Bull Mar Sci. 92(2):191–206. 2016 research paper http://dx.doi.org/10.5343/bms.2015.1041
Catch variation and demographic structure of common dentex (Sparidae) exploited by Mediterranean artisanal fisheries
1 University of Corsica Pasquale Michel Marengo 1, 2 * Paoli, UMR 6134 CNRS-UCPP 3 Science for Environment, 20250 Anthony Pere Corte, France. Bernard Marchand 1, 2 3 2 University of Corsica Pasquale Pierre Lejeune Paoli, UMS 3514 CNRS-UCPP Stella Eric DH Durieux 1, 2 Mare Platform, 20620 Biguglia, France. 3 STARESO research station, 20260 Calvi, France. ABSTRACT.—The common dentex, Dentex dentex (Linnaeus, 1758), is an iconic coastal fish found in the * Corresponding author email: Mediterranean Sea. Despite its ecological and economic
For centuries, artisanal fishing has been an essential economic activity and one of the pillars of Mediterranean culture and diversity (Guidetti 2012). Artisanal fishing is performed along the entire Mediterranean coast, with a considerable number of vessels (about 42,000) and fishers (about 280,000) (Griffiths et al. 2007, Morales-Nin et al. 2010). Artisanal and small-scale fisheries involve commercial fishers operating in small boats (<12 m in length), exploiting areas near the coast (0–200 m deep). They use a large number of gears and techniques, are typically manned by one or two fish- ers, and target a diversity of species (Colloca et al. 2004, Forcada et al. 2010, Lloret and Font 2013).
Bulletin of Marine Science 191 © 2016 Rosenstiel School of Marine & Atmospheric Science of the University of Miami 192 Bulletin of Marine Science. Vol 92, No 2. 2016
Due to the complexity of this activity, very few studies have quantified catch and effort (Maynou et al. 2011). The lack of quantitative, spatiotemporal data hinders op- portunities to: (1) develop management measures required to make fishing methods sustainable (Colloca et al. 2004); and (2) perform effective planning and manage- ment of threatened marine species (Pressey et al. 2007, Cleguer et al. 2015). In the Mediterranean Sea, currently 95% of the assessed stocks are overexploited (e.g., Merluccius merlucius Linneaus, 1758; Mullus spp.; Pagellus erythrinus Linneaus, 1758) and it is estimated that about 98% of the unassessed demersal species captured by bottom trawl are likely overexploited (Chato Osio et al. 2015). The global commer- cial catch of common dentex (Dentex dentex Linnaeus, 1758) has fluctuated over the last 60 yrs, declining substantially since the 1990s (Marengo et al. 2014). Specifically, landings declined steadily from a peak of 10,000 t in 1992 to 1300 t in 2012 (FAO- FISHSTAT 2015). The common dentex is a Mediterranean and Atlantic species. It inhabits the Mediterranean Sea south of 40°N, is found occasionally in the Black Sea, and oc- curs in the Atlantic Ocean, off the British Isles, Cape Blanc, Bay of Biscay, Madeira, Canary Islands, and Senegal (Morales-Nin and Moranta 1997, Marengo et al. 2014). This demersal (0–200 m) sparid grows to a maximum length of 100 cm and a weight of 13 kg, with a potential life span of >20 yrs (Ramos-Esplà and Bayle-Sempere 1991, Morales-Nin and Moranta 1997). As a high–trophic level predator, common dentex holds a key position in coastal marine food webs. Due to its large size, flesh qual- ity, and high commercial value, the common dentex is considered a trophy fish and is targeted by both artisanal and recreational fishers (Marengo et al. 2015). The main artisanal fishing gears used are trammel nets, gillnets, and bottom long-lines (Vandeperre et al. 2006). Fishing occurs throughout the year by modifying and/or combining these gears, depending on the season, habitat, and geographic area (see Marengo et al 2014). The common dentex appears to be a heavily exploited species and is classified by the International Union for the Conservation of Nature (IUCN) as ‘‘vulnerable’’ on the Red List of Threatened Species in the Mediterranean Sea (Abdul Malak et al. 2011). Despite its ecological and economic importance, fisheries data on this species are scarce and little is known of its space-time dynamics at regional scales. The aim of the present study was to obtain new information on the artisanal fishery of common dentex around the island of Corsica (NW Mediterranean) over a 5-yr period. Specifically, we: (1) analyzed the relationship between the catch per unit effort (CPUE, by weight and number) and possible factors driving the trends observed; (2) examined temporal and spatial trends in CPUE; (3) characterized the spatial distribution of fishing effort; and (4) compared the size distribution and age structure of catches.
Materials and Methods
Study Area.—The present study was performed around the island of Corsica (France; NW Mediterranean), which is located southeast of the French mainland and north of Sardinia (Italy) (42°N and 9°E). Corsica is the fourth largest island in the Mediterranean Sea and is characterized by a mountainous landscape and a dis- parate underwater morphology, featuring a steep descent (down to 3000 m depth, 10 km offshore) along the western part of the island (Pluquet 2006), and wide expanses Marengo et al.: Catch variation in common dentex off Corsica 193
Table 1. Sampling data from the artisanal fishing surveys in the Mediterranean Sea from the present study on Dentex dentex for April–September, 2009–2013. Fish and spiny lobster nets are meters of nets sampled. See Figure 1 for locations of Strata.
Strata Number of fishing trip Fish net Spiny lobster net Ajaccio 123 196,810 77,000 Bonifacio 94 119,270 153,280 Calvi 125 177,050 224,500 Cap Corse 82 94,550 145,580 Plaine 61 86,950 32,000 Porto 46 83,300 35,100 of relatively shallow waters (depths of <200 m) along the east coast. Corsica has a surface area of 8722 km2 and a coastline of 1047 km, representing more than half of the French Mediterranean coast. In 2015, 91% of the 195 active boats of the Corsican fleet were included in the artisanal fisheries group. This region is among the least intensively fished areas in the entire northern Mediterranean Sea; yet, to date, it has received little attention from fishery biologists (Relini et al. 1999). Data Collection.—Sampling was conducted from fishing vessels operating from the main fishing harbors and in the main fishing sectors of Corsica between 2009 and 2013 (April to September). Fish catch data were collected by scientific ob- servers onboard fishing vessels, in collaboration with professional fishers. The- on board observers inspected catches as they were hauled aboard. In total, 1425 km of nets were sampled over the 5-yr period, corresponding to 531 observation days (Table 1). In the Corsican artisanal fishery, a fishing trip cor- responds to a day of fishing for a single boat. Observed days and fishing gears were chosen at random. In total, 122 different fishers were surveyed during this study, representing about 63% of the Corsican fleet. Observed fishing trips corresponded to approximately 9% of estimated total number of fishing trips over the period of study. The survey provided information on fishing operations (fishing gear, length of nets, duration, depth, location, total catch, and weight). All the common dentex caught were measured and their total weight estimated. Total length (TL) was measured and size class was used for weight estimates for 2009–2010: Small, 0–40 cm = 0.3828 kg; Medium, 40–60 cm = 2.1053 kg; Large, 60–80 cm = 5.4653 kg (Marengo et al. 2013). During sampling, care was taken by the fishery observers in 2011 to measure the common dentex within 5 cm, then within 1 cm for 2012. For these years, weight estimates were derived using length-weight relationships (W = 0.011 TL3.06) published in FISHBASE (http://www.fishbase.org/; Morales-Nin and Moranta 1997). To standardize the data collected, total catch per unit of effort (CPUE) was ex- pressed for nets (including retained fish and discards) in grams per net section (50 m) (CPUE of biomass: g 50 m−1), and in number of individuals caught per net section (50 m) (CPUE of individuals: ind 50 m−1). In the present study, the “spiny lobster nets” were defined by a duration of the fishing operation (≥2 d at a depth ≥50 m) and all others were defined as “fish nets” (Pere 2012). For spatial analyses, six strata (fishing grounds) were designated as Ajaccio, Calvi, Cap Corse, Plaine, Porto, and Bonifacio (Pere 2012) (Fig. 1). These six strata reflected differing geomorphologic criteria (e.g., differences in depth and habitat type) as rec- ognized by the fishers. 194 Bulletin of Marine Science. Vol 92, No 2. 2016
Figure 1. Map of Corsica (NW Mediterranean, France), showing the six strata delimited by the black horizontal lines (Ajaccio; Calvi; Cap Corse, Porto, Plaine; Bonifacio), fishing grounds and Marine Protected Areas (“Reserve Naturelle de Bonifacio” and “Réserve Naturelle de Scandola,” respectively, in the Bonifacio and the Porto strata).
Age Determination.—Scales were collected from 90 individuals during the sur- vey for the years 2012–2013. For each individual, four scales were selected (from those that are non-regenerated) and cleaned with sodium peroxide to remove organ- ic debris (Panfili et al. 2003). Scales were placed between two glass slides for exami- nation using a stereomicroscope (Zeiss Discovery V20) connected to a camera (Sony XCD-U100CR) and a computer equipped with TNPC 7.0 software, an image analysis application dedicated to sclerochronology (the study of calcified tissues for ageing purposes). Readings were carried out twice by the main reader and independently by a second reader on a subsample of 50%, with no reference to the previous readings or fish dimensions. The second readings of the main reader were used after comparison with the second reader. These estimates agreed with those of the second reader and were therefore considered the most accurate (coefficient of variation = 3.72%, percent agreement = 78.81%). Data Analysis.—Catch data contained numerous zero values (90% of the nets sampled); thus CPUE values were log-transformed [log(CPUE + 1)] to better meet assumptions of normality for multivariate tests. A Student’s t-test was conducted to determine differences in catch rates (weight and number) between the two fishing gears identified (fish nets and spiny lobster nets). Multivariate analysis of variance (MANOVA) was used to test the effect of year (2009–2013), month (April–September), Marengo et al.: Catch variation in common dentex off Corsica 195
Table 2. Multivariate analysis of variance results for testing the effects of year, month, depth, and strata on artisanal fishery catch per unit effort (CPUE; weight and number) of Dentex dentex off Corsica and F-values of the tests used. SignificantP values in bold.
Year Month Depth Strata Fishing gear\factor F P F P F P F P Fish net CPUE of biomass 1.60 0.172 5.81 ˂0.001 2.24 0.014 6.56 ˂0.001 CPUE of individuals 1.17 0.321 3.95 0.001 3.82 ˂0.001 6.77 ˂0.001 Spiny lobster net CPUE of biomass 2.26 0.061 2.07 0.670 3.58 0.003 2.71 0.019 CPUE of individuals 4.11 0.003 3.47 0.004 3.62 0.005 2.28 0.050 depth (0 to >100 m deep), and strata on mean CPUE values (weight and number) for the fish nets and spiny lobster nets. The variables were not normally distributed (Kolmogorov–Smirnov test: P < 0.05); however, MANOVA is robust to skewed dis- tributions and performs reasonably well with non-normal data (Paukert and Wittig 2002, Ridenour et al. 2009). Whenever the MANOVA tests indicated a significant effect, a posteriori univariate ANOVA and post hoc Tukey’s honestly significant dif- ference (HSD) tests were used to identify factor influence on the response variables. Finally, catch and effort spatial distributions were mapped using a geographical in- formation system (GIS, QGIS 2.6.1).
Results
Catch Variation.—The depth distribution of common dentex ranged from 10 to 142 m. There were significant differences between the fish nets and spiny lobster nets for the CPUE of biomass (t-test = −2.31, P < 0.05) and the CPUE of individuals (t-test = −5.75, P < 0.0001). For fish nets, MANOVA results indicated that there were significant differences in the CPUE of biomass between months (P < 0.001), depths (P < 0.05), and strata (P < 0.001) (Table 2). Using one-way ANOVAs with post hoc HSD, CPUE differed significantly by strata (P < 0.001), with Cap Corse being higher than Calvi, Bonifacio, and Ajaccio (Fig. 2A). Seasonal variability in CPUE was observed, with a clear maximum during May followed by a decrease during the summer. May CPUE was significantly higher (P < 0.001) than in June, July, August, and September (Fig. 2B). CPUE (biomass) differed significantly by depth (P < 0.05), with 10–20 m exhibiting higher values than 30– 40 m (Fig. 2C). MANOVA results indicated significant differences in the CPUE of individuals between months (P < 0.01), depths (P < 0.001), and strata (P < 0.001) (Table 2, Fig. 3). For the spiny lobster nets, strata (P < 0.05) and depth (P < 0.01) significantly affected the CPUE of biomass (Table 2). The strata with the highest values were Plaine and Calvi. Two peaks of exploitation were observed for depth, with maxima between 50–60 m and 90–100 m (Fig. 4). The CPUE of individuals varied by year, month, and depth (MANOVA: P < 0.01), whereas it seemed unaffected by stratum. Catches (individuals) were maximal in May, at a depth of 50–60 m, and significantly higher (P < 0.001) in 2013 than in the other years (Fig. 5). 196 Bulletin of Marine Science. Vol 92, No 2. 2016
Figure 2. Dentex dentex fish net (2009–2013) catches off Corsica. Variations in mean log catch per unit effort (CPUE) of biomass (g 50 m−1 of net; ±SE) among (A) strata, (B) months, (C) depths. The values sharing no letter are significantly different (ANOVA + post hoc Tukey’s test: *P < 0.05).
Figure 3. Dentex dentex fish net (2009–2013) catches off Corsica. Variations in mean log catch per unit effort (CPUE) of individual (ind 50 m−1 of net; ±SE) among (A) strata, (B) months, (C) depths. The values sharing no letter are significantly different (ANOVA + post hoc Tukey’s test: *P < 0.05).
Figure 4. Dentex dentex spiny lobster net (2009–2013) catches off Corsica. Variations in mean log catch per unit effort (CPUE) of biomass (g 50 m−1 of net; ±SE) among (A) strata, (B) months, (C) depths. The values sharing no letter are significantly different (ANOVA + post hoc Tukey’s test: *P < 0.05).
Figure 5. Dentex dentex spiny lobster net (2009–2013) catches off Corsica. Variations in mean log catch per unit effort (CPUE) of individual (ind 50 m−1 of net; ±SE) among (A) strata, (B) months, (C) depths. The values sharing no letter are significantly different (ANOVA + post hoc Tukey’s test: *P < 0.05). Marengo et al.: Catch variation in common dentex off Corsica 197
Figure 6. Frequency histogram of exploited Dentex dentex ages (fish nets and spiny lobster nets) off Corsica (2012–2013; n = 90). Vertical dotted lines show approximate age at maturity for both sexes (from Morales-Nin and Moranta 1997).
Age Structure.—In total, 90 common dentex were sampled between 2012 and 2013 (79 individuals by fish nets and 11 individuals by spiny lobster nets). Using scales, six age classes were identified, ranging from 0 to 5 yrs old and older (Fig. 6). The scales of fish >5-yrs old were problematic due to poor definition of the annual rings and the abundance of false rings; therefore, they were classified in an age 5 yrs and over category. Analysis of age structure indicated that the sample was dominated by three age classes (2, 3, 5 and over). Thirty-eight percent of the specimens caught (age classes 0, 1, 2) had not reached the age of sexual maturity (2 yrs old, the size at which 50% of the population was mature, Morales-Nin and Moranta 1997). The fish nets caught a broad spectrum of ages (six classes), while the spiny lobster nets tended to target older specimens (≥2 yrs). Size Distribution.—In total, 140 common dentex were sampled between 2011 and 2013. Ranging 15–90 cm (total length, TL), length-frequency histograms indi- cated that a wide range of life-stages of this species, from juveniles to adults, were caught by the artisanal fisheries (Fig. 7). The analysis of size distribution showed that there was a predominance of four distinct modes (25–30, 30–35, 35–40, and 40–45 cm) (Fig. 7). The majority of sized fish were caught by fish netsn ( = 124, 89%),
Figure 7. Length-frequency histogram for Dentex dentex captured off Corsica (2011–2013; n = 140) by artisanal fisheries (fish nets and spiny lobster nets). Vertical dotted lines show approxi- mate total length at maturity for both sexes (from Morales-Nin and Moranta 1997). 198 Bulletin of Marine Science. Vol 92, No 2. 2016
Figure 8. Map of fishing grounds around Corsica showing Dentex dentex catch per unit effort (CPUE) in (A) biomass (g 50 m−1 of net) and CPUE in (B) number of individuals (ind 50 m−1 of net) (2009–2013). and 43% of the specimens (modes, 15–35 cm) had not reached the length of sexual maturity (35 cm, size at which 50% of the population was mature, Morales-Nin and Moranta 1997). Spatial Distribution of Catches.—The spatial distribution of fishing grounds showed a heterogeneous distribution of common dentex around Corsica. The high- est CPUEs (biomass and individuals) of the two gears combined were mainly located off the northwest of Corsica (Porto, Calvi, Cap Corse) (Fig. 8). These fishing grounds were mainly at depths between 10 and 50 m. The highest CPUEs (biomass) occurred during the spawning period, in 10 fishing grounds (red color in the Fig. 9A) within four stata (Porto, Calvi, Cap Corse, and Bonifacio). Several areas supporting very high common dentex abundance (weight) were located near/inside two Marine Protected Areas: “La Réserve Naturelle des Bouches de Bonifacio” and “La Réserve Naturelle de Scandola.” Analysis of size distribution within these “hot spot” areas showed that the specimens caught were mainly medium to large individuals (between 40 and 100 cm) (Fig. 9A).
Discussion
Understanding the impact of different gears that compete for the same resources is vital for improved management and conservation (Erzini et al. 2006). We found significant differences in the catch rates of D. dentex between the fish nets and spiny Marengo et al.: Catch variation in common dentex off Corsica 199
Figure 9. Map of fishing grounds off Corsica Island showing Dentex dentex catch per unit effort (CPUE) in biomass g 50 m−1 of net: (A) during the spawning period (April–May) for the years 2009–2013 for Dentex dentex caught by artisanal fisheries. Diagrams represent size class compo- sition [total length in cm (Small = 0–40, Medium = 40–60, Large = 80) within “hot spots” areas; (B) Spatial distribution of fishing effort over the sampling period (2009–2013). lobster nets. The fish net is the most profitable gear for common dentex fishing- con sidered in our study. Preference for the use of fish nets was apparent in areas where D. dentex was concentrated, such as rocky bottoms close to the coast, and gener- ally at depths of <50 m or along seagrass meadow [Posidonia oceanica (L. Delile)] edges. These results were consistent with previous studies in the Mediterranean Sea (Morales-Nin and Moranta 1997, Stobart et al. 2012). The artisanal CPUE of D. dentex varied over spatial and temporal scales. The catch rates by gear showed annual stability throughout the time period considered; how- ever, there was a noticeable increase in the number of individuals caught with spiny lobster nets during 2013. Fluctuations in fish abundance likely reflect not only on the effects of fishing or other anthropogenic impacts, but also on key processes such as recruitment, predation, and migration, which may also be linked to environmental fluctuations and climate change (Rijnsdorp et al. 2009). Seasonal variation in catch rates for the fish nets and the spiny lobster nets (both biomass and individuals) were found. The study revealed a seasonal pattern in CPUE for common dentex exploited by artisanal fishing around Corsica. Exploitation peaked during May, with lower rates during the rest of the year. Seasonal variation 200 Bulletin of Marine Science. Vol 92, No 2. 2016 observed in the present study was similar to that reported from other Mediterranean locations (Morales-Nin and Moranta 1997, Chemmam-Abdelkader et al. 2004). This seasonal increase of CPUE in spring was probably related to the reproductive period of this species, i.e., from March until June (Morales-Nin and Moranta 1997). This could be due to mating behavior prior to fertilization when individuals aggregate for reproduction, and/or seasonal migrations toward spawning grounds (Tzanatos et al. 2006, Henriques et al. 2013). Several species concentrate during spawning, which can lead to high CPUE values as fishers exploit this phenomenon (Petitgas et al. 2003). During this season (April–May), common dentex probably become more active or expand their area of activity to find conspecifics, therefore becoming more accessible to fisheries (i.e., catchability increases) (Muñoz et al. 2013). It is possible that increased fishing pressure on these species during this period might have a neg- ative impact on stocks (Tzanatos et al. 2008). Exploitation can contribute to the loss of spatial structure, and can potentially eliminate a stock or a genetic component of a metapopulation complex (Ciannelli et al. 2013). It is primarily the overharvesting of local populations/aggregations (i.e., sub-units; often those closest to ports suffer first) that erodes population structure (Ciannelli et al. 2013). Many aggregations have declined or collapsed due to overfishing, and several aggregating species are at risk of extinction (De Mitcheson et al. 2008). When fishing pressure is low, and a relatively small proportion of spawners is removed, aggregations persist (Sadovy et al. 1994). However, when fishing pressure removes a high proportion of aggregating fish each year, aggregations may quickly decline and cease to form within a few years (Sadovy et al. 1994). Recent evidence has shown that spawning aggregations may exhibit hy- perstability, a situation where CPUE of an aggregation is relatively stable over many years while the abundance of fish within the aggregation declines to critical levels and possible collapse (Sadovy and Domeier 2005). Fishing of spawning aggrega- tions can have a wide range of effects on target species and local fishery productivity (Nemeth et al. 2007). Our results revealed that the size class composition of the captures during the spawning period was mainly composed by mature individuals (Medium and Large). Fishing mortality tends to remove larger and older individuals from the population, especially the “Big Old Fat Fecund Female Fish” (the BOFFFF concept) (Field et al. 2008, Hixon et al. 2014). This often leads to strong truncation of age and size dis- tributions within fished populations (Berkeley et al. 2004). The removal of these -in dividuals can include negative effects: (1) shortening and change in timing of the spawning season; (2) behavioral change in migratory pathways; (3) decrease in the production of eggs and larvae; (4) decrease in the average survival potential of larvae; and (5) reduction in genetic heterogeneity (Berkeley et al. 2004, Field et al. 2008). Such effects may seriously damage the resilience of the populations, without giving a clear signal of decline in total population biomass in fisheries data (Hsieh et al. 2010). Clearly, effective management of fish spawning aggregations requires detailed -un derstanding of the dynamics of spawning behavior and associated fishing activities in both space and time (Sadovy and Domeier 2005, Erisman et al. 2012). The lower CPUE observed after the spawning period (June–September) may be partially due to the fishing-induced mortality of the preceding months, a change in social behavior after reproduction, or to return to normal non-reproductive behav- ior (Goñi et al. 2003). However, seasonal variations in catch rates are influenced not only by the natural behavior of the species, but also by a multitude of factors such as Marengo et al.: Catch variation in common dentex off Corsica 201 recent catch and income, market demand, fishers’ experience, weather conditions, distance to fishing grounds, and cultural/traditional norms (Tzanatos et al. 2006, Battaglia et al. 2010). The length and age frequency histograms of the common dentex showed that ex- ploitation (essentially during June–September) mostly removed small/young and im- mature individuals that had not yet reached first sexual maturity (Morales-Nin and Moranta 1997). Not allowing these smaller individuals to reach maturity may lead to population declines because they are removed from the population before having the opportunity to reproduce and contribute to its growth (Hupfeld and Phelps 2014). The exploitation of juveniles threatens the reproductive potential of the species (Lloret et al. 2012) and may lead to growth overfishing (Jennings et al. 1999), which has impor- tant economic repercussions, inasmuch as individuals could be caught later at larger sizes and thus attain higher prices (Jennings et al. 1999). It should be noted that the bottom long-line is used in Corsica for the exploitation D. dentex. However, since this activity is mostly practiced between September and April (period of low activity), data were not available and therefore not considered in our study. Our spatial approach revealed a heterogeneous catch distribution of common den- tex at the island scale and identified major exploitation areas. It should be noted that the sampling effort was not the same throughout the study area and for certain strata (Bonifacio/Plaine) the number of nets sampled was less consistent. However, the identification of the fishing grounds exploited clearly indicated the existence of “hot spots,” which may represent important habitats, such as spawning or nursery grounds. More than one habitat can be suitable for different phases of life history, especially for species that make ontogentic habitat shifts (Macpherson et al. 1997). The “hot spots” identified for common dentex were mainly located near or inside a marine protected area (MPA). Some MPAs are partially protected, but still permit certain forms of artisanal fishing. Catches of common dentex obtained by these arti- sanal fishers inside the MPAs were higher in biomass and density than outside them. This effect was described previously and was consistent with the results of previous studies, which showed that common dentex responds positively to protection from fishing (Forcada et al. 2010, Guidetti et al. 2010, Marengo et al. 2015). Variability in the recruitment of sparids is high at small spatial scales (Vigliola et al. 1998). Geographical differences may be partially explained by larval dynamics, which in turn depend on: reproductive success at the regional scale; larval mortal- ity; timing of settlement and/or post-settlement survival; and hydrodynamic regimes (García-Charton et al. 2004). A fishery’s impact on its target resources is determined in large part by the dis- tribution of fishing effort (Diogo et al. 2015). The use of GIS applications to allocate fishing effort constitutes a powerful tool that can help managers evaluate decisions taken and assess the effectiveness of policy changes (Caddy and Carocci 1999). The present study generated the first fine scale maps of common dentex exploitation dis- tribution and intensity during the fishing season. The significant differences in catch rates among depths also reflect the complex as- sociation of organisms with their environment. Depth and substrate associations of different fish sizes/ages may be linked to prey preferences and availability (Tzanatos et al. 2008). Previous studies have demonstrated that several environmental vari- ables linked to the bathymetric gradient, such as water temperature, salinity, or 202 Bulletin of Marine Science. Vol 92, No 2. 2016 other habitat features, may also strongly influence of fish assemblages (Demestre et al. 2000, Gaertner et al. 2005). In summary, the present study contributes to the knowledge of several aspects of common dentex exploitation by a typical Mediterranean artisanal fishery. A wide range of life-stages of this species, from juveniles to adults, were caught by artisanal fisheries around Corsica. However, we showed thatD. dentex were mainly caught during the reproductive season, when individuals concentrate in certain “hot spot” areas. A combination of management tools and specific measures to safeguard the repro- ductive potential of this species should be promoted, such as: (1) limiting or banning their capture through seasonal closures during the spawning season; (2) establish- ing a national minimum landing size exceeding the age at first maturity; and (3) the establishment of permanently closed marine reserves in key areas, such as spawning grounds and diversity hot spots (Nemeth et al. 2007, Matić-Skoko et al. 2015). Management will also need to keep in mind that multiple populations may be pres- ent within a single management area and that there is a risk of localized depletion if measures are inappropriate for all the populations within a managed area (Ballagh et al. 2012). One alternative for professional fishers may be to change their fishing activ- ity to another related activity, such as “fishing tourism,” which in other Mediterranean areas is already a well-established alternative source of income (Maynou et al. 2013). Sustainable use of fishery resources may involve a balance between protecting fish when they are most vulnerable to exploitation, while not stifling the need for fisher- ies to operate economically and efficiently (Hilborn 2011). The preferences of artisanal fisheries for “hotspots” identify them as important socioeconomic fishing locations whose sensitivity should be borne in mind in fishery management decisions (Pascual et al. 2013). It would be desirable to engage artisanal fishers in a partnership with scientists, and fishery and MPA managers. The more fishers are engaged in management processes, the more successful the management will be (Di Franco et al. 2014). Our study also shows the importance of continuous monitoring of the artisanal fisheries off Corsica. Little is known of the reproductive movements of the common dentex. Further studies using fishery-independent research (e.g., passive and active acoustic telem- etry, external tags, direct observations) are needed to better understand the repro- ductive behavior of this species over space and time. Understanding the factors influencing behavior during spawning may provide pre- dictable patterns that can be used in management. There is a pressing need to incor- porate knowledge of mating systems in stock assessments (Lloret et al. 2012). Finally, our results will be useful in designing sustainability scenarios, in terms of maximum sustainable yield for artisanal fisheries (Pascual et al. 2013).
Acknowledgments
This research was funded by the “Contrat de Plan Etat Région” (CPER, PhD grant to M Marengo), the “Fonds Européen de Développement Régional” (FEDER), the “Collectivité Territorial de Corse” (CTC), and the “Università di Corsica Pasquale Paoli” (UCPP). Special thanks are extended to the team of the STARESO for collecting data and the effective col- laboration of the fishermen. Marengo et al.: Catch variation in common dentex off Corsica 203
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