Spawning habitat selection by Octopus vulgaris: New insights for a more effective management of this resource
Introduction
Cephalopod landings in Galicia (NW Spain) accounted for 11,633–7190 tonnes in the period 2011–2014 (RAGG, 2014). The most important cephalopod species in Galicia is the common octopus (Octopus vulgaris Cuvier, 1797), which yielded an average annual catch ranging from 3405 to 2586 tonnes between 2011 and 2014, with revenues of over € 12.8–13.7 million in that period (RAGG, 2014).
Galician small-scale creel fishery targeting O. vulgaris is seasonal and includes the Rías. The species is caught on sea bottoms ranging from 5 to 7 m to a maximum depth of 150 m (Otero et al., 2005, RAGG, 2006). Nowadays, 1289 boats have permission to use common octopus baited creels, and a total of 710 boats with permission landed catches during 2013–2014 fishing season (Molares, 2015). Approximately 320 boats operate in the Ría of Vigo and adjacent areas in 2004 (RAGG, 2006), which include the National Park of the Atlantic Islands of Galicia (NAPAIG).
The current management measures for the O. vulgaris fishery in Galicia is complex (RAGG, 2006, Molares, 2015), and they are subject to Annual Management Plans (AMPs). The main management measures to 2014–2015 AMP in the western Atlantic zone of Galicia include: (1) a period of fishing closure from May 31 to July 1; (2) a minimum legal weight of 1 kg per individual; (3) a maximum number of fishing creels that varied from 200 to 600 per crew member depending on boat register tonnes; (4) a schedule of fishing of 6:00 a.m. up to 17:00 p.m. with mandatory rest the weekends; and (4) a maximum daily catch quota of 350 kg per boat (Xunta de Galicia, 2014). Despite these or very similar management measures in each AMP, O. vulgaris landings decreased from 4205 tonnes in 2010 to 2586 in 2014, which represents a drop of 61% (RAGG, 2014). However, other Galician official source valued that landings drop in 50.1% (Xunta de Galicia, 2014). Moreover, the poorly planned anthropogenic impact in the Ría of Vigo (Galicia, NW Iberian Peninsula) caused the disappearance of fertile areas of fishing and shell fishing, the exhaustion (and even the disappearance) of some aquatic living resources, and the loss of biological and cultural diversity (Guerra et al., 2008).
One of the most interesting conclusions of the GESCIES project carried out by Ourens et al. (2010) was the urgent need for new regulation of the fisheries within the NAPIAG. The authors emphasised the regulatory aspect because of the unavoidable need to harmonise the requirement of a marine conservation area with the requirements of sustainable resource exploitation by the fishing industry.
Managing the small-scale fishery of Galicia is difficult due to its great complexity (Freire et al., 2002), and the situation of O. vulgaris is not an exception (Otero et al., 2005). An approach that involves the comprehensive planning and regulation of human activities in the context of a complex set of interacting objectives with the aim of minimising social conflicts, while ensuring long-term sustainability of the O. vulgaris artisanal fishery, would be very appropriate. This Integrated Management (IM) recognises the need for protecting the ecosystem, as well as addressing the implications of multiple uses, and aims at sustainable development (García et al., 2003). One of the difficulties in adopting this type of governance is to define and characterise the habitats and ecosystems to be managed (Forcada, 2007). We consider that this approach could be feasibly applied in those areas for brooding O. vulgaris females, considering that the spawn of this species is commonly set in easily protected rocky crevices (Hanlon and Messenger, 1996, Mangold, 1983).
Understanding the relationship between environmental variation and nurseries, spawning grounds and successful recruitment is an emerging issue and has been undertaken for O. vulgaris using both fishery-dependant and fishery independent data (Faure et al., 2000, Garofalo et al., 2010, Katsanevakis and Verriopoulos, 2006, Moreno et al., 2014, Otero et al., 2009, Pierce et al., 2008). Den ecology of O. vulgaris on soft sediment and the availability and types of shelter were studied by Katsanevakis and Verriopoulos, 2004a, Katsanevakis and Verriopoulos, 2004b. However, other types of requirements for bottom substrate have not been closely examined for their influence on common octopus habitat use, and, except for some few and recent studies (e.g., Mereu et al., 2014), the specifics of this species’ habitat use remain largely unknown. There is, however, some previous information on other octopus species (e.g., Enteroctopus dofleini by Hartwick and Thorarinsson, 1978 and Schell, 2002; Octopus tehuelchus by Iribarne, 1990; Octopus bimaculoides by Cigliano, 1993; Octopus cyanea by Forsythe and Hanlon, 1997; Octopus tetricus by Anderson, 1997; Octopus insularis by Leite et al., 2009) that indicated protection from predators and dominant conspecifics, including den availability, preference for back reef over reef top, fore reef habitat, hard bottom substrates, depth and some hydrographic parameters influence habitat selection and use. However, information on the spawning habitat selection for any octopus species is very scarce (Moreno et al., 2014).
The main goal of this paper is to identify and characterise preferential spawning habitats for O. vulgaris (PSHO) within the NAPAIG. Specifically, we seek to target: (1) the role of bottom substrate characteristics, depth and seasons, and (2) the role of the fishing creels. The hypothesis underlying this work is that common octopus brooding females carefully select the habitat in which to deposit their eggs. We also aim to confer potential insights for management plan achieving sustainable yield of this meaningful marine resource in NW Iberian waters based on PSHO.
Section snippets
Study area
Observations were made in the NAPAIG, which has a total maritime surface of 7285 ha and comprises the islands of Cíes, Ons, Sálvora and Cortegada. Within NAPAIG, our work was carried out around the Cíes Islands whose maritime area covers 2637.77 ha and is located at the mouth of the Ría of Vigo (Fig. 1).
Visual censuses
Four visual censuses were performed monthly, from April 2012 to April 2014, in subtidal areas of the Cíes Islands. The location of each visual census was performed using GPS. Two or three scuba
Results
A total of 93 visual censuses were undertaken off the Cíes Islands (NAPAIG) from April 2012 to April 2014 (Table 1; Fig. 1). These censuses comprised 104.2 net diving hours. The distribution of censuses by geographic location, date and depth are shown in Table 1. A total of 41 cells (28 in the inner side and 13 in the outer side of the islands) were sampled. The average swept area in each census was 1.33 ha and the total swept area was 123.69 ha (Table 1), equivalent to 4.69% of the total marine
Methodology
We were unable to find a standardised protocol to measure the area swept in each visual census (SAC) in the scuba-diving literature. Consequently, we had to develop an original methodology. This methodology provides fairly accurate data on the swept area in each visual census.
The characteristic behaviour of brooding O. vulgaris females, in which they stop leaving the den to forage for food (Hanlon and Messenger, 1996), is the main reason why females are not amenable to fishing gear or
Acknowledgements
We wish to acknowledge Alex Chamorro, Francisco de la Granda (IIM, CSIC, Vigo), Enrique Poza (ECITMAT) and José Castro, José Antonio Fernández-Bouzas, Montserrat Martínez and Mercedes Olmedo and the keepers of the NAPAIG for their invaluable assistance in the development of various aspects of this project. Many thanks also to Jacobo de Uña, Professor of Statistics at the University of Vigo, for his valuable advice and, above all, for enabling Marta Sestelo to join our research team. M.
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