Chapter 9 -Nekton in Estuaries
Sampling Nekton in different habitats
 Fish and invertebrates that swim use and migrate
between different habitats
 We would like to have a quantitative estimate of density
or biomass for nektonic species so that we can
understand and model estuarine ecosystems, but it is still
largely guesswork because of:
 Gear selectivity (net have different sized meshes,
trawls scare nekton)
 Net avoidance by nekton (they move!)
 Habitat interacts with gear (seines and trawls get hung
on oysters)
 Lack of quantitative estimates with some gears
(density in environment unrelated to density in gear)
 Lack of absolute density estimation (relative estimates
are achievable)
 Fish behavior affects gear effectiveness (pelagic vs.
benthic species)
 catch efficiency = recovery efficiency * capture
efficiency
 catch efficiency is estimated by releasing a known number of
nekton in a large enclosed region (a pond or pool or section of the
estuary), then sampling with the gear within the enclosed region
repeatedly. The average catch per m2 is divided by the known
stocked density per m2 to give a catch efficiency
 recovery efficiency is measured by releasing marked nekton inside
the gear and estimating recovered upon examining the catch
 capture efficiency is [catch efficiency/recovery efficiency]
 example: catch efficiency = 0.76, recovery efficiency = 0.90 and
capture efficiency = 0.84 for a 1.0 m2 throw trap
Rozas and Minello (1997). Estimating densities of small fishes and decapod
crustaceans in shallow estuarine habitats: a review of sampling design with a
focus on gear selection. Estuaries 20(1): 199-213.
Towed nets:
Dip nets - Not quantitative; not effective due to vegetation
Otter trawl - easy to use, large area sampled; catch
efficiency is low and variable (17 - 53 % for shrimp; 6 %
for spot; 26 % for croaker; 7 % for anchovies); difficult to
standardize; ineffective in vegetation
Beam trawl - easy to use, large area sampled; catch
efficiency is low and variable; ineffective in vegetation
Epibenthic sled - easy to use, large area sampled; catch
efficiency is low and variable; difficult to standardize;
ineffective in vegetation
Surface trawl - easy to use, large area sampled; catch
efficiency is low and variable; difficult to standardize;
ineffective in vegetation
Seine - easy to use, large area sampled; catch efficiency is
low and variable (53 % for striped killifish; 52 % for
Atlantic menhaden; 40 % for white mullet; 33 % for
mummichog; 23 % for spot) but no corrections can be
made because these numbers are not stable; sampling area
is difficult to define; difficult to use in vegetation, rocky
bottoms
Passive samplers:
Channel net - large sampling area; recovery low for some
species; gear avoidance by some species; catch efficiency
unknown
Fyke net - large sampling area; recovery low for some
species; gear avoidance by some species; catch efficiency
unknown
Flume net - Recovery is high and measurable; large
sample area; restricted to intertidal, near marsh edge;
structure may attract nekton; catch efficiency unknown
Light trap - inexpensive to construct; catch efficiency
difficult to estimate; sampling area undefined; species
selective;
Breder traps - inexpensive to construct; catch efficiency
difficult to estimate; sampling area undefined; species
selective, may not sample all fishes
Heart traps - inexpensive to construct; catch efficiency
difficult to estimate; sampling area undefined; may not
sample all fishes, species selective
may not sample all fishes
Pit traps - Inexpensive to construct; catch efficiency
difficult to estimate; sampling area undefined; may not
sample all fishes, species selective; predators and sediment
are problems
Simulated Aquatic Microhabitats (SAMs) were used by
Kneib (1997) to estimate abundance of salt-marsh nekton in
Georgia (petri-dish in marsh)
 Primary species collected: mummichog Fundulus
heteroclitus
 Correlated actual density with SAM-measured density
 Of fish introduced, 34 % - 72 % were collected by the
SAMs
 a standing stock of 15.8 million young nekton were
estimated to be present in the marsh and could not be
estimated any other way
Encircling gear:
Block nets - Recovery efficiency measurable, but variable,
depending on method of removal; large area sampled; may
not sample all fishes, low capture efficiency; tide
dependent
Throw traps (Wegner ring) - Catch efficiency high (70 76 %); recovery efficiency high (90 % or higher for most
species), but poor for heavy vegetation; destructive
sampling; does not capture large fishes well, small
sampling area
Drop net - Catch efficiency high ( 97 % for shrimp 98 %
for goby; 96 % for penaeid shrimp; 94 - 100 % for fish)
recovery efficiency can be measured, usually high (82 - 98
%); small sampling area; under-samples large fishes
Barrier seine - catches large fishes; catch efficiency has
not been measured; recovery efficiency measured (44 %),
but variable, depending on method of removal; large area
sampled; may not sample all fishes; structures may attract
nekton
Purse seine (haul seine, swipe net) - Recovery efficiency
measurable, but variable, depending on method of removal;
large area sampled; may not sample all fishes, low capture
efficiency
Flume weir - Recovery efficiency is high; large sample
area; added structure attracts nekton.
Pop net - Capture efficiency high (94 - 100 % for fishes);
recovery efficiency is high; inexpensive; added structure
attracts nekton, difficult to deploy in oysters, thick
emergent vegetation
Bottomless lift net - Recovery efficiency is high;
inexpensive; intertidal habitat only; small sampling area
Entangling gear:
Gill nets - useful for collect large adult nekton for stomach
analysis or otolith studies; poor recovery efficiency and
capture efficiency; species selective (catches spiny fishes);
sampling area difficult to define
Trammel nets - useful for collect large adult nekton for
stomach analysis or otolith studies; poor recovery
efficiency and capture efficiency; species selective (catches
spiny fishes)
Other (experimental) approaches:
Acoustics - water is transparent to sound; easy to measure
 passive acoustics useful in identifying certain species of
fishes that produce sounds (Sciaenidae); cannot estimate
absolute abundance at this time (but we are trying to do
this here at ICMR/ECU Biology and Physics);
 active acoustics useful in estimating density and
biomass, and fish size; but not species identity; species
need to have swim bladder; some fish (shad) may avoid
high frequency sounds (echosounders use ultrasonic
frequencies, but fish may hear them because that's what
dolphins use, too). Active and passive together may be
useful.
Video/visual census - great if the water is clear, but it is
almost never clear enough in an estuary to use visual
census techniques.
 Line transects
 point circle counts have been used on coral reefs.
 SCUBA required.
 Visual identification difficult for novices