Herring develop a taste for feared waterflea invader

Fishhook waterflea has not lived up to its advance billing as a threat to the food chain

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By Jukka Perttu
     
      At the beginning of the 1990s, there was much alarm at the arrival in the Baltic Sea of a feared "invasive exotic species", the fishhook waterflea, known to science as Cercopagis pengoi.
      It transpires that this curious creature has found its own niche in the Baltic ecosphere and has not brought doom and desturction after all, at least if we are to believe Maiju Lehtiniemi from the Finnish Institute of Marine Research.
     
The species, which is believed to have arrived here first in the time-honoured fashion of travelling in ships’ ballast tanks, was a source of great concern some years ago.
      Its diet includes the small zooplankton popular with such fish as the Baltic herring, an important economic fish hereabouts.
      It was feared that with the new competitor on the block, the Baltic herring would lose their food supply, and given the voracious appetite of the waterflea, that the phytoplankton would be in short supply, bringing an added adverse reaction in the water quality, since the plankton are themselves an important eater of algae: the Baltic already has more than an ample supply of algal blooms.
     
However, things have not turned out quite as badly as the forecasters suggested.
      For a start, the waterflea has also turned out to be a mouthwatering morsel on the menu of the fish.
      The creatures are wolfed down particularly eagerly by Baltic herring (Clupea harengus membras), by the sprat (Sprattus sprattus), and by the three-spined stickleback (Gasterosteus aculeatus), reports Lehtiniemi.
      And quite a few other species of fish are not averse to eating fishhook waterfleas, including whitefish (Coregonus lavaretus), the European smelt (Osmerus eperlanus), and the little nine-spined stickleback.
     
“It seems that contrary to the warning scenarios, Cercopagis pengoi has not produced any alarming changes in the food chain with its arrival here.
      It looks to have settled into its own ecological pigeon-hole, and we are not anticipating any dramatic changes ahead.”
      “Naturally the situation is fluid, and when things gradually warm up over time, the waterflea will benefit from it”, says Lehtiniemi.
      The fishhook waterflea generally disappears when the waters grow colder, usually around October, and then resurfaces again in June or July.
      Mass sightings of the creature arte largely limited to the short period of relatively warm sea water, generally in August. However, in recent years this has spilled over into September as the early autumn weather has been unseasonably mild.
     
The waterflea has spread rapidly to all corners of the Finnish coastline, and can be found from the eastern end of the Gulf of Finland all the way up to the Bay of Bothnia, the northernmost reaches of the Gulf of Bothnia.
      Even if the fish themselves have possibly welcomed the newcomer, fishermen find the waterflea a real nuisance.
      The flea’s tail-end contains a hook, and so the animal easily gets tangled in nets. If there are enough of them, they can foul up fishing nets, forcing the fishermen to have to wash their nets increasingly often.
      "All the same, the fishhook waterflea is a relatively small factor in fouling nets when set alongside the algae that come from eutrophication of the water", says Kim Jordas, Managing Director the Finnish Professional Fishermen’s Association.
      He does not have any hard and fast information on the impact of the waterflea on fish stocks.
     
With the fishhook waterflea problem apparently receding, Finnish fishermen have another cause for furrowed brows, in the shape of the comb jelly, Mnemiopsis leidyi.
      The question with this exotic invader, which has arrived from the Atlantic coastal waters of North and South America, is once again that it eats zooplankton, but it also takes a fancy to fish fry as well.
      Fishermen are anxious to know how well (or preferably how badly) it will adapt to the northern winter.
      Maiju Lehtiniemi is at present aboard the Institute of Marine Research’s research vessel Aranda examining the comb jelly situation in winter conditions in the Baltic. No results are in as yet.
      One interesting question is whether the comb jellies might eat small fishhook waterfleas. "There is certainly no way the reverse could happen", notes Lehtiniemi.
     
Helsingin Sanomat / First published in print 13.1.2008

More on this subject:
 FACTFILE: An immigrant from the south-east

Previously in HS International Edition:
  Comb jelly poses serious threat to Baltic Sea ecosystem (27.8.2007)
  Invasive comb jellies now found throughout Baltic Sea (17.12.2007)

Links:
  Global Invasive Species Database: Cercopagis pengoi

JUKKA PERTTU / Helsingin Sanomat
jukka.perttu@hs.fi