The Changes Observed in Atlantic Bonito (Sarda sarda) Populations During the Autumn-Winter Migration in the Suthern Black Sea Coast

Transcription

1 The Changes Observed in Atlantic Bonito (Sarda sarda) Populations During the Autumn-Winter Migration in the Suthern Black Sea Coast Click to add title Mustafa ZENGİN Central Fisheries Research Institute, Trabzon, Turkey

2 Introduction Bonito is the most commercially important species among the Black Sea fishes. While total catches of bonito from the countries bordering the Black Sea has reached the maximum of thousand tons in 1969, thereafter no bonito catch was recorded from these countries except Turkey and Bulgaria. Mainly due to pollution in the northwest Black Sea and migration problems occurred in the bonito stocks (BSEP, 1997). Until the mid 197s, the northern part of the Black Sea was the most important area for spawning of commercial fish species, including valuable pelagic species such as Sarda sarda and Pomatomus saltatrix and demersal species (Chaschin, 1995). After end of the 197s input of nutrients and toxic substances from a large catchments basin caused changes in the hydrochemical regime with consequent eutrophication (Shiganova, 1998).

3 Reports of last years show that almost whole bonito catches obtained from the Black Sea belong to Turkish side (Prodanov vd, 1997; BSEP, 3). Ortalama av miktarı (ton) Türkiye Bulgaristan Romanya BDT Yıllar

4 Introduction For spawning and feeding migration, bonito starts to enter into Black Sea from the Aegean Sea and the Sea of Marmara, from the end of April until the middle August. By the end of September it starts to return into the Aegean Sea. The highest level of migration takes place in the months of November and December. During the winter it hibernates at termoklin layer in the Sea of Marmara (Kutaygil, 1979). Studies conducted over the bonito catches during autumn/winter migration in the southern Black Sea costs showed the evidence that apart from the migrated stock of bonito, some small part of it remains in the Black Sea (Nümann, 1955; Artüz, 1958). Bonito migration in the Black Sea is mainly controlled by biological (predation) and oceanographic (water temperature) conditions. Bonito spawns from the end of May until middle July in the most northern parts of the Black Sea (Demir 1957). It is reported that the most suitable water temperature for spawning is 18. ( ) C (Majorova and Tkecheva, 196).

5 Bonitos populations spawning and feeding migration since last years

6 Objective of Studies The bonito populations are very important commercial top predator fish in the Black Sea. And such as their migration, reproduction, feeding and growth characteristics to be necessary known for the management strategies criterions. For the long terms sustainability; its stock nearly to be caught by Turkey fisherman. In this paper, the bonito population and the specifications of landed catch were defined and some comments on the management strategies were discussed.

7 Material and Method Data related to fishing gear and methods used to catch bonito were obtained from the commercial fishers along the coast of Turkish waters (Figure 1). Data concerning other catch parameters were collected by means of sample forms distributed in the main fishing centres. Whole data was recorded on this sample forms. All dates were collected monthly. Population parameters were obtained from the landed catch. The catch per unit effort (CPUE) was used as an index to define the fishing intensity (Phiri and Shirakihara, 1999). Bhattacharya method, which is based on analysis of length sequence, was used to determine the age classes for bonito stocks. This method is mostly applicable to the populations that present rapid growth and short life phenomenon (Sperra and Venema, 1992).

8 Sample area: Along the coast of Turkish waters Sampling stations

9 1-Characteristic of landed catch Resualts Parameters Seine nets Gillnets 1-Vessel length (m) (-48) 8.6 ( ) 2-Engine power (HP) 982 (449-16) 44.8 (8-1) 3-Gear main particulars Length (m) Depth (m) Part length (m) Number of longitudinal parts Part depth (m) Number of vertical parts 8.1 (6-198) ( ) Mesh size (mm) 12,14,, 36, 54, , 72, 76 6, 64, 84 () 5, 56, 6,88, 96 5-Distance from shore (m) ( ) 37.6 ( ) 6-Fishing time (in 1 day period) 24 hours (16-2 ) 7-Operation number (daily) (1-6) 8- Operation duration (minute) 16.5 (6-2) 185 (-7) 9-CPUE (kg/vessel/op.) ( ) 83.1 (.6-967) 1-Catch distribution (%)

10 Resualts 1-Characteristic of landed catch CPUE (kg/boat/op.) Purse seine Gillnet J F M A M J J A S O N D 5 1 Fishing frequency (%) Months

11 1-Characteristic of landed catch Resualts Figure 3. Some fishing specifications for bonito (N=134); 1: The CPUE distribution in the months of September, October and November, 2: The CPUE distribution in 24 hours period, 3: The CPUE by distance from the shore Fishing period (days) (1) CPUE (Kg/boat/op.) Fishing tim e (hours) (2) Distance from shore (m) (3) Gill net Purse seine

12 Resualts 2-Population features By analysing the landed catch composition for the fishing seasons of /1 and 1/2 the length distributions of bonito were found to be (28.1± 2.61, n=492) and (31.2±3.33, n=198) cm, respectively. Length distribution it was seen that the length groups are formed in such that it has two distinct pick points. The average length values for these points were computed to be 26.4±3.68 and.4±3.57 cm (Figure 4). Each of these pick points refer to a year class group (Sparre and Venama, 1992). Although, individuals in the bonito population generally reach the sexual maturity at two years of age, maturity differences observed even within the individuals of the same year class and as well as the variations occurred in water temperatures have caused the spawning time to spread in rather long period. It was noticed that for the whole months in which the sampling took place there were new individuals of zero age to recruit the stock.

13 Resualts 2-Population features Frrquency% 1 5 General N=1759 year class (cohort) 1 year class (cohort) 17,5 22,5 27,5 32,5 37,5 42,5 47,5 Length (cm) Figure 4. Length distribution of bonito taken from the southern Black Sea coasts for the years of, 1Figure 4. Length distribution of bonito taken from the southern Black Sea coasts for the years of, 1 and 2. The pick points of the curve refer to the age groups.

14 Discussion 1-The migrated bonito populations into the Black Sea coasts during the period of autumn/winter period consist of zero and one year of individuals. 2-The bonito reaches the first sexual maturity length at the end of first year old and at the beginning of two years old (Ivanov and Beverton, 1985). It is therefore possible to say that almost whole bonito population occurred into southern Black Sea coasts during the autumn/winter, migration is composed of individuals that have not reached the sexual maturity yet. 3-During this study, only 5 individuals belonging to higher age groups were met. It is thought that the fewer amounts of such individuals in the population are closely related to the migration pattern. 4- Bonito populations have a tendency to form school by being composed of the individuals of the same age. The reproduction period is spread rather extent time. In this study; all of the sampling months were coincided with individual of the age group which were new recruit groups (Figure 5). 5- The bonito s stock can be defined mostly in the southern Black Sea coast, its moving-fishing interactions phenomenon to be needed well known for the purpose of its fishery management.

15 Discussion 6-Bonito is a fish species which is grow up rapidly. 7-During their reverse directions movement it has high growing performance in the southern Black Sea coast. 8-The bonito populations have a complex seasonal migration phenomenon. The mature bonito stocks migrate to eastern Black Sea coast in May-July and their juveniles (gipsy bonito, vanoz) which are age groups recruit stocks and to be form schools among themselves from August on. And they stay in the same area from all fall/winter seasons to beginning of the March. 9-On the other hand 1 age group bonito s population make feeding migration from early autumn on the same region and then migrate towards to the southern region Marmara and Aegean Seas in the end of winter.

16 Frequency % Frequency % Frequency % No data /1 September N=71 Balık boyu (cm) /1 October N=7 Balık boyu (cm) /1 November N=118 Freguency% Freguency% Freguency% Frequency% Fish length (cm) 1/2 August N=37 1/2 September N=497 1/2 October N=696 1/2 Fish length (cm) November N=322 Frequency % Freguency% Freguency% Balık boyu (cm) /1 Decem ber N=66 Balık boyu (cm) /1 January N=16 Balık boyu (cm) /1 February N=28 Freguency% Freguency% 6 1 Fish length (cm) 1/2 December N=43 Fish length (cm) 1/2 January N= Fish No length data (cm) Fish length (cm) Freguency% 1 Balık boyu (cm) /1 March N=33 No data Fish Fish length (cm) Figure 5. Monthly length variation of landed bonito for the periods of /1 and 1/2 (Total lengths were taken)

17 Discussion 1-The periodical variations in the amount of catch were also reported previously by Artüz (1957) and Artüz (1958). 11- It is expected that this variations are due to the increasing fishing power imposed on the populations. A strong evidence of this is the fact that nearly rest of the landed catch of -2 fishing period were composed of 1-year-old individuals. 12-Therefore there is no enough chance for the population to recruit. If the matured stocks decrease insufficient level, the young populations are reduced. 13-This state is a sign of the recruitment overfishing. As to long year s data, it can be seen from Figure 6 that picks formation in the catch and thus recoveries in the stocks require some 5 or 6 years. 14-To sustain the reliable fishing in the Black Sea the total amount of annual catch should not be over 1 thousands tonnes. -Taking this figure as a base it is recommended that the CPUE values should not exceed 166 kg/day/boat and 12 kg/day/boat for small and large fishing vessels, respectively.

18 Resualts Figure 6. Variations in the landed bonito catch from Turkish coasts of the Black Sea in last years and in the future recommended catch. Landing trends (tons) 1 5 Mean catch Reference point Years

19 Local fish market in Trabzon, 2