An Estimation of the Fish Population of the Bay of Famagusta

Ata Atun

3rd International World Fisheries Congress

Beijing (Pekin), China






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

FISHING

Assoc. Prof. Dr. Ata ATUN

 

Near East University, Faculty of Maritime Studies Nicosia, North Cyprus











1.    ABSTRACT - A rough estimation based on the trawl catch of fish for the bay of Famagusta was made. Trawl fishing made by traditional wooden boats of about 15 m. length at 50-100 m. depth on courses parallel to the coastline was taken as the basis of the estimate. In order to be conservative, it was assumed that 90 % of the fish corresponding to the trawl net entrance area escaped either from the net entrance or through the openings of the net. The results were extended to estimate the weightwise population of some of commercially important fish, using a Rayleigh distribution of fish for various depths.

2.   INTRODUCTION

Although the fish population of coastal areas are a subject of discussion in everyday life, almost no numerical information exists in the literatüre. In order to estimate the fish population of the bay of Famagusta at the Eastern shore of North Cyprus, the recorded daily catch of trawl fishermen were used, covering the years 1989-1995. The periods are from September to May, a total of 85 days per year. Systematic observations conducted by the first author[l] indicated that 65 distinct species existed in the waters indicated.

A conservative approach for the estimation of fish population of selected species was used. The area where the sampling was made is the 50-100 m. depth range. The results were extended to cover the entire depth range where the particular species lived, assuming a Rayleigh distribution of fish population on depth basis. The estimate of fish


populatioıı is on weight basis, which can be extended to cover the total number of fish for the area in question.

It should be stressed that the term "fish" refers to marine animal forms caught by the trawl net that are commercially significant, not to "fish" in biological sense. As can be seen later, shrimp (crustacean) and octopus, cuttlefish and squid (cephalopod molluscs) are also refered as fish.

3. EQUIPMENT USED

A trawl arrangement with two doors at 40 m. lateral distance apart and a net entrance of 15 m. was used. The distance between the doors and the net entrance was 120 m. Since the trawl net extended from the bottom to the surface, it was deemed reasonable that the net swept ali the depths where the fish could possibly be. The general sketch of the trawl equipment is shown in Figüre 1.






















Fig 1 : Bottom Trawl details


The swept area at any instant is roughly a trapezoid:

The rate of sweep will then be:




 



Defining an "Escape factor", f(e) as the ratio of number of fish escaped to the total number of fish at the trawl net entrance area, or by the escape of smaller fish through the net openings, the effective sweep rate shall be:









 

 

 

 

The escape factor will be either through the openings of the net or from the entrance of the net. Based on observations, a value of 0.90 was assumed for the escape factor.

The trawling speed was 2.5 knots throughout ali the catches. The trawling routes were almost parallel to the coastline, covering an area extending from 35° 04' 30" N to 35° 17' 24" N and 33° 53' 24" E to 34° 00' 00" E (See the chart at Figüre 2). The total area corresponded to about 8 square nautical miles. At each sweep, a straight stripe of 27.5 m. wide at 50-100 m. depth range was assumed to be swept and the fish caught represented 10 % of the population at that stripe.








4- ANALYSIS OF FISHING RECORDS

The results of trawling for species with commercial value with their Turkish and English names is shown in Table 1 below:

Table 1

Species

Trawling period, hours

Nominal Depth, metres

Fish caught, kilograms

Shrimp

40

86

40.8

Lagos (White grouper)

36

65

62.7

Barbun (Red mullet)

604.5

68

1483.5

Çipura (Gildhead bream)

604.5

68

741,1

Ahtapot (Octopus)

604.5

Variable

326.0

Süpye (Cuttlefish)

604.5

Variable

15.0

Kalamar (Squid)

604.5

Variable

15.7

Total area swept is:






















BBased on the numerical values cited above, the following populations were found for the swept areas:

Species

Area Swept (km2)

Density at 50-100 m. depth range (kg/km2)

Estimated amount in 50- 100 m. range (kg.)

Karides (Shrimp)

0.509

80.1

2200

Lagos (White grouper)

0.458

136.8

3755

Barbun (Red mullet)

7.697

192.7

5285

Çipura (Gildhead bream)

7.697

96.3

2640

Ahtapot (Octopus)

7.697

42.4

1165

Süpye (Cuttlefish)

7.697

1.95

55

Kalamar (Squid)

7.697

2.04

55





These results can be generalised to cover the full depth of ranges that the fish expected to live. The range of depths where the fish are expected to live are taken from the study by Akjîyray [1], Since the slope of seabottom was roughly constant, it was assumed that the fish population showed a Rayleigh distribution with depth: This distribution satisfies the condition that there should be a zero population at zero depth. It is also postulated to model the fact that shallower seas favour a denser population due to more sunlight and therefore a richer fauna and flora. The median of the depth range was taken as the median of the fish distributionwith depth and the Standard deviation was taken to be one-sixth of the depth range.

Rayleigh distribution is formulated by the probability density fiınetionp(x) given by:











 

 

and a cumulative probability fiınetion P(x) given by:

Assuming that the tabulated depth range, (0-R) metres of the particular species accounts for the 99 % of the population, i.e.,

P{R) = 0.99                                                                             (7)

it can be shown that:

a = 0.33 R                                                                               (8)

Therefore, the fish population for the 50-100 m. range tabulated in Table 2 above represents the proportion of the total population in the area under investigation as:



It can be shown that the 50m.-100 m. depth range accounts for, as in Table 3:

Table 3

Depth range

% of the total population

0-100 m.

30.7

0-200 m.

43.3

0-300 m.

28.0

0-400 m.

18.1

0-500 m.

12.3

0-600 m.

8.9


The results can be extended to the fish population at the bay of Famagusta as shown in Table 4:

Table 4

 

Species

Estimated amount in

50-100 m. range (kg.)

Living range (metres depth)

Total amount

in the area (kg.)

Karides (Shrimp)

2200

0-500

17,900

Lagos (White grouper)

3755

4-500

30,500

Barbun (Red mullet)

5285

0-300

18,900

Çipura(Gildhead bream)

2640

0-400

14,600

Ahtapot (Octopus)

1165

0-100

3,800

Süpye (Cuttlefish)

55

0-100

180

Kalamar (Squid)

55

5-200

130








































5- CONCLUSIONS AND RECOMMENDATIONS FOR FURTHER STUDY

It was shown that the fish population at the bay of Famagusta was commercially significant. The methodology can be extended to the estimate of the fish population of other areas. However, crude approximations were made for the estimation of the amount. A more detailed method of analysis taking the inaccuracies of the estimations into account can be developed: More accurate values of "escape factor", a different and more accurate probability density function for fish distribution and the inaccuracies involved by the unevenness of the bottom slope and bottom nature are recommended for further study.

 

References

1.  Ata ATUN, Suna ATUN : "Research on Fish Species at Turkish

Republic of North Cyprus", Coordination Meeting on the Fisheries of T.R.N.C., 22-23 March 1999, Nicosia and Famagusta, North Cyprus.

2.  AKŞIRAY, F. : "Sea Fish of Turkey and Recognition Guide", Publications of İstanbul University Rectorate, No.: 3490, 1987