REARING OF MUGILIDAE
A STUDY ON LOW SALINITY TOLERANCE BY THINLIP GREY MULLET (LIZA RAMADA) RISSO, 1826.

THE IMPORTANCE OF SALINITY ACLIMATIZATION OF MULLET FRY.

By Magdy A. Saleh
(GAFRD)*

* GAFRD, 4, El-Tayaran st, Nasr City, Cairo, Egypt.

Abstract: The effect of aclimatization on reducing post-transport fry mortality was studied during the fry transport operation to Wadi El-Rayan lakes. The results of the study showed that great losses occur in mullet fry especially during the first weak after transplantation if the aclimatization to low salinity was not conducted. Low salinity aclimatization, if carried out correctly, can gratly reduce the overall losses in transplanted fry. The experiment was done under normal field working conditions and an applicable aclimatization system for extensive fry transport was established. A follow up study will be carried out this season to establish a more solid system.

INTRODUCTION

Mullet species are of the most important cultured fish in Egypt. Trials of transplanting mullet, either to fish farms or to inland lakes, may go back to the late 20s which may put mullet as a pioneer aquaculture fish in Egypt. Collection and transport of mullet species fry on a commercial scale was known and applied since more than 50 years to lake Quaroun to substitute the declining nile fish biomass in this lake. Thanks for the work of the pioneers in this field, some Egyptian expertise had attained extensive experience and skills in the field of handling mullet fry on a large scale level.

Most, if not all, of fry were collected from a single fry collection station at El-Max near Alexandria until the late 70s.

With the great expansion of aquaculture in Egypt during the last decade (not less than 100k feddan of registered fish farms) the demand for mullet fry had increased greatly. Mullet fry are now collected from 10 official stations with a recorded catch of 150 mil. fry/year. The expansion in aquaculture and fry collection was not accompanied with an equal development in the knowledge on fry handling and mullet farming. As a result, great losses in fry were observed. Of the main reasons of this losses is the lack of knowledge on the biology of mullet among most of fish farmers, on top of this, is the salinity tolerance by mullet fry. Most of the studies conducted on this topic, either in Egypt of other countries were mainly carried out on a small scale and limited sample size under laboratory conditions and some times by stimulating field condition.

In this study I tried to get a realistic understanding and approach to take this problem through the application of this experiment during the large scale mullet fry El-Fayoum lakes. The results of this study shows the importance of applying low salinity aclimatization for mullet fry to reduce the mortality and designing an applicable aclimatization system for large scale mullet fry transport.

MATERIAL AND METHODS:

Fish : Mullet species fry collected by a fine mesh seine net at Dammytta fry collection station. Experimental system: (1m3 each) habbas made of mosquito net material fixed in nursing pond of the second lake Wadi-El-Rayan with its bottom intermingled with the pond bottom. ICD-WTW brande conductivity meter and a LCD-WTW brand dissolved oxygen-temperature probes.

A DAF, 1600 fry transport truck with built-on 6, 1m3 each double-walled insulated glass-fibre fry tanks supplied with air by a diesel runned air compressor with oil and traps.

EXPERIMENT PROCEDURE:

Liza ramada fry (18 mm + 1.6,0.04 gr) were transported in sea water (37.4 ppt total disolved solids; TDS) in a density of 10k/m3. The truck trip from El-Gerby, Dammytta fry collection station took. 7h and fry were kept in the transport water (oxygen saturated, pH 8.5, temperature 18 ° C and 0.12 ppm total ammonia nitrogen) for three hours with the tank led open and aeration on where temperature equalization with pond water occurs.

Fry were identified after Yashouv & Berner, 1970 and exotic fish fry were excluded.

Nursing pond water (2,6 ppt tds, pH 8.4 and 17.8°C temp.) was pumped into fry tank and TDS was observed by conductivity meter*. The concentration of salt was reduced by 5ppt each/2h. After each 5ppt reduction in TDS, 200 fry (in two patches each of 100 fry) were scooped out by a hand net and placed in two habbas. Fry were kept under observation for days, and mortality was counted each 12 hours.

* TDS was calculated by multiplying conductivity by a factor 0.684

To study the tolerance of L. ramada fry to sudden changes of salinity, a synchronized experiment was conducted L. ramada fry were brought to Wadi El-Rayan lake in two DAF trucks, each with 6 built on fry transport tanks. The experiment fry stock was transported in a single tank in a stocking density of 15k/m3, one tank full of sea water and the other 10 tanks were loaded with then normal fry transport rate. After unloading the trucks, tanks were washed clean and filled with sea water as follows; 1 % × 2,5 % × 2,22 % and 38 % × 2. Pond water was added to give a final TDS of 2.95, 4.34, 6.43, 10 and 15.8 ppt respectively. Two tanks were left full with 100 % sea water. Fry were distributed in 1k patches/tank. Fry were kept under observation and mortality was recorded for 24 hours and compared with those kept in 100 % sea water.

RESULTS AND DISCUSSION:

Aclimatization system examination: Fry which were transfered directly to pond water showed a frenzic reactions showed in fast aimless movements; vertical swimming to the bottom and the surface. After few minutes some animals started to show ill swimming and hyperventilation and increased coughing rate. During the second hour, fry seems as if it started to restore normal reactions, movements became slower but tands to rest near the bottom. Loss of control on buoyancy commences after the third hour where they start to float of sink unvoluntairly with side swimming or swimming with the long axis directed vertically. first mortality was recorded after 2.5 h, then mortality started to increase gradually. Hypermilanosis was observed in all fry just before death especially on the dorsal area.

The same reaction was also observed in fry transfered to pond water from 30,25 and 20 ppt TDS, on the other band, fry showed a moderate reaction when transfered from 15 ppt to pond water, still an overall high mortality was observed. Mild reaction can be observed on transfering the fry from 10 ppt to pond water but the final over all mortality was found tolerable if compared with the other mentioned concentrations. Fry transfered from 5 ppt to the pond water showed normal reaction and started to feed after a very short period. Research mortality during the experiment and the final survival are shown in table 1.

Table 1 : Mortality of fry for each TDS