Αλιευτική βιολογία της γαρίδας Melicertus kerathurus (Forskal 1775) στον Θερμαϊκό Κόλπο

Abstract

Introduction: The prawn Melicertus kerathurus (Forskål 1775) is one of the most characteristic and intensively fished species of Thermaikos Gulf (NW Aegean Sea). Its fishery supports considerably the economy of coastal regions of the gulf. The present thesis was aimed at assembling the first systematic elements on the fisheries biology of M. kerathurus in Thermaikos Gulf with emphasis on the study of its fishery. During the two open fishing seasons 2000/01 and 2001/02 a monthly sampling schedule was followed onboard commercial otter-trawlers in the fishing ground of the outer Thermaikos. During the closed seasons of 2000/01 and 2001/02 and in August 2004experimental hauls were conducted using trawlers and a coastal boat, respectively, in the inner and outer Thermaikos. In total, 99 hauls were conducted, while samples were also obtained from artisanal fishery. Age and Growth: The species exhibits intense growth dimorphism. Females have a wider size range, with a greater size (TL, CL) and weight (BW) than males. Age classes as estimated by Bhattacharya method (1967) varied monthly from 1 to 4 for females and 1 to 3 for males corresponding to 3 generations (0+, 1+, 2+). Young individuals of 0+ recruited in waves mainly at the end of summer and the beginning of autumn up to February. Different cohorts of 0+have approximately a time difference of 1-3 months. The von Bertalanffy growth curve which incorporates seasonal oscillation (Somers 1988) and parameters of growth (CL∞, K, C, WP) have been estimated separately for each sex by: 1. ELEFAN I, and 2. ‘Length at age data’, incorporated in FISAT II, with ELEFAN I analysis producing the most reliable estimates. CL∞, TL∞ and BW∞ were higher in females than males whereas K and Rn values were lower. The two sexes have similar WP values while C was lower in females. The to value, separately for the two sexes, was estimated by an empirical method. Growth performance indices φ', φ and Tmax were higher in females. Yearly size frequency distribution differed in both sexes, with significantly smaller median size during the second year. CL of both sexes increased in parallel with depth. Depth and % presence of young individuals (CL <30 mm) displayed a strong negative correlation. In adult females, the relationship BW-TL exhibits slightly positive allometry, whereas in males and young individuals of both sexes collected off the Axios estuary, a negative allometry is apparent. For adults and juveniles of both sexes, CL increases higher than TL and BW, with the exception of young males, where the relationship CL-TL does not deviate from isometry. Reproduction: In Thermaikos Gulf M. kerathurus exhibits intense reproductive activity and is characterized by a prolonged reproductive period. High percentages of females bearing spermatophore were recorded throughout the year, but their percentages increased considerably from April to July. Females with spermatophore presented all stages of gonad maturity (ST1-ST5), with their percentages increasing with size. High percentages of premature and mature stages (ST3 and ST4) were observed from May up to July. Spent gonads (ST5) were presented from May-October with higher percentages recorded in autumn. A positive relationship exists between increased maturity and % frequency of females bearing spermatophore. Large individuals (CL >40 mm) have higher percentages of spermatophore, appear first to mature and spawn, and, potentially, they may spawn 3 times during the reproductive period: 1. end of April-beginning of May, 2. June-July, and 3. August-October. Wet and dry weight of gonads and Gonadosomatic index (GSI) are differentiated considerably depending on size and the stage of gonad maturity. GSI remains low in winter, increases rapidly in spring with a peak in summer (July). An upward trend was also observed in autumn (October). In general, mean GSI increases with size. Length at first sexual maturity (CL50) based on the presence of spermatophore was lower than that based on the presence of maturity stages ST3 and ST4. Monthly sex ratio (females/males) ranged from 0.6-3.4 exhibiting a seasonal pattern. At the beginning of fishing period sexes were equally represented or a weak female dominance existed. In winter males were dominant whereas in April until August, a female predominance was observed.Fecundity (F) of mature individuals (ST4) is positively related to size. The number of ova ranged from 63,000 to 603,000. Fishery and population dynamics: In the fishery ground of Thermaikos Gulf, 95% of the hauls of the otter trawlers targeting the species are conducted within the geographical points: Cape Atherida - Mavroneri (Pieria) - N. Heraklia (Cape Smitha) (Chalkidiki) - Cape Epanomi (Thessaloniki). These points delimit the main trawling ground of the prawn in the gulf. During the two fishing periods trawlers operated over a depth range between 35 and 70 m, with ≈ 60% of hauls conducted at depths <50 m. The change in the most frequently trawled depth frequency observed between the two fishing periods indicates an inter-annual shift in the bathymetric distribution of the prawn. There was no significant difference in the mean duration of hauls between fishing periods. Nevertheless, haul duration and number of hauls presented an important time differentiation within each period. Haul duration and catch per unit of fishing effort(CPUE) showed a significant negative relationship. Mean CPUE varied significantly with month but not with fishing period or depth. Catches were at maximum at the beginning of the open season in autumn. This is attributed to the migration of young individuals from the shallow-water nursery areas towards the offshore ground characterized by high aggregation of recruits. The sharp decrease of CPUE during the following months is due to the fishing pressure, to prawn dispersal in the fishing ground and to prawn’s tendensy for longer burrowing in the substrate at the winteringperiod. Mean CPUEs of the first three fishing months differed significantly from each other as well from the following months which comprised a group of lowest CPUEs. The effect of depth was revealed when only the most productive months (October, November) were tested separately. Hauls shallower than <50 m showed significantly higher catch rates than deeper ones, while neither month nor period was significant. In the less productive months CPUEs were significantly affected by fishing season with higher catch rates in the second than in the first period. During the closed season for the fishery, CPUE of experimental hauls (5-45 m) varied significantly and the catchwas constituted mainly by young individuals (CL <30 mm). During these months (August, September), CPUE was affected solely by location: hauls were on average more productive in the inner than in the outer gulf. In August 2004 (with the use of a different net) the percentage of young individuals off Delta Axios was ≈100%. The area extending from the Delta Axios to Cape Atherida is one of the nursery areas of the species in the gulf. Commercial CPUE and relative abundance exhibited high positive correlation for all study period. In the region of Thermaikos the autumn rainfall regime differs significantly between years with much higher average in 2000 than in 2001. Thus, low rainfall may be the cause of delayed arrival of recruits on the fishing ground at the second period. For the period 1991-2007 total rainfall during October-December is significantly correlated with the corresponding yield. On the contrary, the relation of annual yield and of total annual rainfall of the previous year for the same period (1991-2007) is not significant. Natural (M), fishery (F) and total mortality (Z) were higher in males than in females. For both sexes F was higher than M. Exploitation rate (E) was slightly higher in males (0.54) than in females (0.52), and implies that the stock is under intense fishing pressure. For the period 1970-2007 the fishery production of M. kerathurus in Thermaikos exhibited important annual fluctuations, and in 2006 and 2007 marked a significant decrease. Thus, it appears that the prawn stock in Thermaikos is vulnerable enough to a series of factors which directly or indirectly affected it, i.e, intense fishing pressure, unfavourable environmental conditions for the reproductive stock and the new generation, anthropogenic interventions in the estuary systems, or, a combination of the above factors. Moreover, since a combination of the above factors can potentially exist, more administrative measures should be taken in the future in order to avoid a collapse of the prawn’s stock.