OVERVIEW OF AN EXPERIMENTAL RELEASE METHOD OF THE SILVER SEA BREAM IN THE LAGOON AT ALBION (PETITE RIVIÈRE BAY)

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1 OVERVIEW OF AN EXPERIMENTAL RELEASE METHOD OF THE SILVER SEA BREAM IN THE LAGOON AT ALBION (PETITE RIVIÈRE BAY) ABSTRACT S Khadun, R Hassea, T Shimizu and H Iwamoto Albion Fisheries Research Centre A programme to follow the movement pattern of marked sea bream in the lagoon at Albion was undertaken under Coastal Resources and Environment Conservation Project, with the assistance of JICA. Fish were marked by branding and were released in selected sites in the lagoon and off lagoon. Six batches with markings on different parts of the body were liberated from 1998 to Three areas of the lagoon were chosen for sampling purposes by seine nets. Marked sea bream were mostly captured from three areas. Information collected from amateur and net fishermen showed that marked fish were being caught in different area of the lagoon and the sea at Pointe Moyenne. The ecology of areas where marked sea bream had been captured should be studied. The release experiment has proved to be successful but trial on larger scale should be carried out to assess effectiveness of release. New method of marking should be tested and applied. Mauritius is the first country in the world where experiment on release of the Rhabdosargus sarba is undertaken. Keywords:Sea bream,marking,movementpattern,recapture,restocking of lagoon, fishermen community INTRODUCTION The silver sea bream, Rhabdosargus sarba, is an important marine fish species in Mauritius. This species is widely distributed throughout the island and has been reported in many areas of brackish water. It is a first cluster group fish and is highly appreciated due to its white flesh and very good taste. It is mostly fished by artisanal fishermen in the lagoon as well as off lagoon around Mauritius. The Albion Fisheries Research Centre (AFRC) with the assistance of the Japan International Cooperation Agency (JICA) started release trial of this species under the Coastal Resources and Environment Conservation Project, which is a five-year programme. The main objective was to follow the movement pattern of fish after their release. Prior to the start of the experiment the marking method was selected and tested at the Centre for a few months. After which, the trial started in December 1998 with the liberation of some sea bream. The lagoon at Albion, Petite Rivière Bay was chosen for that purpose, as sea bream catch has not been reported and as it is near the Centre, it is easier to monitor the trial. It is to be noted that Mauritius is the first country in the world where experiment on release of the Rhabdosargus sarba is undertaken. The potential to propagate the silver sea bream will be assessed at a later stage. Fish were liberated in six batches, four in the lagoon and two off lagoon. MATERIALS AND METHODS General All fish were produced in the hatchery at AFRC in tanks during the winter, which is spawning season of the sea bream. Fish of different sizes were used for trial due to unavailability of a constant size batch. At the beginning of the experiment, bigger size fish were released. Marking In order to follow the movement pattern, marking or tagging of the released species is required. Marking is also a convenient method to distinguish between the hatchery reared fish and those from the wild. From 1998 to 2000 four batches consisting of some sea bream were branded in selected parts of the body by using a hot iron rod. Marking at chosen parts will enable identification of the 231

2 different batches. The branding method of marking is performed at a minimum cost. Marking has been carried out as follows: a) the left side below the dorsal fin (L) b) the right side below the dorsal fin (R) c) the left side below the dorsal fin on the posterior side (LPD) d) the left side below the dorsal fin and at the caudal fin (LDC) e) the right side below the dorsal fin and at the caudal fin (RDC) f) the right side below the dorsal fin on the posterior side (RPD) After marking fish are kept in tanks containing 0.1 ppt of sodium nifurstyrate (Na-NFS) as a prophylactic treatment for one hour. The fish are stocked in a floating cage for more than two weeks to let the fish recover from marked wound. This method is quite effective as fish mortality is less than 5 % and the fish is not stressed. However fish should be above 10 cm in length. The marked areas on the fish body after healing are covered again covered with scales. However, a patch is a clearly visible. Scales from these patches were removed and examined. It was found that all the scales are different in shape when compared to unmarked areas of the fish body. Size of marked sea bream The average total length and average body weight together with the respective length and weight ranges of the released marked sea breams for the five batches are shown in Table 1. Table 1 Marking code, year of marking and size of sea bream Mark Year No. Weight g Length mm Range Average Range Average L R LDC RDC LPD RPD Total Transfer of fish to the lagoon One or two days before a release is executed, the fish are starved to avoid deterioration in water quality during transportation. Fish are carried by truck in one-ton plastic tank. Water in the tank is aerated during the whole trip. Fish are removed by plastic buckets and transferred to the sea or in a small floating cage when released off lagoon. The floating cage is slowly towed to the selected release site by a motorboat. Due precautions are taken during this operation to avoid rupture of the cage net by corals. The advantages of using a floating cage are as follows: i. The fish are less stressed and acclimatized gradually to the new environment while being dragged to the release site. ii. They received dissolved oxygen directly from the water instead, iii. Liberation of the fish is made easy by untying the net only, thus avoiding further stress by handling Under water observation Immediately after liberation the behaviour of the fish in relation to its new surroundings is observed under water for a few hours. The bottom conditions are studied and the presence of predators such as caranx and barracuda, which is an inhibiting factor to the release program, is observed. The fish 232

3 usually stay in one group and move in a circle at the release point for some hours after which they move to different areas inside and outside the lagoon. Collection of data Data and other information on the released fish are collected from amateur fishermen, basket trap fishermen, and net fisheries, off lagoon fishermen and by seining operation carried out by our staff and nurserymen. Physical parameters Physical parameters like temperature and salinity are recorded during seining, which is performed randomly. These parameters will be needed to study any relationship of the released fish to its new environment in the future. Seining in selected areas Seining is undertaken in previously selected sites. These areas have been preferred because it can be easily seined. Three different areas of the lagoon have been chosen. The three areas opted for, are the barachois at Albion, the mouth of the Belle Eau River and the right side of the Albion lagoon and were named area 1,2 and 3 respectively. All sea breams caught during seining, are sampled for length and weight parameters. Stocking effectiveness will also be assessed by this operation but at a later stage of the programme. Result of seining operations Sampling was performed on a monthly basis and was also related to the availability of nurserymen for seining operations. More sea breams were caught from the barachois. Results are shown in Table 2. OBSERVATION Twenty three seining operations were effected in 1999 and twenty two in Some 114 released sea breams were caught, 53 in 1999 and 61 until November From our seining operations, more fish were caught in the barachois at Albion where the water is brackish in nature than the right side of the Albion lagoon and consisted of fish with the six different markings. This indicates that irrespective of the site the fish are liberated, they will gather in the lagoon and barachois. All fish were released again after sampling. The ecology of both areas is different. These areas were surveyed and it the presence of different species of bivalves and gastropods in sufficient amount to attract sea breams was observed. However, from information gathered a large number of released sea bream were caught from the left side of the lagoon by amateur line fishermen and the right side the lagoon by amateur line fishermen as well as professional net fishermen. A small number of fish have also been reported from line fishermen from one area of the open sea at Pointe Moyenne. DISCUSSION At first, we have been relying more on data collected during our seining operations. We have assumed that the released sea bream do not move too far away from the release sites. As more marked sea bream has been captured in the barachois than the Petite Rivière bay, it is believed that the sea bream have a preference for brackish water. However, from the latest information collected from amateur fishermen, released sea bream are being caught near Pointe Moyenne some 5 km away from the Albion lagoon. This is showing that the released fish are wandering into new areas of the natural environment. During seining operations the ecology of sites, where fish have been caught, are different in terms of flora and fauna and bottom conditions. The ecology of these areas needs to be investigated to understand the behaviour of released fish. We are presuming that more than 90% of our first two 233

4 batches of released fish have already been caught by amateur and net fishermen, hence the small number of fish caught by our team during sampling exercises. Table 2 Number of sea bream caught and number of seinings effected in 1999 and 2000 Month Seinings Nunber caught Wild Jan Feb Apr May Jun 3 5 Aug Sep Oct Nov Dec 8 23 Total Results obtained so far from preliminary data are satisfactory. The effect of release has proved to be successful. However, we should now carry out trial with smaller size fish. Sea breams should be liberated in small batches, in order not to aggregate fish after release. Such action may render capture of fish difficult for line and net fishermen. Moreover, fish should be properly adapted to the lagoon conditions prior to release. A fish pen could be set up in the estuary and lagoon for adapting fish before release. The fishermen, local community and the public at large have to be involved in any restocking programme. Seminars and workshops has to be organized for fishers so as to inform them of this activity and the success it achieved in other countries CONCLUSION This trial is aiming at the restocking of the lagoon as presently practiced in many countries. A longterm programme is required to assess its effectiveness and viability. However, the programme is dependent on the progress of the hatchery at Albion. More improvement in seed and juvenile production techniques is necessary in order to carry out large-scale releases. New marking methods such as elastomere paint and coded wire are needed to assess release effectiveness. Trials to condition fish to release situations are needed. The relation between fish size and time of release is to be investigated to increase rate of recovery and recruitment. More experiments on liberation need to be effected in areas around the island to evaluate their potential as site for stock enhancement. The fishermen community at large will be benefited by such projects. REFERENCES FAO Fisheries Circular No : Marine Ranching: Global perspectives with emphasis on the Japanese experience. LEBER KENNETH M and LEE CHENG-SHENG Marine stock-enhancement potential with stripped mullet, Mugil cephalus, in Hawaii. SMITH THEODORE IJ, JENKINS WALLACE E and DENSON MICHAEL R Overview of an experimental stock enhancement programme for Red drum in South Carolina. 234

5 MILLER JOHN M, NEIL WILLIAM H and DUCHON KERRI A An ecophysiological model for predicting performance of released fish. OKOUCHI HIROYUKI, KITADA SHUICHI, TSUZAKI TATSUO, FUKUNAGA TATSUHIRO and IWAMOTO AKIO Return rate and economical estimation of stock enhancement program of flounder in Miyako Bay. American Fisheries Society Symposium, STICKNEY ROBERT R Marine enhancement programs-implementation planning TSUKAMOTO KATSUMI, MASUDA REIJI, KUW ADA HIROSHI and UCHIDA KAZUO Quality of fish for release: behavioral approach. Aquaculture Development in Mauritius, Aquaculture Division, AFRC. under preparation,