A Study on the population structure of carp (cyprinus carpio L.) in Akşehir lake

SUMMARY A STUDY QH THE POPULATION STRUCTURE OF CARP { CYPRINUS CARP 10 L.) IN AKŞEHİR LAKE Akşehir is sn eutrcphlc, tectonic lake, located in the south-west part of Central Anatolia, Turkey. its surface area is 355 kmf altitude is 953 m and maximum depth is 5-6 m. The son concentration of the water relatively high, conductivity averaging 2755 As-rrhos/cm- Major cations are Mg, Ca "r, aniens are HCO3, SC^ and CS~ pK is 9.05 - Macrcphytes (mainly reed) cover 30 % of the iake surface- In the îake economically important species are Carp (Cyprfnus carpio), Northern Pike fEsox luciusl, Dace (Leuclscus iepidusj and Crayf i sh (Astacus leptodactyfus). There are six other fish species aîso wrch are not economical iy irrportant. Recent iy the growth of the carp population has become poor and Its economic vaîue has decreased. !n this study the aim was to investigate the structure, growth; feeding, reproduction character ist 5 c of the carp population and analyze some properties of the îake such, as «ter quality, pîankton and benthos. To do this, water; plankton: benthos and carp samples were taken for analysis between March 19S7.and May 1988 bimonthly. The results can be surrrrarised as follows» The general characteristics of the water quality were: Relatively high ion concentration: high pH value; oxygen depletion in some areas; high turbid! t iy and greenish-grey water colour. Tempe rature ranged between 5-2S C in the study period. The zoop lank ton groups observed in the iake were Cladocera, Copepodaj Ostracoda and Rot if era- Phytopiankton groups were Blue-green Algae, Green Algae, Diatoms and Flagel lata. An algal bloom, caused by Microcystis (Blue-green algae) were observed in the summer and some part of the auturm. The algal bloom coloured water green - Benthic fauna analysis showed that in this lake, benthos blormss had declined from previously published work. In some benthos' sarrpîes no benthic fauna was found and no molluscs were found in any sample. Benthic biomass varied seasonally- 2 the average (chironcmlds and ol igochaets) being 5.76 g/sn - V5£«Age determination! was made from scales and opercular bones. The ages cşf the carp varied between 1 and 14 years. The most abundant age class ves the fourth (27.79 %) then the 5th } gin ancj 3rd age cîaSses in order- The females lived î onger than the males. The population was composed of 53.58 % females and 45.32 % maies. The f emaie:male ratio was (1:0.86). The difference between the percentages was stat ist i cat ry significant (p < 0,05), The fork length (FL) of the carp varied from 10 to 55,5 cm. Sixty-five percent of the population was in the 20-30 cm iength group, în any age group difference of fork lengths between males and females »as not significant (p > 0.05). Tine weight of the carp varied from 20 to 2860 g, the majority of the individuals weighed about 200 g.. Tne weights of male and female in any age cîass (exept 4'-'* ) were the same; i.n the fourth year age group females were heavier than males. The growth of the carp population was calculated absolutly, proportionally and specif icaîiy (instantencusly), The growth of the carp population in Akşehir fake was found to be very poor. The first year growth was relatively normal but afterwards growth ratio decreased. Age-length and age-weight relations were expressed by the Von Bertaianffy growth formula. The equations are:... ".,. -0.0711 (t+2.71)^. »-t= -64.01 ( »-e } for -, ma 1 es. "" "", " -0.0463 (t+2. 85),,.= 89.72 (1-e ) for females. "",. -0.0711 (t+2. 71 }, 2. 78621, W* = 3221.2 (1-e ) for males. «««« « /. -0,0463 (t+2. 85). 2. 82127. W = 9099.0 (1-e ) tor females. î The length-weight relationships were calculated by the feast squares method, the îcgarîtmic equations were found as Log W = -1.51130 + 2.78621 Log FL r=0.999 for males. Log W = -1.55063 + 2.82127 Log Ft. r=0, 999 for females. sxThe '". b values were character îst icaî Sy »ess than 3 and the growth of the population ws al »crotric. No significant differences were found between rraSes and ferrales. The ".'? b " values varied between seasons and ages. Fulton Condition Factor and fork length were used to express condition» The average condition «as found to be 1.541 İ 0.029 - The differences between fishing grounds and sexes not being significant, but condition did vary with season and age» The condition ws best around the end of the feeding season, in the first and second year of life. Scales were mature sexually at ages ill» to IV. and ferrates were mature at IV. to V. " in z&ksehir sake the carp began to spawn at the ®td of Ney v>hen the water îerrpareîure was 17-18 °C, the spawning contining to the end of Juiy. The fishes spawn in sh&Hcvy areas, on rrscrcphytes such as Potaraogeton and Ceratophyj I urn. The devot'oprosnt of the carp gonads occured in auturm and spring, The Gonadosorrat ;c index (GS!) was found to be at a rraxtrrurn at the begining spawning. The fecundity in the population varied from S80 to 363000 eggs/female, average fecundity was calculated as 25083 t 5280 eggs/ female, in corrparisson with other carp populations the fecundity of this population was iow. Fecundity Increased ' wi t h length, weight and age, and varied between years. The best correlation with fecundity v«s found to be the weight of fish. Egg diameters changed from 0.71 to 1.5S rrm, the average was calculated as 1.18S İ 0,047 rnn. No relationships were found between egg diameter and other parameters such as length, weight, age ar^d gonad weight. The food of the carp was ccnpcsed of fish, aquatic insects (larval and adult}, Crustacea, rotifera, otigochaete, mo! lusca, algae and rmcrophytes. By comparison with other studies the percentages of fish and plant (algae and rmcrophytes} in the diet of these carp were relatively high. The rracro invertebrates which, are the most irrportant food of carp elsewhere were found rarely in the diet in this lake. A high percent of enpty stomachs was also found. St is possible that the carp could not find enough food' In this lake. xThe basic reasons of poor growth and decline in fishery yieids in Akşehir lake are: - Decrase in fishing effort for carp as their vaiue decreased in relation to the much more valuable crayfish; Generally ineffective fishing equipments, short catching period and the fact that 30 % of the îake area is covered by reeds. Under these conditions the density cf carp population Increased, - The decs ine of the Northern Pike population reducing predatlon pressure on the carp and on other fishes that compete with the carp causing their populations to increase. - Ecological changes, pollution effects and especially through pressure on the bentle food supply by increased number of fish causing it to decrease. As the fish populations Increased, their food supply decreased in the Sake. Causing the poor growth of the carp, reducing Its marketable value which is related to the decreased fishing effort. Possible methods of improvement the carp fishery Abolition of the close season for Carp and Dace to increase fishery pressure and reduce fish numbers. Changes resulting from, this should be studied. - Improvement of cething equipments and re-Introduction of the fishing method called " »grip 5' (a form of trawling) which was effective in the past. The Northern Pike population should be protected to Increase Its numbers and Its predation on the carp and other fish populations» - At present cutting of reeds during the spawning season Is forbidden» This porhibltion should be abolished because a relatively large reed cover has negative effects on fishing, oxygen content and Sake level» x?