Gillnet catch in estimating the density and structure of fish community—Comparison of gillnet and trawl samples in a eutrophic lake

A comparison between the benthic Nordic gillnet and whole water column gillnet for characterizing fish assemblages in the shallow Lake Balaton

“…Hansson, 1988;Peltonen et al, 1999;CEN, 2005;Helland et al, 2007). Such patterns are however rarely considered in the monitoring of shallow lakes (but see Olin and Malinen, 2003;Olin et al, 2009) and pelagic sampling is not required by the European Standard (CEN, 2005) for lakes shallower than 10 m. However, our results suggest that identification of neither horizontal nor vertical gradients in within lake fish assemblage structure can be guaranteed with the Nordic gillnet even in such shallow lakes, like Lake Balaton. Our study thus corroborates those of others (Mortensen et al, 1990;Lauridsen et al, 2005Lauridsen et al, , 2008 which emphasised the importance of taking pelagic samples even for shallow lakes with a maximum depth of 4.5 m.…”

“…Since most fish species show diurnal activity rhythm and many of them group to shoals, timing and duration of gillnet sampling may influence gillnetting results (Olin et al, 2004(Olin et al, , 2009Vasˇek et al, 2009). In this study fishing was done in the morning and thus the density of fish which are active only at twilight or night (e.g.…”

“…45 (2009) [171][172][173][174][175][176][177][178][179][180]inshore and 332 (¡ 252) g bleak per hour in the offshore areas. Similarly short fishing time was applied by Olin et al (2009) in Lake Hiidenvesi in southern Finland to avoid saturation of nets with fish. On the other hand, it is also suggested that when the fishing time is too short, the catch composition may be effected by differences in the activity patterns of fish (Speczia´r, 2001;Jacobsen et al, 2004;Olin et al, 2004;Vasˇek et al, 2009).…”

“…For example, it has been emphasized that further research is needed to develop more accurate corrections for size selectivity ) and more reliable methods for incorporating benthic and pelagic gillnet data to whole lake CPUE estimations (Lauridsen et al, 2008). One possible problem of the European Standard of gillnet sampling (CEN, 2005) can be that it is based largely upon the sampling of benthic fish assemblages, and some complementary sampling of pelagic fish assemblages is required only in lakes with a maximum depth of >10 m. However, it is well documented that pelagic (or non-benthic) fish species may also play a very important role in large, shallow lakes, and their diversity and density may change considerably both within and among lake habitats (Holmgren and Appelberg, 2000;Olin et al, 2002;Mehner et al, 2005;Jeppesen et al, 2006;Ta´trai et al, 2008;Olin et al, 2009). For example, in Denmark sampling by the European Standard (CEN, 2005) was complemented with more intense random sampling of pelagic habitats for lakes deeper than 4.5 m (Lauridsen et al, 2005).…”

Assessment of Ichthyofaunal Assemblage of Erelu Reservoir, Oyo, Nigeria

“…Several abiotic (water temperature, water fluctuation, dissolved oxygen, transparency) and biotic (predation, food availability, maturation state) parameters (Craig et al, 1986;Linlokken and Haugen, 2006) as well as operational (mesh sizes, net length, set time) factors (Jensen, 1986;Minns and Hurley, 1988) are well known to drive this temporal variability suggesting that certain species become more or less catchable by gillnets in the course of the year (Grant et al, 2004;Olin et al, 2009). Wet season in south west Nigeria is a period characterized by high turbidity, overflow, low temperature and high wind disrupting fishing activities hence, low catch recorded during this period.…”