Experimental fishing was conducted off the port of Quarteira (southern Portugal) from October 2016 to February 2017 using standard trammel nets and modified nets rigged with a guarding net. The commercial catches of trammel nets rigged with a guarding net were 46.1% and 38.0% less than those of the standard net in numbers and economic value. However, there were significantly fewer commercial discards in biomass in the modified trammel nets (68.2%) and by-catch abundance and biomass were also lower in the modified nets (41.8% and 17.3% less, respectively). For the two main fish by-catch species, the modified net caught 62.2% fewer longfin gurnards (
La pesca experimental se llevó a cabo frente al puerto de Quarteira (sur de Portugal), de octubre de 2016 a febrero de 2017, utilizando redes de trasmallo estándar y redes modificadas con una red de protección llamada “faldón”. Las capturas comerciales de redes de trasmallo equipadas con una red de protección fueron 46.1% y 38.0% menores que las de la red estándar en número y valor económico. Sin embargo, hubo significativamente menos descartes comerciales en biomasa en las redes de trasmallo modificadas (68.2%) y la abundancia de las capturas secundarias y su biomasa también fueron menores en las redes modificadas (41.8% y 17.3% menos respectivamente). Para las dos principales especies de la captura secundaria de peces, la red modificada capturó un 62.2% menos de arete oscuro (
By-catch consists of marine species that are caught unintentionally alongside targeted species that live in the same environment, due to a lack of selectivity of fishing gears. By-catch may consist of undersized or juvenile target species as well as non-target commercial species and a wide variety of species of no value that are subsequently discarded. Commercial species may also be discarded for various reasons, including quota limitations, being undersize, or being unfit for sale because of damage or parasites (
Trammel nets have discard rates ranging from 0% to 66% worldwide (
There have been several successful approaches to the management of harvesting commercial catch while decreasing by-catch and discards.
In Izmir Bay (Aegean coast of Turkey), a guarding net was added to the common trammel net used for the commercial prawn
The specific objectives of the present study were to compare a standard trammel net with a modified trammel net rigged with a guarding net, in terms of catch composition (commercial, by-catch and discard), economic yield, time needed to “clean” the nets (removal of by-catch and discard species), and net damage.
Two types of net were rigged: a standard trammel net (T) and a modified trammel net (M), a standard trammel net with a guarding net, consisting of a single layer of netting three meshes high, placed between the trammel net and the footrope (
Experimental fishing took place off the coast of Algarve (southern Portugal), using a fishing vessel belonging to the port of Quarteira (
The damage occurring to the nets was assessed at the end of the experimental fishing trials by counting each visible “hole” or broken piece across the span of the entire 1.5-km net. Four observers checked each section of net for possible damage and recorded the damage according to the two types of net used. Upon observing a hole, a 20-cm ruler was placed within the hole and a picture was taken for later analysis of the diameter. Each hole was classified according to its position (e.g. guarding net, inner small mesh panel or outer large mesh panel; lower or upper half of the net) and size (small if <20 cm, large if >20 cm).
The weight of each fish was estimated using species weight-length relationships parameters. The weight of invertebrates was estimated by the fishermen directly on board. The total catch (kg) of each species was converted to value in euros using the average price per kg obtained from the mandatory first fish auction in Quarteira. The weight of each by-catch species was also recorded to compare the two net types, M and T. Trammel net catches were converted to catch per unit effort in numbers (CPUEn) and biomass (CPUEkg) and mean value (€) per 1000 m of trammel net.
Data were analysed using cluster analysis, multi-dimensional scaling (MDS) implemented in PRIMER 6 (
The standard (T) and the modified (M) trammel nets caught a total of 40 commercial species, 19 of them in common with the two nets (
Class | Modified trammel net (M) | Standard trammel net (T) | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Family | Species | CPUEn | sd | CPUEkg | sd | € | sd | CPUEn | sd | CPUEkg | sd | € | sd |
Actinopterygii | |||||||||||||
Balistidae | 0.78 | 1.34 | 0.46 | 0.77 | 2.1 | 3.57 | 0.67 | 1.48 | 0.62 | 1.42 | 2.9 | 6.62 | |
Batrachoididae | 0.07 | 0.33 | 0.05 | 0.23 | 0.1 | 0.40 | |||||||
Belonidae | 0.08 | 0.34 | 0.02 | 0.10 | 0.08 | 0.07 | 0.33 | ||||||
Carangidae | 0.31 | 0.62 | 0.02 | 0.04 | 0.10 | 0.22 | 0.54 | 0.02 | 0.05 | 0.0 | 0.11 | ||
Centracanthidae | 0.07 | 0.33 | 0.01 | 0.05 | 0.0 | 0.13 | |||||||
Clupeidae | 0.16 | 0.47 | 0.18 | 0.18 | 0.22 | 0.99 | 0.08 | 0.37 | 0.0 | 0.03 | |||
Clupeidae | 0.08 | 0.34 | 0.01 | 0.02 | |||||||||
Congridae | 0.07 | 0.33 | 0.03 | 0.11 | 0.1 | 0.31 | |||||||
Haemulidae | 0.30 | 1.33 | 0.18 | 0.82 | 0.6 | 2.89 | |||||||
Gadidae | 0.78 | 1.67 | 0.07 | 0.16 | 0.3 | 0.61 | 0.89 | 1.39 | 0.07 | 0.13 | 0.3 | 0.51 | |
Merlucciidae | 1.01 | 1.92 | 0.31 | 0.62 | 0.9 | 1.81 | 1.19 | 2.57 | 0.30 | 0.69 | 0.9 | 2.00 | |
Mullidae | 0.23 | 1.02 | 0.05 | 0.22 | 0.8 | 3.29 | 0.15 | 0.46 | 0.15 | 0.14 | 0.7 | 2.08 | |
Phycidae | 0.44 | 1.19 | 0.07 | 0.17 | 0.2 | 0.52 | |||||||
Pomatomidae | 0.08 | 0.34 | 0.04 | 0.16 | 0.07 | 0.33 | 0.03 | 0.13 | 0.0 | 0.01 | |||
Sciaenidae | 0.08 | 0.34 | 0.08 | 0.34 | 0.7 | 3.08 | |||||||
Scombridae | 0.08 | 0.34 | 0.06 | 0.26 | 0.2 | 0.75 | |||||||
Scophthalmidae | 0.15 | 0.46 | 0.07 | 0.22 | 0.9 | 2.88 | |||||||
Serranidae | 0.16 | 0.47 | 0.01 | 0.05 | |||||||||
Sparidae | 0.08 | 0.34 | 0.04 | 0.18 | 0.5 | 2.27 | |||||||
0.23 | 0.74 | 0.01 | 0.04 | 0.2 | 0.49 | ||||||||
0.08 | 0.34 | 0.07 | 0.32 | 0.6 | 2.53 | ||||||||
0.30 | 0.78 | 0.04 | 0.13 | 0.1 | 0.26 | ||||||||
0.23 | 0.74 | 0.04 | 0.12 | 0.2 | 0.56 | 0.30 | 1.03 | 0.07 | 0.28 | 0.3 | 1.28 | ||
0.07 | 0.33 | 0.01 | 0.03 | 0.0 | 0.17 | ||||||||
1.09 | 2.25 | 0.14 | 0.27 | 0.8 | 1.52 | 1.11 | 3.33 | 0.13 | 0.35 | 0.7 | 1.97 | ||
0.23 | 0.74 | 0.20 | 0.62 | 2.4 | 7.46 | ||||||||
0.08 | 0.34 | 0.02 | 0.07 | 0.13 | |||||||||
Soleidae | 4.99 | 9.54 | 0.74 | 1.40 | 6.8 | 12.86 | 13.11 | 27.53 | 1.91 | 3.73 | 17.6 | 35.04 | |
0.86 | 1.51 | 0.17 | 0.29 | 1.2 | 2.05 | 2.22 | 3.24 | 0.52 | 0.72 | 3.7 | 5.12 | ||
0.86 | 1.34 | 0.31 | 0.57 | 3.8 | 6.96 | 2.59 | 3.59 | 1.29 | 1.68 | 15.7 | 20.45 | ||
0.16 | 0.47 | 0.05 | 0.18 | 0.8 | 2.70 | 0.30 | 1.33 | 0.09 | 0.38 | 1.3 | 5.72 | ||
0.23 | 0.56 | 0.14 | 0.39 | 1.7 | 4.70 | 0.22 | 0.54 | 0.25 | 0.61 | 3.0 | 7.46 | ||
Cephalopoda | |||||||||||||
Loliginidae | 0.08 | 0.34 | 0.20 | 0.89 | 2.3 | 10.17 | |||||||
Octopodidae | 0.46 | 0.96 | 0.86 | 1.93 | 4.6 | 10.44 | 0.52 | 0.99 | 1.11 | 2.03 | 6.0 | 10.97 | |
Sepiidae | 1.48 | 2.61 | 1.16 | 2.07 | 6.0 | 10.74 | 1.93 | 2.77 | 1.44 | 2.16 | 7.5 | 11.25 | |
Elasmobranchii | |||||||||||||
Rajidae | 0.55 | 1.13 | 0.55 | 2.51 | 3.1 | 6.52 | 0.37 | 0.81 | 1.08 | 2.39 | 2.8 | 6.20 | |
Torpedinidae | 0.15 | 0.46 | 0.16 | 0.51 | 0.4 | 1.44 | |||||||
Torpedinidae | 0.08 | 0.34 | 0.16 | 0.25 | 0.2 | 0.71 | 0.07 | 0.33 | 0.07 | 0.33 | 0.2 | 0.94 | |
Malacostraca | |||||||||||||
Majidae | 0.31 | 1.06 | 0.09 | 0.36 | 0.5 | 2.11 | 0.44 | 1.09 | 0.19 | 0.47 | 1.1 | 2.74 | |
Nephropidae | 0.31 | 0.34 | 0.37 | 1.60 | 9.5 | 41.35 | 0.07 | 0.33 | 0.42 | 1.88 | 10.8 | 48.43 |
Class | Species | C/NC | Modified trammel net | Standard trammel net | ||||||
---|---|---|---|---|---|---|---|---|---|---|
Family | CPUEn | sd | CPUEkg | sd | CPUEn | sd | CPUEkg | sd | ||
Actinopterygii | ||||||||||
Balistidae | C | 0.08 | 0.34 | 0.15 | 0.63 | 0.23 | 0.74 | 0.14 | 0.44 | |
Callionymidae | NC | 0.08 | 0.34 | 0.01 | 0.03 | |||||
Carangidae | C | 0.08 | 0.34 | 0.03 | 0.15 | |||||
C | 1.01 | 1.11 | 0.08 | 0.12 | 0.70 | 1.25 | 0.04 | 0.07 | ||
Clupeidae | C | 0.16 | 0.68 | 0.03 | 0.13 | 0.31 | 1.06 | 0.08 | 0.24 | |
C | ||||||||||
NC | 0.08 | 0.34 | ||||||||
Congridae | C | 0.23 | 0.56 | 0.01 | 0.06 | 0.08 | 0.34 | 0.05 | 0.24 | |
Gadidae | C | 0.78 | 2.72 | 0.06 | 0.27 | 1.56 | 3.97 | 0.06 | 0.24 | |
Merlucciidae | C | 0.47 | 1.40 | 0.05 | 0.21 | 0.16 | 0.68 | 0.05 | 0.22 | |
Moronidae | C | 0.08 | 0.34 | 0.14 | 0.60 | |||||
Mullidae | C | 0.08 | 0.34 | |||||||
Phycidae | C | 0.08 | 0.34 | 0.08 | 0.33 | |||||
Sciaenidae | C | 0.08 | 0.34 | 0.03 | 0.15 | |||||
Scombridae | C | 0.08 | 0.34 | 0.07 | 0.29 | 0.08 | 0.34 | 0.06 | 0.27 | |
NC | 10.21 | 16.05 | 0.50 | 0.74 | 7.95 | 13.12 | 0.46 | 0.70 | ||
Scorpaenidae | NC | 0.16 | 0.68 | 0.01 | 0.05 | 0.39 | 1.19 | 0.02 | 0.09 | |
NC | 0.08 | 0.34 | 0.02 | 0.10 | ||||||
Serranidae | C | 0.08 | 0.34 | 0.08 | 0.34 | |||||
Sparidae | C | 1.17 | 2.13 | 0.06 | 0.15 | 1.48 | 2.88 | 0.09 | 0.16 | |
C | 0.23 | 0.74 | 0.01 | 0.06 | 0.31 | 0.79 | 0.03 | 0.10 | ||
C | 0.16 | 0.47 | 0.01 | 0.03 | 0.23 | 0.74 | 0.01 | 0.03 | ||
C | 0.16 | 0.68 | 0.01 | 0.07 | ||||||
C | 0.16 | 0.47 | ||||||||
C | 0.23 | 0.74 | 1.09 | 2.41 | ||||||
C | 0.16 | 0.47 | ||||||||
C | 1.25 | 2.17 | 0.07 | 0.16 | 2.73 | 3.17 | 0.20 | 0.30 | ||
C | 0.23 | 0.56 | 0.05 | 0.23 | 0.39 | 0.83 | 0.07 | 0.25 | ||
Soleidae | C | 1.01 | 3.09 | 2.65 | 7.30 | |||||
C | 0.16 | 0.47 | 0.03 | 0.09 | 0.47 | 1.40 | 0.09 | 0.26 | ||
C | 0.00 | 0.00 | 0.39 | 1.38 | 0.09 | 0.30 | ||||
C | 0.08 | 0.34 | 0.08 | 0.36 | 0.08 | 0.34 | 0.03 | 0.14 | ||
Syngnathidae | NC | 0.23 | 0.74 | |||||||
Trachinidae | NC | 6.32 | 10.11 | 0.55 | 1.19 | 9.36 | 14.12 | 0.87 | 1.58 | |
Tetraodontidae | NC | 0.23 | 1.02 | |||||||
Triglidae | NC | 0.39 | 1.38 | 0.62 | 2.38 | |||||
NC | 0.08 | 0.34 | 0.31 | 0.79 | ||||||
NC | 0.08 | 0.34 | ||||||||
NC | 6.16 | 4.79 | 16.30 | 13.97 | ||||||
NC | 0.08 | 0.34 | ||||||||
Uranoscopidae | NC | 0.16 | 0.47 | |||||||
Anthozoa | ||||||||||
Gorgoniidae | NC | |||||||||
NC | 0.08 | 0.34 | 0.08 | 0.34 | ||||||
Hormathiidae | NC | 0.16 | 0.47 | 0.47 | 1.49 | |||||
Veretillidae | NC | 0.23 | 0.74 | |||||||
Ascidiacea | ||||||||||
Ascidiiae | NC | 0.31 | 0.79 | 1.17 | 2.69 | |||||
Asteroidea | ||||||||||
Asteriidae | NC | 0.08 | 0.34 | 0.08 | 0.34 | |||||
Astropecrinidae | NC | 0.39 | 0.67 | 0.47 | 1.11 | |||||
Ophidiasteridae | NC | 0.23 | 1.02 | |||||||
Bivalvia | ||||||||||
Pinnidae | NC | 0.39 | 1.19 | 0.86 | 2.38 | |||||
Cephalopoda | ||||||||||
Loliginidae | C | 0.07 | 0.33 | 0.07 | 0.30 | |||||
Sepiidae | C | 0.08 | 0.34 | 0.70 | 1.34 | |||||
Crinoidea | ||||||||||
Antedonidae | NC | 0.08 | 0.34 | |||||||
Echinoidea | ||||||||||
Toxopneustidae | NC | 0.55 | 1.33 | 6.00 | 10.76 | |||||
Elasmobranchii | ||||||||||
Carcharhinidae | NC | 0.08 | 0.34 | 0.69 | 3.02 | |||||
Myliobatidae | NC | 0.31 | 0.79 | 0.23 | 0.56 | |||||
Rajidae | C | 0.47 | 0.71 | 0.44 | 1.12 | |||||
Gastropoda | ||||||||||
Aplysiidae | NC | 0.08 | 0.34 | |||||||
Muricidae | NC | 0.08 | 0.34 | |||||||
Ranellidae | NC | 0.08 | 0.34 | 0.23 | 0.74 | |||||
Volutidae | NC | 1.09 | 3.42 | 3.04 | 6.60 | |||||
Gymnolaemata | ||||||||||
Bitectiporidae | NC | 0.08 | 0.34 | |||||||
Holothuroidea | ||||||||||
Holothuriidae | NC | 0.31 | 1.06 | |||||||
Stichopodidae | NC | 0.08 | 0.34 | 0.08 | 0.34 | |||||
Malacostraca | ||||||||||
Crangonidae | NC | 0.08 | 0.34 | |||||||
Diogenidae | NC | 0.08 | 0.34 | 0.08 | 0.34 | |||||
Majidae | C | 0.23 | 0.74 | |||||||
Polychaeta | ||||||||||
n.i. | n.i. | NC | 0.08 | 0.34 | ||||||
Scyphozoa | ||||||||||
Rhizostomatidae | NC | 1.09 | 1.90 | 0.94 | 2.22 | |||||
Phylum: Porifera | n.i. | NC | 2.18 | 4.75 | 2.57 | 5.84 |
The M net caught 35% (n), 39% (kg) and 38% (value) of the total landed catch of the two nets. The most lucrative species caught in the M net were
The M:T discard ratios were 0.3 for commercial discards and 0.8 for non-commercial by-catch. The species with the highest discards in weight in M nets were
Regarding by-catch species, 14.5 kg of
Combined catch (commercial catch, commercial discards and by-catch) composition showed high similarities within trips for both abundance and biomass data (
The analyses based on the square root transformed data showed high significance levels in PERMANOVA and ANOSIM. The MDS plots for the abundance and biomass of discards with season as a factor are given in
The MDS plots for the abundance and biomass of discards with target species as the factor are shown in
Figure 6 shows the MDS plots for the abundance and biomass data of the combined catch, with depth as the factor. ANOSIM gave global R values of 0.660 and 0.683, with significance levels of 0.1%, indicating significant differences between the two depth ranges. This is supported by a p-value of 0.001 obtained with PERMANOVA. SIMPER results (Tables S6 and S7 in Supplementary Material) also provide support for strong dissimilarity between the two depth ranges (10-20 m and 20-30 m) for both abundance and biomass.
At a resemblance level of 25% there was similarity among depth ranges (10-20 m and 20-30 m) for both abundance and biomass of combined catch data (
Overall, 23 individuals per species/taxa were used to monitor the average time needed to remove the catch of the six most abundant by-catch species/taxa, which represented 85% of the by-catch in number for the M nets and 73% for the T nets. The removal time for
At the end of the fishing trials, the 1.5 km of trammel net had 127 holes, of which 84 were detected in the M net. Approximately 80% of the holes in the M net occurred in the lower half of the net, about 60% were larger than 20 cm, and 62% of the holes were found in the guarding net. Forty-three (34% of the total) holes occurred in the T net; about 90% of them were in the upper part of the net, 58% were larger than 20 cm in width/diameter, and 50% were in the inner layer and 50% in the outer layer.
Mitigation of by-catch of sea birds, turtles and cetaceans in set nets has been widely studied (e.g.
Compared with previous studies such as those performed in Izmir Bay and Antalya in Turkey (
In the present study the greatest decrease in commercial catch was observed for the sole species, especially the bastard sole
Because the majority of the trammel net sets targeted soles rather than cuttlefish, the landed catches of the modified nets had lower total value per unit effort than the standard trammel nets without the guarding net. This is a not unexpected result, because soles and bastard soles are more likely to come into contact with the nets near the footrope. Thus, the guarding net reduces catch rates of these species as the netting is thicker and more visible than the monofilament trammel net. However, we believe that in years of greater abundance of cuttlefish, which come into contact with and are caught higher in the net, when this species is truly the target species, there would be less difference in the catch value per unit effort between modified and traditional trammel nets. Excluding the three main sole species, the modified net caught nearly the same amount of commercial species as the standard net (ratio of 0.84:1).
In this study it was left up to fishers to decide the fate of the catches. Thus, Atlantic chub mackerel,
By-catch in number of the guarding net was 41.8% less than that of the standard net. By-catch of non-commercial species is a problem in terms of taking up net area that could be catching commercial species, especially if the by-catch is being caught in relatively large quantities. Of the three most important by-catch species, the longfin gurnards (
Previous studies comparing standard trammel nets to modified trammel nets also assessed damage (
As stated above, by-catch removal can be laborious and therefore reduction in catches of discard species would reduce time spent cleaning the net and removing catches.
As in other studies (
Since this study was conducted on a commercial fishing boat, we received feedback on the guarding net from the fishermen as well as an understanding of their willingness to use modified trammel nets. While the fishermen were not convinced by the results, had the fishing trials been carried out during a normal cuttlefish season and had the Atlantic chub mackerel,
We would like to thank the skipper and crew of the fishing vessel
The following supplementary material is available through the online version of this article and at the following link:
Table S1. – Two-way PERMANOVA table with factors net type (Ne) and depth (De), net type (Ne) and season (Se), and net type and target catch (Ta) for abundance of combined catch (includes commercial, commercial discard and by-catch).
Table S2. – SIMPER analysis for the discards (commercial discards and by-catch) data for abundance with the factor season (autumn and winter).
Table S3. – Simper analysis for the discards (commercial discards and by-catch) data for biomass with the factor season (autumn and winter).
Table S4. – Simper analysis for the discards (commercial discards and by-catch) data for abundance with the factor target catch (sole season and cuttlefish season).
Table S5. – Simper analysis for the discards (commercial discards and by-catch) data for biomass with the factor target catch (sole season and cuttlefish season).
Table S6. – Simper analysis for the combined catch (commercial catch, commercial discards, and by-catch) data for abundance with the factor depth (10-20 m and 20-30 m).
Table S7. – Simper analysis for the combined catch (commercial catch, commercial discards and by-catch) data for biomass with the factor depth (10-20 m and 20-30 m).