Elasmo GG Gilnet

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Preliminary data on elasmobranch gillnet fishery in the Gulf of Gabe` s, Tunisia

By K. Echwikhi1,2, B. Saidi1, M. N. Bradai1 and A. Bouain2

1National Institute of Sea Sciences and Technologies, Sfax, Tunisia; 2Sfax Faculty of Sciences, Sfax, Tunisia

Summary

Elasmobranchs are an important catch component in Tuni-

sian artisanal fisheries. Generally, species-specific information

is largely unavailable for artisanal fisheries; an increase in

knowledge thereof is essential to ensure proper management

of these species. The study analyzed the gillnet fishery elas-

mobranch catches in the Gulf of Gabes, whereby 45 fishing

trips were conducted from April to June 2007 and 2008 for

the capture of a total of six species: three Rajiformes andthree Carchariniformes. Rhinobatos cemiculus was the most

important species (52%; 4.588 ind km2 net per day).

Among shark species, Mustelus mustelus was the most impor-

tant (66.8%; 2.21 ind km2 net per day). Discards repre-

sented 6.87% of total catch in weight and 14.28% in number

of specimens caught. Size composition of captures varied by

species, but usually mature, mainly gravid females were

abundant. Further investigations are needed to obtain more

information on such fragile species for the development of

protective measures.

Introduction

Due to their typically vulnerable life cycle characterized by

slow growth rates, late maturity, and low fecundity (Corte s,

2000), elasmobranch fishes are highly susceptible to fishing

mortality. In these animals, overexploitation can occur even

with low levels of fishing mortality (Stevens et al., 2000);

once they begin to decline, populations may need decades to

recover (Anderson, 1990).

Industrial fisheries have been highlighted as a major source

of bycatch and mortality for elasmobranchs (Baum et al.,

2003); however, artisanal fisheries represent a considerable

portion of global shark and ray landings (Bonfil, 1994;

Vannuccini, 1999). Artisanal fishery exploitation can greatly

affect the abundance and size composition of long-lived spe-

cies such as elasmobranchs (Pinnegar and Engelhard, 2008).Although essential for the monitoring of exploited popula-

tions and the development of effective management plans,

species-specific information is unavailable for artisanal elas-

mobranch fisheries (White et al., 2006; Henderson et al.,

2007).

In the Mediterranean Sea, few elasmobranchs are subject

to direct fishery; local fisheries land elasmobranchs mostly as

by-catch (Fowler et al., 2005). Elasmobranchs currently rep-

resent 0.78% of the total landings in the Mediterranean Sea

(FAO-FIGIS, 2006). Between 1970 and 1985, landings of

elasmobranch fishes in the Mediterranean increased from

10 000 tonnes (t) to 25 000 t. Subsequently, reported land-

ings declined to 1000 t by 2004 (FAO-FIGIS, 2006).

In Tunisia, the annual catch of elasmobranchs is about

2000 t, of which over 62% (over 1200 t/year) were from the

Gulf of Gabe` s where elasmobranchs are landed as either

bycatch or target species. Artisanal fisheries targeting elasmo-

branchs are seasonal; they operate in springsummer when

species move to shallow waters. The target species are

Carcharhinus plumbeus, Mustelus punctulatus, M. Mustelus,

Rhinobathos cemiculus and R. rhinobathos (Bradai et al.,

2005a). Although elasmobranchs are a significant component

of the Gulf of Gabe` s fisheries, there is no quantification of

the retained or discarded portion, which is never accountedfor official fishery statistics.

Gillnets are well represented in the Tunisian artisanal fish-

eries and are highly selective for those species captured

mainly by gilling (i.e. captured behind the gill cover) or

wedged (being held by a mesh around their maximum body

girth). Because of the unregulated nature of most gillnet fish-

eries, the information describing their operational character-

istics and associated bycatch in the Mediterranean Sea is

limited. This study is the first field assessment of the ecosys-

tem impact of the small scale fleet using gillnets and operat-

ing in the Gulf of Gabe` s (southern Tunisia; central

Mediterranean Sea) that currently counts among the infre-

quent fisheries targeting elasmobranchs in the entire Mediter-

ranean basin. Specific project objectives were to:(i) determine the species composition and catch per unit

effort (CPUE); and (ii) provide biological information (size

distribution and sex ratios).

Materials and methods

In the Gulf of Gabe` s, gillnets targeting sharks and guitarfish

are used in the spring and at the beginning of summer

(March to July). Gillnets are constructed of polyamide

monofilament netting and consist of several pieces of net

(panels) tied together to form a single curtain positioned

upright in the water. These nets are fixed to the bottom by

means of anchors or ballast sufficiently heavy to neutralizethe buoyancy of the floats. During the fishery season, gillnets

are checked daily and cleared of catch, or pulled and reset.

The investigation was carried out from April to June 2007

and 2008, onboard four artisanal boats associated with the

ports of Zarzis and Jerba (Fig. 1). Although mesh sizes from

14 to 16 cm were observed in the study area, bottom gillnets

consisted of 14 cm mesh and varied between 2500 and

3000 m in length and 2.5 to 3 m in height. Soak time could

reach from two to five days, depending on weather condi-

tions. For each set, the length and height of the gillnet and

the station data (time of set and retrieval, position, depth

and course) were recorded. After retrieval and untangling of

specimens from the nets, fishermen decided whether the fish

were retained or discarded. All individuals were identified

following Compagno (1984) and Fischer et al. (1987) and

J. Appl. Ichthyol. (2012), 16 2012 Blackwell Verlag, BerlinISSN 01758659

Received: February 28, 2012

Accepted: March 19, 2012

doi: 10.1111/jai.12022

Applied IchthyologyJournal of


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total length (TL), disc width of batoids (DW) (to the nearest

mm) and weight (to the nearest 5 g) were recorded. Dis-

carded individuals were processed onboard and returned tothe sea. The maturity status and stages (juveniles and

mature) of specimens were determined based on previous

studies conducted in the area (Sadi et al., 2005; 2008, 2009;

Enajjar et al., 2008, 2012).

There is no generally recognized standard measure of catch

per unit effort (CPUE) for nets. Given this, the CPUE was

chosen taking into account as many parameters as possible.

To assess the relative importance of details such as total net

area and soak time for quantifying the interaction of differ-

ent species with gillnets, CPUE 1 was calculated as the num-

ber of specimens captured per km2 of nets per day:

CPUE 1 n=S d

Where n is the total number of specimens caught, S is the

total surface area of gillnets used in the fishing operations

expressed in km2 and d is the total duration of soak time

expressed in number of days.

In addition, the number of each species captured per set,

CPUE 2, was calculated:

CPUE 2 n/N

Where N is the total number of sets. Then the total cap-

ture was estimated using CPUE 2 and the total number of

fishing sets conducted by the gillnet fishery in the study area

(ET) obtained from the DGPA (General Directorate of Fish-

ing and Aquaculture).

CT CPUE 2 ET

All measured specimens were utilized to determine size

composition and sex ratio of capture. Size data were evalu-

ated using two-tailed non-parametric MannWhitney U tests

(Zar, 1996). Additionally, the assumption of equal sex ratios

(1:1) was tested using chi-square analysis with Yates correc-

tion for continuity (Zar, 1996).

Results

During the study period, 45 fishing sets were analyzed corre-

sponding to a soak time of 108.8 days and a total gillnet sur-

face of 0.627 km2 (Table 1). A total of 602 specimens

belonging to six elasmobranch species were caught: three

Rajiformes and three Carchariniformes (Table 2). All speci-

mens captured were identified to the species level, except for

three individuals that could only be identified as Raja spp.,

due to advanced decomposition. Batoids dominated gillnet

capture, accounting for 62.46% of the overall recorded catch

number.

The blackchin guitarfish, Rhinobatos cemiculus, was the

most abundant species, comprising 52% of catches (4.588

ind km2 net per day), followed by the common smooth-

hound Mustelus mustelus (25.08%; 2.213 ind km2 net per

day), as described in Table 2. Considering values of biomass

caught, R. cemiculus was the most important species com-

prising 62.24% of total weight, followed by M. mustelus

(16.15%) (Table 2). Total captures were estimated (Table 3)taking into account the annual total fishing effort (554.5 sets)

and catch rates (CPUE 2) in the study area.

The main elasmobranch portions caught were retained

(85.72% in number; 93.12% in weight) (Table 2). Among

retained captures, a small portion was for own consumption

(7.22% in number; 0.923% in weight) of fishermen, with the

remainder landed. Discarded elasmobranchs represented

14.28% in number of individuals caught (Table 2). Main rea-

sons for target species discards were damaged conditions

when disentangled (63.95%) and size (10.46%); species with

low or no commercial value, such as Raja radula, were

always discarded (25.58%) (Table 2). Captures in poor con-

dition, e.g. entangled individuals showing advanced decom-

position, were found especially after stormy days (which led

to higher soak times since fishermen failed to retrieve nets

during these periods). Although less important, fish size also

seemed to contribute to discards in that discarded specimens

were generally smaller than retained individuals.

Species-specific size and sex composition were available for

all recorded elasmobranchs. Specimens were often examined

prior to a vessels arrival in the harbor. The observed sex

ratio of R. cemiculus (n = 313) differed significantly from a

1 : 1 female-to-male ratio (v2 = 54.827, d.f. = 1, P < 0.05)

(Fig. 2; Table 4). Size composition of females and males did

not differ significantly (U = 9821.500, P = 0.700). More than

40% of females and 70% of males captured were mature

(Table 4).Of the 41 R. rhinobathos examined, females were signifi-

cantly more common than males (v2 = 4.12, d.f. = 1,

P = 0.04) (Fig. 2; Table 4). Size composition of females and

males did not differ significantly (U = 172.500, P = 0.64).

Among captured individuals, 48% and 57% of females and

males, respectively, were mature (Table 4).

The examined size range of M. mustelus (n = 151) was 620

1580 mm TL (Fig. 2; Table 4). Females were significantly

more abundant, comprising 60.20% of the total number of

individuals documented (v2 = 6.364, d.f. = 1, P = 0.011).

Females were significantly larger than males (U = 1369.500,

P < 0.0001). All females and 90% of males captured were

mature (Fig. 2; Table 4).

The recorded size composition of M. punctulatus ranged

from 530 to 1190 cm TL (Fig. 2; Table 4). The number of

Fig. 1. Gulf of Gabe` s, Tunisia

Table 1Fishing effort in the study area

2007 2008 Total

Number of sets 17 28 45Total surface of gillnets (km2) 0.181 0.445 0.627Total duration of soak time (days) 35.94 72.9 108.8

2 K. Echwikhi et al.


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females and males did not depart significantly from a 1 : 1

ratio (v2 = 0.75, d.f. = 1, P = 0.38). Females were signifi-

cantly larger than males (U = 69.500, P < 0.0001). Mature

females (70%) dominated captures, while males were mainly

juveniles (71.43%) (Fig. 2; Table 4).

Carcharhinus plumbeus individuals ranged from 1310 to

1980 mm TL; size composition of females and males did not

differ significantly (U = 61.500, P = 0.237) (Fig. 2; Table 4).

The overall male to female ratio was not significantly differ-

ent from 1:1 (v2 = 1.81, d.f. = 1, P = 0.178). Most captured

specimens were mature: 70% of males and 60% of females

(Fig. 2; Table 4).

Female R. radula outnumbered males within sampled

catches, which consisted primarily of individuals >220 mm

DW (Fig. 2; Table 4). The sex ratio was not significantly dif-

ferent from 1 : 1 (v2 = 2.909, d.f. = 1, P = 0.08). Size com-

position of females and males differed significantly

(U = 21.000, P = 0.011). The majority of specimens, i.e. 80%

of the females and 57% of the males, were mature.

Discussion

This study represents to date the most detailed evaluation of

elasmobranch capture in the Gulf of Gabe` s. Previous investi-

gations were either limited in scope or generalized, preclud-

ing a detailed understanding of fishery characteristics and

species-specific data (Bradai et al., 2005a).

Results show that R. cemiculus is the common component

of gillnet elasmobranch fisheries in the Gulf of Gabe` s. Previ-

ous information reported that this species was the most

abundant ray species landed in the area, where it was caught

between April and August (Bradai et al., 2005a). According

to catch statistics from the Zarzis fish market, which is the

largest landing port for shark fishery in Tunisia, annual land-

ings by target fisheries of this species are about 250 t (Enajjar

et al., 2012). Because of its shape and schooling behavior,

R. cemiculus appears to be especially susceptible to capture

in bottom gillnets.

Although C. plumbeus is considered to be a target species,

its low catch rate compared to others species could be related

to the fishing sites. In fact, the fishing gear types operate in

diverse habitats (at different depths), where the relative abun-

dances of species may be different. The importance in num-bers and catch rates of Mustelus species reflect their

abundance in the fishing area. These species appear to be

more abundant in the study area, compared to the northern

coasts of the Mediterranean where they are considered as

extirpated locally (Aldebert, 1997; Jukic-Peladic et al., 2001;

Ferretti et al., 2005). Indeed, specific elasmobranch fisheries

already subsist, showing an important exploitation potential.

In addition, their abundance suggests that Mustelus spp. in

the Gulf of Gabe` s find the conditions to reproduce and

develop for consideration as a nursery area (Bradai et al.,

2005a). Diversity of catch composition of gillnet fishery

probably reflects the highly opportunistic tendency of fisher-

men and the mixed-species nature of the fishery. Fishermen

tended to target whatever demersal elasmobranchs were

locally present and retained all, or nearly all, of the species.

In the present study, discards accounting for 14.28% of

the elasmobranch catch were higher than those recorded in

southern Portugal (5.4%) (Coelho et al., 2005) and lower

than on the Portuguese west coast (24.8%) (Baeta et al.,

2010). Differences between discard rates could be related to

local species diversity, environmental constrains, social-eco-

nomic aspects or simply due to random factors (Batista

et al., 2009). Nevertheless, the choice of fishing grounds,

reduction in the soak time as well as the use of larger mesh

sizes may reduce bycatch and discards (Goncalves et al.,

2007). A decrease in soak time during the warmer months

could also contribute to a higher quality of landings, sincefish and invertebrates degrade faster at higher water tempera-

tures and the small fleet does not have refrigerated storage

onboard.

In Tunisia, catch statistics lack information on species

composition, even for the most important elasmobranch spe-

cies in the capture, and only catch trends can be assessed.

Annual elasmobranch production from the Gulf of Gabe` s,

considered as the largest in Tunisia, showed a steady increase

during the late 1990s. The highest annual catch of 1650 t

was attained in 2002. From 2003, landings decreased, falling

to 950 t in 2007 (Fig. 3). The same trend was reported for

shark and batoid production in Zarzis, where gillnet activi-

ties were the most developed (Fig. 3). In fact, more than

70% of elasmobranchs were landed by artisanal fisheries.

The concentration of fishing effort on selected species (and/

Table 2Elasmobranch catches in gillnet fishery. Total catches, discards and relative importance of each species captured (percentages in brackets).CPUE 1 (per km2 of net per day)

Number Weight

N (%) CPUE 1Retained(%)

Discarded(%)

W (kg)(%)

Retained(%)

Discarded(%)

Rhinobatos cemiculus 313 (52.00) 4.588 287 (91.69) 26 (8.30) 2850 (62.24) 2715 (95.26) 135 (4.73)Rhinobatos rhinobathos 41 (06.81) 0.601 34 (82.92) 7 (17.07) 320 (6.98) 275 (85.93) 45 (14.06)Mustelus mustelus 151 (25.08) 2.213 134 (88.74) 17 (11.25) 740 (16.160) 676 (91.35) 64 (8.64)Mustelus puctulatus 48 (07.97) 0.703 39 (81.25) 9 (18.75) 285 (6.22) 253 (88.77) 32 (11.22)Carcharhinus plumbeus 27 (04.49) 0.395 22 (81.48) 5 (18.51) 375 (8.187) 345 (92) 30 (8)Raja radula 22 (03.65) 0.322 00 (00) 22 (100) 9 (0.19) 00 (0) 9 (100)Total 602 8.824 516 (85.71) 86 (14.28) 4580 4265 (93.13) 315 (6.87)

Table 3Catch rate (CPUE 2) and estimated total capture of different species

Species

Numberofspecimens

CPUE 2(Individualsper set)

Totalcapture(Individuals/year)

R. cemiculus 313 6.95 3856.85R. rhinobathos 41 0.91 505.21M. mustelus 151 3.35 1860.65M. puctulatus 48 1.06 591.46C. plumbeus 27 0.6 332.70R. radula 22 0.48 271.08

Elasmobranch captures in Tunisian waters 3


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or specific maturity stages, mainly adults) could have adverse

consequences, such the decreasing trend observed in elasmo-

branch landings in the Gulf of Gabes.

A visual check of histograms of the size distribution for

females, except Rhinobatids, showed that mature specimens

dominated captures. For males, with the exception of

M. punctulatus, mature individuals also constituted the

greater proportion of the catches. Neonates and small size

classes were absent; possibly because of gillnet selectivity for

larger specimens. Size composition of Rhinobatids consisted

of a mixture of mature and medium-sized individuals. The

two Rhinobatid species show significantly different sex ratios:

females dominate, undergo seasonal movements, and are

especially abundant during warm water periods, often for

breeding and nursery functions (Enajjar et al., 2008, 2012).

In the Gulf of Gabes, fishermen target aggregations of gravid

females when they migrate to shallow coastal and insular

waters to pup and breed (Bradai et al., 2005a; Enajjar et al.,

Fig. 2. Length-frequencies distribution of elasmobranch species caught by gillnet, Gulf of Gabe` s, 20072008. [Size at maturity of males ( )and females ( )]

Table 4Size composition and importance of mature individuals caught by gillnet fishery, Gulf of Gabe` s, 20072008: (a) Enajjar et al., 2012; (b) Enaj-jar et al., 2008; (c) Sadi et al., 2008; (d) Sadi et al., 2009; (e) Sadi et al., 2005; (f) Capape , 1974.

Species Sex Number

Size range(mm)

Mean SD Size at maturity

Percentage in catch

Min Max Juveniles Mature

R. cemiculus F 222 520 1730 1278.42 323.17 1381 (a) 127 (57.20) 95 (42.80)M 91 550 1720 1265.82 320.14 1118 (a) 28 (30.77) 63 (69.23)

R. rhinobathos F 27 530 1180 817.40 204.91 790 (b) 14 (51.85) 13 (48.15)M 14 550 1120 846.42 211.42 700 (b) 6 (42.86) 8 (57.14)

M. mustelus F 91 1220 1550 1364.40 103.65 1172 (c) 0 (0) 91 (100)M 60 620 1340 1208.50 179.90 971 (c) 6 (10) 54 (90)

M. puncutulatus F 27 640 1160 1011.11 176.90 956 (d) 8 (29.62) 19 (70.38)

M 21 580 950 739.52 113.68 814 (d) 15 (71.43) 6 (28.57)C. plumbeus F 17 1310 1960 1781.76 159.14 1600 (e) 5 (29.42) 12 (70.58)

M 10 1420 1920 1656.00 254.61 1720 (e) 4 (40) 6 (60)R. radula F 15 220 420 350.00 60.47 340 (f) 3 (20) 12 (80)

M 7 220 370 295.71 57.40 320 (f) 3 (42.85) 4 (57.15)



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2008, 2012). Captured M. mustelus and M. punctulatus

included juvenile and mature individuals of both sexes. As

indicated by the size frequency distribution of both species,mature females were an important component of the elasmo-

branch gillnets capture, whereas juveniles were largely com-

posed of males. Mustelus spp. dominated shark landings in

this region in the spring, when females moved to near shore

waters to reproduce (Sadi et al., 2008, 2009). The capture of

sandbar sharks consisted especially of mature, usually near-

term pregnant females that apparently approached the coast

to give birth in a nursery area with advantageous environ-

mental conditions (Bradai et al., 2005b; Sadi et al., 2005).

Records of sandbar sharks were reported throughout the

Mediterranean, but the species seemed to be more abundant

in the southern coasts, where it found favorable conditions

for reproduction (Bradai et al., 2005b; Sadi et al., 2005).

In the Gulf of Gabe` s, gillnet fisheries are mostly restricted

to shallow waters. This activity is related to the timing

migration of gravid females to nearshore waters to pup and

breed (Bradai et al., 2005a). Investigations conducted along

Tunisian coasts suggested that the Gulf of Gabe` s could con-

stitute a nursery for some elasmobranch species (Bradai

et al., 2005a,b). The targeting of gravid females of exploited

species reduces the productivity, resilience, and sustainability

of such populations (Kokko et al., 2001; Smith et al., 2008).

In addition, Simpfendorfer (1999) suggested that, for some

elasmobranch species, sustainable exploitation might be pos-

sible if the youngest age-classes were targeted and older ages

left unfished.

This study provides the first detailed, quantitative informa-tion on the artisanal elasmobranch fisheries in the Gulf of

Gabe` s, one of the most productive areas of Tunisia in terms

of elasmobranch landings. Captures of elasmobranchs were

substantial during the survey years and probably represent a

considerable source of mortality for exploited populations.

The increasing domestic demand for elasmobranch fishing

permits coupled with the lack of regulatory actions poses a

threat to the elasmobranch stocks harvested in Tunisian

waters.

Successful management of elasmobranch fisheries is com-

plicated by a general lack of fishery information and life his-

tory characteristics of this k-selected group of fishes (Musick,

1999). Conservation of these populations is further con-

founded by the delayed responses in the implementation of

fishery restrictions, typically resulting in the implementation

of management strategies following the overexploitation of

targeted elasmobranch populations (Stevens et al., 2000).

In the study area, for gillnet fisheries an effective manage-

ment plan might include the closure of nearshore and inshore

waters to gillnetting during the migration of gravid females,

or the establishment of mesh-size requirements to target spe-

cific population segments. Successful conservation and sus-

tainable use of this fishery resource requires action:(i) continuance of the collection and survey of species-specific

information on catches and landings, by number and weight;

(ii) improvement in statistical data collection at a specific

level; (iii) delineation of the nursery area in the Gulf of

Gabe` s; (iv) assessment of the impact of fishing mortality on

juveniles and gravid females in elasmobranchs/nursery areas;

and (v) socio-economic studies of the coastal artisanal

fisheries.

Acknowledgements

We wish to thank all fishermen who allowed us to operate

on their boats, all of the crews, as well as the scientists and

onboard observers who participated in this work. We thankanonymous reviewers for their insightful comments, which

led to a much-improved manuscript.

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Authors address: K. Echwikhi, National Institute of Sea Sciencesand Technologies, P. O. Box 1035, 3018 Sfax,Tunisia.E-mail: [email protected]


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