. The mean body weight was 26.6, or more than twentyfold higher than at the beginning. The largest individuals attained 152 mm in length and 36 g in weight. The mean growth rate (GL) reached over the period of 148 days was 0.77 mm·day^-1, while the specific growth rate (G) for this period was 5.72 (Table 7).

From the entire quantity of the pond stocked material (300 000 in late April and 10 000 of summer fingerlings in July), 1.380 fish survived till the end of the rearing cycle and were released to Puck Bay for reintroduction purposes. During May and till the half of June, only single death cases were observed in the pond, up to 100 fish a day. Mass mortality, however, occurred in the second half of June, which left only a few survivors. On the whole, it is highly probable that the juveniles that survived to the end of the trial were the fish stocked to the pond in July. Lower mortality rates were observed after 11 July. If we accept this assumption, survival rate should be 0% for the larvae and about 14% for the summer fingerlings.

Production potential of whitefish fry

Considering production of the zooplankton in the stabilisation pond, the way it was supplied as food, environmental factors, stocking density, and daily length gains during the period May – September 2003, the production potential of juveniles was the highest for the younger stages, from 800 000 fry in May to 250 000 fish in July, and lower for the autumn fry, 12 000 fish in August and 18 000 fish of 20.6 g in September (Table 8). If we consider the actual stocking density in 2003, a large part of the zooplankton remained uneaten by the fish, especially during May – July.

DISCUSSION

Daphnia is usually one of the most important food items in the diet of all age groups of the whitefish inhabiting natural bodies of water [11]. The main component of the zooplankton biomass in the stabilisation pond was Daphnia longispina, which fully corresponded to the feeding requirements of the whitefish fry from May to August. Namely, this cladoceran was a dominant food item during this period, which was indicated by its high index of relative importance. However, such food selection was primarily due to the general concentration of the component, which in this case was D. longispina. It should be added that components smaller in size, such as Brachionus or Chydorus, supplied from the stabilisation pond, also constituted an important food item in the diet of the fry. The whitefish, although primarily planktivorous, started to consume also large quantities of dipteran larvae in July. In September, the juveniles of 20.6 g started to prey on organisms of larger sizes, found only in the fry pond, mainly Neomysis integer, Eurytemora affinis, and Acartia tonsa. These changes in the diet could have resulted both from larger mouth openings of the fish and current availability of such organisms [22]. It has been observed that the stomach contents were dominated by female organisms of the zooplankton, as was the case for Acartia tonsa, Cyclops strenuus, or Mesocyclops, which resulted rather from their higher availability than from the preferences of the foraging fish.

The biomass of the zooplankton in the stabilisation pond is to a large extent a resultant of the quality of the wastewater being treated and the purification technology. Should chemical substances that are noxious to zooplankton be carried with the sewage through the treatment process, not only may the biomass of the zooplankton collapse, but the zooplankton organisms may carry the toxins to the fry pond as well. The treatment technology used at present does not allow early detection and prevention of such events, hence a high risk that such substances may indirectly cause mass mortality of the fry. This hypothesis could apply to the fry mass death event observed in June/July 2003.

Daily food requirement shows us that the biomass of the zooplankton supplied to the pond during the period May – June would be sufficient to feed much more fry than that present in the pond. Therefore, at relatively high water temperature, uneaten part of the zooplankton died and decayed at the bottom of the pond, which consumed much oxygen and released toxic decomposition products with easily detectable odour in the sediment samples collected at the feeding site. Thus, a possibly true hypothesis is that the food surplus decaying during the summer, along with higher temperature of the water and with morning oxygen levels falling below 6 mg·dm³, accompanied by temporarily more sluggish water mixing caused by masses of Spirogyra sp. algae [17], could have been one of the factors underlying the mass fish deaths. Unfortunately, the Municipal Association of Communes failed to order veterinary examination of the killed fish, hence the causes underlying the deaths are difficult to ascertain. Rearing fish in a pond (with still water) is a more risky process in terms of sudden fish deaths than in a system of flow-through ponds, where we can swiftly move the fish from one pond to another; besides, we can administer appropriate pharmaceuticals or disinfecting agents.

It should be stressed that the whitefish fry was fed live zooplankton from the first day and this did not cause any mass deaths in the pond at Swarzewo during the first month, contrary to the observations reported be Eckmann [4], who found that feeding with the zooplankton taken from the environment resulted in a high mortality rate during the first 20 days.

In the future, it is recommended that water volume flowing through the microsieve be reduced, which should allow avoiding extensive surplus of the zooplankton mass and maintaining the amount of feed at the level that reflects the needs of the actual density of fish in the pond, especially during hot temperatures. Due to nutritional requirements of much larger juveniles during the period August – September, the daily zooplankton supplies were much lower, and, although sufficient for the current density, could have been a limiting factor in terms of growth and survival if the pond had been stocked with a higher, previously assumed number of fish. Therefore, other methods of increasing food collections should be considered during this period, e.g. underwater illumination in the area where water is taken to the microsieve in order to attract more zooplankton [2, 7, 16] or manual plankton catches along the banks of the pond.

The mean daily gain over the period of 119 days was 0.82 and was similar to that attained in the same period in 2002, i.e. 0.78 mm·day^–1. Specific growth rate, which is presumably the most authoritative growth measure, was also similar in the two periods, being 6.86 in 2003 compared with 6.53 in 2002 [21]. Due to the fact that an additional stocking was performed with summer fry, which – although from the same material – had been reared on artificial feeds for two months, equivalent evaluation of the growth of the entire fry may be questionable.

The survival rate from the stocking to the autumn fry, which was 14%, is not a high result as compared with the fry fed on artificial feeds under controlled conditions, though still survival may be as low as 1.5% for the summer fry [14].

Analysing the availability of the zooplankton biomass, it is probably possible that a similar level of biomass in the years to come may support a target production of approx. 20 000-30 000 whitefish autumn fry with an average weight of 20 g in Swarzewo, provided the requirements of proper health quality of the zooplankton and satisfactory environmental conditions in the fry pond are met. Such level of production is similar to results of experiments carried out in Swarzewo in 1994 and 1995 [20].

Raising fry in seawater and feeding it on zooplankton filtered from treated sewage, as is the case in Swarzewo, ensures low costs of the cycle and, which is important for stocking, does not require the released fish to adjust to the marine environment, allowing them to immediately forage for and utilise their natural food.

The results obtained allow concluding that it is purposeful to widely utilise treated sewage-grown zooplankton in the production of stocking material of whitefish, rainbow trout, or the species of lower oxygen demand, e.g. roach. This method can also find an application in other facilities, similar to that in Swarzewo.

CONCLUSIONS

1. Daphnia longispina was the most preferred and most important food component of the whitefish fry.

2. From May to July, available quantity of food highly exceeded the needs of the current fry density in the pond.

3. High mortality of the fry can be controlled also by reduced zooplankton supplies during the initial period of rearing.

4. Over a span of 5 months, the fry attained an average weight of 20.6 g and length of 128 mm.

5. The existing facilities of the Swarzewo sewage treatment plant enable obtaining 20 000-30 000 whitefish autumn fry at low production costs, assuming favourable environmental conditions.

ACKNOWLEDGEMENTS

The research was carried out within the Sea Fisheries Institute project no. NB-40 financed by the State Committee for Scientific Research. I wish to express my sincere gratitude to Ms Alina Krajewska-Sołtys, for her great work on zooplankton identification, and to the Employees of the Municipal Association of Communes in Władysławowo (Komunalny Zwiazek Gmin we Władysławowie) and “Swarzewo” Water and Sewage Company in Swarzewo (Spółka Wodno-Sciekowa w Swarzewie), for their co-operation in the accomplishment of this research.

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