Billfish (diverse) | European Tracking Network

Billfish (diverse)

Image: George Schellinger for Large Pelagics Research Center ©


Species description and status: The term Billfish refers to a group of large-predatory fish characterised by prominent bills (long spears or sword-like upper beaks) used as predatory tools for slashing at and stunning schooling-preys[1]. Billfish include marlins, spearfishes, and sailfishes belongin to the family Istiophoridae, and swordfish, sole member of the family Xiphiidae[2]. Despite several species belonging to this group[2], in the present document, only billfish species with a potential distribution around European waters have been selected (Table 1 and Fig. 1).

These fast-growth pelagic species are highly migratory, exhibiting large-scale and trans-oceanic migrations[3]. Billfishes are among the pelagic apex predators exploited by commercial and recreational fisheries. Swordfish is mainly targeted at large- and small-scale by commercial fisheries around the world. Istiophorids are largely targeted by recreational fisheries with a profound economic impact[4], but by-catch from tuna and swordfish pelagic longline fisheries is the source of most fishing-induced mortality (> 90%[5]). These compound to result in overfishing of many billfish stocks, but stock status varies among and within species and ocean basins[6](see International Union for Conservation of Nature-IUCN status in Table 1).

Table 1. Billfish species ETN selected. IUCN status colours indicate: Least concern (green), not assessed (black), Vulnerable (orange) and data deficient (grey).


Figure 1. Selected Billfish distribution: A) the Swordfish Xiphias gladius, B) the Atlantic sailfish Istiophorus albicans, C) the Indo-Pacific sailfish Istiophorus platypterus, D) the Blue marlin Maikara nigricans, E) the Atlantic white marlin Tetrapturus albidus, F) the Roundscale spearfish Tetrapturus georgii and G) the Mediterranean spearfish Tetrapturus belone. Range colours indicate degree of habitat suitability which can be interpreted as probabilities of occurrence. Note the absence of Tetrapturus pfluegeri (given the scarce information about its distribution), a potential ETN-key Billfish species widely distributed in offshore waters of the Atlantic Ocean.


Regions of interest: Billfish are found in all oceans, although these species usually inhabit tropical and subtropical waters. Swordfish also inhabit  temperate waters  (see the distribution of selected billfishes in Table 1). Their spatial ecology is governed by four main physical variables: temperature, light, oxygen, and complex water mixing (e.g. fronts and eddies)[3]. These oceanic species show strong thermal preference for water >22°C (except swordfish, whose deepest vertical migrations can reach waters <10°C). Ambient light levels influence vertical movements, especially those associated with foraging. Mounting evidence suggests that some billfishes actively avoid regions with low dissolved oxygen (<3.5 ml l−1).

Knowledge gaps: Scientific advances on the biology and ecology of Billfish (excluding here swordfish) are constrained by their socio-economic importance (in comparison, for example, with tuna which supports an immensely important worldwide commercial fishery and this in turn results in higher levels of funding for research) and the available logistic facilities to manage these animals under controlled conditions[7]. In this sense, the information about spatial ecology of billfishes is scarce and scattered, even for some species and juvenile phase remain practically unknown. Therefore, regarding spatial ecology, a wide range of knowledge gaps emerge for most of Billfish species: i) identification of hotspots (spawning and feeding areas; ETN Key knowledge gap ii) and how climate change could spatial and temporal shift these essential areas (ETN Key knowledge gap 3 and 5).

Telemetry tools: Satellite and acoustic telemetry could be used to monitor these species. In fact, over 1,000 pop‐up satellite archival transmitting (PSAT) tags have been deployed on Billfish species (Fig. 2), demonstrating both trans-equatorial and trans-basin movements (large-scale horizontal distribution) and how environment play a key role on Billfish distribution and behaviour ecology[3]. However, for the most of these studies, the exceptionally high premature tag shedding rates has limited such information to days, or in the best of cases, some months[3,8].

Acoustic tracking have been successful applied to address short-term vertical distribution patterns as well as how environmental cues govern such spatial ecology (e.g., [9,10]). But, these studies were constrained at short-term (days), given these high-mobility species escape prematurely from the network of detection (e.g.,[9]).

Benefits within the ETN: The transition from local/regional telemetry studies towards a more efficient and integrated pan-European biotelemetry network, embedded in the international context promoted by the ETN, provides an exciting opportunity to address key knowledge gaps in these species through collaborative efforts. Key infrastructures proposed into ETN (e.g., Gibraltar Strait[12]) provide an suitable platform for long-term monitoring of Billfish species,  using the information from satellite tracking, but also providing essential information (e.g., mortality) for the sustainable management of these species[13].

Join the team and contact Dr. Miguel Cabanellas Reboredo (, Dr. Patricia Reglero Barón ( and Dr. Francisco Abascal ( from Spanish Institute of Oceanography (IEO) to be involved in this project/species and being involved in future project proposals to pan-European Calls to address these key knowledge gaps in Billfish.


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[2]Collette, B.B.; McDowell, J.R.; Graves, J.E. Phylogeny of recent billfishes (Xiphioidei). Bull. Mar. Sci. 2006, 79, 455–468.

[3]Braun, C.D.; Kaplan, M.B.; Horodysky, A.Z.; Llopiz, J.K. Satellite telemetry reveals physical processes driving billfish behavior. Anim. Biotelemetry 2015, 3, 2.

[4]Kitchell, J.F.; Martell, S.J.D.; Walters, C.J.; Jensen, O.P.; Kaplan, I.C.; Watters, J.; Essington, T.E.; Boggs, C.H. Billfishes in an ecosystem context. Bull. Mar. Sci. 2006, 79, 669–682.

[5]Cox, S.P.; Essington, T.E.; Kitchell, J.F.; Martell, S.J.D.; Walters, C.J.; Boggs, C.; Kaplan, I. Reconstructing ecosystem dynamics in the central Pacific Ocean, 1952-1998. II. A preliminary assessment of the trophic impacts of fishing and effects on tuna dynamics. Can. J. Fish. Aquat. Sci. 2002, 59, 1736–1747.

[6]Pons, M.; Branch, T.A.; Melnychuk, M.C.; Jensen, O.P.; Brodziak, J.; Fromentin, J.M.; Harley, S.J.; Haynie, A.C.; Kell, L.T.; Maunder, M.N.; et al. Effects of biological, economic and management factors on tuna and billfish stock status. Fish Fish. 2017, 18, 1–21.

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[8]Domeier, M.L.; Ortega-Garcia, S.; Nasby-Lucas, N.; Offield, P. First marlin archival tagging study suggests new direction for research. Mar. Freshw. Res. 2019, 70, 603–608.

[9]Block, B.A.; Booth, D.T.; Carey, F.G. Depth and temperature of the blue marlin, Makaira nigricans, observed by acoustic telemetry. Mar. Biol. 1992, 114, 175–183.

[10]Brill, R.W.; Holts, D.B.; Chang, R.K.C.; Sullivan, S.; Dewar, H.; Carey, F.G. Vertical and horizontal movements of striped marlin (Tetrapturus audax) near the Hawaiian Islands, determined by ultrasonic telemetry, with simultaneous measurement of oceanic currents. Mar. Biol. 1993, 117, 567–574.

[11]Arostegui, M.C.; Braun, C.D.; Gaube, P. Movement and thermal niche of the first satellite-tagged Mediterranean spearfish (Tetrapturus belone). Fish. Oceanogr. 2019, 28, 327–333.

[12]Abecasis, D.; Steckenreuter, A.; Reubens, J.; Aarestrup, K.; Alós, J.; Badalamenti, F.; Bajona, L.; Boylan, P.; Deneudt, K.; Greenberg, L.; et al. A review of acoustic telemetry in Europe and the need for a regional aquatic telemetry network. Anim. Biotelemetry 2018, 6, 12.

[13]Punt, A.E.; Su, N.-J.; Sun, C.-L. Assessing billfish stocks: A review of current methods and some future directions. Fish. Res. 2015, 166, 103–118.