Fluctuations of perch populations in Lake Geneva from 1984 to 2011 estimated from the number and size of egg strands collected in two locations exposed to different fishing practices



Fluctuations in the number and size of spawning perch (Perca fluviatilis) in Lake Geneva were evaluated and compared each year from 1984 to 2011 in two locations. This evaluation was based on: (1) each mature female lays only one egg strand per year and (2) the number and size of mature females are well correlated with those of their egg strands. Egg strands were counted and measured at two locations in France and in Switzerland. The collection of egg strands from either artificial substrates (France) or by direct sampling of the natural lake bottom by a diver (Switzerland) did not seem to affect the results. In most years the number and mean width of egg strands fluctuated in the same way in both locations. In addition, yearly perch catches were correlated with the number of egg strands at both locations. However, more small egg strands were found in France than in Switzerland, whereas the reverse was true of large egg strands. Fishing pressure concentrated to a greater extent on small perch in France than in Switzerland could explain this difference.