MARINE ECOLOGY PROGRESS SERIES Published November 17 Mar Ecol Prog Ser
An experimental study of gill net and trammel net 'ghost fishing' off the Algarve (southern Portugal)
'Unidade de Ciencias e Tecnologias dos Recursos Aquaticos (UCTRA). Universidade do Algarve, P-8000 Faro, Portugal 'IPIMAR CIM-Sul, P-8700 Olhao. Portugal
ABSTRACT. Four 100 m lengths of both monofilament gill nets and trammel nets were deployed at depths between 15 and 18 m off the coast of the Algarve (south of Portugal) between April 1995 and June 1996. The nets were set on a natural rocky bottom with one end cut loose to simulate lost nets. Changes in net structure (net height, effective fishing area, movement, colonisation, wear and tear) and their catches (species, sizes, numbers, and biomass) were monitored by divers. Similar patterns were observed in all the nets, with a sharp decrease in net height and effective fishmg area, and an increase in visibility within the first few weeks. Net movement was negligible except in the case of interference from other fishing gears. Catch rates were initially comparable to normally fished gd.l nets and trammel nets in this area, but decreased steadily over time. No sea birds, reptiles or mammals were caught in any of the 8 nets. Catches were dominated by fish (89% by number, at least 27 species), in particular by sea breams (Sparidae) and wrasscs (Labridae). Under the conditions experienced throughout the study the fishlng hfetime of a 'lost' net is between 15 and 20 wk. Based on an exponential model, we estimated that 100 m lengths of gill net and trammel net will catch 314 and 221 fish respectively over a 17 wk period. However, we consider this to be an underestimate due to high rates of predation and scavenging by octopuses, cuttlefish, moray eels, conger eels, and other fish such as the wrasse Coris julis. When the nets were surveyed in the following spring. 8 to 11 mo after being deployed, they were found to be completely destroyed or heavily colonised by algae and had become incorporated into the reef.
KEY WORDS: Lost gear. Ghost fishing. Gill net. Trammel net. Catches. Incidental mortality
INTRODUCTION a fishing gear to continue to fish after it has been lost and the fisherman has lost control. For example, Breen The impact of lost fishing gear on the environment (1987) estimated that ghost fishing traps caught an has aroused considerable concern in recent years amount equivalent to 7% in weight of the reported (Laist 1995). Of particular concern is the danger posed commercial catch of Dungeness crab. Similarly, lost to rare and endangered species of marine mammals, fish traps were estimated to catch a quantity equi- sea birds and turtles (Degange & Newby 1980, Hen- valent to 3-13.5% of the total Kuwait landings derson 1984, Millner 1985, Carr 1987, Perrin et al. (Mathews et al. 1987), and the decline in Gulf of 1994). It has been estimated that millions of sea birds St. Lawrence Queen crab has been partially attributed and thousands of turtles and marine mammals may die to mortality associated with lost crab traps (Blois 1992). each year when entangled or trapped in lost gear In addition to being a source of mortality, lost fishing (Colema et al. 1983). In commercial fisheries, inciden- gears have the potential to interfere with normal fish- tal mortality due to gear over which the fisherman has ing and diving operations and may even be a major lost control, may be highly significant (Chopin et al. cause of further gear loss. Lost gears may also have 1995). 'Ghost fishing' can be defined as the capacity of negative impacts on the environment, particularly in terms of habitat degradation. Both types of detrimental impact have been observed in Algarve (south Portu-
0 lnter-Research 1997 Resale of full article not permitted 258 Mar Ecol Prog Ser 158: 257-265, 1997
gal) waters, with complaints from commercial fisher- ence of natural reef bottom, permitting the simulated men about the amount of lost gear in certain areas, and loss of nets snagged on the bottom (according to local claims that artificial reefs have become so heavily fishermen, the scarcity of hard bottom in Algarve fouled with lost nets that reef fish may have reduced waters results in concentration of fishing effort around access. such reefs, leading to a concentration of lost gear); While the possibility of partial or total gear loss exists (4) proximity to artific~alreefs, permitting surveillance, in all fishing activities, certain types of gears, in par- and reducing the likelihood of interference from local ticular 'active' gears such as trawls and seines, are not fishermen; (5) existence of fish community data, allow- a major source of concern as their catching ability is ing interpretation of experimental results (Erzini et al. negligible or non-existent once lost. On the other hand, 1996a, b, Santos et al. 1996). 'fixed' or 'static' gears, such as gill nets, trammel nets, The 2 experimental sites are very similar, situated and traps, may continue to fish with significant effi- approximately 1 mile off the Praia de Faro, southern ciency, at least in the short term (Carr et al. 1992, Portugal (36'58' to 36'59'N and 7'59' to 8" 00' W), Kaiser et al. 1996). and less than half a mile apart. Both are rocky out- In southern Portugal, gill nets and trammel nets are 2 crops protruding from a sandy bottom, 200 to 300 m in of the most widely used gears. In 1993 the Algarve length and 50 m wide at the most. Maximum depth is commercial fishing fleet consisted of 3414 boats of approximately 18 m at the base of the reefs, with a which 2996 belonged to the 'local' category (
tem) receiver was used to locate the experimental sites. catches for the experimental gill and trammel nets For net groups A, B, and C, monitoring consisted of 3 over a 17 wk period. This period was chosen since consecutive days of diving, while monitoring took catches after 15 to 20 wk were negligible. place over 2 d in the case of net group D. Estimated lengths were converted to weights using On the first day, 2 divers swam along the gill net weight-length relationships (W = aLb) determined for while the other pair did the same for the trammel net. the species in the Algarve or in Portuguese waters At intervals of 5 m, corresponding to the spacing of the (Canario et al. 1994, Goncalves et al. 1997).For species numbered floats, the height of the net was measured for which weight-length relationships were not avail- as the vertical distance between the leadline and the able from this region, published equations from other headline (Kaiser et al. 1996). All organisms caught areas were used. in the nets were tagged with small aluminium tags (Kaiser et al. 1996). For each organism, the following data were recorded: species, total length to the cm RESULTS below, the state of the catch, and the sector of the net (i.e. between float numbers i and i + 1. A 4 point scale Due to bad weather and/or poor visibility, it was not was used to classify the state of the catch: (1) uniden- possible to obtain data for net groups A and B during tifiable remains, (2) decomposed but identifiable, (3) the first 2 wk after deployment. In general, the same recently dead, and (4) still alive. patterns were observed in both types of nets and for all This procedure was repeated the following day, with 4 groups of nets. Net height decreased dramatically the exception of the measurement of net height. Photo- within the first few weeks after deployment and then graphs were taken of the sections of net between floats stabilised or decreased gradually thereafter (Fig. .l); 3 and 4, 9 and 10, and 16 and 17. Direct observations of For gill nets the mean net height decreased approxi- the degree of wear (i.e.abrasion or tears in the netting) mately 50 % within the first 3 wk, while the reduction and net movement were also made. in trammel net height was between 60 and 70 % during On the third and final day of each sampling period, the same period. Both gill net C and trammel net C, the procedure of the previous day was repeated, which were set at the end of August, decreased more replacing photography with a video recording of the sharply in height than net groups A, B, and D which entire length of each net. In the case of net group D, as were deployed in April and June. monitoring was reduced to 2 consecutive days, 1 rather The reduction in height of the nets led to a rapid than 2 estimates of 24 h catch rates was made. decrease in the effective fishing area after deployment Effective fishing area, catches and 24 h catch rates. of the nets (Fig. 2). For example, net area decreased to Using the height of the net at each float, the approxi- 50% after 1 wk for gill net C, while the area of trammel mate fishing area of each net was estimated using the net C decreased to 39%. After 17 wk, gill nets A and B following formula: and trammel nets A and B were at 21 and 45 %, respec- tively, of their original area. Considerable variation in A,, ,+l = D X (H, + 2 H,+,) 1 (1) net height along the nets was observed throughout where A,, ;+, is the area between floats i and i+l, D is the study. 5 m, and H, and H,,, are distances between the head- Net visibility increased sharply as particulate matter line and the leadline at floats I and i+l. The total area, and detritus accumulated on the monofilament A,, is the sum of all the areas between floats and can be (Fig. 3a). Over time the nets were colonised by various used as an index of the fishing capacity of the experi- species, primarily macrophytes, which after 3 mo com- mental net: pletely blocked the meshes of some parts of the nets. Netting which came into contact with the reef became A,, At = C ,+l (2) heavily overgrown and blended into the background, Tagging of catches on the first and second day making identification difficult. Nets which were sur- allowed animals captured over a 24 h period to be veyed after the winter, less than a year after being recorded. In order to evaluate the long-term impact deployed, were completely destroyed. Only sections of of the experimental nets on both target and non- the leadline and some netting and floats could be target species, exponential models were fitted to the found. These remnants of net that were found were pooled daily catch data in numbers (N, = Noe-'l) and completely colonised by biota (Fig. 3b). in weight (B, = Bee-'l). N, and B, are the 24 h catches Considerable net damage was observed over time, in numbers and weight (g), No and B, are the inter- with large holes in the netting after 3 to 4 mo. For cepts, r is the parameter defining the rate of decrease, example, the leadline, headline and part of the netting and t is the number of days after deployn~ent.The of gill net B were completely torn after becoming above equations were used to estimate the total entangled with fishing gear, most probably a longline. 260 Mar Ecol Prog Ser 158: 257-265,1997
Trammelnet A and B Gill net A and B ""~ ~ T
C 0 20 40 60 80 100 I?"
Trammelnet C Gill netC
Gill net D Trammelnet D
0 J t 0 l t 0 20 10 60 80 100 I20 0 20 40 60 80 100 I20 Days afterdeployment Daysafter deployment
Fig. 1. Mean gill and trammel net height (r2 SD) measured at 5 m intervals over a 120 d penod after net deployment for net groups A and B. C, and D
There was little net movement except at the loose ends, at the points where the nets had been torn through interaction with fishing gear, and at one of the fixed ends which we suspect had been lifted deliber- ately/accidentally by fishermen. The highest 24 h catch rates were made immediately after the nets had been deployed (Fig 4a, b), with gill nets catching more than the trammel nets. Initial catch rates were comparable to normally fished nets; for example, in an ongoing comparative fixed gear selec- tivity study (Erzini unpubl. data) an average of 8.9 fish per net were caught in 105 gill nets (45 to 50 m in length, 60 mm stretched mesh monofilament). Over time, the catches decreased gradually. However, even after more than 4 mo in the water, some fish and other organisms were still being caught. No marine mam- mals, sea birds, or reptiles were caught. Based on the 0 4 I exponential model, the estimated total numbers of fish 0 20 40 60 80 100 120 caught by 100 m lengths of gill net and trammel net Days after deployment during the first 120 d period after deployment were Fig. 2. Effective fishing area of the nets over a 120 d period respectively 344 and 221 (Table 1). after net deployment, with the fitted exponential models. Total catches in terms of weight also decreased in an (a) Gill nets, (b) trammel nets exponential manner over time for the gill nets Erzini et al.. G111 net and trammel net 'ghost fishing' 26 1
Fig. 3 (a) Gill net 4 wk after deployment, showing signs of wear and tear and Increased visibility due to accumulation of partlculate matter. (b) Remains of a gill net less than a year after deployment showing heavy lnfestatlon and colonisation
(Fig. 4c), although variability was greater in comparison while crustaceans and fish accounted for 10.1 and with catches in numbers. For the trammel nets there 88.8% of the catches respectively. Three species of fish was a less pronounced decrease over time (Fig. 4d). accounted for 41.9% of the total catch by numbers: The experimental nets caught a total of 39 species Diplodus bellottii, D. vulgaris, and Scorpaena notata. belonging to 23 families and 4 main groups: molluscs, In particular, Spandae, which are economically valu- gastropods, crustaceans and osteichthyes (Table 2). able, dominated the catch with 33 and 29% for the gill Only 3 individual gastropods were caught (0.6%), and trammel nets respectively. 262 Mar Ecol Prog Ser 158. 257-265, 1997
0 Net B
A Net C
0 20 40 60 80 100 I20 11 20 40 60 80 100 120 L)a!s after deployment Daysafter deployment
Fig. 4.24 h catch rates over a 120 d period after net deployment, with fitted exponential models. Catch in numbers for (a)gill nets and (b) trammel nets, catch in weight for (c) gill nets and (d) trammel nets
DISCUSSION The way in which the nets were deployed, entirely on a natural rocky bottom, with one end fixed and the The results presented represent a limited number other loose, may not be characteristic of normally lost of many possible scenarios of net loss. Due to a nets. Nets lost on soft bottom may have a very different number of constraints, nets were set in relatively evolution and impact. shallow water, corresponding to depths at the lower The effective fishing lifetime of a net lost under the end of the range normally fished by Algarve fisher- experimental conditions in the present study is a men. The relatively shallow depth has important maximum of 15 to 20 wk. During this period, catches implications in terms of changes in net structure and dropped sharply during the first few weeks, with a catch characteristics because of light penetration and gradual decline thereafter. The observed pattern is consequently the rates of colonisation by algae. In due to changes in net shape, increasing net visibility, addition, weather conditions and bottom currents decreasing net height and effective fishing area, and have a stronger influence at these relatively shallow increasing wear and tear, all contributing to a decline depths. in 24 h catch rate per 100 m of gill net or trammel net. The nets were set during periods of good weather As noted above, these results are highly site and sea- conditions. Thus we were not able to evaluate the son specific. Nets set in winter or in deeper waters or effect of storm conditions soon after deployment. Such on soft bottom would no doubt have evolved differ- conditions, which are common during the winter, ently, resulting in different catch rates and patterns. might have brought about considerably different Over the 15 to 20 wk, the cumulat~vecatch in num- changes in terms of the rate of net collapse, and con- bers and in biomdss may be considerable. Our esti- sequently in terms of the catches. mates based on daily catch rates should be considered
Table 1 Parameter estimates of exponential model [Y= a X exp(-r X days after deployment)] fit to net area and to daily catch in numbers and in weight
Net Variable a SE r SE MSE df
Gill Area 129.270 8.649 0.0105 0.00180 435.99 24 Trammel Area 69.760 3.145 0.0062 0.00102 68.12 25 Gill Catch (n) 18.160 1.294 0 0542 0.00710 4.13 2 0 Trammel Catch (n) 5.739 0.711 0.0204 0.00578 2.06 20 Gill Weight (g) 1742.107 284.945 0.0378 0.01214 233844.72 20 Trammel Weight (g) 326.420 156.795 -0.0103 0.00595 348388.61 19 Erzini et a1 Gill net and trammel net 'ghost flshing' 263
Table 2. Specles composition of the expenmental net catches
Group Famlly Specles G111 nets Trammel nets Total A and B C D A and B C D - - - - - Crustaceans Galatheidae Galathea str~gosa 1 1 hdalidae Map squ~nado 1 1 2 3 7 Portunldae Maciop~puspuber 1 I 2 Scyllandae Scylla~us arctus 13 4 8 5 10 4 44 Gastropods Cymatudae Chaionia lampas 1 2 3
Molluscs Volut~dae Cybl LI~olld 1 1 Sepiolldae Sepia off~cinal~s 2 2 Osteichthyes Ballstldae Balistes ca~ol~nens~s 1 2 2 5 Blennlldae Parablenn~usgattorug~ne 2 2 Call~onynlldae Call~onyrnuslyra 1 1 Carangidae Caranx rhonchus 1 3 1 5 Trachurus trachurus 8 7 1 1 1 18 Conger Conger conger 1 1 Gadidae Phycls phyc~s 3 1 4 Trysopterus luscus 2 2 1 5 Haen~uhdae Pomadaisys lnclsus 2 3 2 8 Labrldae Labrus merula 1 1 Symphodus balllon~ 3 4 18 1 27 Symphodus spp 5 2 7 Merlucc~dae i\4erlucc1us merlucc~us 1 1 Mugllldae Mugil spp 1 1 2 Mullidae fi4ullus surmuletus 1 4 5 Scombridae Scomber spp 1 1 Scom berjaponicus 1 9 1 1 12 Scorpaenldae Scorpaena notata 16 44 17 1 25 29 132 Serran~dae Serran us spp 2 2 Serranus cabnlla 3 6 5 2 2 18 Soleidae Buglod~ss~um luteum 1 1 Sparldae Dentex gibbosus 1 1 Dlplodus annulans 1 4 2 7 Dlplodus bellottl~ 14 15 23 1 53 Dlplodus cervlnus 2 2 Dlplodus puntazzo 3 1 4 Diplodus sargus 2 1 1 4 Djplodus spp 2 6 1 2 11 Diplodus vulgar~s 7 2 15 4 4 7 39 Pagellus acarne l 1 Sarpa salpa 1 2 5 11 19 Spondyliossoma cantharus 2 2 4 Not ]dentifled 1 29 14 2 20 6 72 Total 88 148 123 28 73 75 535
underestimates for a number of reasons. Based on our It is interesting to note that the nets eventually experience and observations, there inay be consider- became incorporated into the reefs acting as a base for able predation and scavenging of fish caught in the many colonising plants and animals The colonised nets. Thus some fish may be caught and disappear nets then provided a complex habitat which was completely from the net during the interval between attractive to many organisms For example, large marking of captured fish and quantification of new schools of juvenile fish were often observed in the I catches 24 h later. This is probably particularly true vicinity of these heavily colonised nets, which may pro-
I for soft-bodied fish such as red mullets (Mullus sur- vide a safe haven from predators l muletus). Important predators include octopuses, cut- To date, relatively few experimental studies of the I tlefish, conger eels, moray eels, and wrasses such impact and evolution of ghost fishing nets have been as Coris julis, which were often observed feeding on carried out In a parallel study 90 ni sections of gill and recently caught (live) fishes. Carr et al. (1992) also trammel nets were set inshore in Wales (Kaiser et a1 reported predation by a wrasse, the cunner Tautogo- 1996) These nets immediately caught large numbers labrus adspersus, on fish and lobsters caught in gill of dogfish, which caused the nets to collapse and the 24 h catch rate of fish and the vertical fishing area to 2 64 Mar Ecol Prog Ser
decrease dramatically. The considerable biomass of lost herring gill nets in British Columbia, Canada, con- decaying dogfish attracted large numbers of inverte- tinued to catch fish for 7 yr, while live fish were found brate scavengers, resulting in crustacean catches in 8 yr old g111 nets retrieved In Norwegian waters reaching a peak approximately 6 wk after the nets (D. M. Furevik pers, comm. 1996). Recent studies by had been deployed. the Irish Sea Fisheries Board [Bord Iascaigh Mhara Unlike our study, the specles composition of the (BIM) 19951, involving the deployment of gill nets catches changed over time, with 24 h fish catches inshore (15 to 18 m) and offshore (80 m), showed that approaching zero after 70 d, while invertebrates con- nets set in deeper waters suffered much less fouling tinued to be caught at low rates more than 4 mo after and consequently were more effective in fishing terms, the nets had been deployed. Based on the exponential with a greater long-term impact on fish and mammals. model fitted in both studies to 24 h catch rates against The scale of the ghost fishing problem is difficult to days after deployment, the gill nets in both studies assess given the uncertainties regarding the amount of caught similar numbers of fish during the first 120 d gear which is lost (Chopin et al. 1995). A number of after deployment, while the trammel nets in the Welsh studies have attempted to estimate the amount of lost study caught fewer fish (Kaiser et al. 1996). The equa- nets in a given area using ROVs (Remotely Operated tions reported by Kaiser et al. (1996) are the following: Vehicles) or by retrieving nets from the bottom with (1) fish (gill net), 1nN = 3.08 - 6.061ndays; (2) fish grappling equipment (Barney 1984, Carr & Cooper (trammel net), 1nN = 2.31 - 0.5171ndays; (3) crusta- 1987, Cooper et al. 1987, Carr 1988). Fosnaes (1975, in ceans (gill net), 1nN = 3.53 - 0.407lndays; and (4) crus- Breen 1990) estimated that 5000 Newfoundland cod taceans (trammel net), 1nN = 4.11 - 0.687lndays. Gadus morhua gill nets were lost annually. Way (1977) The results reported by Kaiser et al. (1996) are more reported retrieving 148 and 167 nets in 48.3 and 53.5 h comparable to those of Carr et al. (1992) who deployed of trawling with a grappling device over a 2 yr period. two 100 m sections of 130 mm stretched mesh gill nets Carr & Cooper (1987) estimated that there were 2240 at 20 m depth in Buzzards Bay, Massachusetts, USA, lost nets in a 64 mile2 area of traditional gill netting and monitored them over a 2 yr period. Skates and grounds. According to a more recent review of the dogfish, along with a number of finfish species, were Canadian Atlantic fisheries 8000 gill nets (2 % of all ac- caught in significant numbers early on, while lobsters tive gill nets) were lost per year on average up to 1992, and other crustaceans continued to be caught through- resulting in an estimated loss of between 3000 and out the study. 30000 t of ground-fish in 1992 (Anon. 1995, Chopin et Simulated lost pelagic or drift gill net studies have al. 1995). Globally, for all gill net fisheries, an estimate also been carried out (Gerrodette et al. 1987, Breen of a 1 "h loss rate per year has been suggested (Na- 1990). Gerrodette et al. (1987) monitored 113 mm mesh tional Resources Consultants, Inc. 1990. in Laist 1995). 9 m deep monofilament nets (50, 100, 350, and 1000 m The present study demonstrates that, under specific in length) and found that the nets collapsed soon after conditions, lost nets may continue to catch both target deployment and that relatively few fish or other organ- and non-target species for extended periods. However, isms were caught in the resulting bundle of netting. given the wide range of conditions in which fishing gear Mio et al. (1990) deployed 5 pelagic gill nets, each can be lost, and the likelihood that nets lost in deep 1200 m in length, and reported that after 4 mo all had water may have a longer effective fishing life-span, it collapsed, forming a large mass of netting. will be necessary to extend this study to take into con- A number of authors have reported on the long-term sideration important factors such as depth and bottom impacts of lost nets, particularly in deeper waters type. In conjunction with such studies on the fate and (Breen 1990). Way (1977) found that gill nets lost in impact of lost nets, the gear loss in commercial fisheries Newfoundland, Canada, waters continued to catch fish should be quantified. Useful approaches may include over several years, albeit at a decreased rate compared surveying fishing vessel captains and owners for records with tended nets. He also noted the attraction of crabs of incidents of lost nets in conjunction with towing for lost to fish caught in the nets; a finding which was fre- nets with grappling devices. Only then wdl it be possible quently reported in later studies taking place in tem- to fully assess the range and importance of unaccounted perate waters. High (1985) monitored pieces of lost or incidental mortality associated with ghost fishing nets. salmon gill net and found that, although nets were heavily overgrown with algae within a year, sea birds Acknowledgements. This work was funded in part by the EC and fish were caught over a 3 yr period, while crabs (DG XIV ref. 94/095). We thank all the participants of the continued to be tangled in the nets for a further 3 yr. FANTARED project, and in particular Jorge Gonqalves, Luis Bentes, Pedro Lino, and the staff of IPIMAR who contributed & Carr Cooper (1987) estimated that the catch rates of in many ways. We also thank Michel Kalser and 2 other lost gill nets in a specific Canadian fishery were 15% anonymous referees whose suggestions greatly improved the of those of active gill nets. According to Breen (1990) manuscript. Erzini et al.: Gill net and trammel net 'ghost fishing'
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Editorial respons~bility:Otto Kinne (Editor). Submitted: July 22, 1997; Accepted: September 9, 1997 OldendorfILuhe, Germany Proofs received from author(s): October 20, 1997