|Biología pesquera del besugo Pagellus acarne (Risso, 1826) del mar de Alborán|
Domínguez, J.B. (2000). Biología pesquera del besugo Pagellus acarne (Risso, 1826) del mar de Alborán. Microfichas Inst. Esp. Oceanogr. 14: 1-228
In: Microfichas. Instituto Español de Oceanografía. Ministerio de Agricultura Pesca y Alimentacion: Madrid. ISSN 1135-8491, more
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The quality and quantity of fish stock assessments carried out in the Mediterranean Sea is much poorer than those of the North Atlantic (ICES). This difference is mainly caused by the lack of information stored in databases, as well as the lack of appropriate methodologies adapted to the peculiarities of the Mediterranean fisheries. However, the classical assessment models are sufficiently validated to assess the dynamics of the fish populations exploited there. Moreover, the argument that the state of the Mediterranean fisheries depends on its own particularities rather than those derived from its exploitation -too extended in the Mediterranean case- should not be hindrance to the use of these models. As a case study of this premise, We decided to examine the population dynamics of the axillary seabream Pagellus acarne (Risso, 1826) in the northern sector of the Alborán Sea. The selection of P. acarne as a target species was mainly due to two reasons. First, this species is the most important in the South Mediterranean region in terms of landings. Second, since two different types of fleets exploit this species, it offers another interesting angle, related to the interaction of the fleets and the response of the exploited population to changes in the exploitation patterns. The latter aspect is relevant in the context of the Mediterranean fisheries, and has been insufficiently analysed; the present paper is the first to address this issue in the South Mediterranean region. This report is organised around the requirements of the population dynamics analysis that were used. Consequently, the first chapters describe the fisheries of P. acarne in the study area. Subsequently, details are given on the acquisition of the biological information from which the input parameters were derived for the application of the assessment models. Lastly, the resulting analysis is discussed. Many of the aspects studied differ insofar as the methods and even the materials Used. This is why they are arranged into different sections within each chapter. The results and discussion are likewise separated, with the intention of facilitating a better understanding of each section. The main body of the paper presents an exhaustive study of the species, which begins by analysing the exploitation pattern, describing the different gears and fleets exploiting the resource and its relative importance in terms of catch by type of fleet and their specific size composition With regard to the catch, the major fraction corresponds to the trawl fleet, which intensely exploits a narrower size range, which comprises specimens of smaller sizes (19-23 cm) than the artisanal fleet (20-25 cm). The population is concentrated in two well-defined areas: in the proximity of the Straits of Gibraltar and west of the Gulf of Almeria. There is a bathymetric segregation in relation to size of the specimens, of the bigger-deeper type. The spawners prefer the depths below 90 m, while the juvenile fraction of the population is distributed in shaUower depths. One of the most important sections is related to the species's biology, especially in those aspects necessary for the application of population dynamics. In the first place, the reproductive aspects of the species are considered. Secondly, the growth of the species is analysed, in relation to weight as well as the size-age relationship. In this chapter, special attention is paid to the rates of population decrease provoked by natural causes, as well as those derived from exploitation. A major reproduction-related issue is the resulting sex ratio of the population, which is clearly skewed towards males. This species is a typical protandreous hermaphrodite, which undergoes a change in sex in sizes between 20.5-209 cm. The spawning season in the Alborán Sea is quite extended in time, from spring (May) to autumn (October). The size at first maturity is 19 cm total length. This species is also a serial spawner, with several spawnings during its reproductive cycle With regard to growth, the size-weight relationship was rather different in the two years of analysis (1990 and 1991); hence, a unique relationship cannot be defined for the area. The size-age relationship showed a good fit with the Von BertalanffY equation, obtaining the following growth parameters Loo = 29.62; K = 027 Y to = - 1.36. The third part of the report is dedicated to the current state of the stock and to the population's response to exploitation pattern modifications. The results of the estimates of the fishing mortality vectors and their response to the input parameters, as well as to changes in the fishing effort, are presented. Simulations were carried out taking into consideration changes in the exploitation, using an initial starting point and analysing the results in time in terms of the yields per recruit and the recruit biomass. Lastly, the interaction between the fleets is considered, examining the impact on the population of the different exploitation strategies involving increase or decrease in fishing effort, applied by one or both types of fleets. The results of Virtual Population Analysis (VPA) were found to be highly sensitive to the different input parameters, more intense in the yields per recruit (Y/R) than the biomass per recruit (B/R). No over-exploitation seems to occur, and although it is difficult to establish a maximum sustainable yield, there is certainly an interaction between the fleets that exploit the resource, which is mainly influenced by the trawling fleet The report concludes with two recommendations: further intensification of the current fishing effort should be avoided; and changes in the exploitation pattern should be directed towards the artisanal component, which would lead to better yields without negatively affecting the state of the resource.