566
Square-mesh codend circumference and selectivity
Matt K. Broadhurst and Russell B. Millar
Broadhurst, M. K., and Millar, R. B. 2009. Square-mesh codend circumference and selectivity. – ICES Journal of Marine Science, 66: 566 –572.
Despite the wide-scale assessment and the use of square-mesh codends in demersal trawls, relatively few studies have tested the effects
of configurations other than mesh size on their selectivity. We investigated the consequences of increasing the circumference of
square-mesh codends used in an Australian penaeid fishery from the expected optimal configuration of 33% of maximum
diamond-mesh extension to 56 and 75%. Three square-mesh designs comprised 27-mm polyamide mesh throughout and had
the same length (100 bars, B), but different circumferences (90, 150, and 200 B, respectively). Paired simultaneous comparisons
(using twin trawls) of each treatment codend against a small-meshed control revealed significant effects of circumference on the efficiency of the trawl for a small teleost (pink-breasted siphonfish, Siphamia roseigaster) and commercial size classes of school prawns
(Metapenaeus macleayi). Compared with the 90 codend, pink-breasted siphonfish catches and prawn count (numbers per 500 g)
were both significantly greater in the codends of larger circumference, and these effects are attributed to concomitant (i) convoluted
or reduced lateral openings of meshes and (ii) lesser probabilities of organisms encountering meshes in the posterior section. These
differences would not preclude the use of codends of larger circumference in the fishery, but they do highlight the need to select
appropriate configurations in future studies to reduce the potential for including the confounding effects of different geometries.
Keywords: bycatch, fishing gear technology, gear selectivity, penaeids.
Received 12 June 2008; accepted 31 December 2008; advance access publication 3 February 2009.
M. K. Broadhurst: NSW Department of Primary Industries, Fisheries Conservation Technology Unit, PO Box J321, Coffs Harbour NSW 2450,
Australia. R. B. Millar: Department of Statistics, The University of Auckland, Private Bag 92019, Auckland, New Zealand. Correspondence to
M. K. Broadhurst: tel: þ61 2 6648 3905; fax: þ61 2 6651 6580; e-mail: mbroadhurst@nmsc.edu.au.
Introduction
Demersal otter trawls are used throughout the world and account
for almost a quarter of the global landed marine catch (estimated
at 86 million tonnes in 2004; Kelleher, 2005; FAO, 2007). Otter
trawls retain disproportionately large ratios of incidental (i.e.
bycatch) to targeted catches (for reviews, see Andrew and
Pepperell, 1992; Kelleher, 2005), because of their active fishing
mechanisms, the frequent use of small diamond-shaped mesh
and, for penaeid trawls, deployment across nearshore habitats
characterized by abundant assemblages of small species.
Although the overall design of a trawl influences the type and
quantity of catch that enters the gear, it is generally accepted
that beyond this point, selection is largely determined by (i) the
lateral mesh openings in the codend, and (ii) the probability that
these are encountered (MacLennan, 1992; Wileman et al., 1996).
An obvious prerequisite towards achieving desired selection in
otter trawls is to regulate a suitable mesh size in the codend,
although this is not the only determining factor. Other parameters
known to influence diamond-shaped mesh openings in codends
include twine diameter (Sala et al., 2007) and material (Tokaç
et al., 2004), length of the extension section (Reeves et al., 1992),
towing speed (Dahm et al., 2002), catch weight (Campos et al.,
2003), presence of codend attachments (Kynoch et al., 2004) or
restrictions (Lök et al., 1997), and codend circumference (Reeves
et al., 1992; Broadhurst and Kennelly, 1996; Lök et al., 1997). Of
these factors, the last is among the most important and has been
identified in several studies as having a strong negative relationship
with selectivity that can be explained by an associated reduction in
lateral mesh openings as the surface area of the netting increases
(Reeves et al., 1992; Broadhurst and Kennelly, 1996).
One simple modification to maintain openings in codends is to
orientate the meshes on the bar so that they are square-shaped
(Robertson, 1986). Often, compared with diamond-mesh
codends, those made from mesh of similar or even smaller
size hung on the bar have increased 50% size-at-retention (L50)
and/or reduced selection range (SR) for a variety of species
(Robertson and Stewart, 1988; Halliday and Cooper, 2000;
Broadhurst et al., 2004; Bahamon et al., 2006).
Although square-mesh codends maintain consistently wider
openings than diamond-mesh designs, they are still characterized
by variability in performance, which might be attributed to the
confounding effects of at least some of the factors listed above
(Halliday and Cooper, 2000; Campos et al., 2003; Bahamon
et al., 2006; Macbeth et al., 2007). For example, Campos et al.
(2003) and Macbeth et al. (2007) observed positive relationships
between catch weights in square-mesh codends and the L50 of
blue whiting (Micromesistius poutassou) and the SR of school
prawns (Metapenaeus macleayi), respectively. Further, Halliday
and Cooper (2000) suggested that, as for diamond mesh (e.g.
Tokaç et al., 2004), square-mesh codends made from polyamide
(PA) twine could provide higher values of L50 than those
constructed of polyethylene (PE) twine.
The circumference of square-mesh codends has also been implicated as impacting on selection, but like other parameters, the magnitudes of such effects are less clear than for diamond mesh
(Broadhurst et al., 1999a, 2004). Specifically, Robertson (1986)
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567
Square-mesh codend circumference and selectivity
suggested that if square-mesh codends were constructed with circumferences considerably larger than the expected fishing circumference of the anterior diamond-mesh extension section, their
meshes could convolute, so reducing and/or masking lateral openings. Similarly, during work to examine two square-mesh codends
in a southern Australian penaeid-trawl fishery, Broadhurst et al.
(1999a) observed a greater L50 and lower SR for western king
prawns (Penaeus latisulcatus) in a tapered and slightly narrower
design, possibly because of an increased encounter probability
with the mesh, i.e. (ii) above. The same trend was not evident for
comparable square-mesh designs tested in a different fishery, but
this was partially attributed to the attachment of a novel release
mechanism that reduced available mesh openings, irrespective of
circumference (Broadhurst et al., 2004).
Despite the potential for some confounding effects of circumference, most studies that have compared diamond- and squaremesh codends have ignored the issue. This is demonstrated in
several cases where, apart from the mesh size, no technical specifications on square-mesh designs were provided (Petrakis and
Stergiou, 1997; Ordines et al., 2006). In other studies where such
information is available, there is variability among relative circumferences, which can be illustrated by calculating the maximum circumference of the square-mesh codend as a proportion of the
theoretical maximum, i.e. the mesh size times the number of
meshes around, of the conventional diamond-mesh design or
extension section. Such proportions typically range from 0.3
(Campos et al., 2003; Bahamon et al., 2006; Macbeth et al.,
2007) to 0.5 (Robertson and Stewart, 1988; Halliday and
Cooper, 2000). It remains unclear to what extent, if any, such
differences among relative circumferences would have on the
selectivity or efficiency of square-mesh codends.
We examined this relationship in the New South Wales
(NSW) river penaeid trawl fishery, which consists of up to 204
small vessels (,10 m) targeting mainly school prawns (but also
catching some eastern king prawns, Penaeus plebejus) at three
locations (the Clarence, Hunter, and Hawkesbury rivers).
During the past 20 years, concerns over large bycatches of nontarget, economically important species by these trawlers culminated in the development and legislation of a range of bycatch
reduction devices, such as the Nordmøre-grid, in the trawl extension section, followed more recently by codends made from
27-mm PA mesh (2.25 m diameter, ø, twine) hung on the bar
to replace conventional 40-mm diamond-mesh designs
(Macbeth et al., 2007). The 27-mm mesh was chosen to select
school prawns at industry-preferred categories of between 150
and 180 individuals 500 g – 1. Here, all mesh sizes refer to mesh
opening according to Ferro and Xu (1996).
Based on the fishing circumference of the anterior extension
section (and corresponding to the area of the Nordmøregrid—400 600 mm) of the gear used by NSW river trawlers,
the optimal circumference of the 27-mm square-mesh codend
was calculated as 1.4 m or 90 B (Figure 1; Robertson, 1986;
Broadhurst et al., 2004). Although Macbeth et al. (2007) demonstrated that this codend selected school prawns at appropriate
sizes across a range of trawlers and conditions, some fishers
suggested that larger circumferences would be beneficial for
accommodating greater catch volumes. Given the above, our
aims were to examine the effects on catches associated with
increasing the circumference from 90 (1.4 m) to 150 (2.4 m)
and 200 B (3.2 m); corresponding to relative increases from
0.33 to 0.56 and 0.75 of the maximum circumference of the
extension section.
Figure 1. Schematic diagram of the penaeid trawl, with the Nordmøre-grid and 90 codend attached. PE, polyethylene; PA, polyamide;
N, normals; B, bars; ø, diameter.
568
Material and methods
Equipment used
The experiment was carried out on commercial trawl grounds for
penaeids in Lake Wooloweyah (part of the Clarence River system,
298260 S 1538220 E), NSW, during October and November 2007. A
local trawler towing two Florida Flyer trawls (each with a headline
length of 7.32 m) in a conventional twin-gear configuration was
chartered to conduct the research. Both trawls contained aluminium Nordmøre-grids with bar spaces of 20 mm, located in
an extension section made from 40-mm PE mesh and with a circumference of 100 meshes in the transverse (T) direction
(Figure 1). Zippers (Buraschi S 146R, 1.45 m long) were secured
posterior to the Nordmøre-grids to facilitate changing the
codends (Figure 1).
One control and three treatment square-mesh codends were
constructed for use with the trawls. All codends were made from
dark knotless polyamide mesh netting and had a length of
1.58 m. The three treatment designs (termed the 90, 150, and
200 codends) consisted of 27-mm mesh (2.25 mm ø braided
twine) hung on the bar and had the same length (100 B;
Figure 1), but different circumferences, 90, 150, and 200 B,
respectively. The 90 codend had a maximum circumference of
1.42 m (i.e. 90 15.75 mm centre bar to centre bar), the same
as the estimated fishing circumference of the diamond-mesh
extension section (i.e. 100 T a fractional mesh opening of
0.3 40-mm mesh and accounting for a twine diameter, ø, of
2 mm). The control codend was made from 9-mm (1.5-mm ø
braided twine) mesh hung on the bar and measured 386 263
B. To maintain symmetry, all codends were constructed in two sections, each section consisting of upper and lower panels sewn
together in opposite directions (see Broadhurst et al., 2004, for
details of their construction). Zippers (see above) were attached
to the anterior sections of each codend to allow attachment to
the trawl extension section.
Experimental design and analyses
Between 07:00 and 14:00 on each day of fishing, the three treatment codends were compared with the control in independent,
paired 40-min deployments across a combination of sandy and
mud substrata in depths ranging from 1 to 2 m. Two replicate
deployments of each treatment codend against the control were
made each day, providing a total of 18 deployments for each comparison over 9 d. The daily order of the deployments was randomized within complete blocks, so that one deployment of all three
treatments against the control was completed before starting the
second replicate for each treatment.
After each deployment, the two codends being examined were
emptied onto a partitioned tray. Catches were separated by species
and the following categories of data collected on board for each
codend: the weights of total penaeids and bycatch; the numbers
of all species comprising bycatch; and the total lengths (TLs in
mm) of pink-breasted siphonfish (Siphamia roseigaster, the only
teleost caught in sufficient numbers with sizes small enough to
pass through the square meshes). Random samples of 1 kg of
penaeids from each codend were placed into plastic bags and processed in the laboratory. During this latter work, all individuals
were separated by species. The data collected from each sample
included carapace lengths (CL in mm) and weights of all eastern
king prawns and 250 school prawns (from a random subsample).
These data were then used to calculate the numbers and weights of
M. K. Broadhurst and R. B. Millar
total school and eastern king prawns in each deployment, and the
industry size category of the number of school prawns 500 g21.
For the two penaeid species and pink-breasted siphonfish
caught in the three experimental gears, logistic and Richard’s
selection curves were obtained via the SELECT method (Millar
and Walsh, 1992) by fitting a single combined curve to the collection of size frequency datasets from the individual deployments, as described in Millar et al. (2004). This provides an
estimate of overdispersion to adjust estimated standard errors
so that they incorporate the effects of between-haul variability
and the subsampling of catches. The bivariate form of Wald’s
F-test (applied to the L50 and SR parameter estimates) was
used to test for significant differences between pairs of gears
(Kotz et al., 1982).
In addition to pairwise tests, the hypothesis that all three
codends were identical was tested simultaneously. Millar et al.
(2004) cautioned that the combined curve fits may not be reliable
for multiple gear comparisons when using conventional techniques such as parametric likelihood ratio tests. Instead, a permutation test was used with the likelihood ratio as the test statistic, so
avoiding the need to make parametric assumptions about the distribution of the likelihood ratio test statistic. Under the null
hypothesis, all 54 deployments (i.e. 18 for each of the three treatments and their controls) have the same selection curve. This null
hypothesis was tested under two scenarios: (i) individually varying
the relative fishing efficiencies of each deployment; and (ii) a
common relative efficiency. Under the first scenario, the null
hypothesis is that the selection curves of the three gears are identical, whereas under the second scenario, the null hypothesis is that
the selection curves and relative efficiency of the three gears are
identical (Millar and Fryer, 1999). In all, 1000 permutations
were done for each test, and all the analyses were done using the
freely available R language.
Analysis of variance (ANOVA) was used to investigate the
hypothesis of no differences among catches (numbers and
weights) of the three treatment codends. Data for those variables
that were present in sufficient numbers were ln(x þ 1) transformed (to account for multiplicativity) and used (after preliminary tests for heteroscedasticity) in balanced two-factor ANOVA
(with codends and days considered fixed and random factors,
respectively).
Results
In all, 88 272 organisms, comprising 23 species, were retained by
the three treatment codends, although most of the catches by
number were school prawns (85%), followed by eastern king
prawns (12%). Bycatches consisted of one crustacean and
20 teleost species and were dominated by pink-breasted siphonfish
(1.1% of the total catch by number), silver biddy (Gerres subfaciatus, 1.1%), Ramsey’s perchlet (Ambassis marianus, 0.5%), and
southern herring (Herklotsichthys castelnaui, 0.4%). Of the
species caught, only eastern king (6–27 mm CL) and
school prawns (5 –24 mm CL) and pink-breasted siphonfish
(30–65 mm TL) were small enough to escape through the
codend meshes. All remaining abundant teleosts ranged from
70 to 250 mm TL. The species compositions and the mean
(+s.e.) catches of targeted penaeids (4.87 + 0.25 kg) and
bycatch (0.37 + 0.06 kg) per 40-min deployment were consistent
with those typically experienced in the fishery.
569
Square-mesh codend circumference and selectivity
Size selection
Richard’s and logistic selection curves were converged for school
and eastern king prawns and pink-breasted siphonfish, but based
on the inspection of residuals and tests of deviance, the latter
models were preferred (Figure 2, Table 1). Owing to low catches
of pink-breasted siphonfish in the 90 codend, it was necessary to
fix the value of the relative fishing efficiency parameter, P,
because it could not be estimated reliably. The value used was
P ¼ 0.67, corresponding to the average of the estimated values
of P for the 150 and 200 codends (Table 1). Fixing P precluded
any variance estimates for the parameter vectors or subsequent
Table 1. Lengths in mm and (s.e.) at 50% probability of retention
(L50), SR, and relative fishing efficiencies (P) for school prawns
(Metapenaeus macleayi), eastern king prawns (Penaeus plebejus),
and pink-breasted siphonfish (Siphamia roseigaster) from the 90,
150, and 200 codends.
Parameter
90 codend
L50
SR
P
School
prawns
Eastern king
prawns
Pink-breasted
siphonfish
10.97 (0.34)
4.51 (0.72)
0.48 (0.01)
12.94 (1.86)
7.19 (3.06)
0.54 (0.06)
150 codend
L50
SR
P
10.41 (0.22)
2.67 (0.42)
0.51 (0.01)
9.63 (0.85)
3.37 (1.86)
0.47 (0.02)
58.93 (7.92)
9.28 (2.44)
0.69 (0.25)
200 codend
L50
SR
P
10.57 (0.23)
2.89 (0.42)
0.49 (0.01)
11.97 (0.84)
4.34 (1.72)
0.52 (0.04)
57.24 (7.31)
9.22 (2.98)
0.65 (0.22)
82.98
23.10
0.67
pairwise tests for these data, but they were still amenable to the
permutation tests, because of the absence of distributional
assumptions.
For all three species, the estimated L50s and SRs of the 90 codend
were larger than those of both the 150 and 200 codends (Figure 2,
Table 1). However, bivariate Wald tests failed to detect any significant differences among appropriate pairwise comparisons (p-values
ranged between 0.08 and 0.94). Note that there were seven pairwise
tests (because only the 150 vs. 200 comparison was made for
pink-breasted siphonfish), and using the Bonferonni correction, a
significance of 0.05/7 0.007 would be required to reject the
null hypothesis, but the smallest observed p-value was 0.08 for
school prawns between the 90 and 150 codends.
The permutational test of the hypothesis that the selection
curves were identical provided similar conclusions to the Wald
tests for all three species (p . 0.05). However, when the assumption of identical relative efficiency was added to the analyses, and
using a Bonferonni adjusted significance level of 0.05/6 0.008
for the total of six permutation tests employed, the null hypothesis
of same selection curve and relative efficiency was not rejected for
eastern king and school prawns (p . 0.008), but it was rejected for
pink-breasted siphonfish (p , 0.001).
Species selection
Figure 2. Logistic selection curves for (a) school prawns
(Metapenaeus macleayi), (b) eastern king prawns (Penaeus plebejus),
and (c) pink-breasted siphonfish (Siphamia roseigaster). The shaded
areas encompass relevant size frequency data from the control
codend.
There were no significant interactions between square-mesh
codends and days in the ANOVA models applied to the variables
examined (p . 0.05). Further, except for the number of southern
herring, all interactions had p-values of .0.25, so this term was
pooled with the residual to increase the power for the main
effects of days and the key factor of interest, codends. All variables,
except for the number of southern herring, returned significant
main effects of days (p , 0.05).
There were no significant differences among codends for the
weights and numbers of total penaeids (F2,43 ¼ 0.30 and 1.17,
p . 0.05), school prawns (F2,43 ¼ 0.30 and 0.91, p . 0.05), or
eastern king prawns (F2,43 ¼ 0.49 and 0.29, p . 0.05), or for the
weight of bycatch (F2,43 ¼ 0.18, p . 0.05) and the numbers of
silver biddy (F2,43 ¼ 0.30, p . 0.05), southern herring (F2,16 ¼
0.53, p . 0.05), and Ramsey’s perchlet (F2,43 ¼ 0.64, p . 0.05;
570
M. K. Broadhurst and R. B. Millar
Figure 3. Barplots of mean catch (þs.e.) in the 90, 150, and 200 codends for the weights of (a) total penaeids, (b) school prawns
(Metapenaeus macleayi), (c) eastern king prawns (Penaeus plebejus), and (d) bycatch, and the numbers of (e) total penaeids, (f) school prawns,
(g) eastern king prawns, (h) school prawns per 500 g, (i) pink-breasted siphonfish (Siphamia roseigaster), (j) silver biddy (Gerres subfaciatus),
(k) southern herring (Herklotsichthys castelnaui), and (l) Ramsey’s perchlet (Ambassis marianus). Black barplots represent significant F-ratios.
Figure 3a–g, j –l). However, compared with the 90 codend, the
means of these penaeid and teleost variables were greater and
less, respectively, in the codends of larger circumference
(Figure 3a–g, j– l). Significant differences were detected
among codends for the numbers of school prawns per 500 g (F2,
43 ¼ 5.86, p , 0.01) and pink-breasted siphonfish (F2, 43 ¼ 5.51,
p , 0.01), with fewer catches in the 90 codend than in the larger
circumference designs (Figure 3 h and i).
Discussion
The estimated selectivity parameters for school prawns for each
of the three square-mesh codends were within the range provided by Macbeth et al. (2007) for the recommended, smallestcircumference design tested on ten trawlers throughout the
NSW river fishery. Although this result further supports the
broad utility of square mesh for consistently selecting penaeids
of appropriate size across a range of configurations and fishing
conditions (Broadhurst et al., 1999a; 2004), the potential nevertheless exists for at least some confounding effects of circumference, manifest here as significant differences among codends for
the number of school prawns per 500 g and their relative efficiencies for pink-breasted siphonfish. These differences can be
explained with respect to species morphology and the possible
changes to codend mesh geometry associated with increasing
circumference.
Specifically, at an optimal configuration, the maximum lateral
and diagonal openings of the square-shaped 27-mm mesh were
13.5 and 19.1 mm, respectively. Based on morphological relationships calculated by Dingle (2008), pink-breasted siphonfish at
50 mm TL (a typical size in the catch) have a height and width
of 13.4 and 5.9 mm, so any fish of this size or smaller that were
swimming in the codends and encountered meshes should have
been able to pass through, irrespective of their perpendicular
orientation. Further, even the largest pink-breasted siphonfish
(65 mm TL) recorded in the treatment codends had a maximum
height of ,19 mm and could have penetrated the meshes by
orientating obliquely. This clearly occurred at a significantly and
incrementally greater rate in the conventional 90 codend than
in the 150 and 200 codends, which may be indicative of at
least two possible concomitant departures from optimal mesh
geometry in the latter designs.
First, increasing the codend circumference beyond that of the
extension section in the 150 and 200 codends would have
created surplus netting during fishing, with convoluted and/or
narrower, more rectangular openings (Robertson, 1986), especially
in the anterior sections close to where the circumference was physically maintained at 1.4 m by the Nordmøre-grid (Figure 1). Even
a slight departure from the optimal square shape would reduce the
escape of the larger pink-breasted siphonfish with heights close to
the maximum mesh opening. Second, the codends of larger circumference would have allowed the catch to spread more laterally,
possibly increasing the diameter of their posterior sections
(Broadhurst et al., 1999b). Any pink-breasted siphonfish swimming immediately in front of the build-up of catch in the 150
and 200 codends would have had to travel greater distances
towards the netting than in the 90 codend and, therefore, may
have had a lesser probability of encountering openings. Either of
these effects could account for the significant two- and threefold
increases in the numbers of pink-breasted siphonfish retained in
the 150 and 200 codends.
571
Square-mesh codend circumference and selectivity
The significantly fewer school prawns per 500 g and the nonsignificant, but nevertheless, slightly greater selectivity parameters
and lower catches of both penaeids in the 90 codend compared
with the two larger-circumference designs support the above
geometry-related impacts, though at reduced magnitudes. Such
relatively milder impacts on penaeids may reflect known differences in their morphology and behaviour. Like pink-breasted
siphonfish, most eastern king and school prawns, i.e. ,20 mm
CL, had carapace heights and widths that were less than the
maximum lateral mesh opening (13.5 mm; Dingle, 2008). Unlike
pink-breasted siphonfish, however, both penaeids are characterized by considerable morphological discontinuities and appendages, which probably restricted their passage through the
meshes more than codend geometry. Further, penaeids and crustaceans in general are known to exhibit quite different behavioural
responses to trawl-induced stimuli than fish, which typically
include fewer active attempts at penetration through codend
meshes (Watson, 1989).
In addition to the significant direct impacts of codend circumference on school prawns and pink-breasted siphonfish, there was
some evidence of indirect contrary effects on the other teleosts
caught in sufficient quantities for meaningful analyses, including
silver biddy, southern herring, and Ramsey’s perchlet. All three
species are laterally compressed and were caught at sizes
(.70 mm TL) with corresponding heights well more than the
maximum mesh opening (Dingle, 2008). Such morphology effectively precluded their escape through any of the codend meshes, so
their catches would not be expected to vary with circumference.
However, although not significant, there were fewer of these fish
in the codends of larger circumference, opposite to the situation
observed for pink-breasted siphonfish.
One explanation for this anomaly is that greater drag attributable to the additional netting material and lateral distribution
of the catch in the larger-circumference codends increased the
displacement of water forwards (Broadhurst et al., 1999b) and
assisted some larger silver biddy, southern herring, and Ramsey’s
perchlet to swim out through the escape exit at the top of
Nordmøre-grid (Figure 1). Broadhurst and Kennelly (1996) attributed similar flow-related effects of circumference in diamondmesh codends to a significant increase in the escape of fish of
comparable size through a strategically located square-mesh
panel. As the swimming speed of fish is a positive function of
their size (Beamish, 1978), such effects may not have extended
to the much smaller pink-breasted siphonfish.
Notwithstanding the significant confounding effects of codend
circumference observed in this study, the magnitudes of differences for school prawns and the low overall quantities of small teleosts (such as pink-breasted siphonfish) likely to be selected out of
the mesh would not preclude NSW river trawlers from using circumferences of up to 3.2 m. Nevertheless, these results still have
implications for other trawl fisheries where the quantities of
bycatches are much greater, and particularly those characterized
by large numbers of fish with transverse morphologies similar to
the mesh openings. Considering the observations here for pinkbreasted siphonfish, even a slight increase in the circumference
of square-mesh codends from 0.33 to 0.56 of the maximum circumference of the extension section has the potential to result in
twice as many of these small animals being retained. Conversely,
if the trends observed here for the relatively larger silver biddy,
southern herring, and Ramsey’s perchlet are maintained, then
any increases in the codend circumference in penaeid trawls
might be offset by a greater escape of fish through strategic,
anterior escape openings (Broadhurst and Kennelly, 1996).
Given the results of this study, future research may benefit from
a closer examination of the influence of the circumference of
square-mesh codends on their performance. The same logic
would also extend to other variables that are also likely to influence
selection, including twine diameter and material (Halliday and
Cooper, 2000; Sala et al., 2007). Isolating any confounding
effects of these parameters could facilitate a clearer evaluation of
the underlying effects of the mesh size and shape and, ultimately,
the design of optimal configurations.
Acknowledgements
The work was funded by the NSW Department of Primary
Industries. Shane McGrath, Craig Brand, Stephan Soule, and the
Clarence River commercial fishers are thanked for their assistance.
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