AN ESTIMATION OF THE FISH STOCK OF THE BAY OF FAMAGUSTA BASED ON RECORDED CATCH OF TRAWL FISHING

Ata Atun

 

 

AN ESTIMATION OF THE FISH STOCK OF THE BAY OF FAMAGUSTA BASED ON RECORDED CATCH OF TRAWL FISHING

 

 

Ata ATUN*

Faculty of Maritime Studies, Near East University

Nicosia, T. R. North Cyprus

 

Abstract-  An estimate of the fish stock of the bay of Famagusta was made based on the recorded amounts of fish caught by trawling, within an observation period of seven years and an analysis period of four years. 

Trawl fishing at the bay of Famagusta is made by traditional, locally-built wooden boats of about 15 m. length.  Trawling is performed on courses parallel to the coastline, closely following the isobaths lying between the 50 m.-100 m. depth range.  Since the species of fish caught are of 0.1 kg.- 5 kg. weight range in their adult form, it was arbitrarily assumed that about 70% of the fish population on the trawling track, mainly the hatchlings, escaped through the openings of the net, which is required not to be less than 24 mm. according to Coast Guard regulations.  The results were extended to estimate the weightwise stock of certain species of fish.

 

INTRODUCTION

            Although the fish stock of coastal areas of sea are a subject of discussion in everyday life, there is not a wealth of quantitative information on fish stocks of coastal fishing areas.  In order to estimate the fish stock of the bay of Famagusta on the eastern shore of the North Cyprus, the recorded daily catch of trawl fishermen were used, covering the years 1989-1995.  Fishing season spanned a period from September to May each year, covering a total of 85 days per year.  Systematic observations made by the authors has indicated that 65 distinct species existed in the waters fished[1], which was verified by the recent research performed by the Dokuzeylül University of Turkey[2].

 

            A geometrical-statistical approach for the estimation of the fish stock of selected species was used.  The area of sampling covers the 50-100 m. depth range, the shallower limit enforced by the Coast Guard and the upper limit set by the technical restrictions of the trawling gear used.  The results obtained for the depth range of trawling was extended to cover the entire depth range where the species lived, assuming that the probability of a fish to be at a given depth is uniformly-distributed.  The estimate of the fish stock was made on a weight basis, which can be extended to find the number of fish in the area in question.

 

            It should be stressed that the term “fish” of this research refers to marine animal forms of commercial value, not to fish in the biological sense.  As can be seen, shrimp (crustacean), octopus, cuttlefish and squid (cephalopods) are also referred as fish in this paper.

 

            EQUIPMENT USED AND THE AREA TRAWLED

            A trawl arrangement with two doors at 40 m. lateral distance apart and a horizontal net entrance width of 15 m. was used.  The distance between the doors and the net entrance is 120 m. in the trawling direction.  The trawl net mouth opening is nominally 2.20 m. high, providing a sufficient area for the bottom fish of the area to be caught.  The trawl net openings are 24 mm. by 24 mm. in size, also enforced by the Coast Guard.  A general sketch of the trawling gear used is shown in Figure 1.

           

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1 : Equipment used

 

The area trawled lies between the isobaths of 50 m.-100 m., between the coordinates of 35o 04’ 30”N to 35o 17’ 24”N and 33o 53’ 24”E to 34o 00’ 00” E.  (See the chart of Figure 2).  The total area swept corresponds to about 8 square nautical miles (27.44 km2).  At each sweep, a straight stripe of 15 m. width on the sea bottom, lying between 50 m.-100 m. isobath lines is swept.  The fish caught represents roughly 30% of the fish population of that stripe.  Trawling is assumed to be performed at a nominal speed of 2.5 knots.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2 : Location of Trawling  Area

           

 

GEOMETRIC DATAS

 

The cross section of the sea bed and accordingly the isobath lines are in a very uniform shape as in Figure 3.

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3 : The cross section of the sea bed.

 

The estimated fish distribution, based on the fact that the fisherman sweep the most populated area based on their past experience, but not the whole range allowed by the Coasty Guard as shown in Figure 4. Dependent on the season and randomly occuring underwater currents, the fisherman usually fish on the same spot with in the range of 100 metres for the best catch. 

 

 

 

 

 

 

 

 

 

 

Figure 4 : Estimated fish distribution, from rich to poor.

 

It is observed that the sweeping volume is consisted of  a cubic shape as shown in Figure 5.

 

 

   

 

 

 

 

 

 

 

 

 

Figure 5 : Sweeping volume (Sweep length x 15 m. Stripe x 1.00 m. Height)

 

CALCULATIONS

 

The swept area length is assumed to be a single continuous stripe found by the multiplying the  Trawling period  by Trawling speed as in Figure 6.

 

 

 

 

 

 

 

 

 

 

 

Figure 6 : The swept area length, assumed to be a continuous strip of 15 m. in w

 

 

Sample calculation for  Karides/Shrimp/

 

Data :

Total Trawling period (hrs)     = 40

Average Depth (± 1 m.) (m.)  = 86

Total catch                    (kg)    = 40.800

Speed  on 1600 RPM (knot)   = 2.00

Width of trawled strip  (m.)    = 15

 

Total length of  15 m. wide strip trawled      = 40 (hrs) x 2 (knot)

                                                                     = 80 nautical mile       

                                                                     = 80 x 1.852 (km/nm) = 148.16 km

 

Fish caught per km per 15 m. strip (Fig 6)   = Total catch (kg) / Total length (km)

                                                                     = 40.800 / 148.16

                                                                     = 0.275 kg/ km per strip of 15 m. wide

Visual underwater observations estimates the escaped amount of fish to be around 70% of the population living in that specific depth with in the height range of 1.0 m. from the sea bed. Accordingly the fish population in that volume (Figure 6)  will be  (1/30%) x fish caught (kg/ km per strip of 15 m. wide)

Or :

Fish caught (as above)   =   0.275 kg/ km per strip of 15 m. wide

Fish escaped                   =   0.275 x 70/30 =             0.641 kg/km per strip of 15 m. wide

 

Total estimated amount          = Fish caught + Fish escaped

                                       = 0.275 + 0.641  (kg/km per strip of 15 m. wide)

                                       = 0.916  kg/km per strip of 15 m. wide                                 

 

Estimated fish population at the sea bed is assumed to be a trapezoid of height 1.00 m., base length 2400 m. and  upper edge length 100 m. as detailed in figure 7. 

Figure 7 : Estimated distribution of the fish population at the sea bed

 

Area 1:

Shape           = Triangle

Base length = (2400 m – 100 m) / 2 = 1150 m.

Height          = 1.00  m.         

Population at point A = 0.000 kg/km per strip of 15 m. wide

Population at point B = 0.916 kg/km per strip of 15 m. wide

Amount of strips         = length / strip width

                                    = 1150/15 = 76.67 strips 

Total population          = Area of triangle (Total of populations in each of 76.67 strips)

                                   = 76.67 x (0 + 0.916)/2

                                    = 35.11 kg/ area 1

 

Area 2 :

Shape           = Rectangle

Base length = 100 m.

Height          = 1.00  m.         

Population at point B = 0.916 kg/km per strip of 15 m. wide

Population at point C = 0.916 kg/km per strip of 15 m. wide

Amount of strips         = length / strip width

                                    = 100/15 = 6.67 strips 

Total population          = Area of rectangle (Total of populations in each of 6.67 strips)

                                   = 6.67 x  0.916

                                   = 6.10 kg/ area 2

Area 3:

Shape           = Triangle

Base length = (2400 m – 100 m) / 2 = 1150 m.

Height          = 1.00  m.         

Population at point C = 0.916 kg/km per strip of 15 m. wide

Population at point D = 0.000 kg/km per strip of 15 m. wide

Amount of strips         = length / strip width

                                    = 1150/15 = 76.67 strips 

Total population          = Area of triangle (Total of populations in each of 76.67 strips)

                                   = 76.67 x (0 + 0.916)/2

                                   = 35.11 kg/ area 3

 

Total Population          = Area 1 + Area 2 + Area 3

                                    = 35.11 + 6.10 + 35.11 = 76.31 kg/trapezoid area, base 2400 m.                          

To speak of a constant figure all over the fishing area it is very logical to take the averages and assume an evenly distributed population over the base length of 2400 m., as shown in figure 8.

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 8 : Evenly distributed fish population assumption.

 

Average population of the fishing area per km.= Total Population of trapezoid area / length

                                                                           = 76.31 (kg) / 2.40 (km)

                                                                           = 31.795 kg/km2   

 

Density at 50 m.–100 m. depth range (kg/km2) = 31.795    

 

Total fishing area 8 square nautical miles          = 27.44 km2

 

Estimated amount in the 50 m. – 100 m. range = Fishing area x population per sq. km

                                                                           = 27.44 x 31.795

                                                                           = 872.45 kg.          

 

 

 

RESULTS

 

The results of trawling, in terms of the fish caught with commercial value to be sold at the local markets with their local (Turkish), English and scientific (Latin) names are listed in  Table 1

 

Species

Local (Turkish)

English

Latin

 

 

 

 

Karides

Shrimp

Crangon Vulgaris/Crangon

 

 

Lagos

White grouper

Aeneus/Epinephelus/Serranidae

 

 

Barbun

Red mullet

Barbatus/Mullus/Mullidae

 

 

Çipura

Gildhead bream

Auraba/Sparus/Sparidae

 

 

 

Voppa

Blue whiting

Merlangus/Merlangius/Gadidae

 

 

Hanno

Comber

Serranus cabrilla/Serranus/Serranidae

 

 

Trompet

Trompet fish

Scolopax/Macrorhamphosus/Macrorhamhosidae

 

Other species

 

 

 

 

 

 

Ahtapot

Octopus

Vulgaris/Octopus/Octopadae

 

 

Süpye

Cuttlefish

S. Officinalis/Sepia/Sepioidae

 

 

Kalamar

Squid

Illex coindeti/Coindeti/Teuthoidae

 

 

 

Table 1 : Fish caught with commercial value with their local

Turkish, English and scientific (Latin) names

 

The period of trawling covered a time-period of four years, from 1989 to 1992, at the same order of magnitude of the lifespan of an individual fish of the species investigated.

 

Species

Trawling period (hours)

Nominal depth (meters)

Fish caught, kilograms

Fish caught per km per 15 m. strip

Total estimated amount (100%) per km per 15 m. strip

Total estimated population in the fishing zone (kg/ 2.40 km length)

Density or Total estimated population in the fishing zone (kg/ km)

Total estimated amount in the 50 m.-100 m. fishing zone (kg)

Shrimp

40

86

40,80

0,275

0,918

76,500

31,875

874,650

White grouper

36

65

62,70

0,470

1,567

130,625

54,427

1493,479

Red mullet

604,5

68

1483,50

0,663

2,209

184,057

76,690

2104,386

Gildhead bream

604,5

68

741,10

0,331

1,103

91,948

38,312

1051,271

Blue whiting

604,5

50-80

867,80

0,388

1,292

107,667

44,861

1230,998

Comber

604,5

50-80

6010,40

2,684

8,948

745,707

310,711

8525,919

Trompet fish

604,5

50-80

376,50

0,168

0,561

46,712

19,463

534,076

Other species

604,5

50-80

376,20

0,168

0,560

46,675

19,448

533,650

Octopus

604,5

Variable

326,00

0,146

0,485

40,447

16,853

462,440

Cuttlefish

604,5

Variable

15,00

0,007

0,022

1,861

0,775

21,278

Squid

604,5

Variable

15,70

0,007

0,023

1,948

0,812

22,271

Table 2 : Estimated fish population with in the fishing zone.

The fishing records above are reduced to the following table 3 to indicate the total amount of fish population in the area under investigation based on the calculations detailed in Sample calculation for  shrimp section.

 

The results can be extended to cover the entire depth ranges where the fish of a given species live.  Taking the fact that the fishing season covers autumn, winter and spring, that fishing is performed both at daylight hours and night time, and also taking the diurnal and seasonal changes of dwelling of the fish species, it has been assumed that the fish can be assumed to be uniformly distributed over the depth range at which the given species live.  Therefore, for example, it can be assumed that 50-100 m. depth range hosts one-fourth of the fish population for a species living in the 0-200 m. range.

 

            With those assumptions, the total fish population on the weight basis at the area of investigation has been evaluated as follows:

 

specS                Species

Effective Swept Area (km2)

Density or Total estimated population in the fishing zone (kg/ km)

Total estimated amount in the 50 m.-100 m. fishing zone (kg)

Depth range at which the fish live (meters)

Total population in the Bay of Famagusta

(kilograms)

Shrimp

2,778

31,875

874,650

0-500

8747

White grouper

2,500

54,427

1493,479

4-500

14815

Red mullet

41,983

76,690

2104,386

0-300

12626

Gildhead bream

41,983

38,312

1051,271

0-400

8410

Blue whiting

41,983

44,861

1230,998

0-500

12310

Comber

41,983

310,711

8525,919

4-500

84577

Trompet fish

41,983

19,463

534,076

25-600

6142

Other species

41,983

19,448

533,650

-

-

Octopus

41,983

16,853

462,440

1-100

916

Cuttlefish

41,983

0,775

21,278

0-200

85

Squid

41,983

0,812

22,271

5-200

87

 

Table 3 : Fish population in the bay of Famagusta

 

 

CONCLUSIONS AND RECOMMENDATIONS FOR FURTHER STUDY

 

            It has been indicated that the fish stock of the bay of Famagusta was commercially significant.  The methodology used in this study can be extended to the fish stock estimate of the other areas.  The estimates made are strongly affected by the crude approximations and assumptions used, and by no means can be claimed to be accurate.  Therefore, the numerical results for the fish stock estimate should be taken on “order of magnitude” basis.  However, by the usage of a more accurate value of “escape factor”, a different and more accurate probability density factor for the distribution of each species with depth, also taking the diurnal, seasonal and local effects into account, much more accurate results can be achieved.  Also, the effects of the change of bottom slope, bottom nature and environment-related effects of the fishing areas should be studied with their effect in the fish population.

 

 

 

REFERENCES:

  1. ATA ATUN, SUNA ATUN : “Research on Fish Species at Turkish Republic of Northern Cyprus”,  Coordination Meeting on the Fisheries of T.R.N.C., 22-23 March 1999, Nicosia/Famagusta, North Cyprus. (In Turkish)
  2. CİHANGİR, B., BENLİ, H.A., TIRAŞIN, E.M., KAYA, M., ÜNLÜOĞLU, A.:  “Demersal Fisheries of the Bay of Famagusta, the Turkish Republic of Northern Cyprus”, 
  3. AKŞIRAY, F. :  “Sea Fish of Turkey and Recognition Guide”,  Publication of the Istanbul University Rectorate, No.:3490, 1987.  (In Turkish)