Fisheries Research 93 (2008) 117–124

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Strategies for improving fuel efficiency in the Portuguese trawl fishery∗

Joaquim Parente , Paulo Fonseca, Victor Henriques, Aida Campos

INRB/L-IPIMAR – National Institute for Biological Resources/Fisheries Research Laboratory, Avenida de Brasılia, 1449-006 Lisbon, Portugal

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Article history:Received 31 October 2007Received in revised form 6 March 2008Accepted 6 March 2008

Keywords:Portuguese trawlersFuel savingEnergy economyModelsFlume tank

a b s t r a c t

The recent rise in oil priceparticularly in trawling. Csuch cases, the most succgear and towing conditiofor Portuguese fish trawleperformance. Two trawleproject. Both vessels workto elucidate the actual vestank, which provided theboth vessels using the momaintained their previousthe same commercial traw(NCF) of up to 27% over ththe performance of the necommercial fishing.

1. Introduction

Harvesting fish from the sea requires a large amount of energy,although the energy requirements can vary substantially depend-ing on the type of fishery. Tyedmers et al. (2005) estimated worldfishery fuel consumption at 50 billion (50 × 109) l. While considered‘serious underestimates’, these figures nonetheless correspond toabout 1.2% of the global fuel consumption. On the other hand, atonne of fuel is consumed for each 1.9 tonnes of fish captured, and1.7 tonnes of CO2 is released for each tonne landed. As such, fuelconsumption is both an economical and environmental problem.

Energy saving has been a subject of research since the 1970s oilcrisis, leading to several studies aimed at improving vessel designand power consumption. Special attention has been given to hullresistance and tests in model basins. Benefits were identified fromusing bulbous bows in small fishing vessels, leading to a reductionin fuel consumption of 15–30% during sailing (Kasper, 1983). Gainsin propulsive efficiency between 10 and 17% during free navigationwere also attained using ducted propellers in trawlers (Basanez,1975). Large savings in fuel consumption (up to 28%) could alsobe obtained from this type of propeller by towing at lower speeds(O’Dogherty et al., 1981).

∗ Corresponding author. Tel.: +351 21 3027162; fax: +351 21 3015948.E-mail address: jparente@ipimar.pt (J. Parente).

0165-7836/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.fishres.2008.03.001

brought renewed attention to energy savings in the fishing industry, andl trawlers spend most of their time on fishing grounds near the coast. Inl energy-saving modifications ought to result from changes in the fishinghe purpose of this study was to identify the energy-economy potentialter altering a vessel’s operating conditions and improving its trawl gearmed Tricana de Aveiro and Joao Macedo were selected as subjects in thisgear of similar design and size. Experimental sea trials were carried out

nd gear performance. A model trawl was then built and tested in a flumefor improving the gear design. Full-scale trials were then carried out withd trawls in order to assess changes in gear performance. The new trawlsty to catch species of different ecological groups and consumed less fuel atspeed. An economic study showed potential increases in the net cash flowge of operational navigation and trawling speeds. Having demonstrated

wls, the skippers of both vessels subsequently adopted the new design for

© 2008 Elsevier B.V. All rights reserved.

In addition to vessel design, special attention has also been givento vessel operations. Efficient ship operation is required for long-term fuel economy of the vessel, and entails selecting the best route,draft and trim; adequate maintenance of the hull and machinery;and a rational exploitation of the available systems by well-trained

crews. The choice of the best running point (that is, the vessel’soperating speed that maximizes cash flow), both in trawling andin free navigation, is a major contribution toward energy savingsand must be continuously adjusted according to vessel require-ments.

Trawlers are among the most fuel-demanding fishing vessels.This is due to the high towing resistance associated with the gears;the netting drag alone typically accounts for 60% of the total gearresistance (Wileman, 1984). Reducing the netting surface by usinglarger meshes in the net forepart (wings and square) may signifi-cantly reduce net drag without affecting the trawl mouth area andthus the catch efficiency. This is particularly true for those speciesthat display herding behaviour inside the trawl (Fiorentini et al.,1987).

Other possibilities for reducing the net drag have also beenrecently investigated, such as the use of knotless netting and thin-ner twine. Ward et al. (2005) compared the drag of twin trawlsmade of traditional polyethylene twine with similar trawls ofreduced twine diameters, and reported a drag reduction of 6% andan increase in mouth opening of 10%. Alterations to the gear rigging,such as the number of bridles and their relative length, may also

118 J. Parente et al. / Fisheries Rese

Nomenclature

A mouth area (m2)C total catch per haul (kg)Cw crew variable wages per trip (D )CPUE catch per unit effort (kg/h)D total distance covered per trip (in free navigation)F fuel costs per trip (D )K sum of all fixed costs per trip (capital costs, mainte-

nance, insurance, crew fixed wages, administration,other costs) (D )

m 1.05 (margin for lubricating oil costs as a fraction offuel costs)

n number of haulsNCF net cash flow per trip (D )p unit fish price (D /kg)

pf unit fuel price (D /kg)p (%) crew percentage over the total catch value (0.35 for

all vessels tested)qh hauling fuel rate (l/h)qn navigation fuel rate (l/h)qp harbour fuel rate (l/h)qs setting fuel rate (l/h)qt trawling fuel rate (l/h)R total catch value per trip (D )T total trip duration (h)Th average hauling duration (h)Tn total time spent in free navigation per trip (h)Tp total time navigating inside the harbour (h)Ts average setting duration (h)TT(tr) total trawling duration per trip (h)Tt average trawling duration (h)V free navigation speed (kn)Vt trawling speed (kn)� f fish density per unit volume of water (kg/m3)

strongly affect the net shape and drag and consequently improvethe overall trawl efficiency.

Recent oil price increases have brought renewed attention toenergy-saving methods in the fishing industry (e.g., Project Green

Fish1; Leblanc, 2005), including the use of alternative fuels andlubricants (such as bio-diesel and bio-lubricants). However, dueto the European Commission restrictions on new constructions,the major opportunities for reducing fuel consumption are chieflyrelated to improving vessel operation rather than commissioningnew energy-saving vessels. Fuel-efficient gear design continues tobe a top priority for improving the efficiency of the existing fishingfleet (European Commission, 2006).

The objective of this study was to identify the fuel-economypotential for Portuguese fish trawlers either by changing the ves-sel’s operating conditions or by improving the trawl gear design.Coastal trawlers were chosen for study since they spend most oftheir time trawling near the coast, and thus might expect the great-est energy-saving return from changes of fishing gear.

2. Material and methods

2.1. Choice of vessels

The existence of two primary metiers has traditionally beenassumed for the Portuguese coastal trawl fishery: crustaceans and

1 http://www.peixeverde.org/peixe org eng/index.htm (last accessed2007/10/01).

arch 93 (2008) 117–124

fish. Each corresponds to well-defined fleets of 26 and 70 activevessels, respectively (DGPA, 2004). According to the statistics ofthe General-Directorate for Fisheries (DGPA, 2004), the mean val-ues of gross tonnage and engine power for the fishing fleet are183.8 tonnes (standard deviation S.D., 70.9) and 712 HP (S.D., 285.9),respectively. It is a coastal fleet accustomed to short (3 day average)fishing trips. The main species landed are horse mackerel (Trachurustrachurus, accounting for around 40% of the total catch), followedby blue whiting (Micromesistius poutassou) and other semi-pelagicfish, and finally other cephalopods (squids and octopuses; DGPA,2004).

The present study focused on two trawlers from the Portuguesefleet, since fish trawls usually offer a wide basis for gear mod-ifications. The increase in mesh size in the trawl fore is one ofthe measures usually tested to reduce the net drag in these typesof trawls. This is because most fish species, unlike crustaceans,display herding behaviour within the net area. Such behaviourtranslates to a larger catching efficiency despite the larger meshsizes. The two trawlers studied were the Tricana de Aveiro andJoao Macedo, each of approximately 24 m overall length and 600 HPengines. Each vessel lands a diversified number of species, includinghorse mackerel, other species swimming near the ocean bottom,and benthic species such as octopus and flatfish. These catchesbelong to a well-defined landing profile (homogeneous group interms of species composition) recently defined in Campos et al.(2007).

2.2. Trawl design

The technical drawings and rigging details for trawl T1 (of theTricana de Aveiro) and trawl J1 (of the Joao Macedo) are very sim-ilar. Both trawls are reinforced in the lower belly using a thickerpolyamide (PA) twine that is usually found in trawls of Spanishdesign, and differed mainly in the mesh sizes of the different panels.The footropes are made of steel wire rope covered with polyethy-lene and have extra chain weight protection in the bosom, lowerquarters and at the wingends. The technical drawing for J1, togetherwith the footrope details, are shown in Fig. 1.

2.3. Data collected and measuring devices

A total of eight trials were carried out during the study. For eachvessel, an experimental and a commercial trial were carried out atthe two different phases of the project (before and after trawl gearoptimization) in order to measure fuel consumption under different

vessel-operating conditions. The vessel’s consumables (water andfuel supplies) at the start of the trials were kept the same in bothvessels to ensure identical testing conditions.

A fuel monitoring system was installed in each vessel. The work-ing time duration of the engine, the engine speed, the total fuelconsumption and the instant fuel rate were logged by the sys-tem. Data on the exhaust temperature and vessel speed (overthe ocean bottom) were obtained from vessel instruments (suchas engine temperature gauges and GPS equipment, respectively).Trawl geometry (e.g., the vertical opening at the centre of the head-line, and the wingend and otterboard spread) and the water flow inthe towing direction were measured by hydroacoustic (Scanmar)sensors. In the commercial sea trials, the catch weight was alsoregistered for all commercial species.

A typical round trip for a coastal trawler consists of severaloperating situations for different engine loadings. Fishing vesselswith a controllable pitch propeller have an optimum combina-tion of pitch and propeller revolutions for each operating situation,leading to optimum specific engine fuel consumption. However,during free running, it is common practice to transfer somepower from the main engine to constant displacement hydraulic

J. Parente et al. / Fisheries Research 93 (2008) 117–124 119

Fig. 1. Technical drawings of the J1 trawl (FV “Joa

pumps and AC generators through power take-offs, forcing themain engine to run at a constant speed. Having this in mind,changes in the vessel-operating situation were carried out onlythrough propeller pitch variation, although this is not the bestprocedure to optimize both specific fuel consumption and engineefficiency.

2.4. Experimental sea trials

Data on the above parameters were collected at several pro-peller pitch increments up to the maximum working pitch, both intrawling and in navigation. For each pitch increment, operating con-ditions were kept constant for 15 min. Fuel rate was then recordedunder both conditions as a function of trawling speed and engineexhaust temperature.

Gear geometry was recorded over a range of trawling speeds,including the average speed used in commercial fishing. The main

o Macedo”), together with footrope details.

parameters characterizing the gear performance, namely verti-cal opening, wingend spread and otterboard spread, were alsorecorded as a function of trawling speed (as measured by a speedsensor).

2.5. Commercial sea trials

The vessel performance was evaluated at the different phasesof the fishing trip (Table 1). This allowed for a full characterizationof the average trip for each vessel. A phase is defined as the sum ofseveral sub-phases repeated along the trip (e.g., the trawling phaseis the sum of all trawling operations). The relevant parameters foreach sub-phase are presented in Table 1, and include the workingtime T of the engine, the fuel consumption Q, the average vesselspeed, the average exhaust temperature T (◦C), and the average fuelrate q.

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120 J. Parente et al. / Fisherie

Table 1Characterization of the commercial trip and parameters registered during the differ

Phase Description

Harbour navigation Harbour manoeuvres and conditioned navigation inside the hFree navigation Travel between the harbour and the fishing ground, as well aSetting/hauling Setting and hauling operationsTrawling Trawling operationsMiscellaneous Net repairing, waiting for dawn to set the gear, and other unf

Parameters include T, time duration (h); Q, fuel consumption (l); V, average vessel s

2.6. Alterations to trawl design

When improving trawl efficiency, either a new trawl can beadopted or alterations can be introduced to the existing gear with-out a radical design change. The first hypothesis seemed unlikelysince Portuguese fishermen, as with fishermen in other countries,are typically not receptive to sudden changes in work habits. There-fore, we decided to modify currently used trawls with the aimof decreasing trawl drag without affecting catch efficiency. Modeltests were carried out in IFREMER’s flume tank at Boulogne-sur-Mer. A 1:15 scale model was used based on the original trawl design,which was then further modified and tested according to the new(and hopefully improved) specifications.

Only the net itself was scaled down, together with a small por-tion of the sweeps, in order to obtain accurate measurements of

net drag and geometry. Given the similarities in shape between theoriginal J1 and T1 trawls, with only small differences in the overallmesh size and total twine surface area (J1: 98.2 m2; T1: 104.9 m2),the flume tank tests were carried out over a single trawl modelbased on the J1 trawl design.

The main trawl alteration was to increase the mesh size at thewings and square; this is the optimum method to improve themouth area without increasing net drag. However, an importantpercentage of the total catch for both the J1 and T1 trawls cor-respond to species that do not display herding behaviour (e.g.,octopuses; see Table 2) and this has tempered the amount of allow-able mesh size increase. The former mesh sizes used in the lowerand upper wings (80 mm, full mesh) were increased to 100 and120 mm, respectively, in the new design. The square was dividedinto two sections: the first with 120 mm mesh sizes and the secondwith 100 mm mesh sizes (Fig. 2).

Further changes in the original trawl design included alterationsto the wingends, which were modified into a V-shape by chang-ing the panel cuttings. By doing so, it was possible to eliminatesome useless netting while also fitting a third bridle at the joininglevel of the two faces. The extra bridle decreased the tension on the

Table 2Percent catch, revenues, and CPUE for trawl T1 (vessel Tricana de Aveiro) and trawl J1 (ves

Species Catch (%) CPUE (k

T. AveiroHorse mackerel 41 31.08Pouting 27 20.57Skate 11 7.83Octopus 6 4.71Blue whiting 3 2.55Large sc. gurnard 3 2.04

Total catch 75.54

J. MacedoOctopus 38 10.45Pouting 15 4.18Horse mackerel 14 3.74Small spotted dogfish 7 2.00Common squid 6 1.57Large sc. gurnard 5 1.31

Total catch 27.69

arch 93 (2008) 117–124

b-phases

Parameters registered during the sub-phases

r T, Qation between the fishing grounds T, Q, V, T (◦C), q

T, QT, Q, V, T (◦C), q

n events T, Q

(kn); T (◦C), average exhaust temperature; q, average fuel rate (l/h).

upper bridle and trawl headline, which favoured the vertical open-ing. Alterations to the cuttings were also performed in the panels atthe belly section, while the number of meshes at the codend joiningrow was made equal in order to match their widths.

Following the tests on the optimized model, a new full-scaletrawl J2 (Fig. 2) was tested at sea. Adjustments were made to thelength of the lower bridle during the full-scale trials in order tooptimize the mouth area. Performance comparisons of the twogenerations of full-scale trawls focused on the following criteria:first, the WS/VO ratio, as a measure of the amount of trawl flatten-ing over the ocean bottom. High ratios are characteristic of trawlsadapted to the capture crustaceans and benthic fish, while lowratios lend themselves to catching fish with higher vertical distribu-tions, such as horse mackerels; second, the mouth area (A), roughlyestimated as the product between the vertical opening and the win-

gend spread (that is, VO × WS); third, the trawl resistance, for whichvalues were obtained during the tests with the flume tank.

2.7. Economic analysis

The average parameter values were used as inputs whencomputing the economical analysis of each vessel’s trip. Theseparameters included the total distance covered per trip, D; the totaltrip duration, T; the number of hauls, n; the average duration ofhauling and setting, or Th and Ts, respectively; the total time navi-gating inside the harbour, Tp; the fuel rate during hauling, settingand harbour navigation, or qh, qs, and qp, respectively; the unit priceof fish, p; and the unit price of fuel, pf. Based on those parameters,it was possible to simulate the potential for fuel savings and thenet cash flow (NCF) variation, assuming different combinations ofthe navigation and trawling speeds and the corresponding values offuel rate during navigation and trawling, or qn and qt, respectively.The NCF for a fishing trip is computed as below (see Nomenclaturefor an explanation of the variables):

NCF = R − F − Cw − K, (1)

sel Joao Macedo)

g/h) Revenues (%)

Pouting 30Horse mackerel 25Skate 13Atlantic john dory 7Common sole 7Octopus 5

Octopus 34Common squid 21Pouting 14Horse mackerel 7Atlantic john dory 5Seabass 5

J. Parente et al. / Fisheries Research 93 (2008) 117–124 121

Fig. 2. Technical drawings of the J2 tr

where R is the total catch value per trip, F is the fuel costs per trip,Cw is the variable crew wages per trip, and K is the fixed costs pertrip. Additionally, we have

R = C × n × p, (2)

awl and rigging specifications.

where C is the total catch per haul. According to Dahle (1982), thiscan be simulated by

C = (Vt × 0.5147 × Tt × 3600 × A) × �f, (3)

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the flume tank indicate a reduction of approximately 15% in trawlresistance (within the range of commercial trawl speeds) when thenew trawl was tested (Parente, unpublished data). Overall reduc-tions in fuel consumption of 18 and 13% were obtained using thenew trawls for the Tricana de Aveiro and Joao Macedo, respectively(Table 5). These results hold at the towing speeds usually adoptedby the skippers (3.7 and 4.3 kn, respectively).

The NCF was determined based on the mean value of fish densityper unit volume (� f) estimated with data from the second commer-cial sea trial (Table 6). Both trawls were assumed to fish under thesame conditions, and the value of � f was assumed constant in orderto facilitate the comparison of economical results. A set of param-

122 J. Parente et al. / Fisherie

Table 3Operational parameters under two different working conditions during navigation,and corresponding fuel rate decrease obtained through speed reduction

FV Tricana de Aveiro FV Joao Macedo

A B A B

V (kn) 10.0 9.6 9.7 8.6q (l/h) 96 78 125 92T (◦C) 417 362 387 330

Speed reduction (%) 4 11

Fuel rate decrease (%) 19 26

‘A’ denotes original condition and ‘B’ denotes estimated critical speed.

where Vt is the trawling speed, Tt is the average trawling duration,and � f the fish density per unit volume. Here, � f was calculatedby considering the average parameters for C, Vt, Tt and A obtainedduring the second sea trials of both vessels.

The fuel costs per trip (F) and the crew wages per trip (Cw) wereobtained according to

F = m × pf × [(qt × Tt + qh × Th + qs × Ts)n + qn × Tn + qp × Tp], (4)

and

Cw = p × R, (5)

where m is the margin for lubricating oil costs, Tn is the total timespent in free navigation per trip, and p is the crew’s (fixed) percent-age of the total catch value.

The values for Tn and Tt were calculated according to the fol-lowing equations, where D is the total distance covered per trip infree navigation, V is the free navigation speed and TT(tr) is the totaltrawling duration per trip:

Tn = D, (6)

V

TT(tr) = T − Tn − n × (Th + Ts), (7)

and

Tt = TT(tr)n

. (8)

The objective was to maximize the NCF through the best com-bination of the navigation and trawling speeds, in order to havecomparable results for both trawl versions. The fixed costs (K) werenot considered since they were assumed constant throughout theanalysis.

3. Results

The first set of experiments estimated vessel performance dur-ing free navigation. Values obtained for V, q and T are presentedin Table 3 for the original working conditions (denoted by A) andfor the estimated critical speed (the speed beyond which the fuelrate increases sharply; denoted by B). A reduction in the navigationspeed alone leads to a decrease in fuel rate of up to 26% for thisphase.

Table 4Results obtained in full-scale trials for the J1 and J2 trawls

Speed (kn) VO (m) WS (m) WS/VO Mouth area (m2)

Full scale J1 J2 J1 J2 J1 J2 J1 J2

3.5 2.7 2.8 16.7 14.7 6.2 5.3 45.1 41.24.0 2.5 2.6 16.0 14.8 6.4 5.7 40.0 38.54.5 2.3 2.6 15.8 14.6 6.9 5.6 36.3 38.05.0 2.2 2.6 15.8 14.4 7.2 5.5 34.8 37.4

VO: vertical opening (m); WS: wingend spread (m).

arch 93 (2008) 117–124

Table 5Vessel data obtained from trawling experiments with the old and modified trawlsat the commercial average speed

Vessel Trawlingspeed (kn)

Exhaust temperature (◦C) Fuel rate (l/h) �%

J1 trawl J2 trawl J1 trawl J2 trawl

T. Aveiro 3.7 367 315 78 64 −18J. Macedo 4.3 390 365 120 104 −13

Table 4 presents the results from full-scale trials for both theJ1 and J2 trawls. The best results were achieved with a regulatorychain 1.5 m long, resulting in higher vertical openings (maximum of2.8 m at 3.5 kn). Wingend spread varied only slightly (between 14.4and 14.8 m) over the experimental speed range (3.5–5.0 kn) whileotterboard spread remained constant (between 70 and 71 m). Com-paring these results with those from the first experiments at sea(Table 4), we note that the wingend spread of the new gear is lowerwhile the vertical opening is higher, especially for higher trawlingspeeds. This potentially favours the capture of species that swimhigher in the water column. Gear resistance measurements are notavailable from full-scale trials; however, tension values obtained at

eters (also assumed to be constant) characterizing the activity ofeach vessel was obtained from the commercial sea trial (Table 6).The values A, qt and qn at different speeds, along with the above-mentioned parameters, were entered into equations (2) through(8) in order to estimate the values of R, F, and Cw. Finally, the cor-responding NCF values were determined. Figs. 3 and 4 display thetrends in the NCF as a function of the trawling speed (Vt) for a rangeof navigation speeds (V) within the limits specified in Table 6 foreach vessel and trawl version.

Table 6Characteristic parameters and speed variation for NCF determination

Parameters Tricana de Aveiro (J2 trawl) Joao Macedo (J2 trawl)

Used for � f determinationC (kg) 253.8 158.2Vt (kn) 3.7 4.3Tt (h) 2.964 3.516A (m2) 39.0 38.0

Characterizing each vessel tripD (n.mi.) 71 56T (h) 28.7 25.9n 5 5Th (h) 0.17 0.17Ts (h) 0.15 0.17Tp (h) 1.32 1.23� f (kg/m3) 31 × 10−5 15 × 10−5

qh (l/h) 50 80qs (l/h) 60 87qp (l/h) 80 114p (D /kg) 359 330pf (D /kg) 30 30

Speed variationV (kn) 8.4–10.0 8.1–9.7Vt (kn) 3.4–4.6 3.7–4.5

J. Parente et al. / Fisheries Rese

Fig. 3. Net cash flow (NCF) for J1 and J2 trawls as a function of the trawling (Vt) andnavigation (V) speeds for the ranges specified in Table 6. Vessel: Tricana de Aveiro.

Fig. 4. Net cash flow (NCF) for J1 and J2 trawls as a function of the trawling (Vt) andnavigation (V) speeds for the ranges specified in Table 6. Vessel: Joao Macedo.

3.1. FV Tricana de Aveiro

For a navigation speed of 10.0 kn, the NCF estimates (Fig. 3) arehigher for the J1 trawl at lower trawling speeds, with a highest valuefound at 3.7 kn (D 1604). This figure is similar for both trawl ver-sions at 4.1 kn, substantially increasing for the new J2 trawl at higherspeeds (where it decreases for the old J1 trawl), and reaching a max-imum at 4.6 kn (D 1769). According to these results, with both trawlsworking at their best operating conditions, a 10% increase in the NCFis obtained with the new J2 trawl (Table 7), despite the increase infuel costs resulting from the adoption of a higher (+0.9 kn) trawlingspeed.

Table 7Determination of NCF, fuel costs and percent variation

Tricana de Aveiro Joao Macedo

J1 trawl J2 trawl �% J1 trawl J2 trawl �%

V (kn) 10.0 10.0 9.7 9.7Vt (kn) 3.7 4.6 4.3 4.5F (D ) 366 416 +14 482 447 −7NCF (D ) 1604 1769 +10 383 487 +27

arch 93 (2008) 117–124 123

3.2. FV Joao Macedo

Similarly, for a navigation speed of 9.7 kn, the NCF values arehigher for the old J1 trawl at lower trawling speeds. The best resultsare found at 3.7 kn (D 433). The NCF is about the same for both trawlsat a speed of 4.0 kn, increasing for the new J2 trawl at higher speeds(where it decreases for the old J1 trawl), and reaching a maximum at4.5 kn (D 487; Fig. 4). The best operating conditions with the newJ2 trawl are attained with a slight increase in the trawling speedrelative to the original speed adopted by the skipper for the oldertrawl, leading to a 27% higher NCF (Table 7).

4. Discussion

This study demonstrated that significant improvement in fuelconsumption and net cash flow can be obtained in the short-termfor two Portuguese coastal fish trawlers. This benefit can also beobtained without the need for major changes in overall vessel tech-nology. Fuel savings of up to 26% were obtained by bringing thenavigation speed close to the ‘critical speed’. However, the latterfigure pertains to the navigation phase alone, and data from thecommercial sea trials showed that the time spent at this phase islow when compared to trawling. Even though the duration of thenavigation phase may vary substantially, since it depends heavilyon the strategy adopted by the skipper (such as the distance fromthe coast and time of navigation among fishing grounds, as dic-tated by the abundance of target species), it averages only 24% ofthe whole fishing trip. As such, the percentage of fuel consumed innavigation will always be substantially lower compared to trawling.

The trawling phase therefore emerges as the more importantphase for fuel reduction efforts. Simple changes at the trawl level(such as steeper cuttings in the wings and bellies, and mesh sizeincreases in the respective net sections) demonstrated fuel reduc-tions of up to 18%. Overall, simulations carried out to estimate theoperational conditions that maximized the net cash flow showedthat the NCF could increase up to 27%, with significant dependenceon the specific vessel.

Strong differences were observed in the NCF trends between J1and J2 trawls. This can be explained by the corresponding differ-ences in mouth area and net resistance, which in turn affect the

total catch per haul and the fuel cost per trip. The J2 trawl was moreefficient at higher trawl speeds, while the J1 trawl showed betterperformance than the J2 trawl at lower trawl speeds. In fact, theJ1 trawl mouth area decreased substantially with increasing speed,leading to reduced catches, while for J2 this parameter remainedapproximately constant within the range of commercial trawlingspeeds. On the other hand, the lower net resistance associated withthe use of the J2 trawl was the likely cause of the lower fuel con-sumptions, thus contributing to the increase in the NCF.

The efficiency gains in gear drag resulting from the introduc-tion of technical alterations to the trawls allowed an increase intrawling speed and therefore in ground coverage. This increasedthe fishing yield at the expenses of extra fuel consumption. Thistrade-off proved to be the best option to maximize the NCF as, inboth cases, the maximum NCF was achieved for trawling speedshigher than those originally adopted. Also, the potential savingsfrom bringing the navigation speed close to the ‘critical speed’ wereovershadowed by the advantage of navigating at a higher speed,2

thus freeing more time for trawling and therefore increasing catchyields.

2 However, it can be argued that the differences in the NCF (due to the differ-ence between the critical speed and the navigation speed adopted by both vessels)is small. This would not justify a higher navigation speed since it might increasemaintenance of the engine due from the higher loading.

124 J. Parente et al. / Fisheries Rese

Table 8Percent catch, revenues, and CPUE for J2 trawl: vessels Tricana de Aveiro and Joao

Campos, A., Fonseca, P., Fonseca, T., Parente, J., 2007. Definition of fleet components

Macedo

Vessel Species Catch(%)

CPUE(kg/h)

Revenues (%)

T. Aveiro

Pouting 37 31.72 Pouting 28European hake 19 16.27 European hake 24Horse mackerel 11 9.31 Octopus 10Octopus 7 5.80 Horse mackerel 7Little sole 5 4.12 Skate 4Large sc. gurnard 4 3.10 Common sole 4Common squid 1 1.01 Little sole 2Dogfish 1 0.61 Common squid 2Total catch 85.64

J. Macedo

Pouting 3 1.25 Pouting 2European hake 28 12.51 European hake 28Horse mackerel 16 7.39 Octopus 10Octopus 11 4.78 Horse mackerel 7Little sole 5 2.10 Skate 1Large sc. gurnard 8 3.47 Common sole 4Common squid 9 4.04 Little sole 4Dogfish 8 3.81 Common squid 24Total catch 44.99

At about 4.5 kn, the optimized trawl presented an overall moutharea only slightly higher than the original one, but favoured the

vertical opening (+13%) over the horizontal opening (−7.5%). Thesealterations to trawl geometry may be advantageous at high tow-ing speeds since it increases the trawl efficiency towards smallpelagic, fast-swimming species, while still allowing for simultane-ous capture of a number of benthic and demersal species. This isevidenced when comparing CPUE results in Tables 2 and 8. Also,the overall CPUE was higher for the new trawl in both vessels. Thenew trawl was maintained by both vessels after the conclusion ofthe project, which is a strong indicator of the skippers’ acceptanceof our modifications. Unfortunately, there was no possibility fora follow-up assessment of the skippers’ adherence to the vessel-operating conditions necessary to achieve the expected reductionsin fuel consumption.

At the time of the reported experiments, fuel consumptionissues were not so strongly felt as today. This has since changeddue to the continuous increase in fuel prices. There is currentlya growing awareness among the main stakeholders of the needto ensure economically balanced fishing companies, as well as agrowing awareness from the public for ‘greener’ fishing activities.The latter should be perceived not only in terms of stock sustain-ability, reduction of by-catches and discards, and seafloor impact

arch 93 (2008) 117–124

of gears, but also in terms of the broader perspective of energyefficiency and vessel emissions. In other words, the environmentalapproach should concern the entire ecosystem, rather than just themarine ecosystem. Consequently, the time is ripe for the adoptionof short-term, easily implemented, effective methodologies such asthose reported in this study. These should be further complementedwith the use of high strength synthetic fibres and reduced-dragotterboards, while also continuing research on fuel technology (bio-diesel and bio-lubricants) and higher efficiency engines.

Acknowledgements

The authors are most grateful to Jean-Claude Brabant and thestaff of the flume tank at IFREMER/Centre de Boulogne-sur-Mer fortheir contribution to the modelling and testing of the trawl gears.We also thank Peter Stewart, the referees and the editor, whosecomments greatly improved this manuscript. The work was par-tially financed by the European Union under EU project TE.2.408:‘Fuel Saving in Portuguese Trawlers’.

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