Pengelolaan Berkelanjutan Perikanan Rajungan (Portunus pelagicus) di Lampung Timur.
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Increasing demand for crab meat export and followed by good prices have been increased fisher preference for capturing the blue swimming crab, Portunus pelagicus (Linnaeus, 1758) in Indonesia. One of potential area for wild catch production of this species in Indonesia was at the East Lampung coastal waters, which is an open coast type with generally slight slope of marine topography. However, biological information of this species was rare. Catches productivity as well as caught crabs sizes tend also to be decreased and might be related to high exploitation rate; so this fishery is required to be managed properly. This study aimed to: (1) describe life history characteristics in reproductive biology; (2) distinguish spatial temporal variation of stock structure; (3) describe population dynamic parameters; (4) evaluate of crab exploitation pattern; (5) arrange management measure options for sustainable management of this crab fishery in East Lampung coastal waters. A comprehensive reproductive biology parameters were analysed based on representative samples, which were collected at landing site and conducted monthly from June 2011−Mei 2012. Spatio-temporal of stock structure was analysed based on sampling at three stratifications area from March 2012−February 2013. These stratification areas consist of S1 (water depth <5 m and <4 nautical mile from the shore line), S2 (water depth about 5−10 m and 4−10 nautical mile from the shore line), S3 was offshore direction, whiles fourth sub-areas (A1−A4) at each stratification. Those sub-areas were decided based on morphological characteristics of shore and coastal land coverage. Thus, the unit area of sampling was noted as S1A1−S3A4. Those above samples were also used for population dynamic parameter analysis. Crab exploitation pattern was investigated by interviewing the fishers and guided by questionnaire during December 2012−February 2013 and June−September 2013. Life history characteristics of this species in this area indicated by: (a) the size of males and females reach gonad maturity 50% (Lm50)were at 98 mm and 103 mm carapace width (CW); (b) female’s Lm75 and Lm95 were at the size 111.6 mm and 126.0 mm CW, while reproductive maturity (Lr50) was at 115 mm CW; (c) berried females (BEF) ovaries have dominated by gonad development stage of III; (d) gonad somatic indexes of nonberried female (NBF) were higher than BEF in each month, except in August, November, and February, indicating multiple spawning in the year; (e) females became double breeding or more in the year as well as high potential for spawning and the value of relative percentage of BEF/NBF above 30% at the size larger than 111 mm CW; (f) the potentially reproductive females at the size 111−170.99 mm and reproductively females at 111−155.99 mm CW. Reproductive pattern was partially spawner and spawning occurred thorough in the year, but it was seasonal-continuous with two peaks season. The first peak of spawning season tends during April−June and the second peak was during September− October/November. Both peaks of spawning seasons vary in BEF abundance and fecundity. Fecundity at the 1st stage of embryonic development with BEF’s size between 91.58−168.00 mm CW was between 229,4682,236,355 eggs (mean ± SE = 926,638 ± 30,975 eggs), linearly and positive correlation to carapace width, but positive logarithmic correlation to body weight. Individual fecundity was also high variation at the size larger than 126.0 mm CW and might relate to multiple spawning in the year. BEF with all embryo development stages were found at most sampling areas. Thus, spawning and breeding area seems to be occurred at the S1 with saline water and sandy loam sediment to offshore area. This condition was also indicated by BEF with final stage of embryonic development. BEF distribution area seems to be overlapped with fishing ground in study area. Mean whiles, main nursery habitat at S1 in almost round years and mixed with sub-adult and adult habitats. The proportion of sexually adult crabs by the number of individual and total catch was ~66.5% and ~78.7% at S1, so part of this area toward offshore direction was relatively potential for fishing. Variation in spatio-temporal stock structure might be related to its life cycle, behaviour and habitat preferences. The abundance and biomass were high variation at S1−S3. Abundance and biomass were found high between December-May (west monsoon/wet season), but the highest biomass in March. Sub-area A1 and A2 have high crabs abundance and biomass during February- May. Based on abundance and biomass, the potential area for fishing thorough in the year was at the S2, but it was about eight months at S3. Male and female crabs have an asymptotic carapace width 189.5 mm and 191.50 mm, while both sexes have high intrinsic growth rate and short life span (less than 3 years). Estimated males and females reach Lm50 was ~7.32 and ~8.4 months, while female Lr50 was ~10.0 months. The period of high potential females for spawning was 16.0 months. Recruitment pattern seem to be synchronize to spawning and it was occurred almost round year, but it has two peaks season (AprilMay and AugustSeptember). Both seasons were overlapping and the highest recruitment indicated only once in the years (AugustSeptember). This pattern seems to be the same with peak of crabs abundance and fishing season. Exploitation ratio was 0.76 year-1, so it tends to be over-fishing. This condition might be affected by intensive fishing pressure since the size at first capture (Lc50) less than Lm50; the proportion of caught crab consisted of juvenile and sub-adult ~17.1% and ~7.4% of total individual and catch volume, the proportion of caught BEF/NBF and BEF/total individual ~16.2% and ~6.4% at S1−S3. The Lc50 should be at 115 mm CW under the current exploitation ratio. Optimum of relative yield and biomass per-recruit (Y’/R and B’/R) could be obtained at the Lc50 between 115−135 mm CW. Fishing gears dominated by bottom set gill-net. Fishing season known by the fisher as peak, medium, and low. Peak fishing season at S1−S2 was during December/ January−March/April, medium season during June/July and November-beginning of December, while low season during July/August−October. Peak fishing season at S3 was during February−May, medium season at June-July and November-January, while low season August− October/November. Fishing by the fisher for approximately 20-30 fishing vessels occurred in part of S1−S2 at low season. Fishing enterprises have high benefit. Fishing at S1 was quite feasible and high efficiency compare to fishing at S2 and S3 during peak fishing season. However, caught crabs at S1 comprise of juvenile and subadult (pooled data) ~33.6% and ~21.3% of total individual and catch volume. Fishing enterprises at all stratification areas in the year have high of mean benefit per-month, but it has minus benefit during low fishing season and would be increased fisher debt to middleman or supplier (mini plant owners). Tactical and integrated management measures are proposed in harvest strategy, such as: (a) minimum legal size (MLS) application in accordance to current fisher economic condition at 105 mm CW and would be increased gradually to reach optimum Y’/R and B’/R based on stock status; (b) MLS regulation in conjunction with nursery habitat conservation and using trap fishing gear with escape vents; (c) close fishing season during August−October; (d) fishing gears standardization and fishing ground arrangement; and (e) releasing of live caught BEF. Implementation of management measures above shall be companied by data collection of capture crabs per-unit of effort, alternative livelihood development during closed season, improved fisher cohesion and capacity as well as fisheries management policies improvement.
- DT - Fisheries