1. Introduction
Blue marlin (
Makaira nigricans, Lacepède, 1802) is an epipelagic oceanic species distributed throughout tropical regions, which spends most of its time near the surface at night and at greater depths (25–100 m) during the day [
1]. The blue marlin is the largest species of the family Istiophoridae, reaching weights of up to 625 kg [
2], with a tropical and temperate distribution throughout the world [
3]. Its latitudinal range in the Atlantic extends from about 45° N to about 35° S [
4]. Its total distribution is generally associated with sea surface temperature (SST) isotherms >24 °C [
5]. Even so, in the Atlantic Ocean, adults are commonly found in waters with SST ranging between 22 and 31 °C [
6,
7]. Although this species has a low market value, catches are not negligible, especially due to the frequent accidental catches by pelagic longliners targeting tuna (
Thunnus spp.) and swordfish (
Xiphias gladius Linnaeus, 1758). That said, small catches are also made in coastal fisheries that use gillnets, harpoons, and purseseines [
8]. This phenomenon of accidental bycatch has led to a large decline in blue marlin stocks [
9]. Standardized catch per unit effort (CPUE) indices of abundance for blue marlin depict a monotonically decreasing trend from the early 1960s to the early 2000s [
10]. In the Atlantic, billfish landings represent only 0.76 percent of the combined tunas, swordfish, and billfish species which makes the collection of billfish stock assessment data through formal fishery statistical systems challenging [
10]. Landings of Atlantic blue marlin fluctuated between 3000 and 4000 tonnes during the 2000s, most of them originating from longline operations and gillnetting [
10]. Food and Agriculture Organization (FAO, Rome, Italy) describes significant uncertainties in the state of their exploitation that represent a serious concern. In the Atlantic, blue marlins seem to be overexploited even though they are not generally targeted, while in the eastern Pacific they are fully exploited [
11]. The scarcity of available information has motivated research with the goal of better understanding the biology and conservation of this species [
4,
9].
In addition to being a frequent bycatch species in commercial fisheries, blue marlin is the target of a worldwide recreational fishing industry based on the capture of large pelagics [
10,
12]. This activity is known as “big game fishing,” in which fishers troll with live bait or with jigs to catch a specimen and, in case of hooking, the objective is to draw the catch close to the boat for subsequent release [
2]. Blue marlin, which performs spectacular acrobatics when caught on rod and reel, is a very attractive catch for recreational fishers, making it one of the most important game fish species in the world [
2,
4]. The catch-and-release (C&R) approach to marlin fishery does not allow for the collection of extensive information about the species. A further consequence of this practice is that fish sometimes require resuscitation prior to release or may die during the struggle [
12,
13]. Anglers′ experience differs greatly depending on their handling skill level and C&R behaviors, which influence the short- and long-term physiological consequences for angled fish and, in turn, determine their survival outcomes [
14]. Furthermore, marlins are sometimes landed for measuring when there is a possibility of breaking a record, a situation that disproportionately affects larger specimens.
Other billfishes, mainly the white marlin (Tetrapturus albidus; Poey, 1860), are also sometimes captured. Additionally, some other species, such as wahoos (Acanthocybium solandri, Cuvier, 1831), dolphinfishes (Coryphaena hippurus, Linnaeus, 1758 and Coryphaena equiselis, Linnaeus, 1758), and various species of tuna (mainly bigeye tuna—Thunnus obesus, Lowe, 1839; albacore—Thunnus alalunga, Bonnaterre, 1788, and skipjack—Katsuwonus pelamis, Linnaeus, 1758), are landed for their high gastronomic value.
In the Macaronesian region (eastern Atlantic), some fishing companies and many amateurs practice this type of fishing [
15,
16]. In Madeira, the good weather and the proximity of fishing grounds attract many fishers during the high season, which usually lasts from May to September [
5,
17,
18]. Most vessels fish in this specific season, and, when it is over, the boats are usually taken to dry dock for repairs until the following year. Due to the year-long good weather conditions, some companies extend their activity over almost the entire year, targeting other large fish when marlins are scarce. The presence of blue marlin varies seasonally, and their density is influenced by the interannual variability of oceanographic and environmental factors [
8,
19].
There are significant knowledge gaps regarding this type of fishing, given the effort required, the seasonality, the fleet, and the C&R approach. The aim of the present study was to investigate the state of big game fishing in Madeira and how it affects the population of blue marlin. The specific objectives were to analyze blue marlin captures in the region to know their seasonal and annual variation, the average weight of the individuals in the region and the dynamics of blue marlin in Macaronesian archipelagos, and to evaluate how environmental factors can affect the presence of blue marlin.
4. Discussion
Blue marlin captures registered from 2008 to 2019 showed considerable variability between years, with a high number of individuals being caught in the early years of the period, followed by a strong decrease and far fewer individuals being caught. Finally, in the last year, the numbers almost returned to those of the first year. Unfortunately, it was not possible to obtain the records of fishing trips and the numbers of boats involved for the first few years, and it was therefore not possible to make accurate comparisons. It was only possible to calculate the monthly CPUE from 2017 to 2019.
The obtained CPUE results seem to agree with the basic features of recreational fishing: high fishing effort and low catch rates [
27]. As observed, the probability of catching a blue marlin is quite variable. This is supported by the differences in CPUE obtained for the different boats and their variations from year to year, including the high CPUE value of 0.062 obtained in 2008 for the region by Graça [
13]. This higher value observed in 2008 corresponded to a high number of captures of blue marlin; however, the higher CPUE could also be related to the low number of boats (five) analyzed in that study.
Some previous studies of the catch rates and effort of recreational fisheries also registered a very variable CPUE in other areas of the Atlantic Ocean, such as in the Azores [
28], the Maryland coast [
29], and in the Gulf of Mexico [
30,
31,
32,
33]. These works confirm the high interannual variability in CPUE, which is probably related to the migratory behavior of this species, as blue marlin travel long distances in search of warmer waters and follow the currents, which can vary from year to year.
According to the latitude of each archipelago, the variability in seasonality allows us to infer the probable migratory route of this species in this geographical area. The results from the analysis of catches around different Macaronesian archipelagos suggest the presence of blue marlin in equatorial waters during the early winter and the northward migration in late winter and early spring, reaching Azorean waters in the summer. Therefore, we can assume that as the water temperature rises over the course of the year, this species begins its migration to northern latitudes, probably for reproductive purposes. Our results are consistent with those of previous studies in which similar blue marlin migration patterns were observed in other parts of the world, such as in the Pacific (indicating that this species migrated northward during April–October, and south thereafter [
8]) and in the southern Atlantic region, where blue marlins were present in the south during the austral summer time and then moved north toward warmer equatorial latitudes during the winter [
34,
35,
36].
Recently, several studies on this species have been carried out using pop-up satellite tags [
1,
12,
34,
36,
37], which provide data on movements, distribution, and post-release survival in different areas of the world. The results for this geographic area are currently being analyzed (Freitas et al., unpublished results), and this will allow us to confirm whether the migration movements suggested in this work are correct.
Analysis of blue marlin seasonality in Madeira showed that they always arrive at the same time of the year (end of May, beginning of June), when the waters become warm (19–20 °C). The SST where blue marlins were caught ranged from 18.5 to 24.5 °C. This is similar to the range observed along the south coast of Portugal, where captures were registered between 18.6 and 25.5 °C [
38], but lower than that registered by Crespo [
36] for the Southwest Atlantic. In that region, an SST range between 24 and 29 °C was observed for 90–92% of the cases, and blue marlins moved southward off the Brazilian coast in order to spawn following the displacement of the 25 °C isotherm in the summer [
35].
The results from Madeira reflect that in June, when the catches were most abundant, the average temperature was 20.7 °C, while in September the water reached the highest temperature (with an average of 23.7 °C) but blue marlins were sparse. These data indicate that blue marlins seem to prefer warm waters, but it is not necessarily the case that there is a greater abundance of specimens when the water is warmer. Therefore, this association is probably due to the greater abundance of this species in the summer months, which is when the water is warmer, and thus the probability of catching a specimen is higher.
The SST appears to be positively linked with catch rates of pelagic species in sport fishing in the Pacific Ocean [
39], but it has a relatively minor influence on the CPUE of the Brazilian longline fleet [
40], a detail that could be masked because marlin was considered a bycatch species in that study; direct fishing might yield different results. Additionally, Carlisle et al. [
1] suggested that the horizontal distribution of blue marlin in the central Pacific was influenced by SST and large-scale fluctuations thereof, in particular those associated with strong La Niña conditions, which might influence marlin migratory behavior. The eastern Pacific Ocean’s blue marlin population moves east during El Niño years, as evidenced by catch rates [
8]. This behavior is supported by studies [
41,
42] indicating that the distribution and movement patterns of tuna-like species may be strongly linked with environmental variation, such as El Niño–Southern Oscillation (ENSO) events and related changes in various oceanographic features. However, the present study was carried out in the Northern Hemisphere; therefore, the relation between blue marlin presence and NAO was evaluated instead, as it seemed likely that it would influence blue marlin catches.
NAO is considered the largest source of variability in climate oscillation affecting the North Atlantic region, redistributing air mass from the Arctic to the subtropical Atlantic [
43]. The variability introduced by the NAO affects the ocean by changing many parameters, varying the SST, the depth of the mixed ocean layer, the ocean heat content, sea ice cover, surface current circulation, the intensity and direction of the prevailing winds, and several meteorological phenomena such as rain and storms [
44,
45]. During positive NAO periods, the conditions are cooler and drier than average in the Northwest Atlantic and Mediterranean regions, while conditions in northern Europe, the eastern United States, and parts of Scandinavia become warmer and more humid than average [
44]. Some factors influenced by the NAO increase the presence of nutrients in the sea, thus altering the trophic levels of marine ecosystems and their exploitable resources [
45,
46]. This could affect the catch of large migratory pelagics, as westerly winds originating from a positive NAO could displace the schools eastward towards the European and African coasts, as suggested by Rubio et al. for two species of tuna [
45].
Additionally, higher catches were associated with low cloud cover. This result should be considered with caution, since we have to consider that the fishing season is concentrated in the summer months, and therefore will include few days with cloudy skies. In addition, fewer fishing trips were probably taken on such days due to the possibility of rain. A higher CPUE is associated with the presence of less cloudiness; therefore, the catchability increases with the level of light in the water. This may be related to the fact that blue marlins are visual predators [
47], and it is easier for them to see the lure on clear days. This is exploited by fishers to increase their chances of catching blue marlins, as they adapt their fishing equipment according to the weather conditions. On very calm days, the speed of trolling is higher and the line used is thinner in order to make it invisible, while in bad weather with higher waves the line tends to jump instead of sinking, so a thicker line with larger and heavier lures is used in order to make it sink more efficiently and be more visible to marlins (fisher’s comment).
Finally, it is interesting to note the possible effects of fishing practices on blue marlin populations. Because of the practice of C&R carried out by the big game fishing fleets, we could assume that this fishing practice might not have a significant negative impact on the target species. Until recently, there was an almost complete lack of knowledge on the effects of C&R on the survival of most fish species, but some studies have observed that the mortality of blue marlin subjected to C&R is low (89% survival after tagging [
12]), which suggests that C&R is a viable management option that protects populations [
48,
49,
50]. Nevertheless, it is still a practice that raises some concerns, as handling can cause great stress and lead to subsequent death for fish caught and then released. Various factors, such as hooking on internal organs, the removal of hooks from deeply hooked fish, the depth at which fish are caught, water temperature, and handling time, can all contribute significantly to mortality [
14]. In order to assess the real impact of this practice, it would be useful to carry out a study on the delayed mortality of blue marlin following a normal C&R protocol.
Some studies have shown that the most serious impact on blue marlin is made by longlines used for tuna and swordfish [
4,
29,
51], suggesting that longline fishing should be restricted in seasons and areas with high blue marlin CPUE [
52] so as to reduce the fishing mortality of this species. However, this management measure is difficult to apply in Madeira, since these fishery practices usually coincide in both season and fishing area. The immediate release of the fish after capture would help reduce mortality and increase the resilience of the populations [
29]. Pelagic longlines in Madeira are not very common; a pole and line with live bait is the most frequently used tuna fishing method in the region, while pelagic longlines directed at black scabbardfish are also employed, but these are placed deep in the water, and only deep-water species tend to be captured as bycatch or incidental species [
53,
54]. Therefore, these methods do not pose a significant risk to species such as marlin. This was confirmed by the infrequent landings of blue marlin registered as bycatch in the regional commercial fisheries, where only the equivalent of 1376.2 kg was landed between 2017 and 2019 (Regional Directorate of Fisheries), considerably less than the almost 7 tons (4.2% of the total catch) estimated for the recreational fishery in the same period. These results point to the relatively low mortality of blue marlin in the region, suggesting that regional fisheries may have an overall low impact on this species.
Average weight of blue marlin captured between 2010 and 2019, 290.5 kg (
n = 938), was very close to the value obtained by Graça [
13] for the same region (298.7 kg), and slightly higher than the 277 kg registered for southern Portugal [
38]. If we compare this with the average weights of blue marlin from other parts of the world, it is interesting to observe that larger specimens were caught in Madeira than in either the western Atlantic (with an average of 236.6 kg off the Maryland coast [
29] and 177 kg in the northern Gulf of Mexico [
30]) or the Pacific (with an average weight of 175 kg [
2] and 108.7 kg in Baja California [
32] or 155 kg in New South Wales [
55]).
For the eastern Atlantic, there is little information about this fishery in the Azores [
28] beyond the average weights of blue marlin landed in 1985 (157.3 kg) and 1986 (210 kg). The analysis performed for the Macaronesian region showed that the average weight in Madeira is similar to that in the other archipelagos, suggesting that the individuals captured could be from the same population.
The other Istiophoridae species captured was the white marlin, but fewer catches of this species were registered. This species normally arrives in Madeiran waters a little earlier than blue marlin, usually being captured between May and November, but there was no significant presence as bycatch until June to August period. Some other big pelagic fishes were captured included dolphinfish (with a peak in catches during July and August), some wahoos (mainly during the second half of the year, occurring more frequently from August to October), and different tuna species, the most frequently caught of which was bigeye tuna, which were found mainly from April to September.
Some of these other species captured by the big game fishing fleet (tuna, dolphinfish and wahoos) are usually landed, since many of them are highly valued as food. The results show that a relatively small number of fish were caught, but it would be interesting to study these catches and landed specimens. Only occasionally, as in 2018 with dolphinfish, were large numbers of individuals caught during the same fishing trip, due to small concentrations of this species being found below floating objects and following ships [
56]. It would be interesting to conduct a more detailed follow-up study including the systematic sampling and weighing of these other species in order to better understand certain characteristics that would help to assess the impact of this fishery practice on the target species.
Finally, it is important to highlight the problems associated with data acquisition in the present study, in light of the importance of reliable and consistent data collection for adequate analysis. Unfortunately, the authors were faced with many difficulties when compiling the information for this study. The initial aim was to register fishing trips and catches using data sheets, as this is considered one of the easiest and least expensive sampling methods [
57]. The problem is that their completion is perceived as demanding by fishers [
58], and as a result, there was little acceptance on the part of captains.
Additionally, some crews were unwilling to cooperate, and only agreed to participate after some insistence. It was necessary to establish periodic contact to share the data and build confidence over time, which facilitated the collection of more accurate information. Furthermore, it was difficult to obtain historical data for this fishery due to the absence of logbooks, and only catch registers recorded by one of the authors for personal use were available.
The data obtained in this study should therefore be used only as a very rough indicator of fish population size. In addition, abundance indices obtained from catches (such as CPUE) are difficult to interpret due to the limited understanding of how changes in fishing strategy interact with the behavior and distribution of marlin within the water column depending on the area and season [
59]. Furthermore, the absolute number of recorded fish may be affected by the different amount of effort expended in fishing, which will correlate with the number of boats and the hours trolled and which may vary from year to year.
In conclusion, this is the first study of the practice of big game fishing in Madeira, and it contributes to expanding our knowledge about the seasonal distribution of blue marlin. The presence of blue marlin in Madeiran waters is markedly seasonal, and is probably related to their migratory behavior and some of the environmental factors analyzed. Nevertheless, no determining factor has been found that can satisfactorily explain the greater catches of blue marlin in certain years. Therefore, more studies should be carried out, and a program for monitoring this activity should be implemented. This work shows the difficulties encountered in obtaining data from this fishery and the importance of consistently gathering data on catches over time. People practicing this fishing modality should be involved in these studies as stakeholders, collaborating with authorities and researchers in order to obtain accurate data. This would help in maintaining adequate records and monitoring how the fishery practice and target species evolve over time.