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International Council for. C .M. 1989/J: 23 the Exploration of the Sea Baltic Fish Cornmitee

LERNAEOCERA BRANCHIALIS IN COD (GADUS MORHUA) OF THE BALTIC SEA

by

T. Lang Bundesforschungsanstalt für 'Fischerei Institut für Küsten- und Binnenfischerei Außenstelle Cuxhaven. . Bei der Alten Liebe 1 2190 Cuxhaven Fed. Rep. Germany

ABSTRACT

From 1983-1988 more than 30;000 cod of the south-western Baltic Sea (ICES-Subdivisions 22, 24, 24, and 26) were examined for the occurence of the parasitic . Fishes were only found to be infested in ICES-Subdivisions 22 and 24 with highest infestation rates in cod hetween 20 and 29 cm total length. In contrast, none of the specimens examined smaller than 20 cm was affected. A marked variability could be demonstrated comparingthe prevalences of different years.

INTRODUCTION

The parasitic Lernaeocera branchialis systematically belongs to the order Copepoda, the suborder , and finally to the family Pennilidae (Kabata 1981). Members of this family are cha racterized by extensive morphological chan­ ges during their development and by a two-host-cycle of the . female (Möller and Anders 1986). The developmental stages of this parasite include two free-swimming nauplius stages fol-

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Besides the pre-adults and adult stages of the parasite. also fish exhibiting acute wounds due to remains of freshly det~ tached ware counted as infested cod. However. this phenomenon could be found only at very low frequencies.

RESULTS

Out of the 32.593 cod examined a total of 253 specimens were found to be infested by one to a maximum of four parasites. In more than 90 % of the cases the affected fish were infested by only one copepode.

Concerning the spatial distribution.of infested cod shown in Fig.1 the results of the 6 cruises were nearly identical. With the exception of four individuals. which were found in ICES­ • subdivision 24. all infested cod were caught in the western Baltic Sea (ICES-subdivision 22). Combining the data of the different years an average infestationrate of 4.89 % was calculated for this subdivision.

Table 2 shows the results of the single years of investigation concerning ICES-subdivision 22 in relation to different length groups of cod. Although the sometimes low number of cod examined and the quite different length-frequency distribution have to be taken into account when interpreting the findings. the results • neverthe­ less. indicate the occurence of considerable fluctuations in the mean infestation prevalences during theyears. The lowest prevalence of 2.68 % was found in December 1985. whereas the maximum value reached 12.00 % in the following winter (November 1984) . However. out of the data there is no evidence for· an·up- or downward trend in the occurence of Lernaeocera during.the period of investigation.

The results shown in the table also reveal a certain preference of the parasites to infest particular length groups of Baltic cod. Combining the findings of the different years (Fig.2) it is obvious that none of the small cod (10-19 cm) was found to be infested whilst highest prevalences (8.55 %) were detected in the length group 20-29 cm. In larger fish a clearly decrea-. sing trend in the parasite frequency could be noticed. Cod larger than 70 cm examined were free of Lernaeocera branchia- lis. . -3-

lowed by the copepodid which resembles the first infective stage. Attached to the tips of the gills of the intermediate host using its second antennae theparasite undergoes several moults (4 chalimus stages) which result in thepre-adult .. During this stage the females get fertilized by the males which die soon after copulation. The female pre-adults leave their first host and attack their final host after a second sh~rt free-swimming period (Kabata 1981. Möller and Anders 1986). Attached to the bases of one of the gill arches the copepode develops into the adult female which protrudes its head deeply' into the host tissue near the heart feeding on blood. Düe to its size ( up to 2cm) the parasite can easiiy be seen when the gill covers of the host are spread out (Möller and Anders 1988) . . In shallow coastal waters of the North'Sea the main interme-, diate host of Lernaeocera branchialis isflounder (Plathichthys flesus) whereas mainlY cod (Gadus morhua) and.whiting (Merlan­ • gius merlangus) serve as final hosts. In the'northwestern North Sea their roles are taken over mainly by lemon sole' (Microstomus kitt) and haddock (Melanogrammus aeglefinus) re­ spectively (Möller and Anders 1986).· For the Baltic Sea only few published information exist on the occurence of Lernaeocera branchialis.·Arntz (1972) ,found in cod. of the Kiel Bay caught between January and April 1972 preva­ lences of 0.1 %. In subsequent investigations Müller (1982) reported of average prevalences of 1-2 .%.in the years 1971 ­ 1979 in the same area.

The aim of this paper is to provide current data on the occu­ rence and spatial distribution of Lernaeocera branchialis. in cod derived from 6 routine cruises for the investigation of fish diseases in the southwestern Baltic Sea from 1983 until 1988.

MATERIAL AND METHODS

The cruises were carried out in December 1983. 1985. 1986. 1987 and in November 1984 with'research vessels "Anton Dohrn".

"Wa 1ther Herwig" and "So lea". For. fishing a. 180 I herring bottom trawl was used. The location of all sampling sites within ICES­ subdivisions 22. 24. 25. and.26 is shown in,Fig.1~ At most of. the stations sampling took place every year. Altogether a total of 33.110 cod were measured and examined for the occurence of Lernaeocera branchialis inthe gill chamber. Table 1 contains the number of fish examined per· ICES-subdivi­ sion and year.

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DISCUSSION

The aim of the present study was to obtain some new information on the occurence of Lernaeocera branchialis in thesouthwestern Baltic Sea. SampIes taken in ICES-subdivisions 22. '24. 25. and 26 during the winters of 1983-1988 revealed that ~the majority of infested cod generally seem to concentrate in ICES-subdivi­ sion 22 wheras in more eastern regions affected fish occur only very sporadically. Since it is known that Lernaeocera - at least during its copepodid-stage - reacts sensibly to a decrea­ sing salinity (Möller 1975. 1984). the declining~salinity gra­ dient from the western to the eastern Baltic presumably is res­ ponsible for the absence of the parasite in the eastern Baltic. Arntz (1972) in his study even came to the conclusion. that the infestation of gadids (whiting andcod) inhabiting' the western ~ Baltic Sea (Kiel Bay) probablY does not take place in that area at all due to the assumed absence of the infective Lernaeocera~ copepodids and other factors. He presumed that infested fish • rather were immigrants from more saline regions outside the Baltic Sea. e.g. the Kattegat.~ Due to results of tagging: experiments there is in fact evidence for migrationsof cod between the Kief Bay and the southern Kattegat (Thurow 1985) . However. in this context it seems~noteworthy that on the other hand there have been findings of flounders heavily infested with copepodids of Lernaeocera in the Kiel ,Bay in' late summers (Weber. pers.comm;). Thus. the possibility of an infestation of cod and other gadids in the western Baltic Sea cannot be ex­ cluded in general.

In contrast to results of Arntz (1972) and Müller (1982) who found a mean infestation prevalence of 0.1 % in cod sampled between January and April 1972 and an average prevalence of 1-2 % in the years 1972-1979 respectively. we were able to deter­ mine a much higher mean prevalence of 4.86 % combining the data of our entire investigation period of 6 winters. In November 1984 the prevalence even was as high as 12.00 %. Reasons for these quite different findings cannot be provided; it may be that seasonal influences or real changes in the frequency of infested cod have played a role.

The observed length-frequency distribution of infested cod in general corresponds with that found in the North Sea. In both areas the smallest specimens were found to be the least infes­ ted. whereas the medium length groups showed the highest preva­ lences. In larger fish the frequency of Lernaeocera decreased again. However. in contrast to our findings considerable numbers of North Sea cod belonging to the length group 10-19 cm were already infested (Mann 1952/53). It should be mentioned here that Arntz (1972) in the western Baltic also found infes- ,. .,

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ted cod of 12. 15. 17. and 19 cm. The typical length distribution of infested cod has been rela­ ted to the fact that the youngest and oldest stages of cod - at least in the North Sea - are almost absent from shallow coastal waters where the risk of an infestation with Lernaeocera cope­ podids derived from flounders. which are one of the major fish species in those areas. is very high (Mann 1952/53. Möller and Anders 1986). Another possible reason for the decrease of Ler­ naeocera in larger cod,might be a directly or indirectly in­ duced increasing mortality of infested fish. Möller and Anders (1986) assume. that the infestation with Lernaeocera is one of the most significant factors fornatural mortality in young cod at least in the Waddensea.

• LITERA1URE CI TED Arntz. W. E. (1972). über das Auftreten des parasitischen Cope­ poden Lernaeocera branchialis in der Kieler Bucht und seine, Bedeutung als biologische Markierung. Arch. FischWiss. 23: 118-127 '

Kabata. Z. (1981). Copepoda (Crustacea) parasitic on fishes: problems and perspectives. Adv. Parasit. 19: 1-71.

Mann. H. (1952/53). Lernaeocera branchialis (Copepoda parasi­ tica) und seine Schadwirkung bei einigen Gadiden. Arch. FischWiss. 4: 133-144

Möller. H. (1975). Der Einfluß von Temperatur und Salzgehalt auf Entwicklung und Verbreitung' von Fischparasiten. Ph.D thesis. math.-nat. Fak. Univ. Kiel. 108 PP.

Möller. H. (1984). Daten zur Biologie der Elbfische. Kiel: Verlag Möller. 217 Pp.

Möller. H. and Anders. K. (1986). Diseases and parasites of' marine fishes. Kiel: Verlag Möller. 365 PP.

Müller. H. (1982). Biologie. Verbreitung und Schadwirkung auf­ fälliger Ektoparasiten an Fischen der Nordsee und westlichen Ostsee. Diplom thesis. math.-nat. Fak. Univ. Kiel. 90 Pp.

Thurow. F. (1985). Wohin wandern die Ostseedorsche '? Infn' Fischw. 32 (1): 9-14

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Fig. 1: Location of sampling sites and spatial distribution of Baltic cod infested with Lernaeocera branchialis

(black dots: stations with

o 25 o 26 o o 0 o 0 0 0 o 0 o 0 0° 0 0 o 00 0 0 0 00 0 0 0 0 o o 0 0 rP 00 0 o o o o 0 o o 0 0 o 0 o • 00·. • o 0 0 o 0 • 0 o 0 080 0 o 0 0 0 0 o 0

9°

%

lf) 10 N,...

'

lf) N lf) N 5 lf) ....

Fig. 2: Prevalences of infestation M .... with Lernaeocera branchi­ alis in Baltic cod related o to fish length o 50 100 cm "

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Tab. 1: Number of cod examined in ICES-subdivisions 22. 24. 25. and 26 from 1983-1988

n examined / year ICES-sd '83 '84 '85 '86 '87 '88

22 1736 325 413 2225 282 134 24 1774 463 1347 1476 1301 658 25 4690 3202 3219 3374 1622

26 1408 1642 337 1023 398

Tab. 2: Prevalences of infestation with Lernaeocera branchialis in cod of ICES-subdivision 22 in the years 1983-1988

1983 1984 1985 1986 1987 1988 E length (CI) n I n I n I n I n I n X n I 10-19 14 0.0 1 0.0 93 0.0 3 0.0 - · 2 0.0 113 0.0 20-29 333 3.0 13 15.4 30 3.3 327 13.8 22 18.3 ..- -.- 725 8.6 30-39 1124 3.4 75 24.0 56 7.1 1447 3.4 64 15.6 ,J 0.0 .2769 4.3 40-49 174 1.2 155 11.0 87 1.2 269 3.7 127 9.5 22 22.7 834 5.6 50-59 75 0.0 76 2.6 105 3.8 103 8.7 116 0.9 50 8.0 525 3.8 60-69 14 0.0 5 0.0 31 3.2 50 4.0 11 0.0 41 4.9 152 3.8 70-79 2 0.0 - · 7 0.0 22 0.0 5 0.0 13 0.0 49 0.0 80-89 - . - · 2 0.0 4 0.0 - · 2 0.0 8 0.0 90-99 - . - · - -.- - . - · 1 0.0 2 0.0 E 1736 2.9 325 12.0 411 2.7 2225 5.2 345 7.8 134 8.2 5177 4.9

n : nullbl!r ot cod I!ulinl!d

I : pl!rcl!ntagl! of cod infested

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