Dear Rafhiah,

I would probably check who is able to provide an introduction into electrofishing in your region or conduct the sampling for you or with you.

Other mechanic methods I would assume are too biased in terms of sampling fishes of all size classes and micro habitats.

There are several fish traps available, but you won't find out the bias mentioned.

Kind regards

Claas

http://electrofishing.net/   has some ressources to start with

My experience on subject is limited to a few examples where fishery biologists led.  Sampling may be somewhat dependent on survey goals, do you want to know diversity of species or also want an estimate of productivity.  A variety of techniques were used including electofishing from boats or through wading if not too deep and scouping up shocked fish for entire section, or conducting multiple passes of habitats such as riffles, runs, glides, pools, debris pockets until 3 passes brought in no new species for that habitat type.  Fish can be measured and weighed if needed, kept for further analysis or returned to stream.

A couple of older experiences used rotenone for confined reach to kill section for detailed sampling or preservation, and potassium permangenate was added downstream of survey section to reverse effects.  I was there as hydrologist to gauge flow so calculations could be made on quantities needed.  Mistakes can be especially harmful beyond reach analyzed.  

Scuba diving useful for sight surveys fo identification by memory to forms or if clear water taking pictures or movies of conditions and species found.  In certain fast flowing streams there can be unseen hazards from debris or suction from water passing through boulders or bedrock gaps, and also if fished, line and hooks, portions of nets, etc.  I would expect extra safety and degree of experience considered before considering.  

Sampling of rapids, cascades and falls suggests to me an added degree of experience and expertise is needed on options as these features may include sensitive species and especially hazardous conditions.  Again, a fishery biologist or aquatic biologist experienced in this type sampling should be sought.  I cannot tell you if a waterproof movie camera suspended from cable would yield any reliable results, as the amount of turbulence and bubbles may be too great to see much, and damage or loss of equipment is risky.  Night sampling adds added risks and perhaps specialized equipment and expertise needed, easy to get disoriented, suggest avoiding.

I did fail to mention, a plastic pipe with clear, watertight plexiglass viewer at bottom was sometimes useful in fast and shallow waters to see some darters and sculpins.  These are commercially available, and handles on the plastic pipe help to hold in place or move as needed.  Would not work well in highly turbid or bubbly waters.  The view is limited, but still may be useful in some circumstances.