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All issues Volume 32 (2019) Aquat. Living Resour., 32 (2019) 25 Full HTML
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Aquat. Living Resour.
Volume 32, 2019
Article Number 25
Number of page(s) 20
DOI https://doi.org/10.1051/alr/2019022
Published online 19 November 2019

© EDP Sciences 2019

1 Introduction

Exploited deep-water species often exhibit high longevity, late age of maturity, slow growth, and low fecundity (Koslow et al., 2000; Norse et al., 2012). Many also aggregate on restricted topographic features such as seamounts, and are therefore highly vulnerable to overfishing, and have potentially little resilience to over-exploitation (Koslow et al., 2000). Little is known about the basic ecology of many temperate demersal/deep-water fish species despite the drive towards ecosystem conservation and long histories of commercial fishing in Europe (Johnson et al., 2013). Moreover, such knowledge is rarely based on full assemblage studies, where ecology of the species and interrelationships are considered (Merrett and Haedrich, 1997). In this context, the declining tendency of deep-sea fish stock levels (Norse et al., 2012; ICES, 2018) and the increased knowledge of the impact of bottom fisheries (Koslow et al., 2000; Pinho et al., 2014) highlight the importance of urgently and thoroughly understanding the life history characteristics, ecological aspects, and population dynamics of demersal fishes.

The Azores is a deep-water ecosystem and consists of a group of oceanic islands and seamounts located along the Mid-Atlantic Ridge (MAR), ICES subdivision 10a2. The Azores demersal/deep-water fishery is a multispecies and multi-gear fishery, with dynamics that are apparently driven by the main target species, the blackspot seabream Pagellus bogaraveo (Pinho and Menezes, 2009). However, other commercially important demersal teleosts, such as the blackbelly rosefish Helicolenus dactylopterus, the European conger Conger conger, the alfonsinos Beryx splendens and B. decadactylus, and the forkbeard Phycis phycis, are also caught and the target species change seasonally according to species abundance, fishing restrictions, and market demand. The Azorean fishery is relatively small-scale in which small vessels (∼90% of the total fleet are <12 m) predominate using mainly traditional “stone/buoy” bottom longline and several types of handlines nearby the islands and seamounts (ICES, 2018). In 2016, the Azorean fleet consisted of 762 vessels, which has remained stable since 2011, when 824 vessels were registered (Santos, 2017).

The fishing sector constitutes the main source of human impact in the sea of the Azores, and is an important source of income and development with high socioeconomic relevance in the region. The bottom longline fishery is one of the most important components in terms of the number of vessels (25.6% of Azorean vessels), commercial landings (26.6% of the landings), and economic value (mean = € 1.17M) (Diogo et al., 2015). However, the demersal resources of the Azores are intensively exploited (ICES, 2018). Commercial landings seem to exhibit a decreasing pattern for almost all the important commercial species (Pinho and Menezes, 2009; ICES, 2018). In some cases, the trends observed in the catches may reflect the variation in the fleet operating regime (variation of the target species, area and fishing gear, for example) and not of the real variation of stock abundance.

Fisheries management in the Azorean Exclusive Economic Zone (EEZ) is based on regulations issued by the European Commission (EC), the Portuguese government, and the Azores regional government. Under the EC Common Fisheries Policy (CFP), total allowable catches (TACs) for various species, specific access requirements, and conditions applicable to fishing for deep-water stocks were established in 2003 (Council Regulation (EC) No. 2340/2002 and Council Regulation (EC) No. 2347/2002). In addition, a box of 100 nautical miles (NM) was created under the management of fishing effort of the CFP for deep-water species (Council Regulation (EC) No. 1954/2003). Inside this area, the Member State may reserve fishing to vessels registered in the ports of the Azores archipelago. Technical measures have been implemented by the Azores regional government since 1998 (including fishing restrictions by area, vessel type and gear, fishing licenses based on landing thresholds, minimum lengths and closed seasons), and updated subsequently (Fig. 1). In recent times, fishers are encouraged by the Azorean authorities to exploit alternative deep-water resources to reduce effort on traditional stocks, but the unsatisfactory response of the market has limited such expansion (ICES, 2018).

Even though several management measures (e.g. closed areas, TACs, and minimum landing sizes − MLS) have been implemented, indications of depletion or over-exploitation of some Azorean demersal fish populations highlight that the process is fully ununderstood (Diogo et al., 2015). Species life history characteristics and population dynamics may be important to provide baseline information, which can be used to delineate stock assessment and management strategies (Bergstad, 2013; ICES, 2018), and determinants for the status and the time scales required for recovery to previous abundances of the species (Menezes et al., 2013). This stresses the importance of characterizing the general biological and ecological aspects of the demersal fish species and reconstructing the history of the Azorean demersal fishery. This information may provide valuable insights about the possible effects of artisanal fishing on the demersal fish community and may improve knowledge for better management advice for fisheries.

This study had the following objectives: (1) to update the annotated list of demersal teleost species occurring in the Azores EEZ (ICES subdivision 10a2); (2) to describe their depth distribution and related fishery information; and (3) to evaluate annual changes (1990–2016) in their abundance and length composition. It is hypothesized that demersal fish communities are vulnerable to overfishing, exhibiting: (i) a decrease in abundance indices of commercially important species, and (ii) length distribution of exploited fish populations changing toward small individuals caused by size-selective fishing.

thumbnail Fig. 1

Timeline of the key events and implemented technical measures in the Azorean bottom longline fishery.

2 Materials and methods

2.1 Study area

The Azores is a Portuguese archipelago (36° to 40° N, 24° to 32° W) formed by nine volcanic islands on the mid-North Atlantic (ICES subdivision 10a2) ( Fig. 2). As a volcanic archipelago of recent origin, the Azores islands are characterized by narrow shelves and steep slopes. The seafloor around the Azores is very irregular and rocky, has vast undersea mountain ranges, with around 100 large and 400 small seamount-like features (Morato et al., 2008).

The Azores Exclusive Economic Zone (EEZ) is about 1 million km2 with an average depth of 3000 m, but the exploitable fish habitat (i.e., areas with depths less than 600 m) is small covering only about 1% (7000 km2) of the entire EEZ (Menezes, 2003). The region is dominated by eastwards flows, which represent southern branches of the North Atlantic Current (NAC) crossing the MAR at 42°−48° N, and to a lesser extent the Azores current (Martins et al., 2007). Seasonal sea surface temperature (SST) variability is observed with average values ranging between 15 °C (winter) and 22 °C (summer) (Bashmachnikov et al., 2004; Lafon et al., 2004).

thumbnail Fig. 2

The Azores archipelago (North-East Atlantic Ocean) with depth contour layers and location of the nine islands (in black), and the limit of the Exclusive Economic Zone (EEZ).

2.2 Data collection and analyses

An up-to-date checklist of demersal teleost species from the Azores EEZ was provided based on information compiled from published scientific data (Santos et al., 1997; Harmelin-Vivien et al., 2001; Azevedo et al., 2004; Cardigos et al., 2005; Almeida and Biscoito, 2007; Menezes et al., 2009; Barreiros et al., 2011; Menezes et al., 2012; Porteiro et al., 2013; Ribeiro et al., 2017) and from Azorean spring bottom longline surveys. Each species was classified according to the geographic distribution and climate origin based on FishBase (www.fishbase.org).

Annual spring bottom longline surveys were conducted from 1995 to 2018 during cruises onboard the R/V “Arquipélago” (ARQDAÇO). The surveys followed a stratified random design and covered the Azores archipelago around the islands, banks, and major seamounts. Each sampling area was further divided into depth strata with 50 m intervals down to 800 m depth and for some pre-defined sets, down to 1200 m. The survey gear (bottom longline design for benthopelagic species) was very similar to the one employed by the commercial fishery locally known as “stone/buoy longline”. All fishes caught were tallied by species and strata, measured and weighed. Detailed information on survey design can be found in Pinho (2003), ICES (2018) and Pinho et al. (in prep.).

A relative abundance index (mean catch per unit of effort multiplied by the corresponding area size) was computed for each species collected during the ARQDAÇO by depth strata and statistical area. These abundance indices were summed across depth strata and statistical areas to compute an annual abundance index. A detailed description of the method for abundance estimation can be found in Pinho (2003) and Pinho et al. (in prep.). The length frequency composition was also computed by the statistical area and stratum as a proportion by length of the mean catch per unit of effort weighted by area size. These abundance indices by length were then summed across depth strata and statistical areas to compute an annual length composition.

Demersal teleost species were classified into three demersal fish assemblages according to the bathymetric distribution: shallow (0–200 m), intermediate (200–600 m) and deep (600–1200 m) assemblages. Each species was classified based on the mean depth of occurrence weighted by the relative abundance. This depth-aligned structure and zonation of the species are fully described by Pinho (2003) and Menezes et al. (2006).

The more abundant (proportion of the total abundance >1%) and frequent (frequency of occurrence >20%) species were selected for detailed spatiotemporal analyses. The abundance data were tested for normality using a normal probability plot and homogeneity of variance was tested using a Cochran test. The non-parametric analyses of variance (Kruskal–Wallis and Mood's median test) with Dunn's test as post-hoc comparison technique were performed to explore potential differences on species abundance indices among different depths (Kruskal–Wallis test, α = 0.05), and to test differences on species fork lengths (LF) among depths (vertical size distribution) and years (Mood's median test, α = 0.05). Statistical analyses were performed using STATISTICA (StatSoft, Inc., 2011; STATISTICA, data analysis software system, version 10. www.statsoft.com).

Annual landing data (catch, effort and length composition) from the Azorean commercial longline fleet (European Commission's Data Collection Framework − DCF and Lotaçor S.A. databases) were examined for each selected species. Standardised relative abundance indices were estimated from the fishery data (catch and effort by vessel) collected by inquiry to the captain at the landing (DCF, 1990–2016). This standardised abundance index estimation was based on the generalised linear modeling approach using a hurdle model (Lo et al., 1992, Ortiz and Arocha, 2004, Zuur and Ieno, 2016). The standardisation protocols assumed a hurdle model with a binomial error distribution and logit link function for modeling the probability that a null or positive observation occurs (proportion of positive catches), and a lognormal error distribution with an identity link function for modeling the positive catch rates on successful trips. Detailed information on standardisation procedures can be found on ICES (2018).

3 Results

3.1 Species composition

The updated checklist given in Table 1 details the 213 demersal teleost species recorded for the Azores archipelago. Most species were classified as subtropical (49%) and mainly from the Eastern Atlantic and Mediterranean areas (44%; Tab. 1). Eighty-one species belonging to 38 families were caught in the ARQDAÇO surveys (Tab. 2), which accounted for 81% of the total catch from 1995 to 2018. The Sparidae family was the best represented, with seven species, followed by Moridae and Muraenidae with six species. Nine species were recorded for the first time for Azorean waters: Paraconger notialis, Merluccius merluccius, Laemonema robustum, Gymnothorax vicinus, G. maderensis, Diplodus vulgaris, Pagellus erythrinus, Synodus synodus and Aphanopus intermedius. Thirty-seven species were assigned to a shallow assemblage (0–200 m), 30 species to an intermediate assemblage (200–600 m), and 14 species to a deep assemblage (600–1200 m). Phycis phycis, Pagellus bogaraveo, Conger conger, Helicolenus dactylopterus, Beryx splendens, B. decadactylus, Pontinus kuhlii and Mora moro were classified as the most abundant and frequent species and selected for detailed analysis (Tab. 2).

Table 1

Updated checklist of demersal teleost species from the Azores archipelago. A − the present study; B − Santos et al. (1997); C − Harmelin-Vivien et al. (2001); D − Azevedo et al. (2004); E − Cardigos et al. (2005); F − Almeida and Biscoito (2007); G − Menezes et al. (2009); H − Barreiros et al. (2011); I − Menezes et al. (2012); J − Porteiro et al. (2013); K − Ribeiro et al. (2017). Geographic distribution: CIRCG − circumglobal; CIRCTROP − circumtropical; EA − eastern Atlantic; MED − Mediterranean; END − endemic; AMPHIA − amphiAtlantic; MACR − Macaronesian; WIDE − wide distribution. Climate origin: TROP − tropical; SUB − subtropical; TEMP − temperate.

Table 2

Demersal teleosts recorded in the Azores archipelago (ICES subdivision 10a2), their assigned fish assemblage, relative abundance indices (% of the total abundance) and frequencies of occurrence (FO%) in the Azorean spring bottom longline surveys (1995–2018), and related fishery information (total landed and reference price for the period 1985–2017). Fish assemblage: S − shallow (0–200 m), I − intermediate (200–600 m), D − deep (600–1200 m). Species selected for detailed analysis are in bold.

3.2 Depth distribution of the most abundant and frequent species

3.2.1 Phycis phycis

The forkbeard Phycis phycis was caught up to 600 m, with the highest abundances (H = 1420.2, d.f. = 23, p < 0.001) in shallow waters (0–200 m). No statistical difference in the median L F (χ 2 = 33.1, d.f. = 23, p = 0.079) was observed along the depths; however, it was possible to visualise a bigger-deeper trend in Figure 3.

thumbnail Fig. 3

Relative abundance index (mean ± 0.95 confidence interval) and boxplot of length (L F) of demersal teleost species by depth stratum from the Azorean spring bottom longline survey (1995–2018). Boxes show the quartiles (25–75%), horizontal lines inside each box show the median, and the limits are shown with whiskers. Empty circle symbols identify outliers and asterisks are extreme outliers.

3.2.2 Pagellus bogaraveo

The blackspot seabream Pagellus bogaraveo was caught up to 900 m, with the greatest captures (H = 1412.2, d.f. = 23, p < 0.001) in shallow and mid-waters (0–600 m). Significantly larger individuals (χ 2 = 339.6, d.f. = 23, p < 0.001) were observed with increasing depth (Fig. 3).

3.2.3 Pontinus kuhlii

The offshore rockfish Pontinus kuhlii was caught between 50 and 650 m, with significant highest abundances between 100 and 400 m (H = 1175.8, d.f. = 23, p < 0.001). No statistical difference in the median L F (χ 2 = 10.2, d.f. = 23, p = 0.990) was observed between the depths (Fig. 3).

3.2.4 Conger conger

The European conger Conger conger was caught up to 850 m, with the highest abundances up to 600 m (H = 900.8, d.f. = 23, p < 0.001) (Fig. 2). No statistical difference in the median L F (χ 2 = 14.6, d.f. = 23, p = 0.910) was observed between the depths (Fig. 3).

3.2.5 Beryx spp.

The alfonsinos Beryx splendens and B. decadactylus were caught between 100 and 850 m and 200 and 950 m, respectively. Beryx splendens was significantly more abundant in mid-waters between 250 and 650 m (H = 915.5, d.f. = 23, p < 0.001), and B. decadactylus between 350 and 800 m (H = 853.9, d.f. = 23, p < 0.001) (Fig. 3). A significant bigger-deeper trend was observed for both B. splendens (χ 2 = 63.6, d.f. = 23, p < 0.001) and B. decadactylus (χ 2 = 82.8, d.f. = 23, p < 0.001) species (Fig. 3).

3.2.6 Helicolenus dactylopterus

The blackbelly rosefish Helicolenus dactylopterus was caught between 50 and 1100 m, with the highest abundances (H = 1263.0, d.f. = 23, p < 0.001) in mid-waters (200–800). No statistical difference in the median L F (χ 2 = 49.7, d.f. = 23, p > 0.05) was observed between the depths (Fig. 3).

3.2.7 Mora moro

The common mora Mora moro was caught between 300 and 1200 m, with the highest abundances in deep-waters between 600 and 1200 m (H = 1084.3, d.f. = 23, p < 0.001). A significant increase in the median L F (χ 2 = 256.8, d.f. = 23, p < 0.001) was observed as a function of depth increase (Fig. 3).

3.3 Fishery information and exploited fish population characterisation

Annual commercial landings of the main target species P. bogaraveo increased from 700 t in the middle of the 1980s to approximately 1000 t at the start of the 1990s (Fig. 4). Between 1990 and 2009, the annual commercial landings fluctuated around 1000 t, with a peak in 2005. During the period 2010–2017, commercial landings decreased to an average of 624 t. Standardised relative abundance indices from the fishery also oscillated over time, with an increasing trend until 1996, and a general decreasing behaviour after this period. Survey indices from 1995 to 2017 showed a high value every three years until 2005 and for 2016, 2017 and 2018 (Fig. 4). An overall and slight increasing trend was observed on the annual median LF from the fishery (χ2 = 54.2, d.f. = 26, p < 0.001) and survey (χ2 = 60.9, d.f. = 18, p < 0.001) but with high variability (Fig. 5). On average, 25–75% (Q3) of the individuals from both datasets presented LF between 21 and 40 cm (Fig. 5).

For P. phycis, P. kuhlii and C. conger, commercial landings and abundance indices from the fishery and survey presented a great fluctuation over time (Fig. 4), with a continuous decreasing trend since 2012. Phycis phycis was caught by the commercial fleet in significantly larger sizes in 1992 and 2001 (χ 2 = 157.5, d.f. = 25, p < 0.001), while individuals caught by the surveys did not indicate significant differences in the median L F (χ 2 = 9.6, d.f. = 18, p = 0.943) (Fig. 5). The Q3 of the individuals presented L F between 31 and 60 cm (Fig. 5). Pontinus kuhlii caught by the commercial fleet showed a slight decrease in the median L F (χ 2 = 76.2, d.f. = 26, p < 0.001) over time, while individuals sampled by the surveys did not show significant differences in the median L F (χ 2 = 11.2, d.f. = 18, p = 0.887) (Fig. 5). The Q3 of the individuals presented L F between 19 and 38 cm (Fig. 5). Conger conger showed an increasing trend on the annual median L F from the fishery (χ 2 = 388.9, d.f. = 26, p < 0.001) and considerable interannual variability in the L F data from the surveys (χ 2 = 58.3, d.f. = 18, p < 0.001) (Fig. 5). The Q3 of the individuals presented L F between 61 and 155 cm (Fig. 5).

Commercial landings and abundance indices from the fishery and surveys for B. splendens and B. decadactylus showed bimodal behaviour, with a peak in 1994 and another in 2009 (Fig. 4) and decreasing behaviour afterward. For B. splendens, the annual median L F from the fishery (χ 2 = 78.7, d.f. = 25, p < 0.001) and surveys (χ 2 = 35.7, d.f. = 18, p = 0.008) were significantly higher in the middle of the 1990s (Fig. 5). For B. decadactylus, the median L F from the fishery did not indicate significant annual differences (χ 2 = 33.9, d.f. = 25, p = 0.110), different from the median L F from the survey data, which showed significant interannual variability (χ 2 = 53.3, d.f. = 18, p < 0.001) (Fig. 5). The Q3 of the individuals presented L F between 21 and 36 cm for B. splendens, and between 26 and 44 cm for B. decadactylus (Fig. 5).

For H. dactylopterus, commercial landings and abundance indices fluctuated over time, with a more stable trend since 2001 (Fig. 4). The annual median L F from the fishery did not differ significantly (χ 2 = 39.0, d.f. = 26, p = 0.050), while the median L F from the survey data showed significant interannual variability (χ 2 = 69.5, d.f. = 18, p < 0.001) (Fig. 5). The Q3 of the individuals presented L F between 21 and 38 cm (Fig. 5). Commercial landings and abundance indices for M. moro showed high values at the start of the 2000s, followed by a decreasing trend with a great recovery in the last three years. This species did not indicate significant differences in the annual median L F from the fishery (χ 2 = 26.1, d.f. = 26, p = 0.458), while the median L F from the survey data presented significant interannual variability (χ 2 = 245.2, d.f. = 18, p < 0.001) (Fig. 5). The Q3 of the individuals presented LF between 41 and 63 cm (Fig. 5).

thumbnail Fig. 4

Commercial landings (bars) and relative abundance indices from the surveys (blue color) and derived from the commercial catch and effort (standardized CPUE) data (red color) of demersal teleost species in the Azores archipelago. Both abundance series have been scaled to their respective means. Dotted lines represent 95% confidence intervals for the standardized CPUE.
thumbnail Fig. 5

Boxplot of length (L F) of demersal teleost species by year (1990–2018) from the Azorean spring bottom longline survey (blue color) and from the commercial landings (red color). Boxes show the quartiles (25–75%), horizontal lines inside each box show the median, and the limits are shown with whiskers. Empty circle symbols identify outliers and asterisks are extreme outliers.

4 Discussion

4.1 Species composition

The 81 species caught in the ARQDAÇO surveys represent about 38% of the 213 demersal teleost species referenced for the area. These species are subtropical in origin, mainly from the eastern Atlantic/Mediterranean; or widespread species with global (wide), amphiAtlantic and circumglobal distributions (Tab. 1; Menezes et al., 2006). Their spatial structure is determined by trophic interactions between species (Colaço et al., 2013) and/or by the oceanographic conditions (e.g. bottom type, temperature, depth, water masses; Menezes et al., 2006). The largest abundance of deep-water species observed (i.e., species caught off-shelf and deeper than 200 m; FAO criterion; Pinho and Menezes, 2006) reflects the characteristics of the predominant marine habitats in the Azores EEZ. In this region, only 0.8% (7715 km2) of the area has depths <600 m, while 6.8% have between 600 and 1,500 m (ICES, 2018).

4.2 Depth distribution of the most abundant and frequent species

4.2.1 Phycis phycis

In the shallowest assemblage, the forkbeard P. phycis was the main demersal teleost species caught. Its occurrence is on hard and sandy-muddy bottoms near rocks, primarily between 100 and 650 m (Cohen et al., 1990). In the Azores, this species occurred up to 600 m depth. No information about vertical stratification by the size of this species has been available until now. The fishing gear (bottom longline) may not have been effective at sampling smaller individuals (few specimens under 30 cm were caught in this study; Figs. 3 and 5), but even so, a slight bigger-deeper trend was observed as previously reported for its congener P. blennoides (Cohen et al., 1990). Additional effort should be made towards sampling juvenile forkbeard, possibly with the use of smaller sized hooks or alternative fishing gear to better understand its spatial distribution.

4.2.2 Pagellus bogaraveo

The blackspot seabream P. bogaraveo occurs on various bottom types (rock, sand, mud) at depths of up to 700 m (Pinho et al., 2014). Juveniles (≤13 cm  L F) are found mainly in shallow and coastal zones (nursery areas), pre-adults (<30 cm L F) in intermediate zones, and adults (L F max. = 65 cm) in deeper and offshore zones (spawning areas) (Fischer et al., 1981; Pinho et al., 2014). In this study, this species occurred up to 900 m, and the reported vertical stratification by size was confirmed.

4.2.3 Pontinus kuhlii

The offshore rockfish P. kuhlii is a sedentary species that occurs on the hard substrate up to 650 m depth (Catarino et al., 2013). In the Azores, the ARQDAÇO survey covered the depth range and preferential habitat occupied by this species. However, little is known about the population structure. Our results did not show vertical stratification between juveniles and adults, and it is not reported in the literature. Further studies considering different areas (e.g., coastal and offshore areas) are encouraged to investigate the population dynamics and spatial distribution of this species.

4.2.4 Conger conger

The European conger C. conger is found on rocky and sandy bottoms, with juveniles occupying neritic and moving towards deeper waters upon reaching adulthood (Maigret and Ly 1986). Larger and sexually mature adults can be found in spawning places in mid-waters over 3000–4000 m depth (Lythgoe and Lythgoe, 1971; Bagenal and Kenney, 1973; Hayward and Ryland, 1995). However, this stratification was not observed in the Azores, although the maximum length reported (265 cm, Correia et al., 2009) suggests that almost every length class was present (14–260 cm; Figs. 3 and 5). It is hypothesized that the ARQDAÇO survey design may be unsuitable for this species because juveniles were underrepresented as well as larger and sexually mature males and females (M.R. Pinho, pers. comm.).

4.2.5 Beryx spp.

The alfonsinos B. splendens and B. decadactylus normally live close to rocky bottoms between 200 and 1240 m depth (Maul, 1981, 1986), with variation in the length-frequency distribution as a function of depth. Juveniles grow in demersal and shallower zones than areas typically inhabited by adults, but this reproductive strategy has been confirmed only for the splendid alfonsino B. splendens (see Santos et al., 2019). For the alfonsino B. decadactylus, a bigger-deeper trend was also reported, but its life cycle characteristics remain unknown because studies considering an entire population have not been developed until now.

4.2.6 Helicolenus dactylopterus

The blackbelly rosefish H. dactylopterus is found in soft/rocky bottom areas of the continental shelf and upper slope, usually between 150 and 600 m (Nunoo et al., 2015). In the present study, this species was caught up to 1100 m depth. No vertical stratification by size was observed in the survey data probably due to the gear selectivity that reduced the capture of juveniles. In fact, the recruits of this species preferentially inhabit the shallower waters and migrate down the slope as they become older and larger (Ribas et al., 2006; Consoli et al., 2010).

4.2.7 Mora moro

The deep assemblage was mainly represented by the common mora M. moro. This species occurs on the outer continental shelf and slope at depths ranging from 450 to 2500 m (Cohen, 1986). In the Azores, the depth range sampled may have restricted the occurrence of this species up to 1200 m. However, the relative abundance index from the ARQDAÇO surveys can be considered reliable for the upper fringe of the population and reflecting changes over years in abundance in the shallowest habitat of the species (Pinho et al., in prep.). Despite of this, it was possible to observe that the mean size increases with depth. The length frequency distribution clearly separating the juveniles from the adults is a common feature of this species (Rotllant et al., 2002).

4.3 Fishing exploitation effects and considerations for management

The Azorean fishery takes place at depths up to 1000 m, catching species from different assemblages, with a mode in the intermediate strata (200–600 m) where the most commercially important species occur (ICES, 2018). The commercial landings of deep-water species have shown a decreasing tendency since the mid-1990s as a reflection of the change in the fleet behaviour towards targeting the blackspot seabream P. bogaraveo. Since the 2000s, some local management actions have been implemented like the interdiction of the use of longlines in the coastal areas on a range of 3 NM from the shore (Ordinance No. 101/2002). Consequently, the fishing effort and landings from the MAR and some of the most distant offshore seamounts increased, suggesting that the 3 NM ban was an important trigger for the spatial expansion of bottom longline fishing effort in the offshore areas of the Azorean EEZ during the last decade (Diogo et al., 2015). All these changes in the fishing operation due to the management measures introduced (particularly the TAC/quotas, minimum landing size, license restrictions and fishing area restrictions) may explain the changes in the commercial landings of some species that were more vulnerable to the use of bottom longlines (e.g. P. kuhlii, C. conger, B. splendens, B. decadactylus, H. dactylopterus) or even became a commercially exploited resource (e.g.  M. moro).

The blackspot seabream is the main target species due to its high selling price (Pinho, 2003), and ranks first in terms of total landed value in the Azores region (Menezes et al., 2006; ICES, 2018). Its availability and market demand seem to drive the dynamics of the multi-specific demersal fishery (Pinho et al., 2014). In the Azores area, genetic and tagging studies demonstrated that the blackspot seabream can be considered a stock unit (Stockley et al., 2005; Hermida et al., 2013). This stock is formed by different meta-populations occupying different islands and seamounts separated by deeper areas where they did not occur (Pinho et al., 2014). Exploratory analysis suggested that the stock in the ICES subdivision 10a2 has been exploited unsustainably at levels above optimal and maximum sustainable yield (MSY) (see ICES, 2018). Because of this, several technical measures to limit catch and fishing effort have been implemented in recent years. Like so, the great oscillation in the observed abundance indices and the slight increase trend on the annual median sizes from the commercial landings may be a reflection of the TAC/quotas implemented since 2003 (EC No. 2341/2002) and the MLS afterward (Ordinance No. 1/2010; Ordinance No. 120/2016).

The possible effect of management measures on the observed abundance indices also seems to be true for the alfonsinos. Beryx splendens and B. decadactylus catches have been restricted since 2004 with the implementation of TACs (EC No. 2270/2004). The International Council for the Exploration of the Sea (ICES) currently assesses a single stock comprising both species in the North Atlantic Ocean. However, recent studies indicate that the stocks of these species in the Azores may be considered a discrete fishery management unit for B. splendens, while B. decadactylus should be considered a north-Atlantic stock (Santos et al., 2019).

P. phycis, P. kuhlii, C. conger and H. dactylopterus are other common and commercially exploited demersal teleost species in the Azores area. Additional studies (e.g. stock connectivity, tagging, and body morphometrics) should be developed to clearly understand the stock structure of these species in the North-East Atlantic. They present a relatively sedentary behaviour, supporting the possibility of the existence of local populations constituting different management stocks in the Azores. It does not seem to be true for the offshore rockfish P. kuhlii and the European conger C. conger, which are probably North-East Atlantic stocks (Correia et al., 2012; Catarino et al., 2013). On the other hand, some studies seem to support this assumption for the forkbeard P. phycis (Vieira et al., 2014, 2016) and the blackbelly rosefish H. dactylopterus (Aboim et al., 2005; Swan et al., 2006; Neves et al., 2011; Sequeira et al., 2010, 2012).

No change in the L F distribution toward small individuals caused by size-selective fishing was detected in this study. The high variability observed in the L F data indicate the need for more accurate studies considering alternative statistical analyses (e.g., generalised linear models) to examine the possible factors (e.g. depth coverage, gear configuration, soak time, and fishing area) that cause this variability. On the other hand, the abundance indices of some species appear to have declined overtime (P. phycis, P. kuhlii, C. conger, B. splendens and B. decadactylus), while others appear to be more stable (H. dactylopterus) or even increasing (M. moro). Any depletion in one of these exploited fish populations might be uncompensated by population migration from the other areas, at least at a sufficiently rapid rate to ensure resource sustainability (Silva and Menezes, 1996; Vinnichenko, 2002). Therefore, the management of the Azorean demersal fishery is a priority. Management strategies should be based on scientific information that is used to define control measures of the fishing operation. These actions are directed at maintaining a stock size that gives the maximum sustainable yield (or catch) through various regulations (e.g. TAC/quotas, number of vessels in the fishery, and rules on access to waters) aimed at controlling, either directly or indirectly, the level of fishing mortality (FAO, 2007–2019).

The present study provides valuable insights into population dynamics of commercially important demersal fish species, which are considered basic scientific information necessary for fisheries managers and policy makers to select appropriate management strategies. Further studies to understand more about the population structure and stock limits of demersal species are highly recommended. These also include studies to examine the factors that may bias abundance indices (e.g. depth coverage, gear configuration, soak time, and fishing area) through statistical modeling or field experiments. They will provide a basis for more accurate management of the Azorean demersal fishery.

Acknowledgments

This study was funded by the Azorean Government under the Data Collection Framework of the European Commission and the DEMERSAIS project. Régis Santos was funded by the IMAR Instituto do Mar, through a Post-doc fellowship (ref. IMAR/DEMERSAIS/001-2018). Wendell Silva was funded by the IMAR Instituto do Mar, through a scholarship (ref. IMAR/UNI/MAR/04292/2013 MARE/001-2018). Ana Novoa-Pabon was funded by an FCT Ph.D. fellowship (ref. SFRH/BD/124720/2016). The authors thank all who participated in field surveys and sampling processing onboard the R/V “Arquipélago”. Ricardo Medeiros (ImagDOP/UAz) is thanked for generating the map.

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Cite this article as: Santos RVS, Silva WMML, Novoa-Pabon AM, Silva HM, Pinho MR. 2019. Long-term changes in the diversity, abundance and size composition of deep sea demersal teleosts from the Azores assessed through surveys and commercial landings. Aquat. Living Resour. 32: 25

All Tables

Table 1

Updated checklist of demersal teleost species from the Azores archipelago. A − the present study; B − Santos et al. (1997); C − Harmelin-Vivien et al. (2001); D − Azevedo et al. (2004); E − Cardigos et al. (2005); F − Almeida and Biscoito (2007); G − Menezes et al. (2009); H − Barreiros et al. (2011); I − Menezes et al. (2012); J − Porteiro et al. (2013); K − Ribeiro et al. (2017). Geographic distribution: CIRCG − circumglobal; CIRCTROP − circumtropical; EA − eastern Atlantic; MED − Mediterranean; END − endemic; AMPHIA − amphiAtlantic; MACR − Macaronesian; WIDE − wide distribution. Climate origin: TROP − tropical; SUB − subtropical; TEMP − temperate.

In the text
Table 2

Demersal teleosts recorded in the Azores archipelago (ICES subdivision 10a2), their assigned fish assemblage, relative abundance indices (% of the total abundance) and frequencies of occurrence (FO%) in the Azorean spring bottom longline surveys (1995–2018), and related fishery information (total landed and reference price for the period 1985–2017). Fish assemblage: S − shallow (0–200 m), I − intermediate (200–600 m), D − deep (600–1200 m). Species selected for detailed analysis are in bold.

In the text

All Figures

thumbnail Fig. 1

Timeline of the key events and implemented technical measures in the Azorean bottom longline fishery.
In the text
thumbnail Fig. 2

The Azores archipelago (North-East Atlantic Ocean) with depth contour layers and location of the nine islands (in black), and the limit of the Exclusive Economic Zone (EEZ).
In the text
thumbnail Fig. 3

Relative abundance index (mean ± 0.95 confidence interval) and boxplot of length (L F) of demersal teleost species by depth stratum from the Azorean spring bottom longline survey (1995–2018). Boxes show the quartiles (25–75%), horizontal lines inside each box show the median, and the limits are shown with whiskers. Empty circle symbols identify outliers and asterisks are extreme outliers.
In the text
thumbnail Fig. 4

Commercial landings (bars) and relative abundance indices from the surveys (blue color) and derived from the commercial catch and effort (standardized CPUE) data (red color) of demersal teleost species in the Azores archipelago. Both abundance series have been scaled to their respective means. Dotted lines represent 95% confidence intervals for the standardized CPUE.
In the text
thumbnail Fig. 5

Boxplot of length (L F) of demersal teleost species by year (1990–2018) from the Azorean spring bottom longline survey (blue color) and from the commercial landings (red color). Boxes show the quartiles (25–75%), horizontal lines inside each box show the median, and the limits are shown with whiskers. Empty circle symbols identify outliers and asterisks are extreme outliers.
In the text

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