Istiophorid Billfishes Research Articles

Genomic data resolve long-standing uncertainty by distinguishing white marlin (Kajikia albida) and striped marlin (K. audax) as separate species

Abstract Large pelagic fishes are often broadly distributed and capable of long-distance movements. These factors can promote gene flow that makes it difficult to disentangle intra- vs. inter-specific levels of genetic differentiation. Here, we assess the relationship of two istiophorid billfishes, white marlin (Kajikia albida) and striped marlin (K. audax), presently considered sister species inhabiting separate ocean basins. Previous studies report levels of genetic differentiation between these species that are smaller than those observed among populations of other istiophorid species. To determine whether white marlin and striped marlin comprise separate species or populations of a single globally distributed species, we surveyed 2520 single nucleotide polymorphisms (SNPs) in 62 white marlin and 242 striped marlin across the Atlantic, Pacific, and Indian oceans. Multivariate analyses resolved white marlin and striped marlin as distinct groups, and a species tree composed of separate lineages was strongly supported over a single lineage tree. Genetic differentiation between white marlin and striped marlin (FST = 0.5384) was also substantially larger than between populations of striped marlin (FST = 0.0192–0.0840), and we identified SNPs that allow unambiguous species identification. Our findings indicate that white marlin and striped marlin comprise separate species, which we estimate diverged at approximately 2.38 Mya.

Post release survival of swordfish (Xiphias gladius) caught by a recreational fishery in temperate waters

Swordfish (Xiphias gladius) are large, economically and ecologically important predatory fish with a wide circumglobal distribution. While swordfish are coveted by anglers, development of recreational fisheries has been limited historically, due in part to the species’ tendency to migrate into the mesopelagic zone during the day, effectively out of reach of typical game fishing methods. Recently, however, the adoption of ‘deep-dropping’, targeting swordfish during daytime with baits at > 300 m depth, has expanded access to the fish and led to the emergence of new regional fisheries. In 2014 a deep-dropping recreational swordfish fishery emerged in temperate southeast Australia and attracted international attention after yielding several swordfish line-class weight records. However, information is needed to guide best practices for emerging deep-dropping swordfish fisheries. Here, we present the first assessment of capture-related morbidity and post-release survival of swordfish caught by deep-dropping in southeast Australia. Among swordfish assessed to be in suitable condition for release, affixed pop-up satellite archival tags indicated 85.6% (57.8 – 95.7%; n = 13) survived after release. Including swordfish assessed as moribund after attempted resuscitation as mortalities, the survival rate for landed swordfish was 44% (95% CI 25.1 – 64.8%; n = 25). Severe abdominal distension (a notably protruding abdomen presumably due to swim bladder overinflation) and observed internal hooking injury (in the gut or gill area) were strong predictors of reduced survival (odds ratios 0.008 and 0.015 respectively), while angling duration and fish weight did not have a discernible effect on mortality. While the swordfish fishery is superficially like more common istiophorid billfish game fishing, the unique physiology and behaviour of swordfish and depths at which they are targeted present a unique challenge for stewardship as the typical catch-and-release billfish game fishing ethos may not be appropriate. The results of this preliminary study suggest swordfish caught deep-dropping are a poor candidate for purely catch-and-release angling. Predictors of post-release mortality are readily observable, so fishers should be prepared to humanely dispatch fish exhibiting symptoms of injury from gill or gut hooking, or abdominal distension severe enough to prevent the fish from righting itself beyond a brief resuscitation period.

High-resolution post-release behaviour and recovery periods of two highly prized recreational sportfish: the blue marlin and sailfish

Abstract High recreational catch rates of istiophorid billfishes in the eastern tropical Pacific (ETP) have led to substantial eco-tourism derived economic benefits for the countries in the region, prompting many countries to mandate catch-and-release practices for recreational anglers. Previous estimates of billfish post-release behaviours and recovery periods after these physiologically stressful capture events, however, vary widely depending on the type of tag used. Using high-resolution, multi-sensor biologging tags, we provide a fine-scale, detailed view of the behaviour and recovery periods of blue marlin (Makaira nigricans; n = 9) and sailfish (Istiophorus platypterus, Istiophoridae; n = 9) caught in a typical recreational fishery in the ETP. Angling times ranged from 4 to 90 min, and fish were monitored for periods of 6–70 h after release. Blue marlin showed a characteristic long, deep dive immediately after release, with significantly greater duration associated with longer fight times, a behaviour not typical for sailfish. Diving depths were, however, much shallower than those previously reported for both species due to the shallow thermocline and oxycline present in the ETP. Data from 40 derived metrics from acceleration (i.e. tailbeat period, amplitude, pitch, etc.) and physical parameters (i.e. depth, speed, temperature, oxygen saturation, etc.) used to quantify a recovery period suggest blue marlin and sailfish recover 9.0 ± 3.2 and 4.9 ± 2.8 h after release, respectively. Our high-resolution assessment of post-release behaviour suggests that these billfish are capable of rapid physiological recovery after capture in recreational fisheries, and that catch-and-release practices like those used here can be an effective approach to conserve and sustain billfish populations in the ETP. Predicted climate change caused shallowing of the oxygen minimum zone, however, would increase the vertical habitat compression present in this region, potentially prolonging or inhibiting recovery.

Fossil evidence for earlier radiation in istiophorid billfishes (Teleostei, Istiophoriformes) uncovered by comparative morphology of the caudal vertebrae

ABSTRACT Billfishes include some of the largest pelagic teleostean species, but several aspects about their morphology, paleobiology, and evolution remain ambiguous. Their fossil record is fragmentary and mostly represented by rostral and skull remains. Here, we present a comparative study of the caudal vertebral morphology of extant istiophorid species and we use this information to describe two fossil vertebrae from the Gatun and Chagres formations, both from the late Miocene of Panama. The caudal vertebra from the Gatun Fm. is characterized by the presence of a lateral apophysis and identified as Makaira sp., while the vertebra from the Río Indio Member of Chagres Fm. lacks this structure and its morphology indicates a different genus. The estimated total length of the Makaira sp. specimen from Gatun Fm. is about 5.18 m, the largest size calculated for a marlin, while the Rio Indio specimen was about 2.56 m long. Phylogenies show that the radiation of most living istiophorid species seemingly occurred during the Pliocene. However, our results highlight that the genus Makaira and the taxa lacking the lateral apophyses occurred during the late Miocene, showing that the radiation of istiophorids most likely occurred before the Pliocene (ca. 5 Ma).

Current status and potential contributions of fisheries statistics from artisanal fisheries for managing juvenile istiophorid billfishes in Southern Brazil.

Billfishes are considered important fishery resources and the identification of aggregation sites is imperative for proper management. Here we present evidence of a seasonal aggregation site for juvenile istiophorid billfishes in southern Brazil. We discuss the results as they relate with the need for management at a local scale and participatory monitoring with artisanal fishing communities as a way to ensure access to data on the occurrence and population status in the long term.

Effect of pelagic longline bait type on species selectivity: a global synthesis of evidence

Fisheries can profoundly affect bycatch species with ‘slow’ life history traits. Managing bait type offers one tool to control species selectivity. Different species and sizes of marine predators have different prey, and hence bait, preferences. This preference is a function of a bait’s chemical, visual, acoustic and textural characteristics and size, and for seabirds the effect on hook sink rate is also important. We conducted a global meta-analysis of existing estimates of the relative risk of capture on different pelagic longline baits. We applied a Bayesian random effects meta-analytic regression modelling approach to estimate overall expected bait-specific catch rates. For blue shark and marine turtles, there were 34% (95% HDI: 4–59%) and 60% (95% HDI: 44–76%) significantly lower relative risks of capture on forage fish bait than squid bait, respectively. Overall estimates of bait-specific relative risk were not significantly different for seven other assessed taxa. The lack of a significant overall estimate of relative capture risk for pelagic shark species combined but significant effect for blue sharks suggests there is species-specific variability in bait-specific catch risk within this group. A qualitative literature review suggests that tunas and istiophorid billfishes may have higher catch rates on squid than fish bait, which conflicts with reducing marine turtle and blue shark catch rates. The findings from this synthesis of quantitative and qualitative evidence support identifying economically viable bycatch management measures with acceptable tradeoffs when multispecies conflicts are unavoidable, and highlight research priorities for global pelagic longline fisheries.

Bycatch mitigation of protected and threatened species in tuna purse seine and longline fisheries

Bycatch and mortality in fishing gear poses a conservation threat worldwide to many marine species. Resource managers and conservation scientists face challenges in identifying bycatch mitigation solutions that work for multiple taxa while maintaining acceptable levels of target fish catch. The most successful mitigation measures to address bycatch concerns are those that (1) minimize bycatch with limited or no impact on target species catch, (2) have been proven through at-sea experimental research, (3) are practical, affordable, and easy to use, and (4) do not risk the safety of the fishing vessel crew or the bycaught animals. We conducted a review of mitigation measures in fishing gears that target tuna and tuna-like species and that either prevent capture of non-target species in fishing gear or facilitate alive post-capture release, and evaluated these against 4 defined criteria: effective, proven, practical, and safe. This paper outlines the most effective bycatch mitigation measures, as based upon the best scientific information available, in commercial and artisanal pelagic longline and purse seine fisheries, specifically those that target tuna and tuna-like species. This review includes information on gear and operational changes to fishing practices that reduce bycatch for protected and threatened species across taxonomic groups, with a focus on cetaceans, sea turtles, seabirds, sharks, and istiophorid billfishes. The information provided can guide future research and management efforts in Regional Fisheries Management Organizations that are specific to tuna fishing and that aim to minimize impacts to protected and threatened species while maintaining viable commercial fisheries.

Meta‐analysis of post‐release fishing mortality in apex predatory pelagic sharks and white marlin

AbstractRobust assessments of the effects of fishing require accounting for components of fishing mortality, including post‐release fishing mortality (Fr). Random‐effects meta‐analysis synthesizedFrin seven pelagic shark species captured, tagged and released with 401 pop‐up satellite archival tags compiled from 33 studies and three gears (longline, purse‐seine, rod & reel). The majority ofFroutcomes occurred within days of release, and the summary effect size forFrwas 0.27 [95%CI: 0.19–0.36], ranging from a low pooled effect size of 0.17 for blue shark (Prionace glauca,Carcharhinidae) to 0.38 (silky shark,Carcharhinus falciformis,Carcharhinidae).Frrates in blue shark were consistent over dissimilar spatial and temporal scales, and results from earlier meta‐analysis were replicated, which is the most powerful way to authenticate results. Condition at tagging was a strong predictor, and dichotomized survival outcomes in silky shark and no sex‐, size‐, location‐ or gear‐specificFrrates were demonstrated. Meta‐analyses and sensitivity analyses indicated exposure to risk factors and conditions whilst caught on the gear probably had the largest explanatory effect onFr, rather than stressors incurred during handling and release. Records from 549 tagged istiophorid billfishes (six species, three gears, 43 studies) demonstrated they are more robust to stressors sustained during capture, handling and release than pelagic sharks. Findings from previous meta‐analysis onFrrates in white marlin (Kajikia albida,Istiophoridae) were replicated. SynthesizedFrrates enable prioritizing approaches to mitigate by‐catch fishing mortality, to improve the quality of stock and ecological risk assessments and to expand our knowledge of factors influencing trophic structure.

Movement ecology and stenothermy of satellite-tagged shortbill spearfish (Tetrapturus angustirostris)

The shortbill spearfish (Tetrapturus angustirostris) is an understudied, istiophorid billfish primarily encountered as bycatch in pelagic commercial fisheries of the Indo-Pacific. The species is listed as data-deficient, and little is known of its biology, ecology, and population structure or status. We assessed the species’ movement ecology and thermal niche with telemetry data from the first shortbill spearfishes ever outfitted with pop-up satellite archival transmitting tags (n = 3 with successfully transmitted data). Short (4–15 day) deployments offshore of the Island of Hawai’i revealed that spearfish primarily occupied the mixed layer, spending >90% of each 24-hr period between the surface and 100 m in water temperatures between 24–26 °C. These individuals consistently exhibited vertical activity at night regardless of the prevailing lunar phase. Nocturnal movements throughout the mixed layer may enable shortbill spearfish to forage on mesopelagic species undergoing diel vertical migration and reduce trophic niche overlap with primarily diurnal, pelagic species. The narrow thermal distribution of shortbill spearfish in this study, almost exclusively within 2 °C of sea surface temperature, suggests that they are more stenothermal than extra-generic istiophorid species.

Movement and thermal niche of the first satellite‐tagged Mediterranean spearfish (Tetrapturus belone)

AbstractThe Mediterranean spearfish (Tetrapturus belone) is one of the least‐studied istiophorid billfishes, with little known of its biology, ecology, and behavior. To assess the species’ movement and thermal niche, we analyzed telemetry data from, to our knowledge, the first and only Mediterranean spearfish ever outfitted with a pop‐up satellite archival transmitting tag. Throughout a 29‐day deployment during July and August 2015, the fish travelled in Italian waters of the Tyrrhenian and Ligurian Seas, spending on average 93% of each 24‐hr period above 30 m and exhibiting a diel activity pattern comprised of daytime vertical movement and nighttime near‐surface residency. The preferred thermal niche was 26–28°C, but the spearfish experienced temperatures as low as 14.2°C during descents. Vertical distribution was limited throughout the deployment with more time spent at depth in areas where the thermocline was comparatively deeper and weaker, consistent with habitat compression experienced by other billfishes.

Misidentification of istiophorid billfishes by fisheries observers raises uncertainty over stock status.

We investigated whether commercially landed black marlin Istiompax indica were being misidentified by fisheries observers operating throughout the Pacific Ocean. Of 83 samples reported by observers as I. indica, 77% were genetically identified to be blue marlin Makaira nigricans and 2% to be striped marlin Kajikia audax. The high rate of misidentification by observers places considerable uncertainty over historic catch ratios of Indo-Pacific marlin and stock assessments relying on the validity of these data.

Analysis of whole mitochondrial genome sequences increases phylogenetic resolution of istiophorid billfishes

The billfishes are a group of highly migratory fishes comprising two extant families. The family Xiphiidae is monotypic and contains only the broadbill swordfish (Xiphias gladius Linnaeus, 1758), while the nine-other species of billfishes constitute the family Istiophoridae. The outcomes of past phylogenetic investigations using partial nuclear and mitochondrial (mtDNA) sequences recommended a restructuring of Istiophoridae, by expanding the three existing genera into five: Makaira, Istiophorus, Kajikia, Istiompax, and Tetrapturus. However, several aspects of the Istiophoridae phylogeny remain unresolved, including the position of the genus Istiompax. In this investigation, we sequenced the whole mitogenomes for all istiophorid billfish species. Phylogenetic analyses supported the presence of five genera within Istiophoridae and resolved the placement of Istiompax as the sister taxon to Kajikia + Tetrapturus. Genetic identity between the striped marlin [Kajikia audax (Philippi, 1887)] and the white marlin [Kajikia albida (Poey, 1860)] was found to be higher than between Indo-Pacific and Atlantic clades of both the blue marlin (Makaira nigricans Lacepede, 1802) and sailfish [Istiophorus platypterus (Shaw, 1792)], suggesting recent isolation or ongoing genetic connectivity between K. audax and K. albida. An assessment of mtDNA gene regions identified NADH dehydrogenase subunit 4 (ND4) as best reflecting the phylogenetic reconstruction using the whole mitogenomes, while cytochrome b (Cytb) was the only gene region able to separate all species of billfishes in the present study. Our investigation increased the phylogenetic resolution of istiophorid billfishes and underscores the need for further investigations to fully resolve the complex genetic relationships within Tetrapturus and Kajikia.

Catch rate and at‐vessel mortality of circle hooks versus J‐hooks in pelagic longline fisheries: A global meta‐analysis

AbstractWe conducted a meta‐analysis of literature reporting on the use of circle hooks and J‐hooks in pelagic longline fisheries. Our study included more data than previous meta‐analyses of the effects of hook type, due to both a larger number of relevant studies available in recent years and a more general modelling approach. Data from 42 empirical studies were analysed using a random effects model to compare the effects of circle hooks and J‐hooks on catch rate (43 species) and at‐vessel mortality (31 species) of target and bycatch species. Catch rates with circle hooks were greater for 11 species, including four tuna species, six shark species and one Istiophorid billfish. Catch rates on circle hooks were lower for seven species, including two Istiophorid billfishes and two species of sea turtle. At‐vessel mortality was significantly lower with circle hooks in 12 species, including three tuna species, three Istiophorid billfishes, swordfish (Xiphias gladius) and three shark species. No species had significantly greater at‐vessel mortality when captured with a circle hook rather than a J‐hook. While our general approach increased model variability compared to more detailed studies, results were consistent with trends identified in previous studies that compared the catch rates and at‐vessel mortality (between hook types) for a number of species. Our results suggest that circle hooks can be a promising tool to reduce mortality of some bycatch species in pelagic longline fisheries, although the effects depend on the species and the metric (catch rate or at‐vessel mortality), emphasizing the need for fishery‐specific data in conservation and management decisions.

Genetic identification of istiophorid larvae from the Gulf of Mexico based on the analysis of mitochondrial DNA control region sequences.

Assigning relative importance of spawning and nursery habitats for threatened and endangered teleosts, such as those seen in the Gulf of Mexico (GoM), relies on the proper identification of the early life-history stages of the species of concern. Here, sequencing a portion of the mitochondrial DNA (mtDNA) control region (CR) I as barcodes is recommended to identify istiophorid (billfish) larvae in the Atlantic Ocean because of its high resolution and the intrinsic value of the levels of genetic variation that can be extracted from these data. The universality of the primers employed here demonstrates their utility for not only the positive identification of istiophorids in the GoM, but for any larval teleost occurring in areas recognized as larval hotspots worldwide.

Postrelease mortality in istiophorid billfish

Meta-analysis (inverse-variance, random-effects model) involving 46 studies was used to estimate the effect size of postrelease mortality (Fr) in six istiophorid billfish species (black marlin (Istiompax indica), blue marlin (Makaira nigricans), longbill spearfish (Tetrapturus pfluegeri), sailfish (Istiophorus platypterus), striped marlin (Kajikia audax), and white marlin (Kajikia albida)) following release from recreational, longline, and harpoon fishing gears. The studies involved 400 reporting pop-up satellite archival tags and 64 reporting acoustic (ultrasonic) tags. Despite fish being captured, tagged, and released under widely disparate conditions, locations, and gear types, Frwas homogeneous among species. Variability in Frwas principally due to random sampling error within studies with no evident patterns. Fifteen studies (33% of tags) indicated no mortality, and the overall summary effect size for Frwas 13.5% (95% CI: 10.3%–17.6%). Since the random-effects model decomposed to a fixed-effect model when the between-studies variance T2= 0.00, results were confirmed using exact nonparametric inferential tests and sensitivity analyses. Our results support earlier findings in the Atlantic and substantiate the majority of istiophorid billfish survive when released from recreational and longline fishing gear, clearly implying catch-and-release as a viable management option that permits fishing activity while protecting parental biomass and the fishery.

Seasonal movements and diving behaviour of black marlin (Istiompax indica) in the northwestern Pacific Ocean

Black marlin (Istiompax indica) is a highly migratory apex predator distributed widely throughout the Pacific and Indian Oceans, with seasonal densities occurring in the East and South China Seas and near Taiwan. The peak seasonal presence of black marlin in eastern Taiwan (September to November) has led to the development of a commercially important harpoon fishery that specifically targets this aggregation. The harpoon fishery provides relatively easy access to black marlin for tagging research to monitor their movement patterns with pop-up satellite archival tags (PSATs). Nine black marlin were tagged from February 2011 to March 2012 with PSATs that acquired depth, temperature and ambient light-level data. PSATs remained affixed for a median 95 days (average 132 days±38 SE, range: 10–360 days) which are the highest documented PSAT retention statistics for any istiophorid billfish. No post-release mortalities occurred in these data and the harpoon method of PSAT application, as well as an improved anchor, may promote longer retention times by tagging fish that are not subjected to stress or injury associated with prolonged fishing bouts or handling. Linear displacements ranged from 567 to 1226km from deployment to pop-up locations with speeds of 3.1–122.6kmday−1 (average: 6.4kmday−1, 0.3kmh−1). Most probable tracks calculated by the state-space Kalman filter, augmented with sea surface temperature (SST), suggested that black marlin undergo dichotomous seasonal movement patterns: fishes tagged in spring and summer moved in a northerly direction to the East China Sea and those tagged in winter moved in a southerly direction to the South China Sea. Estimated track models during March–April for tagged fish in southwestern Taiwan coincided with known spawning activity in this vicinity though the tracks and vertical data were not sufficiently detailed to discriminate this putative behaviour. Diving depths ranged from the surface to ∼258m and ambient water temperatures occupied ranged from 30.3°C to 14.5°C. The distribution of time spent at depth and temperature was significantly different between daytime and nighttime diving activity. Tagged black marlin spent the majority of daytime in the surface mixed-layer to ∼50m and exhibited occasional basking behaviour; and at nighttime, they were confined almost exclusively to the surface to ∼20m. Depth distribution appeared to be limited by an 8°C change in water temperature relative to the warmest water (ΔSST). In the context of regional fisheries management and stock assessments, our study reports the first tagging data of black marlin in Southeast Asia and it will be especially important to compare diving behaviour patterns with other specimens tagged using other fishing methods to discern differences. Like many previous electronic tagging studies on istiophorid billfish in the Pacific, the data suggest that the majority of habitat for black marlin in Taiwan may be adequately described by two dimensions (this species spends most of their time in the surface mixed-layer).

Challenges of estimating post-release mortality of istiophorid billfishes caught in the recreational fishery: A review

Istiophorid billfishes are targeted in recreational fisheries throughout the tropical and subtropical waters of the world's oceans. Over the past thirty years, changes in management regulations and increased angler conservation awareness have resulted in an increasing proportion of the billfish catch being released alive, and in many areas the fishery is principally catch-and-release. However, the fate of released fish, and thus the fishing mortality associated with the recreational fishery, is not well understood. Some insights into post-release mortality of billfishes caught in the recreational fishery have been gained from analyses of conventional tag data, hooking location information, and acoustic tracking data. Over the past 10 years pop-up satellite archival tags (PSATs) have been used to specifically estimate post-release mortality for a limited number billfish species. Extrapolation of the results of these limited studies throughout the various recreational billfish fisheries is not prudent due to potential influences of different species, areas, seasons, fishing styles, and hook types. The current high cost of PSATs presents a significant obstacle to evaluating the effect of these variables on rates of post-release survival. Nonetheless, the results of the few studies to date have had an important impact on fisheries management and the behavior of recreational anglers.

Population structure of istiophorid billfishes

The phylogeny of istiophorid billfishes was recently revised based on analyses of nuclear and mitochondrial DNA loci, and a variety of molecular genetic characters has been used to investigate population structuring within several species of billfish. Despite these efforts, the population structure of most istiophorid billfishes is not well understood. This paper reviews genetic insights into the phylogeny and population structure of the billfishes with an emphasis on recent studies. In general, the results of genetic studies indicate significant heterogeneity between Atlantic and Indo-Pacific samples of circumtropical species. Within ocean basins, levels of population structure range among the species from no statistically significant heterogeneity detected among samples to small, but statistically significant genetic divergence between geographically distant samples. The apparent high level of genetic connectivity within oceans for some species seems to contrast with inferences derived from studies employing conventional and satellite tags that suggest limited trans-equatorial or trans-oceanic movements. This disparity may result, in part, from the different time scales underpinning evolutionary and ecological population structure; however, the power of previous genetic studies to elucidate population structuring has been limited by opportunistic sampling. To effectively assess population structure within the highly migratory billfishes, future genetic research will not only require larger sample sizes, both in terms of individuals and molecular markers, but also the development of biologically meaningful sampling designs that incorporate information on the movement patterns and life histories of these pelagic fishes.

Sex ratio and reproductive condition of four istiophorid billfishes in tropical regions of the eastern North Pacific Ocean: with special reference to striped marlin Kajikia audax (Philippi, 1887)

Summary Sexual differences in the distribution pattern, sex ratio and the gonad condition of four istiophorid billfishes were investigated in three different tropical areas of the eastern North Pacific Ocean from September to November 2004. Sex ratios of striped marlin Kajikia audax were equal (female : male = 11 : 11) in the open-ocean area (16–18°N, 118–134°W), biased to males (32 : 62) in the near-continental area (13–16°N, 103–107°W), and biased to females (19 : 6) in the near-equatorial area (5°N, 104–120°W). Sex ratios of sailfish Istiophorus platypterus in the near-continental area and shortbill spearfish Tetrapturus angustirostris in the open-ocean area were not biased from 1 : 1 (43 : 36, 11 : 13), and that of blue marlin Makaira nigricans in the near-equatorial area was biased to females (14 : 1). Reproductively active females were found for striped marlin, sailfish and shortbill spearfish. Striped marlin showed a specific spawning area (the near-continental area), and sexual dimorphism in its temporal distribution pattern, which was presumably related to reproduction. On the other hand, the limited distribution with evidence from spawning of sailfish and shortbill spearfish imply that these species complete their life history within limited areas.

Ocean scale hypoxia‐based habitat compression of Atlantic istiophorid billfishes

AbstractOxygen minimum zones (OMZs) below near‐surface optimums in the eastern tropical seas are among the largest contiguous areas of naturally occurring hypoxia in the world oceans, and are predicted to expand and shoal with global warming. In the eastern tropical Pacific (ETP), the surface mixed layer is defined by a shallow thermocline above a barrier of cold hypoxic water, where dissolved oxygen levels are ≤3.5 mL L−1. This thermocline (∼25–50 m) constitutes a lower hypoxic habitat boundary for high oxygen demand tropical pelagic billfish and tunas (i.e., habitat compression). To evaluate similar oceanographic conditions found in the eastern tropical Atlantic (ETA), we compared vertical habitat use of 32 sailfish (Istiophorus platypterus) and 47 blue marlin (Makaira nigricans) monitored with pop‐up satellite archival tags in the ETA and western North Atlantic (WNA). Both species spent significantly greater proportions of their time in near‐surface waters when inside the ETA than when in the WNA. We contend that the near‐surface density of billfish and tunas increases as a consequence of the ETA OMZ, therefore increasing their vulnerability to overexploitation by surface gears. Because the ETA OMZ encompasses nearly all Atlantic equatorial waters, the potential impacts of overexploitation are a concern. Considering the obvious differences in catchability inside and outside the compression zones, it seems essential to standardize these catch rates separately to minimize inaccuracies in stock assessments for these species. This is especially true in light of global warming, which will likely exacerbate future compression impacts.