Download Cercopagis pengoi - New Important Food Object of the

Internat. Rev. Hydrobiol., 85 (5-6), pp. 609-619, 2000
ALEXANDER ANTSULEVICH1 and PENTTI VÄLIPAKKA2
1Saint-Petersburg State University, Dept. of Hydrobiology, 16 Linia, 29, St.Petersburg, 199178, Russia. [email protected]
2Southeast Finland Regional Environment Centre, P.O. Box 1023, Fin-4501,
Kouvola, Finland. [email protected]
Cercopagis pengoi - New Important Food Object
of the Baltic Herring in the Gulf of Finland
Key words: Baltic Sea, Gulf of Finland, non-indigenous species, Cercopagis pengoi,
Clupea harengus membras, Baltic herring nutrition
Abstract
The non-indigenous predaceous cladoceran Cercopagis pengoi (OSTROUMOV) was
found for the first time in the Gulf of Finland in 1995. After this invasion, the diet of the
Baltic herring Clupea harengus mebras and its seasonal dynamics were investigated in
the coastal waters of SE Finland. The specimens of C. pengoi were not present in
plankton samplings until the end of July when the surface water temperature reached
13.5 °C. From the beginning of August to the end of October Cercopagis was found in all
the plankton samples.
Before the occurrence of Cercopagis, the diet of herring consisted mostly of Eurytemora
affinis and Bosmina coregoni maritima. During August to mid-October, C. pengoi was the
main food source. In November, after the disappearance of C. pengoi from the plankton,
B. coregoni maritima became the main prey object. Both the smallest and largest size
classes of the herrings investigated consumed C. pengoi.
Females with overwintering eggs are the most attractive prey for herrings. The
overwintering eggs of C. pengoi, due to their very hard capsules, seem to pass
undamaged through the stomach and intestines of herring. C. pengoi started to play a
highly important role in the zooplankton community in the waters of SE Finland, and its
abundance is not too dependent upon the annual temperature fluctuations. The Baltic
herring population has substantially changed its diet in this area, and now it prefers the
newcomer C, pengoi.
1. Introduction
Regular appearances of new alien species in the Baltic Sea ecosystem have become
quite common (LEPPÄKOSKI, 1984). The most recent alien species, Cercopagis pengoi
(OSTROU¬MOV) (Crustacea; Cladocera), was found in the Gulf of Finland in 1995 (Kivi,
1995; Hel¬singin Sanomat). The invasion of this large predatory planktonic crustacean
into the eastern Baltic should be regarded as one of the most successful and important of
the last century. Within a few years, this species has became a dominant component in
the planktonic com¬munity of the Gulf of Finland, and its prosperity is not too dependent
upon the inter-annual temperature fluctuations.
The nutrition of the Baltic herring Clupea harengus membras in the Gulf of Finland has
been studied earlier, but these investigations were carried out before the invasion of
Cerco¬pagis (BITJUKOV, 1961; PIDGAIKO, 1971; FLINKMAN et al. 1998). However,
such a tremen¬dous event in the planktonic community should have a significant impact
on the nutrition and prey selection of the herring population.
In the period July-November 1998 in SB part of the Gulf, both the diet of the herring and
its seasonal dynamics were studied, together with the dynamics of the planktonic
com¬munity, paying special attention to the newcomer - C. pengoi.
2. Material and Methods
2.1. Plankton
The study area in the Gulf of Finland covers the most eastern coastal waters of Finland,
from approximately the City of Kotka in the W to the Finnish-Russian border - in the E
(Fig. 1). Plankton was taken using a net with a diameter of 29.5 cm and mesh size 100
µm from the standard horizon 20-0 m which C. pengoi most commonly inhabits
(AVINSKI, 1997).
The material sampling had to cover the entire period of the C. pengoi presence in the
plankton and register the time and environmental conditions of its first appearance and
disappearance during the season. For this reason, samples were taken in the most
eastern part of the study area (near the islands Santio and Ulko-Tammio) during the
period 27 July to 5 November 1998. This material was collected by combining two hauls
in one bottle, which formed the large catch from a volume of 2.74 m3 of water.
For describing the "momental" spatial distribution of the plankton and the environmental
parameters, the expedition on Ms. "Katarina" was undertaken in the late summer 1998
(31 August-3 September). The network of 20 scientific stations covers the entire sea area
of SE Finland (Fig. 1). Seasonal observations with a frequency of two weeks were carried
out at four stations and two additional stations in the Vyborg Bay. Both plankton and
herring were sampled at the same time at each site. A total of 43 quantitative plankton
samples were examined.
The plankton was studied in a laboratory under a stereo microscope. All plankton species
were identified, individuals were directly counted with mechanical counters inside a
labyrinth cup ("Bogorov's cup") with a volume of 10 ml. The biomasses (wet weight) were
calculated from the known individual masses or weight-size relation formulas and
counted densities (SERGEEV el al., 1977; BALUSHKINA and VINBERG. 1978).
2.2. Cercopagis
Special attention was paid to Cergopagis. The dynamics of the structure of the C. pengoi
population was studied during the entire observation period. All the individuals were
divided into four main groups as follows: juveniles (smallest individuals in the first stage of
development with only one pair of caudal spines or the so-called "one-barb stage" and
with uncertain sex); males, parthenogenetic females; and genetic females with sexual
resting eggs. Among the latter, females with one or two eggs in their marsupium were
taken into account. Reproductive potential of parthenogenetic females (the only source of
population replenishment) was estimated by counting the embryos inside the females'
marsupium.
From each selected group. 50 individuals were measured by ocular micrometer. The
body length was measured as a distance between the top of the head and the end of the
acrosoma. The length of an "average individual" in our material (n = 100 in a natural
proportion of age and sex groups) was estimated as 1.6 mm. Individual mass equation
w =qlb was used with the parameters "q" = 0,077 and "b" = 2,911, which has been
calculated for Bythotrephes - a close relative predatory crustacean with the same size
and morphological form as Cercopagis (SERGEEV el al., 1977; BALUSHKINA and
VINBERG, 1978). Thus, for an "average individual" of C. pengoi, a mass 0.3 mg was
accepted for biomass calculations.
The minimum or "starting" size of newborn Cercopagis individuals was a special subject
of interest with respect to their following ontogenetic growth. It was considered that the
minimum value of body length, measured from the plankton juveniles, may be regarded
as such a starting point. For the verification of this consideration, the parthenogenetic
embryos in the last embryonic stage (the eyes had become completely pigmented) were
removed from the marsupium of the females and measured, too.
2.3. Herring
The section of the herring population inhabiting coastal waters, inner and outer
archipelagos of SE Finland, was studied. Herring were collected six times during the
period 28 July to 5 November 1999. Usually, they were sampled at night using gill nets,
placed at a depth of 6-15 m. Three different mesh sizes, 15, 20 and 25 mm, were used in
these nets to provide a sampling of maximum variety in the size-age classes of the fish.
On one occasion, herring were sampled by trawl. To prevent stomach and digestive
system evacuation, the herring were not kept in a net for longer than 1-1.5 hours. Each
sample consisted of 20-30 fish; in total 140 herring were collected and examined. The
herring were almost exclusively represented by individuals aged 2 to 3 years. They
comprised mostly of a cohort aged 3 years (modal length 12 cm, weight 20 g); partly by
2-year-old fish (8-9 cm; 9-11 g) and only one specimen of a 5-year-old.
The fish were fixed immediately using 4-5% water solution of formaldehyde. In specimens
bigger than 15 cm (fork length), ventral cuts were made for faster penetration of the
fixative agent. Before the digestive system was removed and studied, the fishes were
stored in water for 12 hours.
Every fish was weiged with a LS-150 "Sartorius" balance to the nearest 1 mg and
measured to the nearest 1 mm. After disection. whenever possible, the sex and stage of
gonad maturity as well as the amount of mesenteric fat were estimated. The digestive
system was removed as a whole and weighed to the nearest 1 mg. Afterwards, three
parts of this system, the oesophagus, the stomach itself and the intestines (rectum with
pyloris), were studied separately.
The quantitative-balancing method (BORUTSKY, 1974) was used to study the contents
of the digestive system. The separated parts of the digestive system were opened and
the contents were removed using microsurgery tools. The contents were dried to an "airdry condition" with filter paper and scaled to the nearest 1 mg. The three pieces of
stomach content were studied quantitatively by using the same method as for the
plankton (see above). Prey organisms or their parts found in the digestive system
contents were identified to species or genera and counted as individuals.
General and partial (for Cercopagis) indices of the stomach fullness were expressed in
prodecimille and calculated, using the simple formula:
where "m" is the actual mass of all food or its component; and "M" is the mass of the fish.
Selectivity of food objects (Cercopagis) was calculated as:
where "r1 %" is a percentage of the component in the eaten food; "P1%" is a percentage
of the component in the food source (plankton) (BORUTSKY, 1974). Indices were
calculated for every specimen of herring and presented as means with standard errors.
3. Results
3.1. Plankton
A total of 35 species were found in the net plankton of the study area; these belonged to
the following groups: Rotifera - 10 species, Cladocera -11, Copepoda - 9, Ostracoda - 1,
Turbellaria - 1 and benthic organism larvae - 3. In July, before the appearance of
Cerco¬pagis in the plankton, the dominant species by density in the community were the
rotifers Keratella quadrata and K. cochlearis; by biomass - the copepods Eurytemora
ajfinis and the cladoceran Bosmina coregoni maritima. Approximately 90% of the herring
diet con¬sisted of these two crustacean species. During the total observation period, the
dominance by biomass of the species was the same: Bosmina coregoni maritima,
Eurytemora ajfinis, Acartia bifilosa, Daplmia cucullata, Cercopagis pengoi, Podon
(Pleiopsis) polyphemoides and Mesocyclops oithonoides. The maximum biomass of the
zooplankton was observed to be 548,5 mg m-3, the minimum 151,7 mg. m-3.
3.2. Cercopagis
In summer 1998, which was much cooler and therefore less favourable for Cercopagis
development than the previous summer, C. pengoi was first recorded in the plankton on
30 July, when the surface water temperature rose to 13.5 °C, near the Island UlkoTammio with a density of 60 ind • m-3 (Fig. 2). In comparison, we can note that in
summer 1999 the same water temperature occurred much earlier and these cladocerans
were registered in the plankton as early as 9 July. The first appearance of this crustacean
in the plankton, in connection with the development of overwintering eggs left over from
the previous season, was initiated by the water warming up. In spite of the first generation
not being very numerous, С. pengoi with a biomass of 18,0 mg m-3 immediately became
one of the four dominating plankters by biomass. From the beginning of August until the
end of September, Cercopagis was found in every plankton sample. By the middle of
August its density gradually decreased by approximately two times (Fig. 2).
At the end of August and the beginning of September, its density increased again and at
several sites it reached 120-170 ind • m-3. The seasonal maximum was recorded to be
300 ind • m-3 in Vyborg Bay (7 September 1998). This real peak (Fig. 2) is a direct result
of parthenogenetic reproduction. Every parthenogenetic female bears from 6 to 15
(usually 8 to 10) maturing embryos in the marsupium.
Juveniles of Cergopagis (one-barb stage) from the plankton have a length 0.80-1.20 mm;
mature embryos from the females' marsupium 0.79-0.88 mm. So, the value of body
length 0.80-0.88 mm is a starting size for newborn individuals of Cercopagis in the
plankton.
The density of Cercopagis was lower in coastal areas and highest in SE open areas near
the islands Haapasaari, Ulko-Tammio and Santio. The same regularity was mentioned in
the results of the similar Katarina-97 Cruise, however, there is no sufficient explanation
for this (UITTO et al., 1999).
In October Cercopagis was still present in the plankton and disappeared towards the end
of the month. Only juveniles were found on 11 October; the maturation probably stopped
because of the low water temperature. It was not found in the plankton in the samples
from 5 November.
The structure of the Cercopagis population and its seasonal changes are described in
Figu¬re 3. The percentage composition of parthenogenetic and gamogenetic females,
males and juveniles fluctuated a little during the entire period when the species was
present in the plankton (30 July to 11 October). In the beginning (end of July-first half of
August), the percentage of parthenogenetic females was maximal (65-73%) and juveniles
minimal (21-27%). This situation shows the state of the population with a rising density of
indivi¬duals. During the abundance peak (31 August to 7 September), when the
reproduction of the population was at its highest, the percentage of parthenogenetic
females decreased to 34-44% but, in contrast, that of the juveniles increased to 46-51 %
(Fig. 3). This situation was the starting point of the population reduction.
Judging from these results, it seems possible to approximately estimate the current state
of the Cercopagis population (density increasing or declining) on any particular time and
to predict the direction of its further development, which may have a practical importance.
The relationship between the parthenogenetic females and juveniles (F/J) can be used
for this. In our study the value of F/J varies from approximately 3, in an actively rising
popu¬lation to less than 1 when density decreases. The peak in density was observed at
a F/J of about 1.
3.3. Herring
Before the appearance of Cercopagis, the herring diet consisted mostly of Eurytemora
affinis and Bosmina coregoni maritima; usually these two species contributed to more
than 90% of the stomach content. In a few individuals a preference for Podon (Pleiopsis)
poly-phemoldes was discovered - up to 84% of food mass. The involvement of
Cercopagis in the diet of the herring occurred simultaneously with the appearance of
these crustaceans in the plankton.
In relation to the size of the herring, it was noted that both the smallest (length 8-9 cm)
and largest (14-16 cm) fish in our study successfully consumed C. pengoi.
Several parameters describing the herring - Cercopagis food web relationship are
pre¬sented in Table 1. On average, the proportion of Cercopagis mass in the herring diet
was about one half (41-47%), even at such low species densities as 28 and 50 ind • m-3
and as low a partial planktonic mass as 3 and 6%. This corresponds to a rather high
Cercopagis selectivity by the herring (Table 1). It is interesting to look at the protocols of
the plankton samples taken simultaneously with the herring catches. In these the
"selection" of plankton organisms as well as their abundance are presented to
demonstrate from what kind of a "planktonic menu" the herring (Table 2) choose
Cercopagis. The protocols (partly display¬ed here, for crustaceans only) show that such
common herring food objects as Bosmina core-goni maritima, Eurytemora affinis, Acartia
bifilosa and copepodits of senior stages predo¬minate in the plankton by biomass and
particularly by density. Cercopagis held the last posi¬tion among the potential food items,
but undoubtedly the first position in the diet of the her¬ring (Tables 1; 2).
A large individual variety in prey preference was discovered in every herring sample.
Cor¬respondingly, herring do demonstrate a neutral to very high positive prey selectivity
for Cer¬copagis. The maximum amount of C. pengoi found in one stomach (fish length
12,8 cm; weight 29,32 g) was recorded as 1520 individuals (100% of food items) from a
site with a rather moderate natural density of Cercopagis in the plankton, with 88 ind• m3. In addition, some specimens of herring ignored Cercopagis and some fish with empty
stomachs (3-5% from total amount of herring) were observed.
Unfortunately, we have no information about the herring from the peak period of
Cerco¬pagis abundance (end of August-beginning of September), when its density was
between 100 and 300 ind • m-3 in most of the plankton samples. However, judging from
the available data on seasonal dynamics of general and partial (for Cercopagis) indices of
the herring sto¬mach fullness, it was evident during the total period of observations that
the general satia¬tion of a herring was dependent on the participation of Cercopagis in its
diet (Fig. 4).
Due to their external morphology, the tendency of Cercopagis crustaceans to passively
capture various filamentous substrata (algae, fishing nets, etc.), and each other in
particular, the creation of aggregations is very high. Primarily we believed that this fact
might become a problem for a herring's digestion, which was specially detected, when the
stomachs were examined. Usually, when herring have swallowed several tens or more C.
pengoi, they link together inside the stomach, creating a dense packed bundle. This
probably increases the time needed for digestion and makes the evacuation of exuvial
remnants more difficult. However, no evidence of fish sickness was found, and the fish
looked healthy and had quite a lot of mesenteric fat.
Gamogenetic females of C. pengoi are the most rare group in the population; during the
season their percentage composition varied from 0 to 9% (Fig. 3). These females beared
two (sometimes only one) big bright-yellow coloured eggs in a long marsuipium. It was
found that herring demonstrate high selectivity in removing these gamogenetic females
from a water body. This selectivity may reach 100%. For example, in the stomach of
herring #103 (L = 12.5 cm; W = 18.973 g; male; second stage of gonad maturity) we
found 448 specimens of Cercopagis and 738 eggs. This means that the fish only selected
gamogenetic females and consumed all of them from within a 200-m3 water body (see
Table 1 and Fig. 3). How¬ever, released from a female's marsupium or evacuated from
other herring, the sexual eggs may be also swallowed separately from a water body. In
the stomach of little herring #102 (L = 9.2 cm; W = 10.440 g) 15 eggs were found, but no
remnants of adult Cercopagis; these eggs were probably swallowed separately a second
time from the water column. This does not allow for the calculation of the index of the
gamogenetic female's selectivity without assumptions.
It was discovered that the overwintering eggs accumulate in herring stomachs and pass
through the digestive tracts visually undamaged due to their very hard capsules. They
can be swallowed a second time by smaller fish and still remain undamaged.
In October Cercopagis was still present in moderate densities in both the plankton and
the stomachs of the herring. In November, after the disappearance of С. pengoi from the
plankton, B. coregoni maritima became the predominating prey object of Baltic herring.
4. Discussion
C. pengoi now plays a very important role in the zooplankton community in the eastern
Gulf of Finland. In this area its density may reach 300 ind • m-3 in unfavourable years
such as 1998 (current research) and up to 1000 ind • m-3 in years with a warm summer
such as 1997 (UITTO et al., 1999).
A decrease in Cercopagis density during the first two weeks of its appearance in the
plankton (Fig. 2) has also been observed by other authors in the Gulf of Finland
(KRYLOV and PANOV, 1998). This can be explained by the intensive and selective
consumption of Cer¬copagis by plankton feeding fish, mainly (but not exceptionally)
herring.
The Baltic herring has substantially changed its diet in the studied area, and it now
pre¬fers the cladoceran newcomer C. pengoi. We have to note, however, that the
invasion of the Gulf of Finland by C. pengoi in 1995 happened "on time". Special longterm investigations of the changes in zooplankton community composition, Baltic herring
nutrition and the com¬mercial value of this fish have demonstrated that the mean weightat-age of Baltic herring decreased by 50% between 1977 and 1992. The average
stomach fullness index and the amount of mesenteric fat in herring stomachs also
declined during the same period, indica¬ting a long-term failure in feeding success
(FLINKMAN et al., 1998). This was explained mainly by the decimation of the larger
zooplankton organisms, especially neritic copepods, on which the herring selectively
preyed. This process was considered to be a result of the general decrease of the Baltic
salinity level (FLINKMAN et al., 1998). Invasion, dispersal and mass development of C.
pengoi has resolved this problem with the herring's nutrition, and this large cladoceran
has successfully substituted for the lack of large copepods. It is impor¬tant that C. pengoi
is a much more salinity-tolerant species than marine copepods; it is al¬ready widely
distributed in the Baltic and should not be impacted on as strongly by salinity fluctuations
(MORDUKHAI-BOLTOVSKOI, 1967; 1968; OJAVEER and LUMBERG, 1995; AVINSKI,
1997; KRYLOV et al., 1999; GOROKHOVA and ALADIN, in press).
Large crustaceans with big eyes seem to be highly visible and a rather attractive prey
item for herring, especially the females with yellow overwintering (or resting) eggs. These
females were selectively consumed by herring from the water body, as described earlier
for females of other Cladocera and Copepoda (FLINKMAN et al., 1992). Due to their very
hard capsules, the overwintering eggs of C. pengoi seem to pass undamaged through the
stomach and intestines of the herring. After this they probably have the potential for
survival and development, as was observed for copepod egg passage through the
digestive system of Bal¬tic herring (FLINKMAN et al., 1994). This means that the herring
do not dramatically destroy the following year's population of Cercopagis because the
eggs may be saved after "the mother" is eaten.
Sexual (gamogenetic) females with overwintering eggs are exceptionally rare (less than 1
%) in the areas of central Caspian Sea with temperature conditions favourable for
Cerco¬pagis, but in basins with colder or fresher water their number increases
(MORDUKHAI-BOL¬TOVSKOI, 1967; 1968). A larger percentage (1-9%) of gamogenetic
females was found in our study area (Fig. 3). In comparison with our material, the data of
KRYLOV and PANOV (1998) are much more interesting. They present the structure of
local population from the nearby waters of the Russian part of the Gulf of Finland at
Zelenogorsk. According to their data, the mean percentage of gamogenetic females in
mid-August reached 43% with the maximum value of 67% of the total number of adults
(KRYLOV and PANOV, 1998), which is approximately one order of magnitude larger than
in the area of our investigation. Such a surprisingly big difference in the structure of a
Cercopagis population (percentage of gamo¬genetic females) of two neighbouring areas
of the Gulf of Finland may have a simple expla¬nation. These two sites are more or less
similar in hydrological aspects, and the distinctions in salinity do not exceed two per mille,
which is not of a serious importance for so high-tolerance a species as C. pengoi.
However, there is an ecological biogeographical border of Baltic herring distribution in
between these sites (SMIRNOV, 1971). As was demonstrated above, selective predation
of gamogenetic females by herring may have a powerful impact on the local population of
Cercopagis. KRYLOV and PANOV (1998) described the local popu¬lation where a
predation pressure of herring was totally absent.
Besides the undoubted positive role in herring nutrition, some negative impacts of the
Cercopagis mass development were already observable (damage to fishing nets) or just
expected (over-consumption of plankton filtrators and the following possible increase in
phytoplankton abundance). However, we cannot control the density of the Cercopagis
popu¬lation. Normal inter-annual fluctuations of temperature and salinity may also have
no dra¬matic impact. Only the herring's predation may halt the Cercopagis density and
substan¬tially change the structure of its population, as was demonstrated above.
It is now evident that the Baltic herring is currently consuming quite a different diet in
comparison with the previous very long periods in the eastern part of the Gulf of Finland.
The consequences for fish stocks, fisheries, plankton communities and the environment
that may arise thereof will soon be seen.
6. Acknowledgements
The Finnish Ministry of the Environment financed this study. The authors are also grateful
to JUHA-NI VAITTINEN and PETRI PÄIVARINTA for their help in the sampling of the
material and CHRISTIAN FRANKLIN for checking the language.
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Marine Science, 56 Supplement: 49-57.
Manuscript received January 24th, 2000;
revised May 30th, 2000;
accepted June 17th, 2000