Nile perch, fishing pressure and changing habitats

Posted on December 9, 2010

Nile perch were introduced toLake Victoriain the 1960s to boost local fisheries.  Perch numbers increased dramatically, but so did fishing pressure, and now theNileperch stock appears to be over-exploited and in danger of collapse.  Because fishing targets larger fish, there are concerns that decades of intensive exploitation have resulted in selection for smaller, slower growing individuals, which may mean that the stock will be unable to recover to original levels even if fishing pressure is relaxed.  To better understandNileperch ecology, researchers looked for evidence of fishing-induced trait change in a perch population.  The work was done inLake Nabugata,Uganda, a satellite water body ofLakeVictoria.  This location was chosen because fish populations inLakeNabugata, unlike those in the larger lake, have  not experienced the simultaneous impacts of nutrient pollution, which made it easier to identify the effects of fishing.  Between 1995 and 2007, members of the heavily fishedNileperch population in the open waters of the lake became harder to find, and the average body size dropped from 21 to 15 cm.  Both these changes are suggestive of size-selective fishing mortality.  Over the same period, the wetlands on the lake shore were invaded by hippo grass, which increased refuge and feeding opportunities for Nile perch and their fish prey, and by 2007 a greater proportion of the larger perch were found amongst hippo grass than in more open water.   It seems that changes in the size and distribution of Nile perch in LakeNabugaboreflect the combined influence of fishing pressure and shifts in the distribution of aquatic vegetation. 

Reference:    Paterson, J.A. & Chapman, L.J.  2009.  Fishing down and fishing hard: ecological change in the Nile perch of Lake Nabugabo, Uganda.   Ecology of Freshwater Fish 18, 380–394.