Sampling with Active and Passive Capture Techniques

1
Sampling with Active and Passive Capture
Techniques

• Fisheries Techniques, 2nd Edition
• Chapters 6 7

2
In Fisheries we classify sampling gears on the
basis of effort Passive Capture Techniques are
those that require the active movement of the
fish or other aquatic organism to capture
the animal. Active Capture Techniques are those
that require active sampling by the researcher
to capture the fish. EXAMPLES Passive Active
fyke nets electrofishing gill nets
trawling minnow traps seining angling
dredges
3
Passive Fish Capture Methods

• Chapter 6
• Hubert

4
Passive Sampling Gears can be divided into 3
categories based on mode of capture (1)
Entanglement (2) Entrapment (3) Angling gears
5
Advantages of Passive Gears simple gears
generally dont require mechanized assistance
(e.g. winch) easy to standardize units of
effort for indices of abundance but see Box 6.1
for CPUE and statistics.
Disadvantages of Passive Gears selectivity
for sizes or species or sexes -- no gear is
unbiased. Problems with proportionality of
CPUE and abundance (again see Box 6.1 for
normality and abundance). changes in behavior
(season, size, location, diel) affect CPUE and
its interpretation.
6
6.2 Entanglement Gears Gill Nets vertical
panels of net set in a straight line have cork
or plastic or foam core line for floatation on
top line and lead weights or lead core line
for weight on bottom line of net to help keep it
spread and fishing. modern materials include
nylon and monofilament mesh. Often times use
experimental mesh nets with panels of
different sizes for assessments to try and
get around relative gear selectivity of
different sized meshes. Nets can be set in
tandem (e.g. T, H, etc.)
7
Gill nets are selected based on mesh size,
material weight (diameter/strength of mono line),
and density of net per running foot (hanging
ratio). Problems unpopular with fishermen/
public. Usually not species-specific
and are considered a total kill gear.
Gear avoidance by some species (e.g. bass)
and during light conditions (can see
the net). Gear selectivity/size biases.
Ghost fishing issues.
8
Entanglement Gears Gill Trammel Nets 1.
Hold or snare fish in fabric mesh.
9
Factors influencing Gill Net CPUE - weather
conditions - mesh sizes (size selection) - net
color (visibility) - mesh materials (strength,
visibility) - activity of fish (more active
higher capture), this will influence
species-specific CPUE as well as seasonal or diel
CPUE. - duration of gill net sets (CPUE goes up
with time to a point, then declines as fish
accumulate due to fish avoidance of the net
as fish struggle.
10
Trammel Nets Are constructed similar to
gillnets, but are made of 3 panels of mesh
material, a small mesh sandwiched between two
large mesh panels. Fish push through the
meshes and end up bagged. Typical
materials include cotton or nylon webbing.
Nets are set on bottom, similar to anchored gill
nets. Biases and sampling problems associated
with gill nets generally apply to Trammel
nets as well. Trammels are selective for
certain species (some may lead and avoid
them, more so than for gill nets due to
perception).
11
6.3 Entrapment Gears Aquatic animals enter
entrapment gears by their own movement and become
trapped when, by design, they are largely unable
to find their way out of the gear. Often times
the entrapment gears used in fisheries are just
smaller versions of commercial fishing
gear. Examples - Hoop Nets - Lobster pots -
Catfish traps - Crab pots - Fyke nets - Slat
traps - Trap Nets - Minnow traps - Weirs
(explain here!).
12
Entrapment Gears - usually anchored in
shallow waters. - widely used in reservoir
assessment CPUE easy to standardize. - very
effective for some species, do have species
selectivity. - size-selection due to mesh
sizes used in trap. - Is easier on fish and
is usually not lethal.
13
6.4 Pot Gears - Are portable, rigid traps
with small openings for animal entrance.
Animals must pass through a funnel type opening
to enter. - Used for fish crustacean
collection. - Most efficient for bottom-dwelling
species seeking food or shelter. - Many of
these types of gears are baited with fish or
shellfish to enhance capture of target
species. - Gears are very size and
species-specific with little bycatch. - Can
easily be standardized for good CPUE data. -
One of major problems is ghost fishing.
14
Examples of Pot Gears
15
6.5 Angling Gears. - Baited hook and line are
widely used in commercial fishing. - still
used in long lines - still used in tuna
industry (a dolphin-friendly method). - used in
trot lines. - obviously have size species
-specific bias.
16
Active Fish Capture Methods

• Chapter 7
• Hayes et al.

17
Active fish capture methods have the advantage
over passive methods in that a specified
geometric space can be covered. This provides an
accurate measure of effort to calculate an index
of abundance (better than methods like angling,
etc.). Sampling can be nearly instantaneous and
are mobile. This allows greater spatial
flexibility and allows more replicate samples to
be completed in a shorter period of time than
for passive gears. The larger sample sizes
possible with active gears increases the
statistical precision of indices. Disadvantages
include the usual need for a larger or
more specialized vessel to deploy and retrieve
active gears. Also, catches may be lower per set
than for passive gears which is a disadvantage if
large numbers of individuals are required.
18
Examples of Active Gears
19
7.3 Dragged or Towed Gears Bottom Trawls.
20
7.3 Dragged or Towed Gears Midwater Trawls.
used for pelagic species. require powerful
vessel often times with a hydraulic
winch. often used for species
identification for hydroacoustic studies.
Can be difficult to deploy and fish.
21
Among the big issues of midwater and bottom
trawls are gear performance and gear
efficiency. We know the dimensions of a net,
but when it is towed we do not know the fished
dimensions. These dimensions while fishing
are very important in calculating the area swept,
or the volume of water sampled for density or
area density estimates. Similarly, the
performance of the gear is important. If only
40 of a species is captured by a typical bottom
trawl (60 avoid the net), then density estimates
are biased low. Underwater cameras mounted
directly in front of the net help in evaluations.
22
(No Transcript)
23
Other factors for trawls snags.
determining depth fished.
Angle of cable
Length (L) of cable out
Depth COS (angle) L
24
7.4 Dragged or Towed Gears Dredges Dredges
are demersal, heavy-framed samplers designed to
collect invertebrates (oysters, scallops,
crabs) or fishes (flatfish) from the
bottom. One of the concerns are the damage to
bottom that occur with these types of gear
(e.g. destruction of oyster reefs
communities). Like trawls, they can provide
estimates of area sampled for density
estimates, etc. Gear efficiency is still a
question, but it may be addressed with video
cameras or divers. Used in some assessments,
e.g. Virginia dredges in winter for blue crab
population assessment, NMFS sea scallop surveys,
etc.
25
Examples of Dredges
26
7.5 Surrounding or Encircling Nets Includes
beach or haul seines, purse seines, and lampara
nets. Fish are captured by actively encircling
the fish. Range widely in size. Beach
seines can be easily fished by 1-2 persons,
while haul seines and purse seines require a
small army, and/or large vessels and
specialized equipment. Used often times
commercially for schooling or shoaling
species (e.g. Atlantic menhaden).
27
Examples of Surrounding or Encircling nets.
discuss each here...
28
7.6 Other Active Sampling Gears
also, Angling Diving (visual)
29
7.7 Gear Selection How do you determine the
gears to use in a study? 1. Purpose goals of
the study 2. Habitat sampled (all gears dont
work well in all habitats) 3. Behavior and size
of animals to sample 4. Logistics (time,
budget, personnel, vessel resources, environment
al characteristics).
30
Study objectives and Gear Selection-- - There
are 3 broad purposes for fish collections 1.
Determining density or relative abundance 2.
Collecting specimens for live or other
studies 3. Collecting accessory data from fish
(e.g. scales, tissue, or stomach
contents). - Environmental conditions will
also dictate gear selection are there
obstructions for towed gears? are there snags
for drifted gillnets? will aquatic plants
limit gear effectiveness? Is the water too
deep or shallow for a gear? Will current
(e.g. in streams) make control of the gear
difficult?
31
- Animal behavior dictates Gear selection
demersal fishes require demersal gears
pelagic fishes require pelagic gears (e.g.
midwater trawls, purse seines,
hydroacoustics). Fish that like to be near
structures will limit gear choices (e.g.
rockfish in Chesapeake Bay, bass in WV)
Behavior may lead organism to different
habitats at different times of the day or year
which may make a gear effective at one
time/not another or make it accessible with
different gears at different times of the
year.
32
Fish Size and Gear Selection Most fishing
gears are size-selective and species-selective.
Usually this is due to mesh sizes, or openings
in the nets allowing smaller fish to pass
through the net while retaining larger
fish. other factors may influence
recruitment to the net - e.g. fish like tuna
may reach a size where they can outswim the
net. - changes in habitat use or spatial
distributions with ontogeny (age) may also
influence vulnerability of a fish to a
gear. - Such factors may require the use of
multiple gears to assess multiple
age-classes of a species.
33
A gear choice decision tree..
34
Two other factors that influence the gear type
and study design are 1. Sampling variance
(earlier lecture). 2. Sample verification (how
well does data from the survey gear
correspond with independently collected data?)
35
For example our study compared hydroacoustic
data on numbers and sizes of fish, to those
obtained through lock rotenone sampling.
rotenone
acoustic
Percent
Length (mm)
36
Abundance ( per lock) agree.
3 day
1 day