Spear fishing in Galicia is exclusively a breath-hold activity in which the spear fishers catch coastal fish (and some cephalopods) using spear guns. It is a recreational activity and spear fisher catches cannot be legally sold. Furthermore, the spear fishers must pay a fishing licence and their effort is controlled by limiting the number of fishing days and by imposing a bag limit that is currently 5.0 kg spear-fisher^–1 day^–1 (Xunta de Galicia 2009). By way of illustration, in 2007 the number of fishing days was 220, so the annual total allowable catch was 5819 t (5290 licences were granted in 2007).

Spearfishing competitions take place in coastal areas that vary between a few tens of metres and several kilometres long, mainly in the rocky reefs outside the bays and in areas exposed to waves (Pita and Freire 2014). The spear fishers have a limited time to obtain the heaviest catch without complying with the daily quota limit, but some of the rules are more restrictive than those that regulate recreational spear fishing. There are limitations regarding the minimum weight of the fish (1 kg) and species: only one Conger conger (L.), three Balistes capriscus (Gmelin) and five Mugilidae spp. are allowed per spear fisher. Moreover, cephalopods and fish of the superclass Agnatha, the class Chondrichthyes and the orders Pleuronectiformes and Lophiiformes of the class Actinopterygii are not permitted. However, in some of the first competitions, these rules were not strictly followed and some spear fishers made a few anecdotal bycatches of some of these fish species (Pita and Freire 2014).

The FEGAS archive (see Pita and Freire 2014 for a detailed description) contains comprehensive information on the length in hours, the number of spear fishers, the location and the date of each competition. It also contains the number of fish and the weight of the catch per spear fisher (the weight of C. conger was recorded separately) and the weight of the largest fish caught by spear fishers in each competition (trophy fish). In general, most of the trophy fish were identified correctly to species level but some species were occasionally grouped together under the same morphotype, or different morphotypes were used for forms of the same species. Thus, spear fishers differentiated between “pintos” and “maragotas” (see Villegas-Ríos et al. 2013a), which are in fact different forms of Labrus bergylta (Ascanius). The white seabream morphotype grouped together the species Diplodus annularis (L.), D. sargus (L.) and D. vulgaris (Geoffroy Saint-Hilaire) and the mullet morphotype included Chelon labrosus (Risso), Liza aurata (Risso) and L. ramada (Risso). The term species was used generically to refer to morphotype (Table 1).

It was hypothesized that the catches in a competition would be smaller when another competition had been held previously in the same area. In addition, the shorter the time between the competitions, the greater the decrease in the catch. To test this hypothesis, the information in the FEGAS archive was used to identify and group the competitions held in nearby areas. Thereby, 29 zones were obtained, with an average length of 15.8±7.4 (SD) km (Fig. 1).

In each of these zones, the rate of change in the CPUE between pairs of consecutive competitions
(T_C1→C2) was calculated as the ratio between the catches per spear fisher and hour of each pair of competitions:

(T_C1→C2) = CPUE_C2 ÷ CPUE_C1

where CPUE_C1 and CPUE_C2 are the catches (kg) per spear fisher and hour in the first and second competition, respectively.

It was hypothesized that the catches made in a competition would lead to decreases in the fish densities in the area. It would be expected that the shorter the time elapsed since the competition and the larger the catches obtained, the greater the impact on the fish assemblages.

To test these hypotheses, a rocky reef area around the Ría de A Coruña (43°22’N; 8°22’W) was selected and the densities of the fish assemblages were monitored by UVCs for four years (2003 to 2006). UVCs were used due to their low impact on the populations studied (Schmitt 2002), their great flexibility for all types of environments and species (De Girolamo and Mazzoldi 2001), and the fact that was proven to be the most suitable method in a study performed in the area (Pita et al. 2014). Monitoring was conducted in two coastal zones with similar orientations to waves and on both sides of the Ría: Mera, to the northeast, and O Portiño, to the west (Fig. 1). The UVCs were performed every three months, preferably in February, May, August and November.

Wave exposure is one of the physical factors that most determines the ecology of fish communities in shallow waters (Gust et al. 2001, Micheli et al. 2005, Pais et al. 2007). Therefore, in each zone one sector exposed to waves and one unexposed sector were selected and three fixed transects were positioned in each of the sectors. The UVCs were performed in the locations most frequented by spear fishers (an expert spear fisher with extensive knowledge of the area previously identified the locations). Although the UVCs were carried out in pairs for security reasons, the same diver, who had previous experience in carrying out UVCs, made all the observations (see Pita et al. 2014 for a detailed description of the UVC methods used here). Therefore, the intra-observer variability was kept under control. The transect width varied depending on the visibility, which was estimated according to a 1-m-marked guide rope. The depth and temperature were obtained with a dive computer (recorded every 4.0 s). Over the year, the water temperature varied between 9.0°C and 18.4°C and the maximum depth was 19.9 m.

The fish assemblages were censused along the 100-m-long transects. The divers swam slowly (5.0 m min^–1) along the transect, noting down the abundances of the species generally caught by spear fishers (cryptic species and smaller species of the families Bleniidae, Gobiidae, Labridae and Sparidae were not included). Fish were identified to species level, except for the family Mugilidae, which are difficult to identify in low-visibility waters.

In 2003, FEGAS spear fishers were requested to provide information on their fishing effort. A questionnaire was published on the FEGAS webpage (http://www.fegas.net) and in the magazine Espacio Submarino (which was received by all FEGAS members). The questionnaire asked spear fishers whether they participated in spearfishing competitions. It also asked spear fishers to quantify their fishing effort: number of fishing hours per day and number of fishing days per month, for each season of the year.

The index of vulnerability to fishing of each species was obtained from the list provided by Cheung et al. (2007) and from http://www.fishbase.org for those species absent from the Cheung et al. (2007) list. This index is based on ecological characteristics and life history of marine fish (Cheung et al. 2005) and has four levels of vulnerability: low (1-40), moderate (40-60), high (60-80) and very high (80-100). The vulnerability index of the catches of the recreational and the commercial fisheries was also estimated by the arithmetic mean of the vulnerability of the fish species, weighted by their respective catches (see Lloret et al. 2008).

Additive multiple regression models (GAM, Hastie and Tibshirani 1990, Wood 2006) were used to analyse (1) the relationship between the rates of change in the CPUE between pairs of consecutive competitions held in the same area and the time elapsed between them; and (2) the relationship between the densities of the species targeted by spear fishers, determined by UVCs, and the time elapsed since the completion of the last competition in the area. In addition, the influence of the total weight of the catch (kg) obtained in the competition was evaluated (Table 2).

To minimize undesired influences in the models used to analyse the changes in the competition catch rates (1), the maximum period considered between two competitions was one month. For the same reason, a one-month period was also the maximum time considered between UVCs and competitions in the models to analyse the changes in the fish abundances (2). In addition, competitions held close together (less than 30 days) were considered as a single event, the catch was added and the time was considered up to the end of the second competition.

Boops boops (L.) is a very frequent species that lives in highly mobile large schools and does not normally exceed 20.0 cm (Bauchot 1987). For this reason, it is not a target species for the spear fishers (it is absent from the FEGAS archive). Consequently, this species was excluded from the analysis (2) to prevent its natural dynamics from hiding the cause-effect relationships between competitions and the abundances of target species.

The models were fitted, selected and validated with the mgcv package (Wood 2000) of the statistical package R, version 3.2.2 (R Core Team 2008). Different models were assessed with the response variable transformed, with different error structures and link functions and using smoothed functions for the response variables, with penalized thin-plate regression splines (Wood 2003) and the flexibility by default of the mgcv package (Wood 2000). The most appropriate models were selected based on the Akaike criterion (Akaike 1973) whenever possible, and secondarily on the percentage of explained variance.

The gam.check tool of the mgcv package, which plots the deviance residuals against approximate theoretical quantiles of the deviance residual distribution, according to the fitted model, was used to check the residuals (Wood 2006). Models with over-dispersed and/or anomaly-distributed residuals were discarded. The predict tool of the mgcv package was used to obtain inferences from the definitive model.

The annual recreational catches of the spear fishers (i.e. recreational catches obtained out of competitions in 2007) were estimated by combining their seasonal effort, obtained from the questionnaire, with the seasonal average CPUE (kg spear-fisher^–1 h^–1) obtained from the FEGAS archive (N=13427). Thereafter, the estimated annual catch was distributed among the species by using the catch frequency of each species in the trophy fish record of the FEGAS archive, assigning the minimum frequency (0.01%) to the species censed in the UVCs that were not present in the trophy fish record. The catches assigned to morphotypes that grouped different species were distributed equally between the species under the common name (Table 1).

Based on the information contained in the FEGAS archive, 864 spearfishing competitions were held in Galicia between 1953 and 2007, in which 101 t of fish of 23 species (15 families) were caught (Table 1). Competitions lasted 4.7±1.3 h on average and in each of them an average of 22.1±12.9 spear fishers participated. Their mean CPUE was 1.6±1.9 kg spear-fisher^–1 h^–1.

A total of 244 pairs of competitions were analysed in 24 of the 29 studied areas (the maximum period considered between competitions was one month). The number of competitions has increased steadily over time, so most of the pairs of competitions analysed were completed after 2000 (57.4% of total), 38.1% were completed between 1990 and 2000, and only 4.5% before 1990.

The average time elapsed between pairs of competitions was 13.4±8.2 days. The average catch was 87.5±105.6 kg of fish per competition and overall, 21.1 t were extracted. The average rate of change in the CPUE between the pairs of competitions was 1.1±0.8. The model fitted to this rate of change (Table 2) did not show a time effect (p=0.342). Therefore, the catch obtained in one competition did not influence the catch obtained in the following competition in the area, independently of the time that elapsed between them (Fig. 2).

Bad weather and rough sea conditions prevented the completion of 24.0% of the scheduled UVCs. Therefore, the divers carried out 146 UVCs (48.7 h of observation) in the study area and 18757 fish belonging to 13 species (8 families) were counted. The average per UVC was 128.5±132.3 fish and the average density was 1491.2±1271.4 N ha^–1. The most frequent species were L. bergylta (present in 100.0% of the UVC) and Pollachius pollachius (L.) (89.7%), eight species showed medium frequencies (from 13.7% to 49.3%), and Mullus surmuletus (L.), Labrus mixtus (L.) and C. conger were counted only occasionally (from 2.1% to 3.4%). The most abundant species were B. boops (556.4±939.7 N ha^–1), L. bergylta (432.1±440.1 N ha^–1) and P. pollachius (266.9±538.0 N ha^–1). M. surmuletus, L. mixtus and C. conger were the least abundant species (from 0.3±2.7 N ha^–1 to 1.0±7.2 N ha^–1, Table 1).

In the same time and area, 27 spearfishing competitions were held, including a Spanish competition with many experienced spear fishers (Fig. 3). In these competitions, the spear fishers caught 2.39 t of fish (on average 88.4±120.1 kg per competition). On average, 17.2±10.7 spear fishers participated in each competition and obtained 1.0±0.7 kg h^–1 of fish per spear fisher.

Neither the overall density nor the density of each target species, analysed separately, varied significantly with the time that had elapsed since the last competition held in the zone. However, the densities of L. bergylta were negatively influenced by the weight of the catches taken in the competitions (Table 3), decreasing significantly after the competitions (p<0.001). Furthermore, the highest decreases (83.0%) took place after the competitions with larger catches (Fig. 4).

Based on the information obtained in the questionnaires answered by spear fishers (N=54), the annual fishing effort was on average 307.6±134.6 h spearfisher^–1. This fishery showed large seasonal variations: the highest effort was exerted during the summer (149.7±81.1 h spear-fisher^–1), matching the highest CPUE in the competitions (1.8 kg spear-fisher^–1 h^–1). In winter, the spear fishers did not fish for many hours (39.8±30.1 h spear-fisher^–1) and they were less effective (0.98 kg spear-fisher^–1 h^–1, Fig. 5).

Most of the spear fishers participated in competitions (57% of total). They showed a higher annual fishing effort (365.7±135.7 h spear-fisher^–1) than the spear fishers that did not participate in competitions (229.2±85.5 h spear-fisher^–1). They also distributed their effort more uniformly throughout the year (Fig. 5).

The questionnaire was answered by 6.2% of the spear fishers that participate in competitions in Galicia (approximately 500), but spear fishers who did not participate accounted for only 0.5% of the sampling universe. Thus, the annual recreational catch (during 2007) was exclusively estimated for competition spear fishers. Therefore, combining the seasonal fishing effort obtained in the questionnaires with the seasonal average CPUE obtained from the FEGAS archive, their annual recreational catch was 295 t (Fig. 5). On the other hand, if the annual catch taken during competitions was added, their overall catch would be 297 t (Table 1).

The recreational catches of competitive spear fishers accounted for 2.7% of the combined annual catch of recreational and commercial fisheries operating in coastal waters on the 29 species common to both fisheries (Xunta de Galicia 2015, the official fisheries statistics are available at http://www.pescadegalicia.com). However, competitive spear fishers were very selective, concentrating 95.0% of their catches on six of their targeted species: L. bergylta, Dicentrarchus labrax (L.), D. annularis, D. sargus, D. vulgaris and B. capriscus. Thus, recreational catches on L. bergylta (222 t), the most important species for this fishery (74.7% of total), almost equalled commercial catches (266 t). Moreover, recreational catches on Diplodus cervinus (Lowe), Seriola dumerili (Risso), B. capriscus and Muraena helena (L.) were higher than commercials. In addition, spear fishers obtained a large proportion of catches of D. labrax and D. sargus, species with high commercial values (6.6% and 3.0%, respectively, Table 1).

The mean intrinsic vulnerability to fishing of the fish species caught by the recreational spear fishery in Galicia is low (36.7), ranging from low (D. annularis vulnerability is 21) to very high (C. conger is 86). Furthermore, less than a third of the species targeted by spear fishers (24.1%) are highly to very highly vulnerable to fishing pressure. Moreover, with the exception of D. labrax, with moderate vulnerability (42), the most caught species show low vulnerabilities (Table 1).

The mean vulnerability of the commercial catch on the species common to both fisheries is moderate (56.5). However, it should be noted that C. conger, the most caught species (33.9% of the commercial catch) is the only species that shows very high vulnerability (Table 1).