Selectivity of experimental gillnets: does orientation to shore make a difference Matthew E. Kinsey

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Selectivity of experimental gillnets
doesorientation to shore make a difference?
Matthew E. Kinsey and David G. Argent
California University of Pennsylvania Department
of Biological and Environmental Sciences 250
University Ave,, California, PA 15419
Abstract Recent studies indicate that
experimental gillnets, containing panels of
varying bar mesh can effectively sample
large-riverine fishes. However, the ability of
these nets to catch fish when orientation is
reversed (large mesh inshore, small mesh away
from shore - i.e., a non-traditional set) has not
been evaluated. Our objective was to determine
if net orientation to shore affected catch
efficiency as measured by abundance and richness.
We placed 38-m long gill nets in the Monongahela
River consisting of five panels ranging from 2.54
to 12.70-cm bar mesh. Nets were set in pairs in
the evening on either the left (n20) or right
(n20) river bank every 0.8 km in either a
traditional (small mesh tied to shore) or
non-traditional set. We noted the location of
each captured fish within a panel and its
position relative to shore, identified it to
species, measured it, and returned it to the
water. While we found that traditional sets
yielded a greater abundance, a greater species
richness, and more unique species than
non-traditional sets, these findings were not
statistically significant. However, our results
suggest that experimental gill nets set in a
non-traditional fashion may require additional
effort (soak time) to achieve results comparable
to nets set in a traditional fashion.

• We noted the location of each captured fish
  within a panel and its position relative to
  shore, identified it to species, measured it to
  the nearest mm TL, and returned it to the water.
  Statistical comparisons were made using a two-way
  analysis of variance, testing direction and panel
  effects.
• Species richness asymptotes were plotted for each
  net-set orientation to determine if increased
  effort would be needed to achieve the species
  compliment at large, given the different net
  orientation.

Longnose gar

• Results
• We captured 103 fish over the course of our
  study 61 with traditional sets and 42 with
  non-traditional sets. No fish were caught in the
  12.7-cm mesh for any net set and six nets
  produced no fish at all.
• The 5.08-cm bar mesh caught more fish and more
  species of fish than all other panels, regardless
  of set direction. Overall, set direction did not
  have an effect on catch rates across all mesh
  panels we evaluated (P0.05 Figs. 2 and 3).
• Species asymptotes for each directional net set
  varied, indicating that additional effort would
  be needed to capture the species compliment at
  large with non-traditional sets (Fig. 4).
• The length of fish that recruit to a given mesh
  size does not seem to be influenced by
  orientation in the water column (Fig. 5).
• The traditional set yielded several more unique
  fishes than did non-traditional sets, but only
  one unique fish family, Lepisosteidae (Fig. 6).

Introduction Gill nets are commonly used to
sample large-bodied fishes in lakes or slow
moving riverine systems. To catch a wide variety
of fish at large, gill nets can be manufactured
with panels of varying mesh size, length, and
depth (Fig. 1). These nets, known as
experimental gill nets, are often more efficient
at obtaining representative samples of
large-riverine fishes than those of a single mesh
size. However, given their construction it is
unclear if sets with smaller mesh to shore are
more efficient at capturing fish than sets with
smaller mesh set in the mid-channel.

• Discussion
• Species unique to the non-traditional set
  included white bass, rock bass, and saugeye.
  Although absent from traditional net sets, these
  fishes have a similar biology, size, and behavior
  to those fishes represented in the traditional
  sets. Likewise, the traditional sets yielded
  longnose gar, golden redhorse, river redhorse,
  smallmouth buffalo, and hybrid striped bass
  fishes that may have recruited to the
  non-traditional net sets given their morphology.
• Several (4- reversed 2- standard) nets caught no
  fish. Had they been left to soak for longer they
  may have yielded a catch.
• This series of net sets caught several skipjack
  herring, a smallmouth buffalo, a longnose gar and
  a river redhorse all are listed species of
  concern in Pennsylvania. Sampling large rivers
  with gillnets may yield additional information
  regarding the distribution and abundance of these
  rare fishes.

Fig. 1 Diagram of a traditional net set, with
small mesh oriented to shore.

• Objective
• To determine if the orientation (proximity of
  smaller mesh to shore) of experimental gill net
  sets effects fish catch efficiency.

Skipjack herring
Fig. 2 Plot of the number of individuals
captured within each mesh panel evaluated.
Fig. 3 Plot of species richness accumulated
within each mesh panel evaluated.

• Materials and Methods
• Experimental gill nets were placed in the
  Charleroi pool of the Monongahela River 46 times
  from Brownsville to Allenport, between 11 October
  and 1 November 2005. Nets, consisting of five
  connected 7.62-m panels with bar meshes of 2.54,
  5.08, 7.62, 10.15, and 12.7 cm, were set in pairs
  (left bank/right bank) every 0.8 km and allowed
  to fish for approximately four hours.
• Twenty three nets were set with the 2.54-cm mesh
  tied to shore (traditional set Fig. 1), while
  the other 23 were set with the 12.7-cm mesh tied
  to shore (non-traditional set).

• Conclusions
• Experimental nets set in a traditional direction
  did not yield results that differed from a
  non-traditional set, but may require slightly
  more effort to achieve similar species
  asymptotes.
• Length recruitment to a particular mesh size did
  not differ with respect to distance from shore.
• Additional soak time and an increased sample size
  may be required to better determine the effect of
  net orientation.

Fig. 4 Accumulated species richness by net set
orientation across all mesh panels.
Fig. 5 Mean length (mm) of fish captured within
each mesh panel. Error bars denote standard
deviations.