PREDATION ON HERRING LARVAE BY THE COPEPOD CANDACIA ARMATA.

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1 This paper not to be eited without prior rcferenee to the author. INTERNATIONAL COUNCIL FOR THE EXPLORATION OF THE SEA C.M : 20 Pelagie Fish Committee Ref. Bio!. Oceanogr. Cltee PREDATION ON HERRING LARVAE BY THE COPEPOD CANDACIA ARMATA. by A. Corten Netherlands Institute for Fishery Investigations P.O. Box 68,1970 AB IJmuiden The Netherlands.

2 'l'hin paper not to be cited without prior reference to the INTERNATIONAL COUNCIL FOR TllE EXPLORATION OF TllE SEA <luthor. C.M. 1983/ll : 20 Pelagic Fish Committee Ref. Biological Oceanography Committee PREDATlON ON llerring LARVAE BY THE COPEPOD CANDACIA ARMATA. by A. Corten Netherlands Institute for Fishery Investigations Postbus AB IJmuiden The Netherlands Introduction. Apart from the lack of suitable food. predation is probably the main cause of the extremelyhigh mortality during the early larval stage of many fish species. Year to year differences in the mortality caused by various predators could be responsible for the variations in recruitment. and thus have a strong impact upon the fishing industry. A study of the various predators may ultimately help usto predict fluctuations in recruitment at a very early stage. and thereby increase our capability to manage the fisheries. With respect to predators, one normally thinks that animnls completely devour their prey. and therefore have to be bigger than the prey organism. However. there are also predators that not necessarily eat the whole prey, but only take a small bite, after which the prey organism dies. This was described by Lillelund & Lasker (1971) for 3 species of marine copepods that attacked young anchovy larvae in aquarium experiments. The same was found by Westernhagen & RosenthaI (1976) for the amphipod Hyperooche medusarwn, which appeared to attack yolk-sac larvae of Pacific herring, also under laboratory conditions. In the case of North Sea herring, most of our research into larval mortality has been directed towards large predators of the first kind. Arecent observation on field sampies of herring larvae, however, indicates that predation by small plankton organisms mayaiso be an important cause of natural mortality in larval North Sea herring. Materials and Methods. The present observations were made on plankton sampies collected during a routine herring larval survey by R.V. "Tridens" in the Shetland/ Orkney area from 30 August - 3 September The sampling gear was a modified Gulf 111 sampier, as described in Anon., SampIes were collected in double oblique hauls from surface to bottom, at a towing speed of 5 knots. Preservation was in 4% formalin. Results. During the routine sorting of the plankton sampies it was noted that sometimes a herring larva was being held very tightly by a copepod. At first this was consid~red to be a coincidence, but after several of these combinationn had been encountered, it was decided to keep them apart and study them in more detail.

3 - 2 - In all cases, the copepod concerned was Candacia armata, Boeck, a species known to be carnivorous. The larvae were always hooked tightly between its fangs, mostly near their head, but sometimes halfway their body (figure 1). In most cases, the larva seemed to have been attacked by the predator just recently, as the latter had not yet consumed a significant part of its prey. Only in one case, the copepod had ingested the tail end of the herring larva. The size of the larvae attacked by copepods was 6-7 mm; only one larva of 10 mm was found to be attacked. Specimens of Candacia occurred in the sampies over a large size range (1-3 mm), but only the largest individuals were found attacking herring larvae. A total of 6 sampies were investigated for Candacia preying on herring larvae, and the results are presented in table I. It is seen that the phenomenon was fairly common in the (limited) number of sampies studied, although the frequency at which it occurred varied considerably. The first impression is that predation occurs particularly in the evening and at night; the herring larvae with their good eye-sight probably being able to escape the copepods during daytime. A noteworthy observation was that in one of the sampies investigated, many herring larvae showed signs of decomposition (figure 1). The first explanation was that the sampies had been badly preserved, due to an insufficient concentration of formalin (the jar containing the sampie had been filled for more than the prescribed 1/3 of its volume with plankton). Another explanation, however, is that some of the larvae had been dead already for some time at the moment of capture, possible as a result of earlier attacks by Candacia. Discussion. The first question that arises concerning the above described observations is whether the attacks of Candacia on herring larvae occurred under natural conditions, or whether they happened when both organisms were concentrated in the cod-end of the plankton sampier. There are 3 arguments to support the hypothesis that the attacks had already occurred before the organisms were caught: 1. In one case, the copepod had partly ingested the larva. It is unlikely that this would happen in the net, as it must be a timeconsuming process. 2. The frequency of attacks seems to depend on the time of the day. It is unlikely that this would play a role if the attacks occurred in the net. 3. Herring larvae are almost the only prey found in combination with Candacia. Only in one instance, a smaller copepod was found to be attacked by Candacia. If the attacks had occurred in the crowded cod-end of the sampier, one would expect a more random selection cf prey organisms. If the observed copepod/herring larvae combinations represent true cases of predation under field conditions, the real frequency of this phenomenon could be much higher than observed in our sampier. Lillelund & Lasker mention that copepods which have caught a larva normally drop their prey when they are preserved in a formalin solution. Predator and prey could also become disengaged due to turbulence in the cod-end of the high-speed sampier. or by the manipulation of the plankton when.it is washed down the sampier, transferred to jars, and

4 - 3 - finally when it is sorted. If the number of attacks observed in preserved sampies is only a fraction of the real frequency, the mortality inflicted upon larval herring by Candacia could be very high indeed. This would particularly be the case if not all prey organisms are completely consumed, but only killed and dropped after a short time. Lillelund & Lasker showed that the copepod Labidocera trispinosa frequently attacked anchovy larvae, only to drop them again without eating them. The attacked larvae in all cases did not recover from their injury and died. The same was found by Westernhagen & RosenthaI for the amphipod Hyperoche medusarum. The lowest density of predators and prey tested by Lillelund & Lasker was 1 predator + 10 prey organisms in seawater. This resulted in 100 %predation after 24 hours. In the water masses we sampled, the average density of Candacia must have been times lower than the predator density in the above experiment, and the prey density must have been lower by a factor However, both organisms were probably concentrated in a certain depth layer, which would increase the chance of encounter. Acknowledgements. Thanks are due to Dr. G. Fransz for his help in identifying the copepods and the background information he provided on the biology of these organisms. References. Anon., Report of the Working Group on Herring Larval Surveys south of 62 on. ICES C.M. 1979/H : 4. Lillelund, K. & R. Lasker, Laboratory studies of predation by marine copepods on fish larvae. U.S. Fish. Bull., 69(3): Westernhagen, H. von & H. RosenthaI, Predator - prey relationship between Pacific herring, CZupea harengus pazzasis larvae and a predatory hyperiid amphipod, Hyperoche medusarum. U.S. Fish. Bull., 74(3):

5 TABLE I - Density of herring larvae, Candacia, and frequency of predation by Candaciaupon herring larvae in some selected sampies. All sampies collected by R.V. "Tridens" during the cruise of 30 August - 3 September SampIe No Position 59 45'N 'N 59 45'N 59 45'N 59 35'N 59 35'N 02 30'W 02 10'W 02 10'W 01 50'W 02 10'W 01 50'W Depth (m) Time Volume riltered (m 3 ) Fraction subsampled 1/2 1/4 1/2 1/2 1/16 1/2 Herring larvae intaci Herring larvae decomposed Total number of large Candacia (3 mm) Number of herring larvae attacked by Candacia

6 ,_., lmm Figure 1. Top-, middle-, and bot tom left pictures: herring larvae being attacked by Candacia; bottam right: normal herring larvae and larvae showing signs of decornposition.