Spatial considerations for the Dakhla stock of Octopus vulgaris: indicators, patterns, and fisheries interactions
1 INRH, 2 rue Tiznit, 20200 Casablanca, Morocco
2 IRD, CRHMT, rue Jean Monnet, BP 171, 34203 Sète, France
Correspondence to A. Faraj: tel/fax: +212 22 484542; e-mail: faraj{at}inrh.org.ma or abdelmalekfaraj{at}yahoo.fr
Faraj, A., and Bez, N. 2007. Spatial considerations for the Dakhla stock of Octopus vulgaris: indicators, patterns, and fisheries interactions. – ICES Journal of Marine Science, 64: 1820–1828.The common octopus (Octopus vulgaris) is the target species of the cephalopod fishery that exploits two stocks, Dakhla and Cap Blanc, off southern Morocco (26°N 21°N), an area commonly referred to as the Saharan Bank. Octopus vulgaris is also one of the most abundant demersal species in this highly productive area, and plays a key role in the upwelling ecosystem. Spatial patterns of the main phases of the Octopus vulgaris life cycle of the Dakhla stock are described, using trawl surveys carried out twice a year from 1998 to 2003. Using geostatistics and spatial indicators, mature females and juveniles are analysed and mapped to characterize the main features of the spawning and recruitment phases. There are clear distinctions between the spatial patterns of the spawning and recruitment phases: juveniles are more coastal, less spatially dispersed, more anisotropically distributed, and more patchy. Our results suggest that the spatial pattern of the Octopus vulgaris Dakhla stock is different from that of the same species in other ecosystems such as the Mediterranean. GIS reveals that the spawning–stock biomass is globally more accessible to the industrial fleet than to the artisanal one, a finding contrary to contemporary thinking and with important resource management implications.
Keywords: accessibility indices, cephalopod fishery, Dakhla stock, geostatistics, GIS, Octopus vulgaris, recruitment, spatial indicators, spatial patterns, spawning
Received 7 April 2006; accepted 3 October 2007.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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The common octopus (Octopus vulgaris), a coastal cosmopolitan species, is abundant around the whole Northwest African coast (Guerra, 1981), from 10°N to 26°N (Hatanaka, 1979). The northern stock, also known as the Saharan Bank stock, is biologically very productive because of the upwelling system, and the area is recognized to be one of the richest fishing grounds in the world and supporting an intense fishery (Balguerías et al., 2002). Over the past three decades, O. vulgaris has often been the most abundant of >100 species of the demersal community exploited in the region. According to Balguerías et al. (2000), the reason was probably the heavy trawling activity that removed the species' main predators and competitors. Despite its importance in terms of abundance and productivity, and its key role in the ecosystem, the ecology of O. vulgaris is not well known. In particular, better understanding of the spatial aspects of its ecology would help to explain its interactions with other components of the upwelling ecosystem foodweb, and ultimately improve stock assessment and fisheries management.
Hatanaka (1979) distinguished two stocks within the fishing grounds off Morocco, the Dakhla stock (north of 22°00'N) and the Cap Blanc stock (between 22°N and 19°30'N; Figure 1). His analysis, based on 30'x30' annual catches of Japanese trawlers operating from 1964 to 1975 was the basis for current understanding of the spatial distribution of O. vulgaris and its seasonal and annual variability. However, spatial patterns inferred from commercial catch data, which are biased by the prevalent fishing strategy, do not reflect the intrinsic characteristics of the spatial patterns in the O. vulgaris life cycle. To eliminate such biases, a demersal stock assessment and monitoring programme was initiated in 1980 by the Institut National de Recherche Halieutique (INRH, Casablanca, Morocco) with the objective of quantitatively evaluating the status of the stocks and the impacts of fishing (Idelhaj, 1984a). The surveys permitted unbiased observations of annual and seasonal fluctuations of abundance, as well as observation of spawning and recruitment peaks reported by Hatanaka (1979). The main spawning peak is in spring (February–May), and a secondary one in autumn (October–December); recruitment peaks in autumn (September–November) and to a lesser extent in spring. However, the analyses are based on spatially aggregated data and do not describe the spatial characteristics of the development stages of octopus.
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Until the early 1990s, O. vulgaris was fished by EU (until the end of 1999) and Moroccan freezer trawlers using two types of gear, the so-called "Spanish trawl" and "Korean trawl". Nowadays, the cephalopod fishery has three sectors: industrial, artisanal (wooden boats using passive selective gears like pots and hand jigs), and coastal (ice-trawlers using an "atomic trawl"). As a consequence of the development of inland-based infrastructure and high prices, the last two sectors have burgeoned since 1993. They now account for more than half the nominal catches (INRH, 2002), and competition between the three sectors is intense, so in an attempt to manage the stock sustainably and because it is difficult to reduce fishing effort, a global quota system was implemented for each sector in 2002. Implementation of this system immediately led to a "race" for the resource, each sector trying to catch more than the others in the least time. Another consequence of the competition was disagreement on fishing areas. Because there are spatial interactions between the sectors, permanent conflicts persist and make the management system very complex. These conflicts underscore the need for the scientific community to appraise better the spatial patterns of the O. vulgaris life cycle.
To this end, a regional project led by the FAO was initiated in 1996 to characterize the spatio-temporal variations of demersal resources, and to define spatial and temporal management units on a biological basis (FAO, 1996). The project has demonstrated an area of good recruitment north of Dakhla (around 24.5°N) and has allowed the impact of the small-scale fishery on the parent stock to be evaluated by overlapping the area of activity of small-scale boats on the seasonal distribution maps of the spawning adult octopus (Taconet et al., 2000).
In this context, the objective of this study is to update previous work, using simple indicators to improve the description of the spatial pattern of the O. vulgaris life cycle and particularly of the Dakhla stock spawning and recruitment phases in spring and autumn. The spatial pattern is also compared and contrasted with that described for other areas (Canary Islands and the Mediterranean). Finally, the accessibility and sensitivity of the parent stock to the industrial and the artisanal fleets is re-evaluated.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Data surveys
Data are derived from the 1998–2003 monitoring programme trawl surveys carried out since 1980 by the INRH. A latitudinal profile of the mean standardized densities involving all surveys since 1984 (Boumaaz et al., 1997) has been developed. Since 1998, two surveys (RV "Charif al Idrissi"; grt 397 and 1100 hp; "Spanish" type bottom trawl, Idelhaj, 1982, adapted for cephalopods) have been carried out each year covering the continental shelf between 20°50'N and 26°N, from the coast to 100 m deep (Figure 2). A geostatistical stratified random sampling design (Conan et al., 1988) has been applied (Figure 2), each sample being located randomly inside a cell of 11 x 11 nautical miles. The average number of hauls per survey is 90, at an average rate of seven tows per day. New technology has allowed the area swept to be computed on the basis of horizontal trawl opening and towing speed. Hauls generally lasted 30 min, and average area swept was 65 000 m2, except between 1998 and 2000, when haul duration was reduced to 12 min and average area swept to 25 000 m2. Octopus yields were divided by swept area and expressed in terms of density, i.e. number of octopus per unit area (nautical mile2).
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Using the estimated mean area swept from the 1998–2003 surveys, data from the 1984–1998 period were converted into O. vulgaris densities. All data (1984–2003) were pooled, standardized by the survey average density, and averaged over rectangles of 2 x 2 nautical miles. This process was used under the assumption that standardization reduces the influence of the surveys on resulting densities, so generating a latitudinal profile of octopus densities from which the study area could be selected.
During the surveys, the catch of each tow was weighed and measured, and the sex and stage of maturity were noted. Four female stages were identified (Idelhaj, 1984b), stages 2–4 being mature (Boumaaz et al., 1997). In the study area, spring is the main spawning season and autumn the main recruitment season, so the spatial pattern of spawning was inferred from the density of mature females during spring surveys (Table 1), expressed as the number of mature females (nomf) per square nautical mile. The recruitment pattern is depicted as the density of juveniles during autumn surveys (Table 1), expressed as the number of juveniles (nojuv) per square nautical mile, juveniles being assumed to be the small commercial size categories, i.e. categories Tako 8 and Tako 9 according to Japanese classification (Boumaaz et al., 1997). Landing these categories is prohibited by Moroccan fishery legislation.
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Spatial indicators
Because of the spatial heterogeneity in densities and its interaction with the geometry of the study area (e.g. density decreases towards its offshore border), statistics per individual were used (Bez and Rivoirard, 2001). In this framework, which is derived from transitive geostatistics (Matheron, 1970), the key concept is to move from unweighted to weighted statistics where the weights are proportional to the number of individuals present at any location. As recommended for irregular sampling designs (Bez and Rivoirard, 2001), we also weighted the various statistics by the surface of influence Si of each sample i
[1,N], where N is the number of samples available for each survey. Octopus density (e.g. the number of mature females, nomf, per square nautical mile) was interpreted as a regionalized variable z(x), x denoting a point in geographical space. Sample values are denoted zi.
The average position of the population (
) was estimated by the centre of gravity of fishing locations weighted by octopus density (Bez and Rivoirard, 2000):
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The ratio of the smallest axis to the largest was used as a proxy for quantifying isotropy, i.e. the existence of preferred directions. When individuals are distributed in all directions and there is no preference, the two axes have the same length and the index of isotropy is 1. In contrast, when individuals are dispersed along a single axis, the index is 0.
The index of aggregation, Ia (Bez and Rivoirard, 2001), expressed in inverse units of surface, i.e. nautical mile–2, was used to measure the statistical heterogeneity of sample values (it is not, per se, a spatial indicator):
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) was quantified by
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Mapping
As acknowledged for many other species, octopus density distributions are skewed; a few large values at specific locations make any stationary hypothesis unacceptable. Octopus density is also highly dependent on the geometry of the field, indicating that habitat geometry could be the main driver of spatial structure (i.e. autocorrelation) in the species; a preliminary study on octopus (not reported in detail here, but unpublished) indicated such dependence. The octopus autocorrelation function and habitat geometry are therefore strongly connected. This led us to use a transitive geostatistical approach (Matheron, 1989; Bez et al., 1995), in which (transitive) covariogram models represent in rows the inner structure of octopus and its habitat geometry, the two being indistinguishable.
The covariogram (Matheron, 1970; Bez, 2002) of the regionalized variable z(x) is the function of a distance vector h, expressed here in nautical miles, equal to
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z(x)dx. The relative covariogram is expressed in inverse units of surface, i.e. nautical mile–2. It decreases universally from a maximum value gr(0), the index of aggregation, to 0 at far distance, referred to as the range, and quantifies the maximum diameter of the population distribution area in a specific direction.
To estimate the covariogram, we used the approach suggested by Bez et al. (1995) for stratified random sampling. Directions of computation are given in trigonometric angles, so hereafter "shoreline direction" corresponds roughly to 60° and "depth direction" to 150°. Distances were computed after coordinates were projected by simple cosine transformation in a rectangular reference system. Empirical relative covariograms were fitted automatically by non-linear regressions minimizing the sum-of-square residuals. The models used are constructed of a discontinuous component (8; also referred to as the nugget effect) to take into account the discontinuity at zero distance, and a continuous component of several nested spherical functions (9):
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Interactions between fishing areas and the spawning stock
To estimate the accessibility of the artisanal and the industrial fleets to the parent stock, we calculated the overlap between the spawning areas and the potential fishing areas (Figure 3), yielding accessibility indices. The spawning area (SR) was defined as the area where mature female density (nomf) exceeded 100 per nautical mile2:
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Artisanal fishing areas (SA) were defined as the combination of circles of 20 nautical mile radius centred on landing sites, and the industrial fishing area (SI) as the area offshore of the 12 nautical mile line without known rocky areas (Figure 3). By definition, SA and SI were fixed arbitrarily according to management rules (for the industrial fleet), and information on the landings made by INRH during sampling surveys for the artisanal fleet, and SR changed from year to year. We then calculated the ratio of the overlapping area as a percentage of the spawning area, SR
SA/SR and SRSI/SR. Owing to the uneven spatial distribution of octopus, we also calculated the percentage of total mature female abundance inside the two fishing areas by integrating spatially the kriging maps over the corresponding areas: |
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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The two stocks and the Dakhla stock area
The geographical profile based on all available data (Figure 4) shows a clear octopus density discontinuity between two areas, consistent with a two-stock hypothesis, the two separated by a transition area between 22°30'N and 23°N (Cap Barbas). We arbitrarily chose a limit at 22°43'N to select data which we assigned to the Dakhla stock. As the Cap Blanc stock is represented by too few samples and is possibly a northern component of a stock extending from the south, it is not considered further here, and we focus our attention on the Dakhla stock.
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Time-series of spatial indicators
For the study period, the time-series of spatial inertia (Figure 5) indicates that mature females were on average twice as dispersed as juveniles (p = 0.03). The convergence of the two life stage trends to a similar dispersion level in 2003 is mainly attributable to the extremely low O. vulgaris abundance during the final autumn survey and the consequential dominance of areas of low concentration. Both juvenile and mature female densities were strongly anisotropic (indices of isotropy <0.55; Figure 5), mainly because of the shape of the continental shelf they inhabit and the variability in O. vulgaris density, which tends to be smaller in a direction parallel to the shore than perpendicular to it. Indices of isotropy were smaller for juvenile than for mature females (p = 0.09), and decreased over the study period, down to 0.3 in 2003. The difference in the indices of isotropy is consistent with the idea of diffusion of the concentrated juvenile stage to a more widely spatially distributed mature female stage. Indices of aggregation (Figure 6, Table 2) for juvenile density were on average (7 x 10–4 nautical mile–2) higher than for mature females (3.6 x 10–4 nautical mile–2; p = 0.09). Juvenile densities were greatest in shallow water (20–40 m), and mature females more prevalent between 20 and 100 m (p = 0.03; Figure 6). Although time-series abundances are represented by symbols is proportional to size, abundance is not taken into account in computing the p-values. Finally, ellipsoids representing the centre of gravity and dispersion of the individual octopus show the systematic coastal preference of juveniles compared with the more dispersed and offshore distribution of mature females (Figure 7).
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To summarize (Table 2), juveniles are more coastally distributed (shown by the centre of gravity), less spatially dispersed (inertia), more anisotropically distributed (index of isotropy), and more patchy (index of aggregation).
Spatial distribution and its variability
Kriging maps derived from juvenile surveys (Figure 8) showed, for most years, large concentrations of juveniles near the coast, mainly in the area 24–25°N. Occasionally too, there was a concentration of juvenile octopus south of Dakhla (around 23°30'N), but at lower density than in the northern area. The relative importance of the southern area increased when northern density decreased. This phenomenon is very clear for autumn 2003.
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In contrast, kriging maps of mature female density showed great annual variability in spawning areas over the period 1999–2003 (Figure 9). The maps revealed that mature female densities were sometimes widely distributed over the shelf (2000–2002), but sometimes localized either in the southern part of the survey area (1999) or in its northern part (2003). In the last two cases, the pattern of densities was less dispersed and more aggregated (with areas of high concentration), similar to the pattern for juveniles. Comparing the empirical covariograms by depth (Figure 10) emphasized the fact that the spatial structure of mature females (spring surveys) was less stable from year to year than that of juveniles (autumn surveys).
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Artisanal fishery vs. industrial fishery
Accessibility indices estimated for the artisanal and industrial fleets showed that the spawning stock was more accessible to the industrial fleet than to the artisanal fleet, except in 1999 (Table 3, Figure 11). For that year, 98% of the parental stock was within the artisanal fishing area and just 4% inside the industrial fishing area, mainly because of the stock's apparently unusual compact, coastal distribution (Table 3). For all other years, the accessibility indices for the industrial fleet were in the range 60–93% and for the artisanal fleet in the range 29–43%. For four years out of five studied, therefore, the accessibility indices for the parent (spawning) stock were higher for the industrial fleet.
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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The transition area between the Cap Blanc and Dakhla stocks is discussed in Balguerías et al. (2002), using fishery data, and by Murphy et al. (2002), using genetic analyses, and is confirmed here on the basis of the entire time-series of available surveys. The transition area is characterized by a very low presence of O. vulgaris and by physical characteristics such as the width of the continental shelf and the timing of maximum upwelling intensity (Roy, 1991). Between Cap Blanc and Cap Barbas (Figure 2), the upwelled waters partially contain South Atlantic Central Water (SACW), which distinguishes the area from the northern Dakhla region and makes it among the most productive parts of the continental shelf off Northwest Africa (Minas et al., 1982; Jacques and Treguer, 1986).
For the Dakhla area, the results show a clear distinction between spatial areas of spawning and recruitment. Spawning took place over the whole continental shelf up to 100 m deep, with sometimes more spawning close to the coast, whereas recruitment seemed systematically to be concentrated at the coast, shallower than 50 m. Further, the area used for spawning was highly variable annually throughout the study period between 1998 and 2003, whereas the areas of recruitment were more stable.
We therefore present a hypothesis for the spatial patterns of O. vulgaris in Moroccan waters. The species spawns over the whole continental shelf; although there is not as yet direct scientific evidence of the distribution of paralarvae, the broad distribution of spawning suggests that O. vulgaris paralarvae will be found all over the continental shelf. Recruitment seems to be exclusively coastal; the reason for this is not clear, but it could involve transport to and retention in favourable areas, and/or passive selection of those favourable areas. However, it is very likely that recruitment success is strongly dependent on environmental conditions during the pelagic larval stage (Balguerías et al., 2002), which is fairly long (about two months). Indeed, such a link was demonstrated by Demarq and Faure (2000), who demonstrated the link between indices of retention and the extent of O. vulgaris recruitment off Mauritania and Senegal, and by Balguerías et al. (2002), who showed that high catch rates of O. vulgaris coincided with low temperature, which is itself indicative of strong upwelling. During the recruitment season in autumn, most of the O. vulgaris stock is concentrated in coastal upwelled waters, so presumably the juveniles disperse towards deeper water (Hatanaka, 1979) before ultimately settling (an ontogenic migration). For this hypothesis to be confirmed, however, the ontogenic migration of octopus needs to be confirmed by independent study.
The life cycle of O. vulgaris off Morocco seems to be different from that of the same species off the Canary Islands (Hernandez-Garcia et al., 1997). According to those authors, maximum catches correspond to concentrations of adult animals, which are recorded near the coast during the spawning periods of April/May and from September to November. Such high concentrations of O. vulgaris at the coast during a spawning season would be supported by a coastward spawning migration (mature animals moving from deeper water towards the coast to spawn). The same spatial pattern, i.e. a concentration of spawners near the coast and an extensive large distribution of recruits, is observed and described by Quetglas et al. (1998) and Sanchez and Obarti (1993) for the Mediterranean O. vulgaris population. In Spanish waters deeper than 50 m, octopus do not exceed some 11–12 cm mantle length, whereas in shallower waters, such a size falls at the lower limit of the length frequency distribution observed. According to Quetglas et al. (1998), the pattern is explained by a coastward spawning migration from offshore. In summary, therefore, there appear to be different spatial patterns of O. vulgaris distribution (Figure 12): in the Moroccan coastal upwelling system, a large spawning spatial distribution is supported by coastal recruitment and ontogenic migration; in the Mediterranean and Canary Islands systems, recruitment is spatially more diffuse and coastal spawning is associated with a spawning migration towards the coast.
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In this context, the conflict between the artisanal and industrial fishing sectors working on the Dakhla stock needs to be revisited. Traditionally such a conflict is based mainly on two issues. First, the pot, the main fishing gear used by artisanal fishers (Srour, 1992), is considered by the industrial sector to be targeting spawning animals with eggs, on a selective basis. However, analysis of the maturity stage of catches taken by pots for scientific purposes off Mauritania showed that pots do not target just spawning animals (Jouffre et al., 2002). The pot would be a species-selective gear for O. vulgaris, but a non-selective one for size. Second, the artisanal fishery, which operates inside 20 nautical miles from the coast, is also blamed by the industrial sector for targeting and destroying spawning stocks because it is traditionally believed that mature octopus migrate towards the coast to spawn, as in the Mediterranean (Mangold-Wirz, 1963). Converging to the main artisanal fleet fishing grounds, mature females would then be overexploited by that sector. Such a pattern is, however, still unsupported in Mauritanian waters (Caverivière et al., 2000; Jouffre et al., 2000), and is not supported by our results in the case of the Dakhla stock. Indeed, in the case of the Dakhla stock, as shown by the average values of the accessibility indices (Table 3), half the spawning stock is available to the artisanal fleet within its coastal fishing grounds, and 57% of the spawning stock is available to the industrial fleet on its offshore fishing grounds. These results agree with the seasonal synoptic maps (for 1984–1995) of Taconet et al. (2000), who estimated that half the parent stock was accessible to the artisanal sector. Although the parent stock was on average equally available to the artisanal fleet and to the industrial fleet over the study period, year-on-year fluctuations of the accessibility indices showed that, from 2000 to 2003, the spawning stock was more available to the industrial fleet, and that it was not exposed to greater fishing pressure by the artisanal sector than by the industrial sector.
If we consider that the octopus life cycle ends with an ontogenic migration that allows juveniles to colonize the shelf from their recruitment area (the hypothesis advanced here), the main developmental stage sensitive to fishing pressure would be the coastal juvenile stage, because it represents the bulk of the annual octopus stock. From this argument, juveniles concentrated near the coast during the recruitment period would need to receive some protection from fishing by all sectors, allowing them to colonize the shelf area widely. Currently, Moroccan fishing regulations do not allow fishing during September and October to protect recruitment. Taking into account the dynamics of the dispersion process for juveniles from their recruitment areas, this fishing regulation would ideally be complemented by a measure allowing the fishery to be opened in a staged manner, both spatially, from offshore towards the coastal recruitment areas, i.e. opposite to ontogenic migration, and temporally, i.e. small parts of the fishery at a time. Such measures would allow fishing first to take place in areas far from the source of recruitment, i.e. on animals that had time to grow and disperse.
In conclusion, we believe that this study has provided spatial insight into the life cycle of O. vulgaris. The spatial distribution pattern has been outlined and a modified approach suggested for exploitation of octopus off Morocco. Our belief is that currently the parent stock could potentially be overfished as much by the industrial fishery as by the artisanal fishery, if not more. In addition, we conclude that coastal O. vulgaris recruitment is crucial and should receive enhanced protection from exploitation. Comparison of the spatial pattern of octopus in the Dakhla area with spatial patterns of the same species in other areas has revealed clear differences which are likely to be linked to local hydrodynamic conditions. Therefore, a study of physical processes such as enrichment, concentration, and transport would enhance our understanding of the various strategies adopted by species such as octopus in the different areas.
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Acknowledgements |
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We thank our colleagues at INRH involved in stock assessment and the monitoring programme, particularly those who participated in the surveys, and the crew of the RV "Charif Al Idrissi" for support. Various colleagues and reviewers, especially Hassan Moustahfid, are acknowledged for the valued input during the drafting of this manuscript.
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References |
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