Comparison of Two Ichthyoplankton Sampling Gears with Notes on Microdistribution of Fish Larvae in a Large River

Abstract

A pilot comparison of two ichthyoplankton sampling gears was conducted in the lower Mississippi River, Louisiana. The two gears were (1) a 1-m-diameter conical plankton net towed at the surface and (2) two 0.5-m-diameter, bridleless, conical nets pushed at the surface from either side of the bow of a boat. Sampling was conducted on two different days, during light and dark periods, at three stations representing swift surface currents, sluggish currents, and slack water. Ichthyoplankton were more abundant in townet collections at the fastwater station during daylight hours and at the slackwater station at night, whereas pushed nets were more effective at the slackwater station during daylight hours and at the fastwater station at night. Shads Dorosoma spp. and freshwater drum Aplodinotus grunniens accounted for 94% of all fish larvae collected.Relative abundance of shads was greatest in daylight pushnet collections whereas drum were most abundant in daylight townet collections. Major additional trends: (1) shads, particularly protolarvae, were concentrated in the upper 50 cm of the water column in daylight collections, but were more uniformly distributed at night; (2) freshwater drums were more abundant in the 50-100-cm stratum during daylight collections, but were more uniformly distributed at night;(3) older freshwater drum larvae were more abundant in nighttime collections, whereas protolarvae were more abundant in daylight samples. References

Evaluation of plankton surface pushnets and oblique tows for comparing the catch of diadromous larval fish

“…Other studies have compared the effectiveness of towed and push nets and have observed no differences between the two gear types (Gallagher and Conner, 1983;Hale et al, 1995). However, our results may differ from the previous studies because different methods were used to tow the plankton nets.…”

“…Bow mounted pushnets (Kriete and Loesch, 1980;Gallagher and Conner, 1983) and obliquely towed plankton nets (McGovern and Olney, 1996) are commonly used to sample ichthyoplankton. To determine whether larval catch differed between gear types we compared larval density estimates of three common diadromous fishes along the U.S. Atlantic coastline -river herring Alosa sp., white perch Morone americana, and striped bass Morone saxatilis -collected using a bowmounted surface pushnet and obliquely towed plankton nets.…”

Spawning areas, dispersion and microhabitats of fish larvae in the Anavilhanas Ecological Station, rio Negro, Amazonas State, Brazil

“…These results are similar to many others of diurnal distribution and abundance of ichthyoplankton in tropical rivers (Baumgartner et al, 1997;Araújo-Lima et al, 2001;Bialetzki et al, 2004) and in temperate regions (Gale & Mohr, 1978;Hergenrader et al, 1982;Gallagher & Conner, 1983;Corbet & Powles, 1986;Gadomski & Barfoot, 1998). Baumgartner et al (2004) studying larvae of Hypophthalmus edentatus, Plagioscion squamosissimus and species of Characiformes in the upper Paraná River mentioned the interaction of several factors that act in the highest capture at the surface at night, but they affirm that the feeding activity would trigger the whole process of vertical distribution.…”

Distribuição das larvas de Mylossoma aureum e M. duriventre (Pisces: Serrasalmidae) nas margens do rio Solimões, AM

“…Por outro lado, no rio Volga, cuja transparência alcança 40-46 cm, as larvas de Cyprniformes tiveram seu comportamento afetado pela variação na intensidade de luz entre o dia e a noite (Pavlov et al, 1977). Outros autores afirmam que, no turno da noite, as larvas tendem a se concentrar na superfície ou no fundo, dependendo da espécie, mesmo em ambientes de águas turvas (Gale & Mohr, 1978;Tuberville, 1979;Hergenrader et al, 1982;Gallagher & Conner, 1983;Thayer et al, 1983;Crisp, 1991). As protolarvas de Aplodinotus grunniens, por exemplo, durante o dia concentram-se na superfície, enquanto à noite distribuem-se homogeneamente na coluna d'água.…”