Search

Knowledge of the biology of economically important marine resources is essential in fisheries management. However, the methodologies developed by scientists are usually not applicable for assessing small-scale fisheries (SSFs), as they are too complicated and/or too costly. Classic methodologies taken from industrial fisheries are usually very data demanding and need of highly specialized personnel in modelling and stock assessment; both things are usually not present in most of the small-scale fisheries. In Galicia, shellfishing (that could be considered as SSFs) is an important economic and social activity that is co-managed between the Autonomous Administration (Xunta de Galicia) and 63 fishers’ guilds (called “cofradías de Pescadores”). Most guilds count with a Technical Assistant in Shellfisheries Management (TA), that serves as liaison between fishers, managers and scientists and is in charge of designing, implementing and supervising management plans (Macho et al., 2013). However, these experts count with scarce time and limited resources, being necessary simple assessment and management procedures. The present work aims to provide easy tools to monitor the species biology in order to incorporate it in the daily fishery management. We took as a model the sword razor clam (Ensis magnus (Schumacher, 1817), syn. E. arcuatus (Jeffreys, 1865)) because despite being the most important commercial species of Solenidae in Europe and one of the most important shellfisheries in Galicia (370 t in 2013) (http://pescadegalicia.com. accessed 4 January 2013), little is known about its biology. Furthermore, the fishers and the TAs from Galicia have observed some problems that could be related to the biology of the species. The most important one is that harvested razor clams break by the foot in a high proportion during winter, which could lead to a loss of up to 30-40% of the catch since these individuals have to be discarded. In the light of these problems, the specific objectives of this work were 1) to carry out mesoscale studies (~10 km) of the reproductive biology in order to adapt the possible variability to the management strategy (rotation scheme and reproductive close season), 2) to estimate the size at first maturity, 3) to construct a simple tool to easily monitor the gonad development stage of the resource, 4) to determine the cause of the foot breakage in order to reduce it as much as possible, 5) to study the spatial variability in recruitment for evaluate whether should be established seasonal closures for fishing in areas/periods where recruitment is more intense, 6) to ascertain which method was the most efficient in estimating the age of this species (counting external shell rings by eye vs acetate peels of the internal shell microstructure) and to test the possibility of using external rings as an easy management tool and 6) to evaluate the growth variability throughout the ria. To achieve the presented objectives, different samplings were conducted between 2008 and 2011 in three fishing beds of the Ría de Pontevedra (Galicia, NW Spain) (Brensa, Bueu and Ons located at the inner, middle and outer part of the ria) by scuba-diving at depths between 5 and 15 m. Thus, 20 samples of E. magnus were collected fortnightly/monthly during two years in order to study reproduction using the four following methodologies: gonad coverage (established by visually determining the percentage of coverage of the gonad), gonad smear (a new scale was created describing the follicles), gonadal condition index (GCI: gonad fresh weight/dry shell weight) and histology. The proportion of broken clams was recorded by the divers. Besides, 113 sites separated 50 m between them and distributed throughout the main beds of the ria were sampled by scuba diving in the shallower points and using a box corer in the deeper ones. The collected sand was sieved and processed by binocular microscope to calculate the recruit (size < 2cm) density at each site. Finally, 110 samples were collected in each bed to estimate the age. The reproductive cycle was characterized by a resting stage during summer and early autumn, initiation of gametogenesis in autumn and a period of successive spawning interspersed with gonad recovery during winter and spring (Fig. 1). However, differences in the reproductive cycle were detected between study sites. Thus, a 15-day to one month delay in advanced stages of gametogenesis and maturation was observed between the inner and the outermost fishing beds of the ria, as well as an extended spawning period in the outermost bed. In view of these results, the razor clam fishery in the Ría de Pontevedra has adapted the rotation scheme to the reproductive cycle, moving the harvesting from the inner to the outer bed during the maturation period and from the outer to the inner bed at the end of the spawning period. Size at first maturity was determined in 79 mm, lower than the commercial size established for the species (100 mm). A simple tool to monitor the razor clam gonad development stage was created based on a correspondence table which linked the easiest and fastest reproductive study methods (gonad coverage and gonad smear) to the most accurate but more time consuming methods (GCI and histology). Gonad coverage was confirmed as a good descriptor of the reproductive cycle of the sword razor clam because it changed considerably along the sexual development, following exactly the same evolution as the GCI used. Moreover, the gonad smear could be used to identify the maturation period, complementing the gonad coverage method. Using these methods, the reproductive stage of the razor clams could be monitored by TAs. Besides, a relationship between gonadal development and the proportion of broken razor clams was observed as most broken razor clams appeared during maturity, postspawning and gonad recovery (Fig. 1). This information could be incorporated in the daily management of the fishery since the Galician harvesting plans allows adaptive management according to stock characteristics, such as the reproductive state, at any time (Fig. 2). Monitoring the gonad development stage could prevent the highest razor clam breakage and contribute to reduce as much as possible the discards. Although the razor clam recruits were not easily found due to the burrowing behaviour and the final recruitment density was too low (average of 120 recruits per m2) to explain the razor clam production (34 adults per m2), the recruitment was higher (390 recruits per m2) at the shallowest points of the innermost bed of Brensa. The acetate peel technique proved to be the most suitable method for growth estimate and equivalence between methods is not possible (Hernández Otero et al. 2014). Following the acetate peel method, the growth of E. magnus is faster during the first three years of life, declines at about four–six years old and almost ceases in subsequent years, with the organisms entering into an asymptotic phase around the age of eight-nine years (Fig. 3). The mean population length is attained at four-five years. As observed by the fishers, growth differences were found between beds with the slower growth observed in the middle area (L∞ = 140.4, k = 0.40) followed by the innermost one (L∞ = 151.91, k = 0.40) and the outermost one (L∞ = 172.7, k = 0.33) reaching commercial size in 2.8, 2.3 and 1.7 years respectively. Some of the findings of the present study are being applied since 2011 in the razor clam fishery of the Ría de Pontevedra leading to a more efficient and sustainable exploitation and contributing to its certification as a sustainable fishery by the Marine Stewardship Council (MSC). Fig. 1 Temporal distribution of gametogenic stages of E. magnus and variation in the proportion of broken clams (black) at Brensa throughout the study period. Histological stages: (0) sexual resting, (1) start of gametogenesis, (2) advanced gametogenesis, (3) maturity, (4A) postspawning, (4B) gonad recovery and (5) exhaustion. Foot breakage degree: (0) No broken razor clams, (1) less than 20% of broken razor clams, (2) 20 to 40% of broken razor clams, (3) 40 to 60% of broken razor clams, (4) 60% to 80% of broken razor clams and (5) more than 80% of broken razor clams. Fig. 2. Management procedure proposed. Dark grey polygons indicate the stakeholders involved in shellfishing. Light grey polygons and white boxes show the recommended management actions. Fig. 3 von Bertalanffy growth curves fitted for E. magnus using growth marks revealed in the acetate peels.